[Federal Register Volume 65, Number 214 (Friday, November 3, 2000)]
[Notices]
[Pages 66444-66482]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 00-27924]



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Part III





Environmental Protection Agency





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Revisions to the Methodology for Deriving Ambient Water Quality 
Criteria for the Protection of Human Health (2000); Notice

  Federal Register / Vol. 65, No. 214 / Friday, November 3, 2000 / 
Notices  

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ENVIRONMENTAL PROTECTION AGENCY

[WH-FRL-6893-6]


Revisions to the Methodology for Deriving Ambient Water Quality 
Criteria for the Protection of Human Health (2000)

AGENCY: Environmental Protection Agency (EPA).

ACTION: Notice of Availability.

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SUMMARY: EPA is announcing the availability of final revisions to the 
Methodology for Deriving Ambient Water Quality Criteria for the 
Protection of Human Health (2000) (hereafter ``2000 Human Health 
Methodology'') published pursuant to section 304(a)(1) of the Clean 
Water Act (CWA). The 2000 Human Health Methodology supersedes the 
existing Guidelines and Methodology Used in the Preparation of Health 
Effect Assessment Chapters of the Consent Decree Water Criteria 
Documents, published by EPA in November 1980 (USEPA, 1980) (hereafter 
``1980 AWQC National Guidelines'' or ``1980 Methodology''). Today's 
Notice is intended to support the requirements of section 304(a)(1) of 
the CWA that EPA periodically revise criteria for water quality to 
accurately reflect the latest scientific knowledge on the kind and 
extent of all identifiable effects on health and welfare that may be 
expected from the presence of pollutants in any body of water, 
including ground water. These revisions are prompted by the many 
significant scientific advances that have occurred during the past 20 
years in such key areas as cancer and noncancer risk assessments, 
exposure assessments, and bioaccumulation assessments. These revisions 
are not regulations and do not impose legally-binding requirements on 
EPA, States, Tribes, or the public.

DATES: Technical Support Documents (TSD) on exposure assessment 
guidance and bioaccumulation guidance applicable to the 2000 Human 
Health Methodology are expected to become available early in calendar 
year 2001.

ADDRESSES: The 2000 Human Health Methodology is published in the 
document entitled, Methodology for Deriving Ambient Water Quality 
Criteria for the Protection of Human Health (2000). This document is 
available on the EPA website at www.epa.gov/OST/humanhealth. A 
Technical Support Document (TSD) volume on risk assessments applicable 
to the 2000 Human Health Methodology is also available from the 
website. Materials in the public docket will be available for public 
inspection and copying during normal business hours at the Office of 
Water Docket, 401 M St., SW, Washington, DC 20460 by appointment only. 
Appointments may be made by calling (202) 260-3027 and requesting item 
W-97-20. A reasonable fee will be charged for photocopies.

FOR FURTHER INFORMATION CONTACT: Denis R. Borum, Health and Ecological 
Criteria Division (4304), U.S. EPA, Ariel Rios Building, 1200 
Pennsylvania Avenue, NW, Washington, DC 20460; (202) 260-8996; 
[email protected].

SUPPLEMENTARY INFORMATION: This Supplementary Information Section is 
organized as follows:

I. Background Information
    A. What are human health ambient water quality criteria?
    B. How is the Human Health Methodology used?
    C. Why was the Methodology revised?
    D. What specific scientific advances have occurred since 1980?
    E. What process did EPA follow in revising the Methodology?
    F. What are the major revisions to the Methodology?
    G. How will EPA use the Human Health Methodology?
II. Implementation Issues
    A. How does EPA use its recommended 304(a) water quality 
criteria?
    B. What water quality criteria must a State or authorized Tribe 
adopt into its water quality standards?
    C. May States and authorized Tribes adopt water quality criteria 
based on local conditions?
    D. What cancer risk level should States and authorized Tribes 
use when establishing water quality criteria?
    E. How does the Review and Approval of State and Tribal Water 
Quality Standards rule affect water quality criteria adopted by 
States and authorized Tribes?
    F. While EPA is re-evaluating a 304(a) criterion, what criterion 
is in effect?
    G. What design stream flow should be used to implement human 
health criteria?
    H. What is the relationship between the Agency's recommended 
Section 304(a) water quality criteria and drinking water standards?
    I. How are health risks to children considered in the 
Methodology?
III. Summary of Comments Received on the 1998 Draft Methodology 
Revisions and EPA's Responses
    A. Implementation
    1. Application of Human Health Criteria Within Mixing Zones
    2. Application of Human Health Water Quality Criteria to Marine 
Waters
    3. Cancer Risk Range
    4. Coordinating the Human Health Methodology With Other EPA 
Programs
    5. Designated Uses
    6. Developing National 304(a) Criteria
    7. Developing Organoleptic Criteria
    8. Establishing EPA's Most Recent Federally Recommended Water 
Quality Criteria
    9. Flows
    10. Implementation on a Waterbody Basis
    11. Proposed Chemical List
    12. Publishing Existing 304(a) Criteria Information
    13. Revising Existing 304(a) Criteria
    14. State Evaluation of Data Supporting Criteria
    15. Streamlined Approach to Developing Criteria Documents
    16. Treaty Rights and Trust Obligations/Government-to-Government 
Relations
    B. General Policy
    1. AWQC Derivation Equation Errors
    2. Chronic Human Health Effects Assumption
    3. Protectiveness of the Methodology
    4. Setting Criteria to Protect Both Fish and Drinking Water 
Versus Fish Only
    5. Setting Criteria to Protect Against Multiple Exposures From 
Multiple Chemicals
    6. Uncertainty with the Derivation of 304(a) Criteria
    7. Toxicity Equivalency Factors (TEFs) for Dioxin-like Compounds
    C. Cancer
    1. Acceptable Risk Level for Carcinogens
    2. ED10 (central estimate) versus LED10 (lower bound on dose)
    3. Group C Contaminants
    4. Guidance on Carcinogen Risk Assessment
    5. Hexachlorobutadiene (HCBD)
    6. Integration of Analyses for Cancer and Noncancer Effects
    7. Margin of Exposure (MOE) Analysis
    8. MOE Approach to Applying Uncertainty Factors (UFs)
    9. MOE and MOP
    10. Oral Scaling Factor for Dose Adjustment
    11. Toxic Endpoints
    12. Weight-of-Evidence Narrative and Classification System
    D. Noncancer
    1. Benchmark Dose Methodology
    2. Categorical Regression
    3. Integrated Approach
    4. Integrated Risk Information System (IRIS)
    5. NOAEL/LOAEL Approach
    6. Nonthreshold Approach for Noncarcinogens
    7. RfD Range
    8. Severity of Effects
    9. Stochastic Modeling
    10. Synergistic Effects
    11. Target Population Adjustments
    12. Uncertainty and Modifying Factors
    13. Use of Less-Than-90-Day Studies in Determining an RfD
    E. Exposure Assessment

Default Intakes

    1. Assumption That All of the Drinking Water Consumed Is 
Contaminated at the Criteria Level
    2. Assumption That All Fish Consumed Is Contaminated at the 
Criteria Level and All Fish May Come from One Waterbody
    3. Body Weight Assumptions
    4. Combining Consumption Intakes and Body Weights
    5. Combining Fish Intake and Body Weights

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    6. Default Drinking Water Intake Rates
    7. Default Fish Intake Rates
    8. Effect of Cooking on the Contaminant Concentration
    9. Inclusion of Marine Species in the Default Rate
    10. Precision of the Drinking Water Parameter
    11. Redesignation of Salmon as a Marine Species
    12. Studies on Sportfishers and Subsistence Fishers
    13. USDA Continuing Survey of Food Intake by Individuals (CSFII)
    14. Use of Uncooked or As Consumed Fish Weight for Default 
Intake Rates

Relative Source Contribution (RSC)

    15. Default Percentages and RSC Floor of 20% and Ceiling of 80%
    16. Duplication of Fish Intake Assumptions
    17. Exposure Route Differences
    18. Need for an RSC Factor/Considering Multiple Routes of 
Exposure
    19. Use of RSC With Carcinogenic Effects Based on Linear Low-
Dose Extrapolation
    20. Use of Subtraction or Percentage Methods in RSC 
Apportionment
    F. Bioaccumulation
    1. Use of Bioaccumulation Factors (BAFs) in General
    2. Guidance for Deriving Field Bioaccumulation Factors (BAFs)
    3. Use of Biota-Sediment Accumulation Factors (BSAFs)
    4. Dissolved Organic Carbon (DOC) and Particulate Orgain Carbon 
(POC)
    5. Fish Lipid Content
    6. Use of Food Chain Multipliers (FCMs)
    7. Fish Tissue Criteria
    G. Literature Cited

I. Background Information

A. What are Human Health Ambient Water Quality Criteria?

    Human health ambient water quality criteria (AWQC) are numeric 
values for pollutant concentrations in ambient waters considered to be 
protective of human health. The criteria are developed under section 
304(a) of the Clean Water Act (CWA) and are based solely on data and 
scientific judgments on the relationship between pollutant 
concentrations and environmental and human health effects. Protective 
assumptions are made regarding the potential human exposure intakes. 
These criteria do not reflect consideration of economic impacts or the 
technological feasibility of meeting the chemical concentrations in 
ambient water. Section 304(a)(1) of the CWA requires EPA to develop and 
publish, and from time to time revise, criteria for water quality 
accurately reflecting the latest scientific knowledge. The criteria are 
used by States and authorized Tribes to establish water quality 
standards and ultimately provide a basis for controlling discharges or 
releases of pollutants. The criteria also provide guidance to EPA when 
promulgating Federal regulations under CWA Section 303(c) when such 
actions are necessary.
    In 1980, we published AWQC (i.e., Section 304(a) criteria) for 64 
pollutants/pollutant classes and provided a methodology for deriving 
the criteria. The 1980 AWQC National Guidelines for developing human 
health AWQC addressed three types of endpoints: noncancer, cancer and 
organoleptic (taste and odor) effects. Criteria for the protection 
against noncancer and cancer effects were estimated by using risk 
assessment-based procedures, including extrapolation from animal 
toxicity or human epidemiological studies. Basic human exposure 
assumptions were applied to the criterion equation. When using cancer 
as the critical risk assessment endpoint, which was assumed not to have 
a threshold, the AWQC were presented as concentrations associated with 
specified incremental lifetime risk levels. When using noncancer 
effects as the critical endpoint, the AWQC reflected an assessment of a 
``no-effect'' level, based on an assumption of a threshold for 
noncancer effects.

B. How Is the Human Health Methodology Used?

    The Methodology is used by EPA to derive or revise its section 
304(a) criteria. It provides the detailed means for developing the 
water quality criteria, including systematic procedures for evaluating 
cancer risk, noncancer health effects, human exposure, and 
bioaccumulation potential in fish. This Methodology is also guidance 
for States and authorized Tribes to help them establish water quality 
criteria to protect human health. States and authorized Tribes must 
develop water quality standards that include designated uses and water 
quality criteria necessary to support those uses.

C. Why Was the Methodology Revised?

    EPA periodically revises water quality criteria to ensure that they 
reflect the latest scientific knowledge on the kind and extent of all 
identifiable effects on health and welfare that may be expected from 
the presence of pollutants in any body of water, including ground 
water. Since 1980, many significant scientific advances have occurred 
which prompt revisions to the Methodology. Specifically, advances in 
such key areas as cancer and noncancer risk assessments, exposure 
assessments, and bioaccumulation make the revisions appropriate at this 
time. We therefore updated the Methodology to provide States and 
authorized Tribes with the most current procedures to reflect these 
changes in risk and exposure assessment. States and authorized Tribes 
can use the Methodology to modify their water quality criteria, as 
appropriate, to ensure that their criteria are protective of designated 
uses.
    Another reason for these revisions is the need to address 
differences in the risk assessment and risk management approaches used 
by the EPA Office of Water for the derivation of AWQC--under the 
authority of the CWA--and Maximum Contaminant Level Goals (MCLGs)--
under the authority of the Safe Drinking Water Act (SDWA). Three 
notable differences in these revisions include the treatment of 
chemicals designated as Group C possible human carcinogens under the 
1986 Guidelines for Carcinogen Risk Assessment (USEPA, 1986a), the 
consideration of non-water sources of exposure when setting an AWQC or 
MCLG for a noncarcinogen, and cancer risk ranges.
    1. Group C Chemicals. Chemicals classified as Group C--i.e., 
possible human carcinogens'under the existing (1986) EPA cancer 
classification scheme have been typically classified as such for any of 
the following reasons.
    (1) Carcinogenicity has been documented in only one test species 
and/or only one cancer bioassay, and the results do not meet the 
requirements of ``sufficient evidence.''
    (2) Tumor response is of marginal statistical significance due to 
inadequate design or reporting.
    (3) An agent causes benign, but not malignant, tumors and no 
response in a variety of short-term tests for mutagenicity.
    (4) There are responses of marginal statistical significance in a 
tissue known to have a high or variable background rate.
    The 1986 Guidelines for Carcinogen Risk Assessment (hereafter 
``1986 cancer guidelines'') specifically recognized the need for 
flexibility with respect to quantifying the risk of Group C agents 
(USEPA, 1986a). The 1986 cancer guidelines noted that agents judged to 
be in Group C, possible human carcinogens, may generally be regarded as 
suitable for quantitative risk assessment, but that case-by-case 
judgments may be made for them.
    EPA has historically treated Group C chemicals differently under 
the CWA and the SDWA. It is important to note that the 1980 AWQC 
National Guidelines for setting AWQC under the CWA predated EPA's 
carcinogen classification system, which was proposed in 1984 and 
finalized in 1986 (USEPA, 1984, 1986a). The 1980 AWQC National 
Guidelines did not explicitly

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differentiate among agents with respect to the weight of evidence for 
characterizing them as likely to be carcinogenic to humans. For all 
pollutants judged as having adequate data for quantifying carcinogenic 
risk--including those now classified as Group C--AWQC were derived 
based on cancer incidence data. In the November 1980 Federal Register 
Notice, we emphasized that the AWQC for carcinogens should state that 
the recommended concentration for maximum protection of human health is 
zero. At the same time, the criteria published for specific carcinogens 
presented water concentrations for these pollutants corresponding to 
individual lifetime cancer risk levels in the range of 10-7 
to 10-5 (ranging from one case in a population of ten 
million to one case in a population of one hundred thousand).
    In the development of national primary drinking water regulations 
under the SDWA, EPA is required to promulgate a health-based MCLG for 
each contaminant. Our policy has been to set the MCLG at zero for 
chemicals with strong evidence of carcinogenicity associated with 
exposure from water. For chemicals with limited evidence of 
carcinogenicity, including many Group C agents, the MCLG was usually 
obtained using a Reference Dose (RfD) based on its noncancer effects 
with the application of an additional factor of 1 to 10. If valid 
noncancer data for a Group C agent were not available to establish an 
RfD, but adequate data were available to quantify the cancer risk, then 
the MCLG was based upon a nominal lifetime excess cancer risk 
calculation in the range of 10-6 to 10-5 (ranging 
from one case in a population of one million to one case in a 
population of one hundred thousand). Even in those cases where the RfD 
approach has been used for the derivation of the MCLG for a Group C 
agent, the drinking water concentrations associated with excess cancer 
risks in the range of 10-6 to 10-5 were also 
provided for comparison.
    It should also be noted that in actions taken under the Federal 
Insecticide, Fungicide, and Rodenticide Act (FIFRA), EPA's pesticides 
program has applied both of these methods for addressing Group C 
chemicals and finds both methods (quantified ``C's'' and nonquantified 
``C's'') applicable on a case-by-case basis. Unlike the drinking water 
program, however, the pesticides program does not add an extra 
uncertainty factor to account for potential carcinogenicity when using 
the RfD approach.
    The EPA is in the process of revising its cancer guidelines, 
including its descriptions of human carcinogenic potential. Once final 
guidelines are published, they will be the basis for assessment under 
this Methodology. In the meanwhile, the 1986 cancer guidelines are used 
and extended with principles discussed in EPA's 1999 Guidelines for 
Carcinogen Risk Assessment--Review Draft (hereafter ``1999 draft 
revised cancer guidelines''). These principles arise from scientific 
discoveries about cancer made in the last 15 years and from EPA policy 
of recent years supporting full characterization of hazard and risk 
both for the general population and potentially sensitive groups such 
as children. These principles are incorporated in recent and ongoing 
assessments such as the reassessment of dioxin, consistent with the 
1986 cancer guidelines. Until final guidelines are published, 
information is presented to describe risk under both the 1986 
guidelines and the 1999 draft revisions. To bring in new science and 
characterization principles, draft revisions have weight-of-evidence 
narratives for hazard characterization that use consistent descriptive 
terms (USEPA, 1999a). In order to provide some measure of consistency 
in an otherwise free-form, narrative characterization, standard 
descriptors are utilized as part of the hazard narrative to express the 
conclusion regarding the weight of evidence for carcinogenic hazard 
potential. There are five standard hazard descriptors: ``carcinogenic 
to humans'', ``likely to be carcinogenic to humans'', ``suggestive 
evidence of carcinogenicity but not sufficient to assess human 
carcinogenic potential'', ``data are inadequate for an assessment of 
human carcinogenic potential'', and ``not likely to be carcinogenic to 
humans.'' Each standard descriptor may be applicable to a wide variety 
of data sets and weights of evidence and are presented only in the 
context of a weight-of-evidence narrative. Furthermore, more than one 
conclusion may be reached for a pollutant. For instance, using a 
descriptor in context, a narrative could say that a pollutant is likely 
to be carcinogenic by inhalation exposure and not likely to be 
carcinogenic by oral exposure.
    In the 2000 Human Health Methodology, we quantify those pollutants 
considered ``carcinogenic to humans'' or ``likely to be carcinogenic to 
humans.'' In practice, even though the terminology of the 1999 draft 
revised cancer guidelines differs, this is the approach currently used 
by the EPA pesticides program.
    2. Consideration of Non-water Sources of Exposure. The 1980 AWQC 
National Guidelines for setting AWQC recommended that contributions 
from non-water sources, namely air and non-fish dietary intake, be 
subtracted from the Acceptable Daily Intake (ADI), thus reducing the 
amount of the ADI ``available'' for water-related sources of intake. In 
practice, however, when calculating human health criteria, those other 
exposures were generally not considered because reliable data on those 
exposure pathways were not available. Consequently, the AWQC were 
usually derived such that drinking water and fish ingestion accounted 
for the entire ADI (now called RfD).
    Through the mid-1980s, the drinking water program generally used a 
similar ``subtraction'' method in the derivation of MCLGs, albeit 
inconsistently. More recently, the drinking water program has used a 
``percentage'' method in the derivation of MCLGs for noncarcinogens. In 
this approach, the percentage of total exposure typically accounted for 
by drinking water is applied to the RfD to determine the maximum amount 
of the RfD apportioned to drinking water reflected by the MCLG value. 
This percentage is called the relative source contribution (RSC). In 
using this percentage procedure, the drinking water program also 
applies a ceiling of 80 percent of the RfD and a floor of 20 percent of 
the RfD. That is, the MCLG cannot account for more than 80 percent of 
the RfD, nor less than 20 percent of the RfD.
    The drinking water program usually takes a conservative approach to 
public health by applying an RSC factor of 20 percent to the RfD when 
adequate exposure data do not exist, assuming that the major portion 
(80 percent) of the total exposure comes from other sources, such as 
diet.
    The 2000 Human Health Methodology includes guidance for routine 
consideration of non-water sources of exposure [both ingestion 
exposures (e.g., food) and exposures other than the oral route (e.g., 
inhalation)] via an approach called the Exposure Decision Tree. RSC 
estimates will be made by EPA using this approach, which allows for use 
of either subtraction or percentage methods, depending on chemical-
specific circumstances, within the 20 to 80 percent range described 
above.
    3. Cancer Risk Ranges. In addition to the different risk assessment 
approaches discussed above for deriving AWQC and MCLGs for Group C 
agents, there have been different risk management approaches by the 
drinking water and

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ambient surface water programs on using lifetime excess risk values 
when setting health-based criteria for carcinogens. The surface water 
program historically derived AWQC for carcinogens that generally 
corresponded to lifetime excess cancer risk levels of 10-7 
to 10-5. The drinking water program has set MCLGs for Group 
C agents based on a slightly less stringent risk range of 
10-6 to 10-5, while MCLGs for chemicals with 
strong evidence of carcinogenicity (that is, classified as Group A 
(known) or B (probable) human carcinogen) are set at zero. The drinking 
water program is now following the 1999 draft revised cancer guidelines 
to determine the type of low-dose extrapolation based on mode of 
action.
    It is also important to note that under the drinking water program, 
for those substances having an MCLG of zero, enforceable Maximum 
Contaminant Levels (MCLs) have generally been promulgated to correspond 
with cancer risk levels ranging from 10-6 to 
10-4. Unlike AWQC and MCLGs which are strictly health-based 
criteria, MCLs are developed with consideration given to the costs and 
technological feasibility of reducing contaminant levels in water to 
meet those standards.
    The 2000 Human Health Methodology states that EPA will publish its 
national 304(a) water quality criteria at a 10-6 risk level, 
which we consider to be appropriate for the general population. Again, 
consistent with the 1999 draft revised cancer guidelines, there are no 
more alphanumeric categories. We will only quantify those considered 
``carcinogenic to humans'' or ``likely to be carcinogenic to humans.'' 
We are increasing the degree of consistency between the drinking water 
and ambient water programs, given somewhat different requirements of 
the CWA and SDWA. We will use the same hazard characterizations of 
dose-response.

B. What Specific Scientific Advances Have Occurred Since 1980?

    Since 1980, EPA risk assessment practices have evolved 
significantly in all of the major Methodology areas: cancer and 
noncancer risk assessments; exposure assessments; and bioaccumulation. 
EPA first published guidelines on cancer risk assessment in 1986. EPA 
published Proposed Guidelines for Carcinogen Risk Assessment in 1996 
(hereafter ``1996 proposed cancer guidelines''; USEPA, 1996a). These 
were recently revised following review by the Agency's Science Advisory 
Board (SAB) and receipt of their comments in May 1999. The most recent 
document is the July 1999 draft revised cancer guidelines (USEPA, 
1999a). The 1999 draft revised cancer guidelines discuss the use of 
mode of action (MOA) information to support both the identification of 
carcinogens and the selection of procedures to characterize risk at 
low, environmentally relevant exposure levels. They also address the 
development of new procedures to quantify cancer risks at low doses to 
replace the current default use of the linearized multistage (LMS) 
model. In noncancer risk assessment, we are moving toward the use of 
the benchmark dose (BMD) and other dose-response methodologies in place 
of the traditional NOAEL approach to estimate an RfD concentration or 
other point of departure (POD) divided by an uncertainty factor (UF). 
In addition, several risk assessment guidelines have been published. 
For example, in 1986 EPA published Guidelines for Mutagenicity Risk 
Assessment (USEPA, 1986b). In 1991, EPA published the Guidelines for 
Developmental Toxicity Risk Assessment (USEPA, 1991a), and in 1996, it 
published the Guidelines for Reproductive Toxicity Risk Assessment 
(USEPA, 1996b). In 1998, EPA also published the Guidelines for 
Neurotoxicity Risk Assessment (USEPA, 1998a). In May 1999, EPA 
published the Draft Guidance for Conducting Health Risk Assessment of 
Chemical Mixtures (USEPA, 1999b). In addition, the Agency is developing 
a framework for cumulative risk assessment, and the Office of Pesticide 
Programs has developed draft guidance for assessing cumulative risk of 
common mechanism pesticides and other substances.
    In 1986, EPA made available to the public the Integrated Risk 
Information System (IRIS). IRIS is a database that contains risk 
information on the cancer and noncancer effects of chemicals. The IRIS 
assessments represent EPA scientific consensus positions across the 
Agency's program offices and regional offices.
    In exposure analysis, several new studies have addressed water 
consumption and fish tissue consumption. These exposure studies provide 
a more current and comprehensive description of national, regional and 
special population consumption patterns that we reflected in the 1998 
Draft Water Quality Criteria Methodology: Human Health (hereafter 
``1998 draft Methodology revisions''; USEPA, 1998c). In addition, more 
formalized procedures are available to account for human exposure to 
multiple sources when setting health goals such as AWQC that have 
previously addressed only one exposure source. The Exposure Factors 
Handbook was updated in 1997 (USEPA, 1997a). In 1992, we published the 
revised Guidelines for Exposure Assessment (USEPA, 1992a), which 
describe general concepts of exposure assessment, including definitions 
and associated intake rate parameters, and provide guidance on planning 
and conducting an exposure assessment. In 1986, the Agency published 
the Total Exposure Assessment Methodology (TEAM) Study: Summary and 
Analysis, Volume I, Final Report (USEPA, 1986c), which presents a 
process for conducting comprehensive evaluation of human exposures. The 
Agency has recently developed a revised relative source contribution 
(RSC) policy for assessing total human exposure to a contaminant and 
apportioning the RfD among the media of concern for use in deriving or 
revising AWQC. In 1997, we developed Guiding Principles for Monte Carlo 
Analysis (USEPA, 1997b). Also in 1997, we published the Policy for Use 
of Probabilistic Analysis in Risk Assessment (USEPA, 1997c; see http://www.epa.gov/ncea/mcpolicy.htm). The Monte Carlo guidance document can 
be applied to exposure assessments and risk assessments. The Agency has 
moved toward the use of a bioaccumulation factor (BAF) to reflect the 
uptake of a contaminant from all sources (e.g., ingestion, sediment) by 
fish and shellfish, rather than just from the water column as reflected 
by the use of a bioconcentration factor (BCF) in the 1980 Methodology. 
We have developed detailed procedures and guidelines for estimating BAF 
values for use in deriving or revising AWQC.

C. What Process Did EPA Follow in Revising the Methodology?

    We began by developing (along with other Federal agencies, State 
health organizations, Canadian health agencies, academies, 
environmental and industry groups, and consulting organizations) an 
issues paper that described the 1980 Methodology, discussed areas that 
needed strengthening, and recommended revisions. The paper was 
distributed for review and comment and was examined at a national 
workshop, where more than 100 participants discussed critical issues. 
Based on individual expertise, attendees were assigned to specific 
technical workgroups. The workgroups' topics included cancer risk, 
noncancer risk, exposure, microbiology, minimum data and 
bioaccumulation in fish.
    A summary document based on the workshop recommendations was 
submitted for review and comment by the EPA SAB. Once final comments 
and

[[Page 66448]]

revisions were received from the SAB, the recommendations were again 
reviewed at a meeting of the Federal-State Toxicology and Risk Analysis 
Committee, where state representatives presented their opinions on the 
preliminary draft recommendations. (A more detailed chronology of this 
process was provided with the 1998 draft Methodology revisions.)
    EPA subsequently developed the 1998 draft Methodology revisions 
(USEPA, 1998c) and the Ambient Water Quality Criteria Derivation 
Methodology Human Health Technical Support Document (TSD) (USEPA, 
1998d) that provides greater detail on the Methodology guidance--
including case study examples, data tables, and other supporting 
information. These were published in the Federal Register in August 
1998. A four-month public comment period followed. In May of 1999, a 
fifteen-member independent peer review workshop was held, and a public 
stakeholder meeting followed. The 2000 Human Health Methodology 
reflects, in part, the input received from the public and peer review 
experts, in addition to more recent scientific information and science 
policies since the 1998 draft publication.

F. What Are the Major Revisions to the Methodology?

    The major revisions are in four assessment areas: Noncancer, 
cancer, exposure and bioaccumulation. Equations have been developed for 
deriving AWQC, which include parameters relevant to those four 
assessment areas. These parameters are derived from scientific 
analysis, science policy and risk management decisions.
    For noncarcinogens, the process for deriving a level of exposure 
considered to be without appreciable risk of effect--known as the 
Reference Dose (RfD) value--has evolved over time.
     EPA has developed guidance on assessing noncarcinogenic 
effects of chemicals and for the RfD derivation.
     The Methodology revisions recommend consideration of other 
issues related to the RfD process including integrating reproductive/
developmental, immunotoxicity, and neurotoxicity data into the 
calculation.
     EPA is recommending the use of quantitative dose-response 
modeling for the derivation of RfDs.
     EPA has provided additional guidance (in its Risk 
Assessment TSD) to allow States and authorized Tribes greater 
flexibility in conducting their own risk assessments.
    For carcinogen (cancer) risk assessment, more sophisticated methods 
for determining the likely mechanism that causes human carcinogenicity 
are being recommended, as well as consideration of all biological 
information (rather than just tumor findings) and full risk 
characterization for the general population as well as sensitive groups 
such as children.
    Changes in the area of exposure assessment include the following.
     States and authorized Tribes are encouraged to use local 
studies on fish consumption that better reflect local intake patterns 
and choices.
     EPA will recommend default fish consumption values for the 
general population, recreational fishers and subsistence fishers.
     A factor to account for other sources of exposure, such as 
food and air, is included when deriving AWQC for noncarcinogens and for 
carcinogens based on a nonlinear low-dose extrapolation (i.e., water 
and fish consumption are not the only exposures considered).
    The 2000 Human Health Methodology places greater emphasis on the 
use of BAFs compared to the 1980 Methodology for estimating potential 
human exposure to contaminants via the consumption of contaminated fish 
and shellfish.
     BAFs reflect the accumulation of chemicals by aquatic 
organisms from all surrounding media (water, food, sediment). Compared 
with BCFs, which reflect chemical accumulation by aquatic organisms 
from water only, BAFs are considered to be better predictors of 
chemical accumulation by fish and shellfish for chemicals where 
exposure from food and sediment is important (e.g., highly persistent, 
hydrophobic chemicals).
     EPA gives preference to the use of high quality field data 
over laboratory or model-derived estimates of BAFs, since field data 
best reflect factors that can affect the extent of bioaccumlation 
(e.g., chemical metabolism, food web structure).

G. How Will EPA Use the Human Health Methodology?

    Our future role in developing AWQC for the protection of human 
health will include the following.
     Further refinement of the Methodology as the science and 
EPA's science policies evolve;
     Development of revised AWQC for pollutants of high 
priority and national importance (including, but not limited to 
chemicals that bioaccumulate, such as PCBs, dioxin, and mercury); and
     Development or revision of AWQC for some additional 
priority pollutants.
    We plan to fully update the most environmentally important criteria 
developed in 1980 (or those updated as part of the 1992 National Toxics 
Rule (NTR)). Partial updates of substantially more criteria may be 
warranted. We encourage States and authorized Tribes to use the 2000 
Human Health Methodology to develop or revise AWQC to reflect local 
conditions. EPA believes that AWQC inherently require several risk 
management decisions that are, in many cases, better made at the State 
or Tribal level (e.g., selection of specific fish consumption rates or 
target risk levels). We will continue to develop and update necessary 
toxicology and exposure data needed for the derivation of AWQC that may 
not be practical for the States or Tribes to obtain. More information 
on implementation issues and the effect of the 2000 Human Health 
Methodology on States and authorized Tribes is discussed below.

II. Implementation Issues

    Water quality standards consist of designated uses, water quality 
criteria to protect those uses, a policy for antidegradation, and 
general policies for application and implementation. As part of the 
water quality standards triennial review process defined in section 
303(c)(1) of the CWA, States and authorized Tribes are responsible for 
maintaining and revising water quality standards. Section 303(c)(1) 
requires States and authorized Tribes to review, and modify if 
appropriate, their water quality standards at least once every three 
years.

A. How Does EPA Use Its Recommended 304(a) Water Quality Criteria?

    EPA's recommended 304(a) water quality criteria form the basis for 
Agency decisions, both regulatory and nonregulatory, until superseded 
by EPA publication of new or revised 304(a) water quality criteria. For 
example, these criteria are used in the following ways: (1) As guidance 
to States and authorized Tribes in adopting water quality standards; 
(2) as guidance to EPA in promulgating Federal water quality standards; 
(3) in establishing National Pollutant Discharge Elimination System 
(NPDES) water quality-based permit limits, where the criteria have been 
adopted by a State or authorized Tribe or promulgated by EPA; and (4) 
for all other purposes of Section 304(a) criteria under the Act. It is 
important to emphasize again two distinct purposes which are served by 
the 304(a) criteria. The first is as guidance to the States and Tribes 
in the development and adoption of water quality criteria which will 
protect designated uses. The second is as the basis for promulgation of 
Federal water

[[Page 66449]]

quality standards for States or authorized Tribes when such action is 
necessary.

B. What Water Quality Criteria Must a State or Authorized Tribe Adopt 
Into Its Water Quality Standards?

    States and authorized Tribes must adopt water quality criteria that 
protect designated uses. Such criteria must be based on sound 
scientific rationale and must contain sufficient parameters or 
components to protect the designated uses. Criteria may be expressed in 
either narrative or numeric form. States and authorized Tribes have 
four options when adopting water quality criteria for which EPA has 
published 304(a) criteria. They can establish numerical values based on 
304(a) criteria, 304(a) criteria modified to reflect site-specific 
conditions, other scientifically defensible methods, or establish 
narrative criteria where numeric criteria cannot be determined. (See 40 
CFR 131.11.)
    EPA's recommended 304(a) water quality criteria for States and 
authorized Tribes to use as guidance in adopting water quality 
standards consistent with Section 303(c) of the Act and the 
implementing Federal regulations at 40 CFR part 131 are contained in 
EPA's last compilation of National Recommended Water Quality Criteria 
(USEPA, 1998e) (corrected in USEPA, 1999c). In the future, we will be 
publishing new and revised 304(a) water quality criteria based upon the 
2000 Human Health Methodology for pollutants of high priority and 
national importance. Because the revision of existing 304(a) human 
health criteria to reflect the 2000 Human Health Methodology will take 
time, EPA encourages States and authorized Tribes to make appropriate 
changes to their existing numerical, pollutant-specific criteria in 
their water quality standards to reflect this new Methodology prior to 
publication of a revised 304(a) criteria where they determine that such 
actions are necessary. For example, a pollutant of concern in a 
particular State may not be a high priority on the national level and 
revision of the national 304(a) criteria may not occur for several 
years. In this case, the State or a group of States, might choose to 
use this new Methodology to revise their water quality standards prior 
to EPA publication of a revised 304(a) criteria for that pollutant. EPA 
will recognize criteria that are revised pursuant to the 2000 Human 
Health Methodology as scientifically defensible and promptly approve 
such revised criteria as enforceable elements of State or Tribal water 
quality standards.
    Once a new or revised 304(a) criteria reflecting this new 
Methodology is published, EPA expects States and authorized Tribes to 
reassess their water quality standards and, where necessary, establish 
new or revised water quality criteria consistent with one of the four 
options described above. Because of the critical role that human health 
ambient water quality criteria play in protecting human health, EPA 
will work with States and authorized Tribes to revise existing water 
quality standards promptly following EPA publication of revised section 
304(a) criteria.

C. May States and Authorized Tribes Adopt Water Quality Criteria Based 
on Local Conditions?

    In keeping with their primary responsibility in establishing water 
quality standards, we encourage States and authorized Tribes to develop 
and adopt water quality criteria to reflect local and regional 
conditions. States and authorized Tribes will have access to EPA 
regional, laboratory, and headquarters staff when help is needed to 
interpret today's Human Health Methodology and to make critical risk 
assessment decisions. For the purpose of deriving criteria based on the 
2000 Human Health Methodology, EPA is publishing default values for 
risk level, fish intake, drinking water intake, and body weight. 
Default BAF values and RSC factor values will be published as chemical-
specific criteria are developed or revised. (Other RSC estimates will 
be made when data are adequate to make them.) We believe these default 
values result in water quality criteria protective of the general 
population, and we will use these values when deriving 304(a) criteria. 
States and authorized Tribes may use other values more representative 
of local conditions if data have been collected supporting the 
alternative values. However, when establishing a numerical value based 
on a 304(a) criterion modified to reflect site-specific conditions, or 
water quality criteria based on other scientifically defensible 
methods, we strongly caution States and authorized Tribes not to 
selectively apply data in order to ensure water quality criteria less 
stringent than EPA's 304(a) criteria. Such an approach would 
inaccurately characterize risk.

D. What Cancer Risk Level Should States and Authorized Tribes Use When 
Establishing Water Quality Criteria?

    In deriving 304(a) criteria based on the 2000 Human Health 
Methodology or when promulgating Federal water quality standards under 
section 303(c) of the CWA, EPA intends to use a 10-61 cancer 
risk level, which we believe reflects an appropriate target risk level 
for the general population. EPA acknowledges that at any given cancer 
risk level for the general population, those segments of the population 
that are more highly exposed face a higher relative risk. For example, 
if fish are contaminated at a level allowed by criteria derived on the 
basis of a risk level of 10-6, individuals consuming up to 
10 times the assumed fish consumption rate would still be protected at 
a 10-5 risk level. States and authorized Tribes have the 
flexibility to adopt water quality criteria that result in a risk level 
higher than 10-6, up to the 10-5 level. EPA 
recommends adoption of such criteria if the State or Tribe has 
identified the most highly exposed subpopulation within the State or 
Tribe, has demonstrated that the chosen cancer risk level is protective 
of the most highly exposed subpopulations, and has completed all 
necessary public participation. EPA notes that special scientific 
circumstances and assessment of natural contaminants may lead to 
numbers outside the 10-6 to 10-5 risk range. (For 
additional discussion on this issue, including restrictions on 
selection of a cancer risk level, refer to the response on the comment 
for cancer risk ranges summarized in Section III of this Notice, 
below.)

E. How Does the Review and Approval of State and Tribal Water Quality 
Standards Rule Affect Water Quality Criteria Adopted by States and 
Authorized Tribes?

    Consistent with the Review and Approval of State and Tribal Water 
Quality Standards rule revision (USEPA, 2000a), water quality criteria 
adopted into law or regulation by States and authorized Tribes prior to 
May 30, 2000, are in effect for CWA purposes unless superseded by 
replacement Federal water quality standards (see, for example, the 
National Toxics Rule, 40 CFR 131.35; Water Quality Standards for Idaho, 
40 CFR 131.35). Water quality criteria adopted into law or regulation 
by States and authorized Tribes after May 30, 2000, are in effect for 
CWA purposes only after EPA approval of any new or revised water 
quality standards.

F. While EPA is Re-Evaluating a 304(a) Criterion, What Criterion Is in 
Effect?

    Until such time as EPA reevaluates the 304(a) criteria, subjects 
the criteria to appropriate peer review, and subsequently publishes 
revised 304(a) criteria, the existing 304(a) criteria remain in effect 
for the purposes of EPA review of State and Tribal water quality 
standards under section 303(c). Where EPA has not published a revision 
of a

[[Page 66450]]

304(a) criteria reflecting the 2000 Human Health Methodology, EPA will 
not require the revision of State water quality standards to reflect 
this new Methodology. As noted above, however, EPA will assist those 
States or Tribes that choose to use the new Methodology to revise their 
existing water quality standards prior to publication of a revised 
criteria under section 304(a).

G. What Design Stream Flow Should Be Used to Implement Human Health 
Criteria?

    Human health criteria represent ambient pollutant concentrations 
that are acceptable based on a lifetime (70 years) of exposure. 
Accordingly, discharges of pollutants should be regulated such that 
criteria will not be exceeded under stream conditions that represent 
long-term average conditions. Current EPA guidance recommends the use 
of the long-term harmonic mean flow to implement criteria for 
carcinogens and the 30Q5 flow to implement criteria for noncarcinogens 
(USEPA, 1991b). The harmonic mean flow is the sum of the reciprocals of 
individual flow measurements divided into the total number of 
individual flow measurements, and the 30Q5 flow is defined by the 
lowest 30-day average that has an expected return frequency of once 
every five years. With today's Human Health Methodology, EPA is 
revising its guidance to recommend harmonic mean flow be used to 
implement both carcinogen and noncarcinogen human health criteria. 
Harmonic mean flow should be used to implement human health criteria 
because, by and large, human health criteria are designed to protect an 
individual over a lifetime of exposure. As stated in the 1998 draft 
Methodology revisions, we are not recommending the development of 
additional water quality criteria similar to the drinking water health 
advisories that focus on acute or short-term effects. These are not 
seen as routinely having a meaningful role in the water quality 
criteria and standards program because the chronic health effects 
associated with chemical contaminants are usually the most sensitive 
health endpoint. Human health criteria based on cancer potencies and 
risk levels are based on models that extrapolate animal data to a human 
lifetime. Similarly, a human noncancer criterion is based on an RfD, 
which is an acceptable daily exposure over a lifetime. Therefore, we 
have attempted to match the longest stream flow averaging period (using 
harmonic mean) with the criterion which is protective over a human 
lifetime.
    In rare instances where a human health criterion or value is based 
on a short-term toxicological effect (i.e., the critical effect upon 
which the criterion/value is based is significantly less than lifetime 
and may be an acute effect), the design flow should be adjusted 
accordingly. This does not pertain to RfDs in which a short-term study 
has been used as the RfD basis and an uncertainty factor has been used 
to account for less than lifetime study results; that is, the short-
term study has been used to estimate a lifetime RfD value. This 
pertains only to those situations where the critical effect is a short-
term effect (and where no additional uncertainty factor has been used 
to account for less than lifetime exposure). A good example of this is 
EPA's RfD for nitrate. The critical effect, upon which the RfD is 
based, is toxicity to infants after a short-term exposure. In this 
case, harmonic mean flow would be an inappropriate design flow for such 
a short-term effect. In this case, a 7Q10 or a 4Q3 design flow may be 
more appropriate.

H. What Is the Relationship Between the Agency's Recommended Section 
304(a) Water Quality Criteria and Drinking Water Standards?

    EPA recommends that States and authorized Tribes use this 2000 
Human Health Methodology to develop their own AWQC for all pollutants 
of concern using the latest scientifically defensible data and 
principles. Sources of scientifically defensible data include published 
toxicological literature or recent EPA assessments, including those 
that underlie IRIS values, the most recently published recommended 
Section 304(a) water quality criteria or the most recently promulgated 
SDWA MCLGs.
    When adopting water quality criteria to protect CWA Section 101(a) 
fishable uses, States and authorized Tribes need to ensure such 
criteria adequately address fish consumption as an exposure route.
    When States and authorized Tribes do not develop their own AWQC, 
EPA recommends that States and authorized Tribes use the most recently 
published recommended Section 304(a) water quality criteria for ``water 
and organisms'' based on this new Human Health Methodology to protect 
CWA Section 101(a) fishable uses and waters designated for drinking 
water. This ensures that the water quality criteria adequately address 
fish consumption, bioaccumulation and drinking water uses.
    When EPA publishes the annual compilation of new and revised 
national recommended Section 304(a) water quality criteria, those 
criteria represent the Agency's most current recommended Section 304(a) 
water quality criteria and should be used by States and authorized 
Tribes when reviewing their water quality standards.
    When States and authorized Tribes do not develop their own AWQC, 
and there are no recommended Section 304(a) water quality criteria for 
a pollutant of concern, or the recommended Section 304(a) water quality 
criteria have not yet been revised based on this new Human Health 
Methodology \1\:
---------------------------------------------------------------------------

    \1\ New criteria and criteria revised under this new Methodology 
are published annually as the ``Compilation of National Recommended 
Water Quality Criteria and EPA's Process for Deriving New and 
Revised Criteria'' at www.epa.gov/ost/standards/.
---------------------------------------------------------------------------

    1. For a pollutant for which EPA has published a recommended 
Section 304(a) water quality criterion for ``water and organisms'' 
based on the 1980 Methodology and for which EPA has not promulgated an 
MCLG, EPA will recognize the current Section 304(a) water quality 
criterion, or a criterion that is developed or revised pursuant to the 
2000 Human Health Methodology and approved by EPA.
    2. For a pollutant for which EPA has published a recommended 
Section 304(a) water quality criterion for ``water and organisms'' 
based on the 1980 Methodology and for which EPA has more recently 
promulgated an MCLG, EPA generally recommends the MCLG for 
noncarcinogenic pollutants, or a criterion derived by recalculating the 
MCLG at an acceptable cancer risk level (i.e., a level within the range 
of 10-6 to 10-5, as specifically discussed in 
Section II.D, which notes that special scientific circumstances and 
assessment of natural contaminants may lead to numbers outside the to 
10-6 to 10-5 risk range).
    3. For a pollutant for which EPA has not published a recommended 
Section 304(a) water quality criterion for ``water and organisms'' and 
for which EPA has promulgated an MCLG, EPA generally recommends the 
MCLG for noncarcinogenic pollutants, or a criterion derived by 
recalculating the MCLG at an acceptable cancer risk level (i.e., a 
level within the range of 10-6 to 10-5, as 
specifically discussed in Section II.D, which notes that special 
scientific circumstances and assessment of natural contaminants may 
lead to numbers outside the 10-6 to 10-5 risk 
range).
    EPA no longer recommends that an MCL be used where consideration of 
available treatment technology, costs, or availability of analytical 
methodologies

[[Page 66451]]

has resulted in an MCL that is less protective than an MCLG.
    States and authorized Tribes continue to have the flexibility to 
adopt water quality criteria that are more protective than EPA's 
recommendations, as long as such criteria are protective of the 
designated uses and scientifically defensible.

I. How Are Health Risks to Children Considered in the Methodology?

    In recognition that children have a special vulnerability to many 
toxic substances, EPA's Administrator directed the Agency in 1995 to 
explicitly and consistently take into account environmental health 
risks to infants and children in all risk assessments, risk 
characterizations and public health standards set for the United 
States. On April 21, 1997, President Clinton signed Executive Order 
13045, ``Protection of Children From Environmental Health Risks and 
Safety Risks,'' which assigned a high priority to addressing risks to 
children. In May 1997, EPA established the Office of Children's Health 
Protection to ensure the implementation of the President's Executive 
Order (E.O.). Circumstances where risks to children should be 
considered in the context of the 2000 Human Health Methodology are 
discussed in the Noncancer Section (in terms of developmental and 
reproductive toxicity) and in the Exposure Section (for appropriate 
exposure intake parameters).
    All of EPA's risk assessment guidelines should be consulted when 
conducting a risk assessment to ensure that information from studies on 
carcinogenesis and other health effects are considered together in the 
overall characterization of risk. This is particularly important in the 
case in which a precursor effect to tumor is also a precursor or 
endpoint of other health effects and is used in dose-response 
assessment. The overall characterization of risk will be the basis for 
carrying out assessments of instances in which fetuses, infants, or 
children are at risk.

III. Summary of Comments Received on the 1998 Draft Methodology 
Revisions and EPA's Responses

A. Implementation

1. Application of Human Health Water Criteria Within Mixing Zones
    Comments--Commenters stated that human health criteria should start 
with the local relevant fish consumption rates and then make 
adjustments to reflect the actual relevant fish consumption rate 
related to the discharge and the mixing zone. It was also suggested 
that implementation in the NPDES program inherently needs a translator 
mechanism to adjust the standards to reflect actual consumption 
associated with allowed mixing zones.
    Response--Application of human health water criteria within a 
mixing zone is not within the scope of this Methodology. At this time, 
EPA's current recommendations regarding the application of human health 
criteria within mixing zones are contained in the Technical Support 
Document for Water Quality-Based Toxics Control (USEPA, 1991b) and the 
Water Quality Standards Handbook (USEPA, 1994). We also note that 
mixing zones are an optional policy that not every State and authorized 
Tribe has adopted into their water quality standards. For States and 
Tribes that have authorized mixing zones, the designated uses of a 
waterbody as a whole must be maintained and protected.
2. Application of Human Health Water Quality Criteria to Marine Waters
    Comment--A question was raised as to whether human health water 
quality criteria are applicable to marine waters, given the vastness of 
most marine waters.
    Response--EPA believes human health water quality criteria should 
be applied to near-shore waters (specifically within a three-mile 
limit) wherever dischargers are located to protect aquatic food 
organisms, but not to include the drinking water consumption parameter. 
These water quality criteria are then used to derive permit limits that 
will ensure water quality criteria are not exceeded within the vicinity 
of an outfall. This protects organisms that are sessile and other 
organisms that may be attracted to the effluent and that are food 
sources. In the absence of data specific to the coastal site indicating 
that particular marine species are impacted by those discharges, we 
recommend our human health criteria to protect coastal waters. [Note: 
EPA's recommended national default fish intake value, which excludes 
marine species, supports this position. Estuarine species that are more 
likely to be found in near-shore waters are included in the default 
intake value. Potential exposure from open-ocean marine species are not 
ignored; the marine species exposure pathway can be accounted for as 
part of the RSC factor.]
3. Cancer Risk Range
    Comments--Many comments were received on the appropriateness of the 
cancer risk range. Numerous commenters stated that the permissible 
range and recommended default of 10-\6\ are appropriate and 
approved of the range's consistency with other Agency programs. EPA was 
asked to reconcile the statements that both 10-\6\ and 
10-\5\ are acceptable for the general population, that 
10-\6\ is appropriate for promulgation of Federal water 
quality standards under Section 303(c) given that we have said 
10-\5\ is appropriate for the Great Lakes, and that a 
10-\5\ risk level along with a 17.8 g/day fish intake 
assumption will protect the highest consumers at a 10-\4\ 
risk level. Other comments are listed as follows.
     The Methodology should use a 10-\5\ risk level.
     10-\6\ represents a change in the acceptable 
risk level.
     The 10-\6\ risk level represents a binding 
regulatory constraint that will provide no State flexibility.
     10-5 is used by most States, and EPA should 
retain this default because the Agency has not determined that it is 
inadequate.
     A range of 10-4 to 10-5 is 
advocated.
    In addition, we received comments that allowing highly exposed 
groups to potentially experience cancer risks an order of magnitude 
higher than the general population is unjust and disregards Native 
American treaty rights. A commenter supported the idea that a 
10-4 risk level can be protective and believed highly 
exposed populations are few in number. Another stated that the cancer 
risk range should apply to total contaminants (i.e., a cumulative 
cancer risk ceiling). It was cautioned that the concept of relative 
risk could result in selection of inappropriate target populations and 
intake rates. Others agreed that States and authorized Tribes should 
have the flexibility to select cancer risk levels as risk management 
decisions and requested that EPA explicitly state that it will support 
risk levels chosen by a Tribal authority, while another requested the 
flexibility without requiring involved demonstrations specific to the 
subpopulation at issue. A commenter recommended changes in EPA's 
Methodology to ensure that the resulting water quality criteria are 
more applicable to exposed populations. Others asked EPA to indicate 
the percentile of the exposed population that would meet the 
10-6 risk level.
    Response--With the 1980 Methodology, EPA presented three separate 
304(a) criteria for carcinogens at risk levels corresponding to 
10-7, 10-6, and 10-5 for States and 
authorized Tribes to choose from. However, the 10-7 risk 
level has not been used by any State or authorized

[[Page 66452]]

Tribe when adopting water quality standards. Furthermore, since that 
time, EPA's guidance and regulatory actions have utilized a 
10-6 risk level as an appropriate target risk for the 
general population.
    With the 2000 Human Health Methodology, our position is that both 
10-6 and 10-5 are appropriate targets for health 
protection of the general population and that highly exposed 
populations should not exceed a 10-4 risk level. We also 
note that special scientific circumstances and assessment of natural 
contaminants may lead to numbers outside the 10-6 to 
10-5 range. However, we are not automatically assuming that 
10-5 will protect ``the highest consumers'' at the 
10-4 risk level. One commenter referred to specific data 
indicating high intake levels that would not satisfy such an 
assumption. Nor are we advocating that States and authorized Tribes 
automatically establish criteria based on assumptions for highly 
exposed population groups at the 10-4 risk level. We 
acknowledge that fish consumption rates vary considerably, especially 
among subsistence populations, as is evident from the studies 
summarized in the Exposure TSD. Indeed, it is the variation of fish 
consumption among these population groups that could make either 
10-5 or 10-6 protective of those groups at a 
10-4 risk level. Specifically, if a State adopted a 
criterion based on a 10-5 risk level and a 17.5 g/day 
consumption rate, a high-end subsistence consumption of 1,750 g/day 
would exceed a 10-4 risk level.
    It is important to understand that criteria for carcinogens are 
based on chosen risk levels that inherently reflect, in part, the 
exposure parameters used to derive those values. Therefore, changing 
the exposure parameters will also change the risk. Specifically, the 
incremental cancer risk levels are relative, meaning that any given 
criterion associated with a particular cancer risk level is also 
associated with specific exposure parameter assumptions (i.e., intake 
rates, body weights). When these exposure values change, so does the 
relative risk. As we have previously indicated for a criterion derived 
on the basis of a cancer risk level of 10-6, individuals 
consuming up to 10 times the assumed fish intake rate would not exceed 
a 10-5 risk level. Similarly, individuals consuming up to 
100 times the assumed rate would not exceed a 10-4 risk 
level. Thus, for a criterion based on EPA's default fish intake rate 
(now 17.5 g/day, based on the most recent survey data) and a risk level 
of 10-6, those consuming a pound of fish per day would 
potentially experience between a 10-5 and a 10-4 
risk level (closer to a 10-5 risk level). Even if a 
criterion were based on high-end intake rates and the relative risk of 
10-6, then an average fish consumer would not exceed a 
cancer risk level of approximately 10-8. The point here is 
that the risks for different population groups are not the same.
    EPA believes that the adoption of a 10-6 or 
10-5 target risk level, both of which States and authorized 
Tribes have historically chosen, represents a generally acceptable 
health protection decision, noting again that special scientific 
circumstances or assessments of natural contaminants may necessitate 
additional considerations. EPA recommends adoption of water quality 
standards that include water quality criteria based on either the 
10-5 or 10-6 risk level if the State or 
authorized Tribe has identified the most highly exposed subpopulation, 
has demonstrated that the chosen risk level is adequately protective of 
the most highly exposed subpopulation, and has completed all necessary 
public participation. States and authorized Tribes also have 
flexibility in how they demonstrate this protectiveness and obtain such 
information. A State or authorized Tribe may use existing information 
as well as collect new information in making its determination as to an 
appropriate level of protection. In addition, if a State or authorized 
Tribe does not believe that the 10-6 risk level adequately 
protects highly exposed subpopulations, water quality criteria based on 
a more stringent risk level may be adopted. However, we are now adding 
that a generally specific analysis should be made and presented to 
ensure that highly exposed groups do not exceed a target 
10-4 risk level. In cases where fish consumption among 
highly exposed population groups is of a magnitude that such a 
10-4 risk level would be exceeded, a more protective risk 
level should be chosen. These determinations should be made by the 
State or authorized Tribe and are subject to EPA's review under Section 
303 of the CWA. Guidance on choosing appropriate exposure parameters is 
discussed in both the 2000 Human Health Methodology and the Exposure 
Assessment TSD.
    Given the relatively significant variation in fish consumption 
rates, EPA intends to derive Section 304(a) criteria at the 
10-6 risk level, based on an intake rate of 17.5 g/day. We 
believe that basing our 304(a) criteria on general U.S. population 
exposures is most appropriate, given their use as a default value for 
the nation as a whole. Most States have, in fact, already adopted a 
10-6 risk level with their criteria for carcinogens, not the 
10-5 risk level claimed by one commenter. This default 
would, in turn, be protective for fish intakes of up to 1,750 g/day at 
the 10-4 risk level. However, in the Exposure Assessment 
TSD, EPA has recommended that States and authorized Tribes give 
priority to identifying and adequately protecting the most highly 
exposed population by adopting more stringent criteria, if the State or 
authorized Tribe determines that the highly exposed population would 
not be adequately protected by criteria based on protecting the general 
population. States and authorized Tribes have the option to derive 
their criteria at a 10-6 risk level, as EPA will do with its 
304(a) criteria. They also have the flexibility to combine the 
10-6 risk level with fish consumption rates for highly 
exposed population groups. Thus, States and authorized Tribes may 
choose to adopt criteria that are more protective than EPA's 304(a) 
criteria. We intend to support the health protection decisions made by 
States and authorized Tribes as long as they use the risk range that 
EPA has stated here and in the 2000 Human Health Methodology. EPA has 
made reasonable and conservative assumptions in choosing exposure 
parameters with the goal of protecting the majority of the population. 
However, we do not believe it is possible to calculate the exact 
percentile of the population that would be protected at a given risk 
level in terms of the overall combination of exposure parameters. We 
emphasize that the criteria are derived to be protective, not 
predictive of an exact percentile of the total population that is 
protected.
    Regarding the use of a 10-5 risk level in the Great 
Lakes Water Quality Initiative (GLI), the criteria values were based on 
fish consumption estimates that reflected intake data among 
sportfishers, a group that consumes more fish than the general 
population. Again, we recommend that States and authorized Tribes base 
their criteria on more highly exposed population groups, if they would 
not be adequately protected by criteria based on intake rate estimates 
for the general population. Regarding the application of a cumulative 
cancer ceiling, the commenter has misunderstood EPA's policy when 
setting 304(a) criteria for carcinogenic effects based on linear low-
dose extrapolation. With these carcinogens, the AWQC are set with 
respect to the incremental lifetime risk posed by the substance in 
water and are

[[Page 66453]]

not being set on an individual's total cancer risk from all sources of 
exposure.
4. Coordinating the Human Health Methodology With Other EPA Programs
    Comments--Numerous commenters recommended that the Methodology 
revisions be coordinated with the drinking water program (specifically, 
MCLs/MCLGs required under the SDWA) and believed that the drinking 
water portion of AWQC and MCLGs should be equivalent. Several 
commenters stated that the burden of achieving health goals should be 
borne by dischargers and other polluters, not by water users or the 
environment. Commenters also recommended that EPA use MCLs when AWQC 
are less protective or for chemicals when AWQC do not exist. Another 
recommended that an additional margin of safety be included if the MCL 
were used, in particular for chemicals not effectively removed by 
conventional drinking water treatment, and also stated that neither the 
availability of MCLs or MCLGs should deter development of AWQC. Some 
commenters believed that the use of an MCLG is an acceptable 
alternative for chemicals of drinking water concern because, like the 
AWQC, it is a health-based value. However, others recommended that 
MCLGs not be used when they are more stringent than AWQC because they 
are not regulatory standards. Two commenters stated that EPA should not 
abandon its policy of setting AWQC for carcinogens at zero for 
``maximum protection of human health'' and recommended that the ``Group 
C'' chemicals also have AWQC set at zero (referring to non-zero MCLs as 
inconsistent with the intent of a zero MCLG). However, other commenters 
recommended that AWQC be set at one-half of the MCL when the MCLG is 
zero, at a 10-6 risk level, or by calculating both and 
choosing the lower of the two. Two commenters urged EPA to unify the 
national Human Health Methodology with the GLI guidance. Another 
discussed microbial pathogens and, in addition to recommending 
development of criteria for specific microbial contaminants, 
recommended coordination with the drinking water program [i.e., the 
SDWA's Candidate Chemical List (CCL)] and stated that microbial 
criteria need to be set for more than recreational waters.
    Response--EPA intends to continue deriving AWQC that include a 
drinking water pathway, applicable to waters that are potential sources 
of drinking water, and agrees that the drinking water component of AWQC 
should be consistent with the MCLG (if one has been established). 
Therefore, we intend to use a similar methodology for deriving AWQC and 
MCLGs. We also intend to coordinate with the Agency's safe drinking 
water program when prioritizing chemicals for AWQC derivation/revision 
(see also response to Comment A.11, Proposed Chemical List). Regarding 
the relationship between AWQC and the drinking water MCLs and MCLGs, we 
have clearly stated our position in the Federal Register Notice for the 
1998 draft Methodology revisions (USEPA, 1998c) on this relationship 
and our approach to considering when an MCL or MCLG may be appropriate 
to use in lieu of AWQC. That discussion is excerpted in the 2000 Human 
Health Methodology document, along with clarification of our policy on 
the circumstances and limitations under which either should be used. We 
do not necessarily assume that a chemical's concentrations in ambient 
waters and drinking water are equivalent but are aware that chemicals 
may not be effectively removed by conventional drinking water 
treatment.
    Commenters who referred to EPA's abandonment of its policy of 
setting AWQC for carcinogens at zero have substantively misstated our 
policy based on both the 1980 Methodology for deriving AWQC and our 
1998 draft Methodology revisions, and are directed to the Federal 
Register Notice cited above. We did state in our 1980 Methodology that 
for the maximum protection of human health from potential carcinogenic 
effects, the ambient water concentration should be zero, based on an 
assumption of a linear dose-response relationship at low doses. The 
1980 Methodology also indicated that zero levels may not have been 
attainable at that time. This remains the case at present. The 
combination of background levels of carcinogens from natural sources 
and global background levels from anthropogenic sources make attainment 
of zero levels for many potential carcinogens impossible. In addition, 
more recent and sophisticated toxicological information on 
carcinogenicity suggests modes of action for carcinogens that would 
lead to nonlinear low-dose extrapolation. Note that the 1980 
Methodology preceded the Agency's original 1986 cancer guidelines, 
which are now being revised. We are maintaining our policy to derive 
AWQC for carcinogens to correspond to incremental lifetime cancer risk 
levels, applying a risk management policy that ensures a reasonable 
level of protection for the general population.
    When EPA developed the methodology to derive human health criteria 
for the waters of the Great Lakes System, the Agency was mindful of the 
need for consistency with the planned changes in the Human Health 
Methodology presented today for deriving national AWQC for the 
protection of human health. Throughout the 1998 draft Methodology 
revisions, references were made to comparisons of the two 
methodologies, especially whenever differences occur due to regional 
exposure assumptions made for the Great Lakes System. The GLI guidance 
consisted of water quality criteria, detailed methodologies to develop 
criteria for additional pollutants, implementation procedures, and 
antidegradation policies and procedures tailored to the Great Lakes 
system; these reflected the unique nature of the Great Lakes ecosystem. 
Those States and authorized Tribes are to use the GLI methodology to 
establish criteria for the waters of the Great Lakes system, which 
allows appropriate flexibility to States and authorized Tribes to 
develop equitable strategies to control pollution sources and to 
promote pollution prevention practices. The 2000 Human Health 
Methodology is undertaken pursuant to Section 304 of the CWA, and is 
independent of, and does not supersede, the GLI. Although consistency 
in State water quality standards programs is an important goal for EPA, 
we also recognize that it is necessary to provide appropriate 
flexibility to States and Tribes, both Great Lakes States and non-Great 
Lakes States, in the development and implementation of place-based 
water quality programs. Recognition of a general need for flexibility 
is not incompatible with the requirements for the Great Lakes States 
and Tribes established in Section 118(c)(2) of the CWA. We have 
harmonized the two, where appropriate, while maintaining parameters and 
provisions that are appropriate for Great Lakes-specific criteria.
    EPA has identified development of microbial water quality criteria 
as part of its strategy to control waterborne microbial disease, by 
controlling pathogens in waterbodies and by protecting designated uses, 
such as recreation and public water supplies. The program fosters an 
integrated approach in order to protect both ground-water and surface 
water sources. EPA plans to conduct additional monitoring for 
Cryptosporidium parvum and Escherichia coli, and determine action plans 
in accordance with the results of this monitoring.
5. Designated Uses
    Comments--Commenters indicated that designated uses for waterbodies

[[Page 66454]]

that cross State boundaries and that fail to take into account 
downstream uses may effectively prohibit downstream waters from being 
used as a water supply; the AWQC should reflect the use of a waterbody 
as a drinking water source unless the use patterns of the entire 
waterbody indicate that this is not a current or future possibility.
    Response--EPA regulations at 40 CFR 131.10(b) state:

    In designating uses of a water body and the appropriate criteria 
for those uses, the State shall take into consideration the water 
quality standards of downstream waters and shall ensure that its 
water quality standards provide for the attainment and maintenance 
of the water quality standards of downstream waters.

We believe this requirement is sufficient to address the concerns 
raised by the commenter and to ensure downstream uses are maintained 
and protected.
6. Developing National 304(a) Criteria
    Comments--Commenters stated that EPA should not derive national 
304(a) AWQC and stated their preference for regional measurements, and 
that national 304(a) criteria could be overly stringent or 
underprotective from State to State. Instead, they recommended that EPA 
simply provide specific ``algorithms'' to force States to develop their 
own criteria. However, they also said that EPA should develop a single 
criterion for each chemical based on the most relevant toxic endpoint 
and appropriate target population. A commenter recommended that EPA 
develop criteria for both cancer and noncancer endpoints because their 
comparative protectiveness may not be clear until permit limit design 
flows are determined. Another commenter stated that relying on default 
parameter values would inhibit the process for developing criteria/
implementing standards because the regulated community will not accept 
such criteria. Two commenters stated that the amount of information on 
adverse impacts to water quality, fish, birds, wildlife, and human 
health warrants regulatory action to eliminate those toxicants. They 
recommended that EPA include all biotic pathways using the water 
source, including wildlife and plant life, and advocated protecting 
cultural and religious uses. A commenter stated that limited 
information exists for development of criteria in arid regions and that 
resources would be better spent gaining knowledge on the impacts of 
chemicals in regional watersheds. Another questioned how AWQC can be 
derived when ambient levels are below analytical detection limits. 
Several commenters supported the derivation of fish tissue criteria.
    Response--Section 304(a) of the CWA requires EPA to develop 
national water quality criteria recommendations for States and 
authorized Tribes to use as guidance in adopting water quality 
standards. It is not an option for EPA to ignore this requirement. As 
such, the national 304(a) criteria that EPA periodically publishes are 
generally applicable to the nation's waters. Although we encourage 
States and authorized Tribes to use the Methodology to develop criteria 
based on local/regional information and believe that water quality 
criteria reflecting such local conditions are desirable, we have not 
abandoned our obligations under the CWA. The commenter should be aware 
that States have adopted EPA's recommended 304(a) criteria. 
Furthermore, in contrast to another commenter's suggestion, under the 
CWA, 304(a) criteria are not enforceable regulations; these criteria 
are guidance and do not impose legally binding requirements.
    States and Tribes always have the option to undertake their own 
evaluations to develop water quality criteria, as long as such criteria 
are consistent with the CWA and the implementing Federal regulations. 
States have derived water quality criteria for their waters in the 
absence of EPA guidance and may continue to do so. However, the 
recommended criteria serve as guidance to States and authorized Tribes, 
and EPA cannot force States or Tribes to conduct their own evaluations. 
We are well aware that the resources and expertise within States and 
Tribal authorities vary greatly and, while encouraging them to pursue 
their own criteria development programs, we anticipate that many will 
continue to rely on our expertise and recommended 304(a) criteria. We 
included guidance on site-specific modifications for States and 
authorized Tribes to derive their own water quality criteria and will 
expand this information as part of the TSD volumes for the 2000 Human 
Health Methodology.
    Although we have provided numerous default parameter values for 
different population groups, we intend to derive or revise AWQC based 
on the most sensitive health endpoint and the population group most 
relevant for that endpoint. Regarding measurable levels of chemicals in 
the water column, the CWA clearly states that limitations in analytical 
methods will not be considered when deriving AWQC. Rather, the AWQC 
represent health-based considerations only. However, analytical method 
limitations are taken into account in the implementation of water 
quality standards. We believe that deriving AWQC based on fish tissue 
concentrations may be appropriate in some instances to overcome this 
problem when there is a health concern for that chemical (for greater 
discussion of fish tissue criteria, see response to Comment F.7). 
Regarding cancer versus noncancer endpoints, it is EPA policy to 
develop criteria for the most sensitive endpoint in order to be 
protective of both potentially relevant cancer and noncancer effects. 
EPA intends to continue this practice. Regarding design flows, see the 
response on this issue under Comment A.9. Finally, these Methodology 
revisions apply to the protection of human health only. Other EPA 
efforts to develop methods and criteria for the protection of birds or 
other wildlife are not part of this guidance and will not be addressed 
here. Considerations such as religious or cultural uses cannot be 
quantitatively factored into the AWQC equation for setting pollutant 
criteria values.
7. Developing Organoleptic Criteria
    Comments--Commenters suggested that EPA should provide guidance for 
States to develop organoleptic criteria for ambient waters that are 
sources of drinking water, and develop specific organoleptic criteria. 
Taste and odor are strongly associated with consumer perceptions and 
confidence in water quality. They suggested that EPA should provide 
organoleptic criteria and allow States to make decisions about their 
use. Others stated that organoleptic criteria should not be developed 
because they are not relevant to protection of human health and because 
they should only be considered for drinking water standards.
    Response--The 2000 Human Health Methodology is focused on deriving 
toxicity-based criteria because they, not organoleptic criteria, are 
directly related to potential adverse human health effects. We have 
received much support for our position on this issue since initiating 
the Methodology revisions. EPA acknowledges that if organoleptic 
effects (i.e., objectionable taste and odor) cause people to reject the 
water and its designated uses, then the public is effectively deprived 
of the natural resource. EPA encourages the development of organoleptic 
criteria when States and Tribes believe they are needed to protect 
designated uses and have indicated this in the 2000 Human Health 
Methodology.

[[Page 66455]]

8. Establishing EPA's Most Recent Federally Recommended Water Quality 
Criteria
    Comment--A commenter stated that the proposed California Toxics 
Rule (CTR) established EPA's most recent federally recommended water 
quality criteria, and because EPA did not propose to promulgate arsenic 
in the CTR, there is no federally recommended water quality criterion 
for arsenic.
    Response--With regard to arsenic and the Agency's policy on 
applicable 304(a) criteria, EPA clearly stated in the 1998 draft 
Methodology revisions that until such time as the Agency re-evaluates a 
chemical and subsequently publishes revised chemical-specific 304(a) 
criteria, the existing criteria remain in effect. Although the 2000 
Human Health Methodology represents improvements to the 1980 
Methodology, EPA believes that the existing 304(a) criteria are 
fundamentally sound from a scientific standpoint. We have long 
supported this position. Our recommended water quality criterion for 
arsenic remains the value published in EPA's Goldbook in 1986 (USEPA, 
1986d) and promulgated in 1992 as part of the NTR. Federal 
promulgations for individual States take into account the needs of the 
individual State and site-specific conditions of waterbodies within the 
State. Federally promulgated water quality standards for a State may 
not always result in water quality criteria that are nationally 
applicable. We understand there has been some confusion regarding the 
current recommended water quality criteria in light of State-specific 
promulgations, and as a result, in 1998, we published National 
Recommended Water Quality Criteria (USEPA, 1998b) to clarify our 
national recommendations. This list will be updated approximately on an 
annual basis to contain our most current recommended water quality 
criteria for States and authorized Tribes to use as guidance in 
adopting water quality standards.
9. Flows
    Comment--Comments received suggested that EPA should adequately 
consider and account for regional differences, such as highly variable 
flows, lower exposures, and lack of fish habitat due to no-flow 
conditions in many Southwestern washes (i.e., waterbody flow only 
following a storm event).
    Response--EPA believes there is sufficient flexibility in the 
current regulatory program for States to modify designated uses and 
water quality criteria to protect those uses to address the conditions 
that exist in waterbodies such as intermittent streams and washes. 
Modifications to the water quality standards program are unwarranted at 
this time.
10. Implementation on a Waterbody Basis
    Comment--Commenters stated that human health criteria should be met 
within the waterbody on a long-term average basis instead of short-term 
maximums never to be exceeded. It was recommended that States be able 
and even encouraged to develop site-specific standards for waterbodies 
to reflect relevant fish consumption rates.
    Response--The 2000 Human Health Methodology incorporates long-term 
exposure into the development of water quality criteria. Determination 
of when human health criteria are met within the waterbody is beyond 
the scope of this document. However, EPA guidance addresses this issue 
(USEPA, 1991b). We recommend harmonic mean flow to calculate permit 
limits and taking the geometric mean of ambient water samples to 
determine attainment. Both of these recommendations account for the 
long-term exposure effects of chemical water quality criteria.
    EPA recommends that States develop site-specific water quality 
criteria to reflect relevant fish consumption rates. We have published 
default fish consumption rates in the Methodology as recommendations to 
States and Tribes in adopting water quality standards when a State or 
Tribe lacks information on local fish consumption rates. EPA's 
preference, however, is that States and Tribes adopt human health 
criteria reflecting local fish consumption rates.
11. Proposed Chemical List
    Comments--Commenters suggested that EPA integrate the AWQC 
prioritization process with the drinking water program (i.e., with the 
Candidate Contaminant List). Other comments suggested that EPA's short 
list of pollutants (for revision) would result in a greater burden for 
States that will need to develop more criteria. EPA was asked to 
strengthen efforts to develop criteria for persistent chemicals and to 
add endocrine disruptors. It was pointed out that the short priority 
list published in the 1998 draft Methodology revisions includes 
numerous banned pesticides. Additional chemicals and microbial 
contaminants for EPA to consider in its prioritization of criteria to 
revise/develop are suggested, as follows:

Atrazine
Benzo(a)pyrene
Chlordane
Cryptosporidium parvum strains
Cyanazine
Endrin
Giardia lamblia
Heptachlor
Heptachlor epoxide
Hexachlorobenzene
Methyl-tertiary-butyl-ether (MTBE)
Lead
Other PAHs (specifically advocated use of Relative Potency Factors)
Total Organic Carbon (TOC)
Toxaphene

    Response--We will evaluate all suggested pollutants based on the 
following factors: relative toxicity; occurrence in fish tissue, water, 
and sediments (frequency as well as concentration levels); and for 
chemicals, information on the chemical's bioaccumulation. This 
strategy, previously published in the 1998 draft Methodology revisions, 
received general support, and we will consider these suggestions along 
with priorities identified by both the Office of Pesticide Programs and 
the Office of Ground Water and Drinking Water, and other input received 
from States and Tribes. Regarding a State's need to revise more 
criteria, see the response to Comment A.13, Revising Existing 304(a) 
Criteria.
12. Publishing Existing 304(a) Criteria Information
    Comments--EPA received support for its proposal to occasionally 
publish a list of its criteria and information on revisions or new 
criteria in progress. Some commenters stated that EPA should publish a 
list in the Federal Register annually, and one suggested that EPA post 
any changes during the interim on the Agency's website. It was also 
suggested that EPA should identify which criteria were changed and why. 
One commenter stated that a timeframe of 3 to 5 years is more 
appropriate because little is likely to change in just one year. 
Another commenter expressed support for publishing an annual list of 
EPA drinking water regulations and health advisories.
    Response--EPA believes that regular updates on its website are the 
most efficient way to make accurate information available to the 
public. We hope this will be helpful for States and authorized Tribes 
in reviewing and revising their water quality standards during the 
triennial reviews required under 40 CFR 131. We will consider further 
the circumstances and frequency with which Federal Register 
publications may be used. The commenter who referred to drinking water 
standards and health advisories misunderstood EPA's intention, which is 
to publish a list annually on the

[[Page 66456]]

304(a) water quality criteria similar to that done for the drinking 
water program.
13. Revising Existing 304(a) Criteria
    Comments--EPA received support for revising its Methodology and for 
providing clear indication of the scientific components versus the 
science policy components. Commenters supported the idea of EPA 
revising criteria based on partially updated components of the criteria 
equations. One expressed a preference for comprehensive revisions but 
also stated that partial updates should be done as soon as possible, 
referring to components such as fish consumption rates and 
``interspecies conversion of doses'' as those that can automatically be 
inserted, thereby enabling revision of all criteria within a week of 
effort. [Note: It is unclear whether the commenter is referring to the 
new body weight/surface area scaling factor or something else by the 
term ``interspecies conversion of doses,'' because it is not 
specified.] A commenter stated that as any component is updated, so 
should the criteria. Another suggested that EPA partially revise all 
criteria for the components that current information would allow. On 
the other hand, a commenter stated that EPA should not revise criteria 
based on the new scaling factor or other pieces of data, but should 
conduct literature searches for new available data applicable to the 
Methodology. Other comments were that priority should be given to 
chemicals with significant new toxicity information; the use of partial 
updates is not scientifically sound, will produce overly conservative 
criteria, and restricts the public's right to comment; and all revision 
actions should be subject to public review and comment.
    Response--EPA ideally seeks to conduct re-evaluations of every 
component used in the derivation of 304(a) criteria before revising any 
criteria. However, we have discussed updating a limited number of 
304(a) criteria over the course of the next several years based on one 
or more components of the criteria equation (a ``partial update'') 
rather than a complete set of components, realizing that updating some 
of these (e.g., the BAF, the exposure parameters) is not as time-or 
resource-intensive as completing a toxicological evaluation. Recent 
actions taken by EPA represent this option; both the NTR and the GLI 
were partial updates. We intend to focus our limited resources on 
revising (either partially or completely) those pollutants that we 
consider highest priority in terms of both toxicological concern and 
frequency of occurrence.
    EPA has indicated that it does not believe it is desirable to 
revise criteria based on piecemeal information, such as the 
interspecies scaling factor, when there may be other information (e.g., 
new toxicity studies) that could also change the risk assessment and, 
thus, the criteria. We have also cautioned the States and Tribes not to 
selectively apply data or methods that would inaccurately characterize 
risk (e.g., in order to ensure a water quality criterion that is less 
stringent than an EPA 304(a) criterion). For a water quality criterion 
revision based on a partial update to be considered acceptable to EPA, 
a component of the criterion (e.g., the toxicological risk assessment) 
would need to be comprehensive (e.g., a new or revised RfD or cancer 
dose-response assessment, as opposed to simply a new scaling factor), 
should stand alone and be based on new national or local data. A 
toxicological update should be on a weight-of-all-of-the-evidence 
basis, as called for under EPA's risk assessment guidelines. This 
should incorporate the latest published toxicological literature and 
risk assessment approaches. States or authorized Tribes seeking to 
establish ambient water quality criteria are urged to continue using 
the IRIS noncancer and cancer risk assessments if they cannot conduct a 
complete evaluation to update toxicological values.
    The Agency has developed an improved process that it intends to use 
when deriving new criteria or conducting a major reassessment of 
existing criteria. The process is intended to provide expanded 
opportunities for public input and to make the process more efficient. 
When deriving new criteria or when initiating a major reassessment of 
existing criteria, we will publish a notice in the Federal Register and 
on the EPA website announcing our assessment or reassessment of the 
pollutant. References relied on will be provided, and we will solicit 
additional data or information useful in deriving new or revised 
criteria. After input is received and evaluated, we will develop draft 
recommended water quality criteria. Next, EPA will initiate an 
independent external peer review of the draft criteria. The public will 
also be able to submit views on issues of science pertaining to the 
information used in deriving the draft criteria. We will then revise 
the draft criteria as necessary, incorporating peer review and public 
input, and announce the availability of the final water quality 
criteria in the Federal Register and on the EPA website. In addition to 
developing new criteria and conducting major reassessments of existing 
criteria, EPA also from time to time will partially revise criteria 
based on new information pertaining to individual, stand-alone 
components of the criteria. Because such recalculations normally result 
only in changes to single parameters of the criteria (not in the 
underlying scientific methodologies) and reflect peer-reviewed data, 
EPA will typically publish such recalculated criteria directly as the 
Agency's recommended water quality criteria. If substantial revision is 
done, we will follow the process of peer review and public input 
outlined above. Further discussion of this process can be found in the 
Federal Register Notice compilation of recommended water quality 
criteria and notice of process for new and revised criteria (USEPA, 
1998e).
14. State Evaluation of Data Supporting Criteria
    Comment--One commenter asserted that ``states should be allowed to 
critically evaluate all data and disregard data that, for one reason or 
another, are unrepresentative or unreliable'' and further asserted that 
States should be allowed to critically review EPA's published 304(a) 
criteria and to decline to adopt any criteria they feel are 
inappropriate.
    Response--EPA disagrees with underlying assumptions of the comment. 
EPA's 304(a) criteria are guidance. States and authorized Tribes may 
develop their own scientifically defensible, peer-reviewed criteria. 
Moreover, States and any other interested parties have the opportunity 
to participate in development of water quality criteria published under 
Section 304(a) of the Act. Prior to publishing any new or revised 
304(a) criteria, EPA provides stakeholders with an opportunity to 
review and provide scientific views. EPA maintains that at the time of 
publishing of new or revised 304(a) criteria, the criteria are 
scientifically defensible and establish guidance to States for adopting 
water quality standards under section 303(c) of the Act. Under 40 CFR 
131.11, States continue to have the option of adopting water quality 
criteria based on 304(a) criteria modified to reflect site-specific 
conditions, or other scientifically defensible methods.
15. Streamlined Approach to Developing Criteria Documents
    Comment--EPA received support for the streamlined format used in 
the example criteria documents published in 1998.
    Response--We acknowledge this support.

[[Page 66457]]

16. Treaty Rights and Trust Obligations/Government-to-Government 
Relations
    Comments--Commenters recommend EPA fully incorporate treaty rights 
and Federal trust obligations to Indian tribes in its national AWQC 
guidelines. It was reiterated that EPA has an obligation to maintain 
government-to-government relations with Tribal Governments.
    Response--As stated in the 1998 draft Methodology revisions, ``risk 
levels and criteria need to be protective of tribal rights under 
federal law (e.g., fishing, hunting, or gathering rights) that are 
related to water quality.'' We believe the best way to ensure that 
Tribal treaty and other rights under Federal law are met, consistent 
with Federal trust responsibility, is to address these issues at the 
time EPA reviews water quality standards submissions.

B. General Policy

1. AWQC Derivation Equation Errors
    Comments--Commenters pointed out that the term ``RSC'' (relative 
source contribution) in the Linear Cancer Effects equation of the 1998 
draft Methodology revisions was incorrect and should have been ``RSD'' 
(risk-specific dose).
    Response--The commenters are correct; this was a misprint and 
should have been RSD for the linear equation.
2. Chronic Human Health Effects Assumption
    Comments--EPA received support for its assumption that, by and 
large, AWQC are set to protect against long-term (chronic) human health 
effects.
    Response--We acknowledge the commenter's support.
3. Protectiveness of the Methodology
    Comments--A commenter stated that inherent uncertainties in EPA's 
risk assessments make them useless and that EPA must adopt the most 
conservative methodologies in order to protect human health, while also 
acknowledging the presence of uncertainties in assessing adverse health 
impacts. They suggested that EPA should tighten regulations for 
chemicals of national priority, develop criteria for additional 
priority chemicals, and take the most conservative approach regarding 
reproductive and developmental effects. Other commenters advocated that 
EPA incorporate pollution prevention policies into its risk assessment 
methodologies. One commenter asked EPA to provide guidance to States 
for developing AWQC less restrictive than AWQC for the general public, 
and suggested that engineering and administrative controls could reduce 
exposures. Another stated that the population groups identified 
represent appropriate categories and that the corresponding default 
parameter values are reasonable. The same commenter advocated use of 
the same percentile value for each default parameter (``e.g., 95th 
percentile''). Another commenter recommended that EPA determine 
distributions of exposure in order to assess whether a significant 
subgroup is more highly exposed than the general population, especially 
in the context of the chosen exposure parameter values. Others stated 
that the general population should not be targeted and that EPA should 
instead target the population group most at risk, or that protection of 
health should apply to all humans. Commenters also expressed 
uncertainty over the segment of the population that the AWQC are 
designed to protect, and questioned whether EPA would evaluate all 
subpopulations for all chemicals. Two commenters requested an analysis 
of the overall impact that each parameter has on the criteria and how 
that relates to the conservativeness of the estimated risk, with one 
criticizing EPA for not conducting probabilistic analyses of exposures 
or other methods to evaluate the interaction of exposure parameters. 
This commenter stated that the Agency has used ``high confidence-
level'' values for all parameter values and, therefore, the AWQC are 
``inordinately conservative.'' Furthermore, EPA should specify the 
level of protection within the high-end proportion of the general 
population (e.g., ``the 95% level'') and adjust the exposure parameter 
values within ``their defined distributions.'' Concern was expressed 
that the flexibility regarding infants and children (i.e., for 
developmental effects) conflicted with the fact that chronic lifetime 
effects cover persons when they are children and adults. A commenter 
recommended consideration of tissue effects, as well as organ-level 
effects. Another stated that increasingly strict criteria/discharge 
limits represent regulatory environmental injustice, and that 
discharges in effluent-dependent streams are necessary for trees, 
vegetation, and wildlife.
    Response--EPA believes that it has made appropriately conservative 
assumptions in conducting risk assessments where uncertainties exist. 
Furthermore, for this effort we will rely on the Agency's peer-
reviewed, published risk assessment methodologies, which incorporate 
procedures to address uncertainties in the risk assessments. We will 
continue to make the most appropriate risk management decisions when 
developing or revising criteria, including determining pollutants of 
high priority. EPA does consider tissue-level effects in addition to 
organ-level effects when conducting its risk assessments. We 
acknowledge the comment regarding integrating pollution prevention 
policies with our risk assessment methodologies and specifically 
discuss this in the context of CWA goals in the 2000 Human Health 
Methodology. We also believe that we have selected appropriate default 
parameter values. Regarding the idea of criteria that are less 
restrictive than EPA's 304(a) criteria, a State or authorized Tribe 
would have such flexibility as long as it could clearly demonstrate 
that the criteria it calculated would be protective of its population. 
Such alternate assessments and the resulting proposed State or Tribal 
standard would be subject to EPA's triennial review process. 
Furthermore, the AWQC are health-based criteria, and therefore 
potential effects of engineering and administrative controls are not 
part of criteria.
    By and large, the AWQC are derived to protect most of the overall 
population from chronic adverse health effects. However, States and 
authorized Tribes also need to understand that there are RfD's based on 
developmental or other short-term adverse health effects, perhaps where 
an exposure of one day could result in the effect. Long-term averaging 
of exposure would not be appropriate in such circumstances. States and 
authorized Tribes are also encouraged to consider protecting population 
groups that they determine are at greater risk and, thus, would be more 
protected using alternative exposure assumptions. We do not intend to 
derive multiple criteria for all subpopulation groups for every 
chemical. The commenter who discussed probabilistic analyses has 
misunderstood EPA procedures. We have used median and mean values, and 
percentile estimates, not high confidence-level values, as suggested by 
the commenter. We also disagree that the resulting criteria represent 
inordinately high levels of conservativeness. In general, we are doing 
what the commenter recommended about targeting the overall protection 
at the high end of the general population, even though the criteria 
have not been subjected to an assessment of whether a 95% level has 
been achieved (as recommended by the commenter). Although we have not 
subjected the parameter values chosen

[[Page 66458]]

to a rigorous analysis, we have not used high-end percentiles for all 
parameters. The assumed body weight value used is an arithmetic mean, 
as are the RSC intake estimates of other exposures, when data are 
available. The BAF component data values are based on median (i.e., 
50th percentile) values. The drinking water and fish intake values are 
90th percentile estimates. We believe this will result in water quality 
criteria that will be protective of a majority of the population. That 
is our goal. The commenter has not provided a method that would allow 
us to determine the overall percentile associated with the criteria 
calculations. EPA has provided additional language in the 2000 Human 
Health Methodology to clarify the population the AWQC are intended to 
protect.
    Finally, if EPA determined that pregnant mothers/fetuses or young 
children are the population basis of a chemical's RfD or POD/UF, then 
we would derive our 304(a) criteria using exposure parameter values for 
that subgroup. This would be relevant only for less-than-lifetime 
exposure situations and, therefore, does not conflict with the fact 
that chronic health effects potentially reflect a person's exposure 
during both childhood and adult years.
4. Setting Criteria to Protect Both Fish and Drinking Water Versus Fish 
Only
    Comments--EPA received strong support for deriving one AWQC value 
to protect both drinking water and fish intakes and another to protect 
for fish intakes only, given that the designated uses of waterbodies 
vary and drinking water may not be a designated use. One commenter 
stated that in addition to these two types of criteria, EPA should also 
develop criteria for water ingestion only. They indicated that waters 
may exist where fishing and consumption of fish are not relevant but 
water ingestion is relevant. Furthermore, they pointed out that EPA's 
Advanced Notice of Proposed Rulemaking for Water Quality Standards 
discussed protection for aquatic life and, therefore, stated that 
flexibility is needed so that fish consumption is not inappropriately 
applied to all waters. A commenter questioned whether ambient waters 
that are fished are also sources of drinking water, and whether 
contaminant levels in the two water types could be equivalent. Others 
stated that the drinking water pathway should not be included in the 
AWQC, given the way AWQC are implemented (e.g., AWQC apply to waste 
water discharges and MCLs apply to public drinking water system 
exposures) and that MCLs may consider affordability and treatability. A 
commenter stated that AWQC to protect fish/shellfish are not justified 
and should be dealt with under other regulatory programs (e.g., the 
Food Quality Protection Act).
    Response--EPA believes that AWQC should include a drinking water 
pathway to protect waters designated as potable water sources. (Also 
see EPA's response to Comment A.4 regarding the relationship between 
MCLs/MCLGs and AWQC, Coordinating the Human Health Methodology With 
Other EPA Programs.) EPA strongly disagrees that AWQC to protect humans 
exposed through consumption of fish/shellfish should not be developed. 
Ensuring the protection of human health from consumption of 
contaminated fish and shellfish is clearly within the requirements of 
the CWA. We do not believe that 304(a) criteria to protect drinking 
water uses only are particularly useful, because by and large, State 
and Tribal standards for human health are set to protect waters with 
multiple designated uses, not merely drinking water use. The water 
quality standards program also protects aquatic life. The 2000 Human 
Health Methodology will not change our requirement to apply aquatic 
life criteria to protect aquatic species where they are more sensitive 
(i.e., when human health criteria would not be protective enough) or 
where human health via fish or water ingestion is not an issue.
5. Setting Criteria to Protect Against Multiple Exposures From Multiple 
Chemicals
    Comments--Several commenters thought EPA should consider multiple 
chemical exposures when setting AWQC and consider these exposures 
additive, at a minimum, while using information on synergistic impacts 
from the combination of chemicals. Commenters also suggested that 
certain Native American Tribes may have significant confounding factors 
(not specified) to be considered with any synergistic assessment. A 
commenter suggested that the cancer risk range apply to total 
contaminants or that a cumulative cancer ceiling be established. 
Another stated that the suggested alternate approach to account for 
inhalation and ingestion exposures (via the RfD and RfC equation) 
regardless of the target organ/endpoint was inconsistent with EPA's 
guidance on the use of hazard indices (HIs) and hazard quotients (HQs) 
to evaluate multiple noncarcinogenic toxicants. Commenters also 
questioned whether all exposure routes exhibit the same toxicity or 
stated that inhalation exposures should be disregarded if the pollutant 
in question does not affect the same endpoint.
    Response--Assuming that all multiple exposures from multiple 
chemicals are additive, as the commenters suggest, is not 
scientifically sound unless they exhibit the same toxic endpoints and 
modes of action. We are aware of the complex issues and implications of 
cumulative risk and are developing an overall approach at the Agency-
wide level. In particular, the Agency's program offices are engaged in 
ongoing discussions on how to address the great complexities, 
methodological challenges, data adequacy needs, and other information 
gaps, as well as the science policy and risk management decisions that 
will need to be made, as we pursue developing a sound strategy and, 
eventually, specific guidance for addressing cumulative risks. As 
previously indicated, the Agency is developing a framework for 
cumulative risk assessment, and the Office of Pesticide Programs has 
developed draft guidance for assessing cumulative risk of common 
mechanism pesticides and other substances. We have added discussion 
about the concept of cumulative risk and the state of the science in 
the 2000 Human Health Methodology and its TSDs. As a matter of internal 
policy, we are committed to refining the Methodology as advances in 
relevant aspects of the science improve. Regarding the alternate 
approach to use the HI/HQ equation (combining RfDs and RfCs), we do not 
intend to use this approach to combine chemicals when deriving criteria 
at this time. We requested comment on this as an alternate method to 
consider inhalation exposures for a given chemical, but would not 
consider its use in situations where existing information indicates 
that ingestion exposures and inhalation exposures affect different 
target organs. EPA intends to consider the comparative toxicity between 
exposure routes for Section 304(a) water quality criteria and has 
encouraged States and Tribes to do so. For the recommended national 
304(a) criteria, cumulative risk approaches will not work since the 
mixture of pollutants present in water is inherently site-specific.
6. Uncertainty with the Derivation of 304(a) Criteria
    Comment--Comments suggested that cumulative uncertainty guidance 
should be included in the Methodology, including a maximum acceptable 
uncertainty level.
    Response--Establishing a maximum level of acceptable uncertainty is 
not part of the Methodology and will not be

[[Page 66459]]

factored into the decision of whether to develop or revise 304(a) 
criteria. However, issues regarding uncertainties with the risk 
assessments, exposure assessments, and bioaccumulation assessments will 
be addressed in the risk characterization sections of future criteria 
documents.
7. Toxicity Equivalency Factors (TEFs) for Dioxin-like Compounds
    Comments--Several commenters addressed the use of TEFs for dioxin-
like and other mixtures and classes of compounds. They believed the TEF 
approach has only limited application in risk assessment. Commenters 
indicated that complexities of the biology argue strongly against any 
more than limited and very cautious use of the TEF approach for 
assessment of human health from exposure to dioxin-like compounds.
    Response--EPA agrees that there is a limitation to TEF use and that 
caution should be exercised when using it. More guidance can be found 
in the Guidance for Conducting Health Risk Assessment of Chemical 
Mixtures (USEPA, 1999b) and the Health Assessment for 2,3,7,8-
Tetrachlorodibenzo-p-dioxin (TCDD) and Related Compounds, Internal 
Review Draft, February 14, 2000; Part II, Chapter 9: Toxicity 
Equivalency Factors (TEFs) for Dioxin and Related Compounds (USEPA, 
2000b).

C. Cancer

1. Acceptable Risk Level for Carcinogens
    Comments--Comments were received suggesting that regulations should 
be tightened or that AWQC for all carcinogens including the Groups C 
compounds (possible human carcinogens) should be set at zero, while 
others believed that cancer potency factors may overestimate actual 
risk. Some suggested the actual risk may be much lower, perhaps as low 
as zero, particularly for chemicals for which human carcinogenicity 
information is lacking. Comments also addressed the EPA cancer risk 
range for deriving AWQC.
    Response--Regarding the permissible cancer risk range, see response 
to Comment A.3, Cancer Risk Range.
2. ED10 (central estimate) versus LED10 (lower bound on dose)
    Comments--Several commenters preferred the use of ED10 over LED10 
as the POD or BMD.
    Response--The 1999 draft revised cancer guidelines provided a 
rationale for the selection of PODs. EPA's 1999 draft revisions provide 
for the use of the LED10. The EPA Science Advisory Board (SAB) suggests 
harmonization of the LED10 between the BMD approach for noncancer 
assessments and cancer assessments. The SAB also recommends reporting 
both the LED and ED (see USEPA, 1999d).
3. Group C Contaminants
    Comments--One commenter stated that Group C compounds are treated 
differently under the SDWA and the CWA and wanted clarification on 
development of AWQC for Group C contaminants. Also, an ``integrated 
approach'' was suggested in evaluating nonlinear carcinogen and 
noncarcinogen assessments. However, the commenter's approach was to 
determine tentative AWQC for the contaminant as both a noncarcinogen 
and a carcinogen at 10-\6\ risk, and then choose the lower 
of the two values (i.e., RfD vs. 10-\6\ risk) for setting 
the AWQC. Another commenter stated that integrating nonlinear and 
noncarcinogen assessments proposed by EPA is reasonable and it may be 
possible to replace this in the future with the categorical regression 
approach.
    Response--The 1999 draft revised cancer guidelines require risk 
assessors to use the best science and consider mode of action in 
selecting an appropriate model to use. Under the 1999 draft revised 
cancer guidelines, Group C will no longer exist. The linear approach is 
used when there is insufficient information on mode of action, or the 
mode-of-action information indicates that the dose-response curve at 
the low dose is or is expected to be linear. The default approach for 
nonlinearity is to use a margin of exposure analysis. However, when the 
mode of action suggests both linear and nonlinear approaches, then both 
methods will be applied and considered. As for the integrated approach, 
EPA currently is working to increase the harmonization of both cancer 
and noncancer risk assessments. In the 2000 Human Health Methodology, 
we will only quantify cancer risks for those chemicals considered 
``carcinogenic to humans'' or ``likely to be carcinogenic to humans.''
4. Guidance on Carcinogen Risk Assessment
    Comments--Several commenters supported EPA's 1996 proposed cancer 
guidelines. They endorsed the proposed guidelines for considering all 
scientific data and using the latest information, including weight of 
evidence, mode of action, margin of exposure, and a nonlinear approach 
for certain contaminants. They thought the new approach is more in line 
with recent advances in understanding carcinogenesis. However, they 
requested more guidance on how and when to apply the cancer guidelines.
    Response--We will provide more guidance when the guidelines are 
finalized.
5. Hexachlorobutadiene (HCBD)
    Comments--Comments stated that EPA should not propose AWQC for HCBD 
before the 1999 draft revised cancer guidelines are final. Furthermore, 
for HCBD, there is inconsistency between the statement in the 1998 
Federal Register (Appendix VI) and that in the example HCBD criteria 
document.
    Response--The Agency is considering the comment and will postpone 
completion of the AWQC for HCBD until more recent data can be 
incorporated. In reference to the risk assessment of the chemical, the 
discrepancy is minor. The 1998 Methodology states that both linear and 
nonlinear approaches will be used by EPA. The criteria document 
presents both approaches.

    Note: EPA also will postpone completion of the criteria for 1,3-
dichloropropene. Because of the large volume of new scientific 
information available for acrylonitrile, additional effort will be 
necessary to review the material. Therefore, EPA will not complete 
the criterion for acrylonitrile at this time. For the same reason, 
we are not addressing the comments on this chemical at the present 
time.

6. Integration of Analyses for Cancer and Noncancer Effects
    Comments--Commenters supported integration and harmonizing 
procedures for risk assessment of cancer and noncancer effects in 
ambient water and drinking water programs.
    Response--EPA agrees that it is a good idea to use an integrated 
approach to assess both cancer and noncancer effects. Currently, EPA 
has Agency-wide efforts to investigate harmonization of cancer and 
noncancer risk assessments.
7. Margin of Exposure (MOE) Analysis
    Comments--Commenters requested that EPA provide more guidance on 
how to do MOE analysis and how to select the MOE. They also requested a 
comparison of the BMD with the LED10.
    Response--Guidance will be provided either in the final Guidelines 
for Carcinogen Risk Assessment or in a separate document from the 
Agency's Risk Assessment Forum in the future.

[[Page 66460]]

8. MOE Approach to Applying Uncertainty Factors (UFs)
    Comments--A commenter disagreed with the proposal to apply a UF to 
account for the severity of a precursor effect. Another commenter 
opposed applying a UF of no less than 0.1 when humans are less 
sensitive than animals.
    Response--The Agency will develop more specific guidance on the MOE 
approach, as recommended by the SAB in 1999. The guidance will be peer 
reviewed and published separately as part of the Agency's 
implementation activity for these guidelines.
9. MOE and MOP
    Comments--Commenters seemed confused regarding MOE and MOP 
(``margin of protection,'' as defined by a commenter). They defined MOE 
= MOP = POD/RfD and claimed that the calculated MOEs for chemicals 
based on nonlinear low-dose extrapolation are 100 times higher than 
those for carcinogens based on linear low-dose extrapolation, and 
claimed that the MOE is implicitly linear and, thus, is an inadequate 
approach to dealing with ``nonlinear'' carcinogens.
    Response--There is a significant misunderstanding on the part of 
the commenters. The MOE is defined as the POD (i.e., NOAEL or LOAEL or 
LED10) divided by the environmental level of interest (actual exposure 
or possible criterion). The MOE approach is recommended for chemicals 
that have a nonlinear low-dose response. For carcinogens with a linear 
low-dose response, we estimate the slope of the line drawn between zero 
and the LED10, and use the equation presented in the Methodology to 
estimate the concentration in water for human heath protection 
(10-6 is the recommended risk level). EPA does not recommend 
using any formula such as the one presented [i.e., MOE = (POD) /(RfD)] 
to estimate MOE for carcinogens with a linear low-dose response.
10. Oral Scaling Factor for Dose Adjustment
    Comments--Several commenters endorsed EPA's use of the body weight 
raised to the three-quarters power as the scaling factor. It was also 
suggested that, if available, chemical-specific data should take 
precedence over the generic default scaling factor.
    Response--EPA agrees.
11. Toxic Endpoints
    Comments--A commenter stated that EPA should make clear in its 
Methodology that it intends to take into consideration the toxic 
actions of the individual chemicals for which criteria are being 
established so that an appropriate target population and consumption 
rate can be selected. The commenter suggested that if the critical 
toxic endpoint of a chemical is cancer or other chronic disease, then 
use of the adult population and long-term consumption rates are 
appropriate to develop the AWQC. However, if the most sensitive toxic 
endpoint of a chemical of interest is acute reproductive effects, it 
may be more appropriate to use short-term consumption rates and 
exposure parameters that are relevant for women of childbearing age in 
developing the AWQC.
    Response--EPA agrees.
12. Weight-of-Evidence Narrative and Classification System
    Comments--A commenter expressed support for the use of narrative 
statements, but found the guidance on the weight-of-evidence narrative 
to be overly general and confusing. They suggested that some sort of 
classification system such as the alphanumeric should be retained. They 
also stated that without such a system, practical use of the weight-of-
evidence approach will be more difficult, particularly for States that 
do not have strong expertise and sufficient resources in the 
application of health-based risk assessment.
    Response--Current revisions to the cancer guidelines and the use of 
descriptors and narratives have been endorsed by the SAB and other 
commenters and will be included in assessments and final guidelines 
because they provide important information to the risk manager that a 
number or letter cannot convey.

D. Noncancer

1. Benchmark Dose Methodology
    Comments--Commenters supported the flexibility of having the NOAEL/
LOAEL/UF, categorical regression, and benchmark options for derivation 
of an RfD but pointed out a variety of concerns or factors for EPA to 
consider as it revises the BMD guidance.
    Commenters suggested that the BMD methodology will eventually have 
a prominent role in risk assessment, but checks and balances need to be 
set to ensure that it is applied intelligently and with a healthy 
scepticism for its results, especially those that vary significantly 
from the results of the conventional NOAEL/LOAEL approach. The 
following specific recommendations were presented for EPA's 
consideration:
     Prohibit extrapolations without some mechanistic 
foundation. Permit interpolation only within the experimental dose 
range, for example, between NOAELS and LOAELS.
     Present a range of BMD estimates from the use of multiple-
dose models, including models with thresholds just below LOAELS; 
estimates with the high-dose results dropped sequentially from the 
analysis; and multiple response rates (i.e., 1%, 5%, and 10% response 
rates as well as the response rate associated with the experimental 
detection limit).
     Estimate the BMD using several confidence bounds.
     Compare the results of the alternative modeling approaches 
and reconcile discrepancies.
    Other comments are summarized in the following paragraphs.
    The BMD methodology lacks a mechanistic basis. There is no 
connection between the mechanisms of action that underlie the observed 
responses. Because the methodology is devoid of a mechanistic basis, 
its use needs to be restricted to the observable range. Extrapolations 
below the lowest nonzero dose of a study have no scientific foundation. 
However, it is acknowledged that some extrapolation of the data below 
the observable range is inevitable.
    An additional critique was that high-dose effects influence low-
dose estimates. The curve fitting involved in estimation of the 
mathematical dose-response relationship permits the responses at the 
high end of the dose range to influence the estimated responses at the 
low end of that range. This will occur whether or not the high-dose 
observations are mechanistically related to the responses at low doses. 
Furthermore, response and dose estimates are model dependent. In some 
cases, both central estimates and lower-bound estimates of doses 
associated with various response rates are known to be highly unstable 
and fluctuate significantly in response to minor data manipulations or 
assumptions.
    More research is needed on implementation of the benchmark model. 
Guidelines for selecting appropriate models/benchmark responses, 
handling lack of fit, or selecting a single benchmark dose when more 
than one is calculated should be developed by EPA to assist States and 
other users in implementing this methodology.
    The central estimate rather than the lower bound on dose should be 
used as the POD for benchmark modeling. Such an approach provides 
greater opportunity to compare effect doses among chemicals. 
Uncertainty associated with wide confidence limits

[[Page 66461]]

can be accommodated in other portions of the risk assessment process. 
Furthermore, the most recent peer review of the BMD methodology (USEPA, 
1996c) recommended use of the ED10 rather than the LED10.
    Use of the benchmark model could introduce additional conservatism 
into the derivation of an RfD. Certain benchmark models as applied to 
developmental toxicity endpoints are substantially more conservative, 
on average, than the corresponding NOAELs. Using the benchmark approach 
in such a circumstance will introduce additional unjustified 
conservatism in the standard-setting process.
    Caution should be taken when using different methods for RfD 
determination; that is, the degree of human health protection should be 
comparable from different methods. Because the BMD and categorical 
regression are relatively new methods, more studies are needed to 
compare the RfDs derived using the typical NOAEL/UF approach and those 
derived using the BMD and categorical regression methods.
    EPA should closely coordinate adopting BMDs for noncancer endpoints 
under the Human Health Methodology with other Agency programs so that 
the policy is implemented identically throughout the Agency. However, 
because the benchmark approach makes better use of all data, the Agency 
should continue to work on its development.
    Response--EPA agrees with the concerns regarding widespread 
application of the benchmark approach without consideration of the many 
factors addressed by commenters. The AWQC guidelines do not prescribe 
use of the benchmark approach in the derivation of an RfD. The 
guidelines allow the use of either the NOAEL/UF, benchmark, or 
categorical regression approaches. The risk assessor can select the 
approach most suitable to the available data. Accordingly, if the data 
do not support derivation of a BMD, then the NOAEL/UF approach can be 
selected for the RfD derivation rather than the benchmark approach. In 
addition, when selecting the appropriate equation for derivation of the 
BMD, one should consider goodness-of-fit along with the impact of high 
doses on the model results, confidence interval domains, and 
consistency of the dose-response pattern with the mode of action.
    We do not anticipate that either of the new approaches, benchmark 
or categorical regression, will soon completely replace the NOAEL/UF 
approach. Both of the new approaches require more extensive data than 
the NOAEL/UF approach, and in many cases the data required to apply the 
methodology will not be available.
    EPA is developing technical guidance that will assist in 
determining whether or not a particular data set is compatible with the 
BMD approach. Use of BMD methods involves fitting mathematical models 
to dose-response data obtained primarily from toxicology studies. When 
considering available models to use for a BMD analysis, it is important 
to select the model that best fits the data and is the most 
biologically appropriate. EPA has developed software following several 
years of research and development, expert peer review, public comment, 
subsequent revision and quality assurance testing. The software (BMDS, 
Version 1.2) can be downloaded from http://www.epa.gov/ncea/bmds.htm. 
BMDS facilitates these operations by providing simple data-management 
tools, a comprehensive help manual and online help system, and an easy-
to-use interface to run multiple models on the same dose-response data.
    As part of this software package, EPA has endorsed sixteen (16) 
different models that are appropriate for the analysis of dichotomous 
(quantal) data (Gamma, Logistic, Log-Logistic, Multistage, Probit, Log-
Probit, Quantal-Linear, Quantal-Quadratic, Weibull), continuous data 
(Linear, Polynomial, Power, Hill) and nested developmental toxicology 
data (NLogistic, NCTR, Rai & Van Ryzin). Results from all models 
include a reiteration of the model formula and model run options chosen 
by the user, goodness-of-fit information, the BMD, and the estimate of 
the lower-bound confidence limit on the benchmark dose (BMDL). Model 
results are presented in textual and graphical output files which can 
be printed or saved and incorporated into other documents.
2. Categorical Regression
    Comments--Commenters expressed reservations regarding use of the 
categorical regression methodology. They stated that the methodology 
presents difficulties in that it requires distinction of diverse 
endpoints and definition of severity categories, not as they apply to 
the animal studies, but as they apply to human health effects. 
Commenters also stated that categorical regression would allow the 
Agency to consider several endpoints simultaneously rather than use 
data for only the most sensitive endpoint. Some commenters believed 
that the major limitation of the approach is the need for classifying 
effects into categories (mild, moderate, frank).
    Other commenters believed regression analysis offers attractive 
advantages but does not seem well enough developed at the present time 
to be incorporated into the Methodology. They suggested that because 
the approach makes better use of all data, the Agency should continue 
to work on its development. They also stated that when the data 
indicate that one of the new methodologies is clearly superior to the 
NOAEL/LOAEL/UF approach, it should be utilized.
    Response--As stated in the response on BMD above, EPA does not 
anticipate that either of the new approaches, benchmark or categorical 
regression, will soon replace the NOAEL/UF approach. Both new 
approaches require more extensive data than the NOAEL/UF approach, and 
in many cases the data required by the methodology will not be 
available. We agree that the categorical regression methodology is less 
well developed than the benchmark method. However, we also anticipate 
that the number of chemicals evaluated with this approach will grow 
over time. Including the categorical regression methodology among the 
available options in the 2000 Human Health Methodology provides an 
opportunity for its application in appropriate situations.
3. Integrated Approach
    Comments--Commenters stated that an integrated approach to 
assessing both cancer and noncancer effects for substances that are 
carcinogenic has merit, particularly when the systemic effects of 
concern occur at very low doses. However, they believed it is unclear 
how the nonlinear cancer assessment and the noncancer assessment would 
differ if the tumors were considered secondary to the systemic toxicity 
upon which the RfD is based. They stated that such considerations 
become more important when the systemic toxicity is unrelated to tumor 
formation, as in the case of lead and mercury. Some indicated that 
because EPA recommends different design flows to account for exposure 
scenarios that are appropriate for carcinogenic and systemic effects, 
the Methodology should develop and adopt similar criteria for both 
carcinogenic and systemic effects when appropriate. Some further stated 
that for some waters and pollutants, it will not become clear whether 
the systemic or carcinogenic criterion is more protective until the 
limits are developed using the different design flows. This was not 
previously a concern because a single human health design flow was used 
in most locales.

[[Page 66462]]

    Response--The 2000 Human Health Methodology is not a stand-alone 
methodology. It depends on established or proposed Agency risk 
assessment guidelines for cancer and noncancer endpoints. We do not 
have the latitude to change Agency-wide risk assessment guidelines 
through the AWQC Methodology. Any changes must first be made to the 
supporting documents (e.g., 1999 draft revised cancer guidelines, RfD 
methodology).
4. Integrated Risk Information System (IRIS)
    Comments--Concern was expressed that EPA does not update the IRIS 
files in a timely manner. States use these assessments for their risk 
assessment work and do not have the resources to perform the types of 
detailed consensus risk assessment done under the IRIS process, 
according to comments received. They additionally pointed out that many 
IRIS assessments are more than 10 years old and suggested that EPA 
should update these assessments on a 3- to 5-year cycle.
    Response--We realize the importance of the IRIS program and 
dedicate a portion of our resources to preparation of IRIS 
documentation for regulated chemicals. However, competing priorities 
throughout the Agency limit the effort that can be expended on IRIS by 
program offices and by the IRIS program.
5. NOAEL/LOAEL Approach
    Comment--A commenter called attention to the facts that the NOAEL/
LOAEL/UF approach is the current approach for establishing an RfD and 
that many present regulatory values are based on this approach. They 
stated that use of newer techniques that account for severity of 
effects and sample size seems reasonable, as long as the new techniques 
have been extensively reviewed and have wide acceptability among 
practitioners. However, the commenter also said that in some cases, the 
data needed to use the newer techniques may not be available, in which 
case it seems entirely appropriate to use the NOAEL/LOAEL approach as a 
default.
    Response--See our responses to Comments D.1 and D.2, the benchmark 
dose and categorical regression comments, respectively.
6. Nonthreshold Approach for Noncarcinogens
    Comments--The Agency requested comments on the suitability of using 
a nonthreshold approach for noncancer endpoints. Although open to the 
concept, commenters stated that a threshold should be considered the 
norm and a nonthreshold approach should be applied only if there are 
substantial scientific data supportive of a nonthreshold mechanism of 
toxicity. They stated that when receptor interactions are a component 
of the response, it is important that EPA differentiate between the 
receptor binding that might be without a threshold and subsequent 
biological responses such as enzyme induction or frank toxicity that 
would be expected to exhibit threshold dose-response relationships.
    An additional concern was the use of nickel as an example of a 
chemical without a threshold. It was pointed out that double-blind 
studies indicate that there is a threshold for dermatological responses 
to nickel even in sensitized individuals.
    Response--The Agency made modifications to the recommendations 
regarding a threshold approach for noncarcinogens, most specifically 
using lead as an example rather than nickel. We incorporated the 
commenters' suggestions in making the revisions.
7. RfD Range
    Comments--The concept of establishing a range around the calculated 
RfD from which an alternative RfD might be selected in certain 
circumstances received considerable comment from the public. The 
primary criticism was the lack of a scientific basis for the breadth of 
the range and its correlation to the net uncertainty factor/modifying 
factor (UF/MF) product. The comments are summarized below.
    The span of the range as described by EPA seems to be arbitrary and 
without any scientific support. It would be useful for the Agency to 
analyze a substantial number of past RfD determinations using the 
ranges the Agency has proposed to see whether they make practical 
sense. The Agency should provide more examples on how the factors that 
are to be considered in selecting a point within the range (i.e., 
bioavailability differences, sensitive populations, and slope of the 
dose-response curve) are related to the magnitude of the proposed 
range. Scientific data should be gathered and presented to support the 
use of these factors in influencing the range.
    The Agency should give serious consideration to the possibility 
that the ranges of uncertainty surrounding the point estimate are not 
symmetrical. In particular cases, it may well be that most of the RfD 
uncertainty is on the high side of the point estimate.
    The proposal to use a range is inconsistent with the purpose of the 
RfD. The proposal to use a range rather than a point value for the RfD 
would lead to the potential for double counting uncertainty. The UFs 
and MFs presently applied in calculation of the RfD allow for many of 
the factors that are presented as justifying selection of a point 
within a range as an alternative to the calculated RfD.
    The range for the RfD would create more problems than it would 
prevent. The RfD, by nature, cannot be used to calculate the risk at a 
given level of exposure and is essentially a safety estimate that 
should be expressed as a single point estimate. The definition of the 
RfD recognizes the uncertainty in this assessment. The proposed 
approach would be difficult to implement, create unnecessary confusion 
and controversy regarding the RfD, and could result in prolonged 
unproductive debates between parties with differing interests.
    If EPA chooses to define a range, the range should be developed by 
the scientists undertaking the RfD development. If a range is used, it 
is also strongly recommended that it be accompanied by detailed 
guidance on the factors for choosing a point estimate within the range. 
The uncertainty surrounding the point estimate of an RfD will be 
different for each chemical and study and should be clearly stated in 
any revised RfD.
    An advantage of the range is that it would make more apparent to 
States the uncertainty in the RfD and the flexibility that now exists 
surrounding its use in the regulatory context. However, it is 
preferable to retain the presentation as a single point value but 
provide in accompanying text substance-specific information such as 
steepness of the dose-response curve that States can use in deriving 
standards based on other than the default single point RfD.
    A range is useful to a risk manager or other decision-maker because 
actions can be taken with greater confidence in how likely it is that 
adverse health effects will be manifest at a particular point 
concentration. For example, slightly exceeding the MCL of 1 mg/L for 
nitrite with a UF of 1 is more likely to result in adverse health 
effects upon exposure than slightly exceeding a guideline of 70 
g/L for MTBE with a UF of 10,000.
    Some of the factors EPA recommends in selecting a point within an 
RfD range should be used in determining the RfD itself rather than for 
deviating from it after it is derived. These include the seriousness 
and reversibility of the effect, whether it is based on a LOAEL,

[[Page 66463]]

and bioavailability within humans. The issue of considering the 
presence or absence of sensitive segments of the population is 
impractical and inappropriate in deriving an ambient water quality 
standard. EPA should delete this option and understand that States 
generally set water quality standards on a statewide level. It is 
impractical to ascertain whether infants or pregnant women live near 
and consume fish or water from a particular waterbody. It is not 
practical from an administrative standpoint to set different, separate 
standards for each waterbody.
    The Agency should provide guidance regarding the development of 
scientific rationales for departure from the default RfD. The Agency 
should provide a methodology for deriving the range, along with 
supporting examples, and subject that methodology to peer review before 
using the concept in developing AWQC.
    Response--EPA agrees that the method used to quantify the range 
from which an alternative to the calculated RfD can be chosen is not 
based on specific scientific or statistical data. It is purely an equal 
partitioning of a default, 10-fold uncertainty factor into four equal 
quarter log segments.
    It is important to note that the range around the calculated RfD 
only establishes a domain from which a risk assessor can select a 
single point to use as an alternative to the RfD for a specific 
circumstance. The 2000 Human Health Methodology criteria for using a 
point within the range other than the calculated RfD when calculating 
AWQC clearly require the State to provide a detailed justification for 
that decision.
    One example of a situation where a point other than the calculated 
RfD might be applied would be where there is a difference in the 
bioavailability of the contaminant in the water component of the AWQC 
as opposed to the fish component. In such an instance, the decreased 
bioavailability from fish tissues could be used to support selection of 
an alternative value greater than the calculated RfD if the critical 
study were one where the contaminant had been administered through 
drinking water. Most inorganic contaminants, particularly divalent 
cations, have bioavailability values of 20 percent or less from a food 
matrix, but are much more available (about 80 percent or higher) from 
drinking water. Accordingly, the external dose necessary to produce a 
toxic internal dose would likely be higher for a study where the 
exposure occurred through the diet rather than the drinking water. As a 
result, the RfD from a dietary study would likely be higher than that 
for the drinking water study if equivalent external doses were used.
    The exposures considered in deriving AWQC include fish (food) and 
water. Thus, one might be able to justify an alternative value to the 
RfD point estimate that was slightly higher than the RfD estimate in 
cases where the NOAEL that was the basis for the RfD came from a 
drinking water study, but slightly lower than the RfD estimate if the 
NOAEL came from a dietary study.
    Several commenters suggested that there would be value in applying 
the range concept to several relevant RfD values and then to evaluate 
the results. The range concept was considered in the peer review of the 
1998 draft Methodology revisions, and the peer reviewers had many of 
the same concerns regarding the range. The revised Risk Assessment TSD 
gives examples of how one could justify an alternate RfD value that was 
lower or higher than the RfD estimate.
8. Severity of Effects
    Comments--Several commenters supported consideration of severity of 
effects in determining AWQC, although there was considerable diversity 
in the opinions expressed, as follows.
    Some believed that there was no science behind use of different UFs 
(i.e., 3, 10) in making intraspecies decisions based on severity of 
effect. Some stated that EPA should provide a methodology that will 
define a severity scale prior to adopting use of severity in deriving 
RfDs and associated AWQC. Others commented that the severity scale 
could be alphanumeric, similar to that used for carcinogens under the 
EPA 1986 cancer guidelines, and the severity rating could be presented 
along with the RfD value. However, any severity scale must also 
consider whether it is consistent with the definition of an RfD as a 
dose below which no adverse effects are anticipated to occur in exposed 
populations.
    Other commenters believed that making adjustments in the RfD value 
for severity of effects only confounds regulatory policy with 
toxicological science, and the Agency should explore alternative 
approaches to the problem of differences in severity of various 
toxicological endpoints. The Agency should not have considered severity 
in calculating an RfD because this practice could result in double 
counting of uncertainty. Severity should be considered in selection of 
a UF only when the RfD is based on a LOAEL. If the NOAEL were used, 
concerns for severity should be reflected in the MF.
    Response--There are several situations in which EPA has considered 
the severity of effect in selection of the UF. The Risk Assessment TSD 
cites zinc as an example. The LOAEL used in establishing the RfD for 
zinc was a change in the activity of the enzyme superoxide dismutase. 
This effect compromises the ability of the individual to avoid damage 
to macromolecules, such as proteins and polynucleotides, in the 
presence of free radical oxygen. Although clearly adverse, this effect 
is not as severe as tissue necrosis or impaired organ function. Thus, a 
UF of 3 was used rather than the default of 10 for the adjustment of a 
LOAEL to a NOAEL. The nutritional requirements for zinc relative to the 
RfD supported the use of a UF of less than 10 in this instance.
    As monitoring of molecular biomarkers of toxicity increases, the 
number of situations will most likely increase in which a LOAEL is 
early enough in the progression toward overtly adverse effects that 
factors of less than 10 can be used for the RfD calculation and will be 
supported by mode of action data. Past EPA practice is consistent with 
the suggestion that severity be considered where the RfD is based on a 
LOAEL and that an MF be used, if the data warrant, when calculating 
from a NOAEL.
    We do not believe that establishing a scale for severity is 
necessary at this time. It would be extremely difficult to establish a 
scale for rating toxicological endpoints that could be easily applied 
to the spectrum of endpoints monitored in more recent toxicological 
studies. The present flexibility in UFs and MFs provides ample 
opportunity for severity adjustment.
9. Stochastic Modeling
    Comments--Commenters encouraged EPA to use a stochastic approach 
(Monte Carlo and/or Latin square modeling) for setting RfDs. The 
commenters stated that this would allow EPA to better ``quantify the 
uncertainties and separate them from the variability in the data.'' 
They believed such methods would provide a sounder, more quantitative 
approach to determining whether a range of RfD values is needed.
    Response--The guidelines for determination of the RfD are based on 
previously published, Agency-wide guidelines. The suggestion to use a 
stochastic approach has been noted and will be considered in the 
context of the Agency revisions to its risk assessment guidelines. 
Revisions to fundamental Agency guidelines are beyond the scope of the 
AWQC Methodology.

[[Page 66464]]

10. Synergistic Effects
    Comment--Several commenters encouraged the Agency to consider 
multiple exposures to various chemicals and persistent bioaccumulative 
toxicants when establishing AWQC. For substances that do not persist or 
bioaccumulate in the environment, or do not cause reproductive, 
developmental, or neurological effects, EPA's risk assessment 
methodologies were deemed in need of reconsideration. However, as part 
of the reconsideration, EPA was asked to apply best science on 
synergistic impacts from exposures to a combination of chemicals. Other 
comments suggested sensitive subpopulations, such as Native American 
Tribes and other susceptible populations, may have significant 
confounding, underlying health problems that must be recognized with 
any synergistic assessment.
    Commenters also stated that EPA should give specific attention to 
certain categories of contaminants: persistent organic pollutants and 
endocrine disruptors. The commenters identified two aspects to consider 
in applying this recommendation: (1) Individual contaminants with a 
similar mode of action whose cumulative effects may reach an 
unacceptable level; and (2) selection of specific biologic endpoints to 
use as the basis of an RfD. They also believed that tissue effects are 
valid measures of injury and should be used in addition to organ-level 
effects in people and biota. It was also considered important to 
include immunological, reproductive/developmental, and neurological 
effects to derive RfDs.
    Response--The Risk Assessment TSD encourages States to consider 
synergistic and additive effects of individual chemicals in mixtures 
when establishing AWQC. The HI approach is suggested and described for 
situations where the chemicals have the same effect by similar modes of 
action. The Risk Assessment TSD also acknowledges that methods are not 
presently available for evaluating risk from mixtures where the 
individual chemicals have dissimilar health effects and recommends that 
chemicals in such mixtures be evaluated individually. Specific 
recommendations are found in EPA's Draft Guidance for Conducting Health 
Risk Assessment of Chemical Mixtures published in May 1999 (USEPA, 
1999b).
    The 2000 Human Health Methodology accommodates concerns regarding 
persistent bioaccumulative toxicants primarily through use of 
bioaccumulation factors in the calculation. Situations in which ambient 
waters may contain a group of chemicals that are persistent and 
bioaccumulative and have additive or synergistic effects can in some 
cases be factored into the HI approach. The description of the 
treatment of mixtures in the TSD was expanded to encourage States to 
consider persistence, bioaccumulation, and mixtures concerns in their 
risk assessments. The references to Agency mixtures guidelines were 
updated to include the most recent draft of the mixtures guidelines.
11. Target Population Adjustments
    Comments--EPA was asked to consider the characteristics of the 
target population when determining AWQC. Commenters suggested that when 
the chemical is a carcinogen, it is appropriate that the target 
population consist only of residents of the United States. In cases 
where the effect is an acute reproductive effect, the commenters 
believed it is appropriate to specify adult women as the target 
population and to use short-term consumption rates and exposure 
parameters.
    Response--The default input parameters for determining AWQC for 
human health apply to lifetime exposures and the adult population of 
the United States. However, the equations used for the calculation 
provide the flexibility to use body weight, water intake, and fish 
intake parameter values that are specific to other target populations.
12. Uncertainty and Modifying Factors
    Comment--Additional guidance was requested on factors to consider 
in selecting UFs, particularly a UF for an incomplete database.
    Response--In revisions to the Risk Assessment TSD for the 2000 
Human Health Methodology, we increased the number of examples given to 
illustrate how UFs were selected in establishing RfDs included in the 
IRIS.
    Comment--The suggestion was made to replace the interspecies UFs 
with a body weight to the three-quarters power and thereby harmonize 
the cancer and noncancer approaches.
    Response--The peer reviewers of the 1998 draft Methodology 
revisions also suggested harmonizing the cancer and noncancer 
approaches with regard to the use of the body weight to the three-
quarters power. This can be accomplished only through changes to the 
Agency documents on which the methodologies presented in the 2000 Human 
Health Methodology are based. The Agency currently is working on 
harmonizing the cancer and noncancer methodologies.
    In addition, as pointed out by the peer reviewers, a body weight to 
the three-quarters power conversion adjusts for allometric differences 
between laboratory animals and humans. It does not reflect 
toxicodynamic differences between species that must still be included 
when adjusting for interspecies differences. The use of the scaling 
factor cannot totally replace the interspecies UF.
    Comment--Another comment requested EPA to adopt more rigorous 
quantitatively supportable methods such as PBPK models to replace the 
more arbitrary and less well founded use of numerical scaling factors 
identified in UFs and MFs.
    Response--The revisions to the Methodology clearly support use of 
toxicokinetic modeling when the data are available and use of the 
modeled data in lieu of the toxicokinetic portion of the interspecies 
UF.
13. Use of Less-Than-90-Day Studies in Determining an RfD
    Comments--In general, commenters agreed with the scientific review 
board that false-negatives might result from use of less-than-90-day 
studies to develop an RfD. It was suggested that EPA evaluate data sets 
for groups of chemicals for which there are both chronic and less-than-
90-day studies and compare RfDs. Any comparison of chronic and less-
than-90-day studies should consider the purpose for which the less-
than-90-day studies were conducted and whether they provide evidence 
relevant to the results of longer term experiments. A commenter agreed 
with the scientific review board that any RfD based on a less-than-90-
day study should be used only temporarily.
    Other comments pointed out that the Great Lakes methodology allowed 
use of less-than-90-day studies for determining an RfD but required a 
duration UF of 30 rather than 10. This factor when combined with a 10 
for intraspecies variability and a 10 for interspecies variability 
would yield a total UF of 3,000, the maximum that is said to support 
RfD derivation. The commenter believed very few situations would 
qualify to use less-than-90-day studies, but their use should be 
allowed as long as the total UF is 3,000 or less.
    Additional comments stated that reproductive, developmental, 
immunotoxicological, and neurotoxicity data provide an appropriate 
basis for determining an RfD even if they come from studies of less-
than-90-day duration. However, one commenter also urged that data must 
be collected using

[[Page 66465]]

methods of sufficient accuracy and validity. It was also emphasized 
that evaluations should be conducted to determine how dose-response 
relationships developed for these toxic effects, particularly 
immunotoxicity, are related to modifications in function and evidence 
of overt pathology.
    Response--In several instances, the Agency has developed an RfD 
based on data from studies of less-than-90-day duration (e.g., nitrite, 
zinc), particularly where the data were from humans and evaluated 
endpoints of chronic as well as acute significance. Data from less-
than-90-day studies of reproductive, developmental, 
immunotoxicological, and neurotoxicity data are also considered 
appropriate for an RfD if they identify the critical effect. However, 
such data are used for RfD determination only when supported by a 
rather complete database and a good understanding of the mode of 
action. The Agency does not use data from less-than-90-day studies 
purely because they are the only available data. When the database is 
inadequate to support an RfD determination, no RfD is calculated.

E. Exposure Assessment

Default Intakes

1. Assumption That All of the Drinking Water Consumed Is Contaminated 
at the Criteria Level
    Comment--A commenter questioned the assumption that all drinking 
water consumed has been contaminated to the maximum extent allowed by 
the criteria.
    Response--Refer to response on this same issue for Comment E.2, 
Assumption That All Fish Consumed Is Contaminated at the Criteria 
Level.
2. Assumption That All Fish Consumed Is Contaminated at the Criteria 
Level and All Fish May Come from One Waterbody
    Comments--Commenters questioned the assumption that all fish 
consumed have been contaminated to the maximum extent allowed by the 
criteria. They state the assumption that all of the 17.8 g/day (now 
17.5 g/day) could come from one source is unrealistic, and that EPA 
should specify ways to adjust the fish intake rates to reflect a 
contaminated fish consumption rate.
    Response--As required under Section 304(a) of the CWA, EPA develops 
water quality criteria that reflect the latest scientific knowledge on 
effects of pollutants on human health. The Agency's recommended 304(a) 
water quality criteria are used by States and authorized Tribes to 
adopt enforceable water quality standards including designated uses of 
a waterbody consistent with Section 101(a) of the CWA (e.g., fishing, 
swimming, propagation of aquatic life, recreation). In developing the 
2000 Human Health Methodology, we have made assumptions about exposure 
to contamination from eating fish taken from surface waters of the 
United States. The purpose of the assumptions is to ensure that if 
criteria are met in a waterbody designated with the uses specified in 
Section 101(a) of the CWA, fish consumers can safely eat fish from that 
waterbody. In addition to the assumption that 17.5 g of fish are 
consumed per day based on the most recent U.S. Department of 
Agriculture (USDA) survey data (a value reflecting the 90th percentile 
of the general population), EPA also assumes that fish and shellfish 
are taken from water with pollutants present at the criteria level. In 
order to ensure that people can safely eat fish from waters designated 
with Section 101(a) uses, it is necessary to assume that all of the 
consumed fish is taken from waterbodies at the criteria level (i.e., 
contaminated to the maximum safe level).
    We recognize that fishing patterns (i.e., extent and location of 
fishing) and the degree to which fish and shellfish bioaccumulate 
contaminants from waters across the United States may differ from the 
exposure assumptions used to calculate national 304(a) water quality 
criteria. However, the degree and frequency of such variation are not 
clearly known, and these potential differences do not relieve EPA from 
its CWA obligations to develop national water quality criteria (which 
States and authorized Tribes may modify) that are protective for the 
general population. Furthermore, we note that not all of these 
differences would lead to less restrictive (higher) AWQC. For example, 
some subpopulations may consume fish at a higher rate than the 17.5 g/
day assumed in the national 304(a) criteria, and bioaccumulation might 
occur to a higher degree than the central tendency assumptions used in 
calculating the national default BAF. As indicated above, EPA believes 
that the data do not exist to enable us to account reliably for the 
myriad of spatial and temporal differences in fishing patterns and 
bioaccumulation and subsequent differences in exposure to fish 
contaminants at the national level. In addition, we have not received 
information from any stakeholder that would allow us to make such fine 
distinctions. Our goal is to ensure that populations who rely on a 
particular waterbody as the predominant source of their fish and 
shellfish are adequately protected, thus protecting the designated use 
of that waterbody. For these reasons, we believe that these assumptions 
are appropriate for the development of 304(a) criteria. Where States 
and Tribes have concerns regarding the level of protection afforded by 
EPA's national 304(a) criteria, we encourage States and authorized 
Tribes to make appropriate adjustments to reflect local conditions 
affecting fish consumption and bioaccumulation. Guidance for making 
such modifications is provided in the 2000 Human Health Methodology.
3. Body Weight Assumptions
    Comments--Numerous comments were submitted on issues regarding the 
adequacy of the body weight default values recommended in the 1998 
draft Methodology revisions and what age-based body weight categories 
are appropriate. Several commenters stated the proposed default body 
weights were appropriate and that the 70 kg default for adults is 
appropriate. One commenter stated that the difference between 70 kg and 
the 65 kg value for women of childbearing age is so small that to 
distinguish between the two is unimportant. Another believed that the 
recommended children's body weights are sufficient and that finer age 
categories would not be useful at this time. However, other commenters 
addressed the potential need to use finer age-category body weights if 
it is known that the adverse health endpoint affects a particular age 
group sensitive at that developmental stage, and one commenter stated 
that the broad-age default (i.e., for 0- to 14-year-olds) would be 
inappropriate for an infant. Another commenter pointed out that the 
default assumption for children ages 1 to 3 (i.e., 10 kg) is too low 
compared with data from EPA's Exposure Factors Handbook. Other comments 
advocated that EPA specifically define the percentile value associated 
with the defaults or recommended that EPA not specify default body 
weights for children.
    Response--We believe it is useful to provide default parameters for 
various population groups of concern, where possible, and have received 
support for this from States and from the recent peer review workshop 
panel. The difference between the general adult default body weight and 
the weight for women of childbearing age is statistically significant 
and, therefore, we are providing this value for situations where the 
critical health endpoint is an in utero developmental effect. All 
parameters used for an exposure evaluation should reflect the specific 
population group of concern.

[[Page 66466]]

As stated in the 1998 draft Methodology revisions, EPA has not provided 
finer age group defaults for children because the fish intake data do 
not permit breakouts other than the broader age category. However, in 
spite of this limitation, we have included finer age group body weights 
for State and Tribal use (when they have local or regional fish intake 
data that allow for their use) in the Exposure Assessment TSD. In most 
cases, we have indicated the specific percentile from each data source 
for the default value chosen (based on the surveys used and not in the 
context of the total population because data are not available to 
conclusively describe the entire population), but we have clarified 
this in the 2000 Human Health Methodology. Associating a derived 
criterion with a specific percentile is not possible because such a 
quantitative descriptor would require more detailed distributional 
exposure and dose information than is available.
    EPA acknowledges that the proposed value of 10 kg for a child ages 
1 to 3 is lower than the values reported in the Exposure Factors 
Handbook (USEPA, 1997a). The 2000 Human Health Methodology uses default 
body weight values based on the more recent NHANES III data. Contrary 
to the one commenter's suggestion, the data were not chosen to 
overestimate exposures; we intended to choose the average body weight 
as a default. In all cases (i.e., for the adult, childbearing woman, 
children aged one to ten, and infant/toddler categories), we chose 
average (mean) body weight values as defaults and do not believe these 
are overly conservative.
4. Combining Consumption Intakes and Body Weights
    Comments--Several commenters stated that when possible or where 
appropriate, the intake values and body weight data should be combined 
to generate a ratio/correlation of consumption to body weight, in order 
to provide better estimates. One commenter requested that EPA consider 
deriving a 95th percentile value of the water consumption to body 
weight ratio as the basis for the national 304(a) criteria. However, 
the opposite opinion was also expressed; that is, several commenters 
supported the use of separate parameters in the derivation equation. 
One commenter stated that, based on mean intake and body weight rates 
in EPA's Exposure Factors Handbook, differences in fish and water 
intakes between pregnant women and adults in general are so 
insignificant that they are not worth distinguishing. Opinion was also 
expressed that differences in intake rates per unit body weight can be 
more significant for children. EPA was cautioned to make sure that if 
differences in body weight are considered for different age groups, 
then the variation of intake by each specific group also needs to be 
considered.
    Response--EPA agrees that the intake rates and body weights for the 
specific population groups should match (e.g., a body weight for women 
of childbearing age should be matched with a drinking water intake 
assumption for women of childbearing age). However, we believe that the 
exposure parameter choices should be based on the population of 
concern, regardless of how small the change in the resulting criterion 
might be compared with a general adult population default. We also 
believe that there is not always a direct relationship between 
consumption and body weight. When EPA presented the issue for review by 
the Agency's SAB, they provided the following advice:

    In theory it would be better to develop standards on a per 
kilogram body weight basis. However, in practice the results are not 
different enough to make much difference in the magnitude of AWQCs. 
In particular, data should not be rejected because individual body 
weights are not available, and funds should not be allocated for 
collecting such data since no conceivable benefit would accrue.

EPA has also received input from its State stakeholders regarding 
potential confusion over combining the two parameters. Most believe 
that the difference in accuracy is negligible but that the difficulty 
in associating the units of mg/kg-BW/day with a meal size, especially 
for public communication and understanding, is great and, therefore, 
not particularly useful. Several stakeholders believed that if the data 
were combined as part of a study, or if a strong, demonstrated 
correlation between intake and body weight exists, the combined 
parameter should be used. We have evaluated recent information on both 
drinking water intake and fish intake from the 1994 to 1996 CSFII data 
and have assessed the differences between the two units of measure--
including an emphasis on the differences that result with smaller age 
categories and drinking water consumption rates for children when mL/
kg-BW/day are used (USEPA 2000c,d). [Note: SAB's comment on the 
unavailability of individual body weights is not an issue with the 
CSFII; that is, this information is available.] EPA intends to base its 
national 304(a) criteria on the separate intake values and body weights 
because of the strong input received from its State stakeholders. 
However, we have also provided tables in the final Exposure Assessment 
TSD of all fish/population categories for both g/day and mg/kg-BW/day, 
if States or Tribes prefer their use. The TSD will also provide 
examples on deriving criteria using either, including identifying 
situations where the latter estimate may provide substantively more 
accurate estimates. Additionally, the TSD will provide tables listing 
comparable values in mg/kg-BW/day (fish) or mL/kg-BW/day (drinking 
water).
5. Combining Fish Intake and Body Weights
    Comments--Several commenters recommended the use of separate fish 
intake and body weight assumptions because of clarity, familiarity 
among the States, and data availability. Specifically, the option of 
combining these values was not considered practical because most 
studies do not provide such information, even if potentially more 
accurate. Furthermore, it was suggested that this complicates the 
derivation process or introduces error (an example was cited), and 
States and Tribes have the flexibility to use intake values other than 
the default values provided. Another commenter stated that there is a 
direct proportional relationship between fish consumption and body 
weight and that selection of the 90th to 95th percentile value of fish 
consumption per unit body weight is an appropriate basis for deriving 
the criteria.
    Response--EPA agrees that the use of separate fish intakes and body 
weights is more easily understood and provides reasonable and 
protective default estimates. For additional discussion, see our 
response to Comment E.4, Combining Consumption Intakes and Body 
Weights. We do not agree that there is necessarily a direct 
relationship between fish intake and body weight, especially in the 
context of intake on a per-unit-body-weight basis.
6. Default Drinking Water Intake Rates
    Comments--One commenter stated that EPA has overestimated the 
amount of untreated surface water consumed by the population. However, 
another commenter believed that the 2 L/day rate is reasonable. A 
commenter stated that drinking water intake rates in hot, arid climates 
may be higher than the recommended default rate. Numerous commenters 
stated that incidental water ingestion should not be considered in 
deriving AWQC or that it is unimportant. One called for empirical data 
to support its use and believed that

[[Page 66467]]

EPA has implied that incidental ingestion occurs every day. However, 
other commenters believed that this route should be considered for 
waters not designated as drinking water sources. One of these requested 
that EPA provide additional guidance on incidental ingestion relevant 
to acute toxicity and exposures. Another recommended that EPA evaluate 
the circumstances to determine whether the incidental ingestion rate 
would make a difference. A commenter recommended that EPA use a 30 mL/
hour assumption in cases where short-term effects may be considered in 
criteria derivation. One commenter stated that the 10 mL/day value 
would be too restrictive for use in all nonpotable waterbodies and 
would conflict with existing State guidance on incidental ingestion.
    Response--EPA acknowledges that much of the population consumes 
water from public water supplies that receive treatment. However, we 
intend to continue including the drinking water exposure pathway in 
deriving AWQC for the reasons clearly stated in the 1998 draft 
Methodology revisions. Refer to that discussion for clarification on 
this issue [see Federal Register Notice, August 14, 1998; Appendix III, 
C.1.(b)]. We encourage States and Tribes to use alternative intake 
rates if they believe that water consumption is higher in arid climates 
than the recommended default rate. We have not assumed that incidental 
ingestion occurs every day. We have estimated an averaged rate based on 
available study information. When initiating the process to revise the 
methodology, several stakeholders identified recreational or accidental 
water ingestion as a potential health concern. A couple of States have 
indicated that they already have established incidental ingestion rates 
for use in developing water quality criteria. EPA agrees that the 
averaged amount is negligible and will not have any impact on the 
chemical criteria values representative of both water and fish 
ingestion. The lack of impact would likely also be true for chemical 
criteria based on fish consumption only, unless the chemical exhibits 
no bioaccumulation potential. However, we believe that the issue could 
be important for the development of microbial contaminant water quality 
criteria, and for either chemical or microbial criteria for States 
where recreational uses such as swimming and boating are substantially 
higher than a national average would indicate. Although we will not use 
the incidental ingestion intake parameter when deriving our 304(a) 
national chemical criteria, we will leave the guidance language in the 
final Exposure Assessment TSD in order to assist States and authorized 
Tribes that face situations where this intake parameter would be of 
significance.
7. Default Fish Intake Rates
    Comments--EPA received strong support for its hierarchy of 
preferences regarding fish intake values; that is, use of local or 
regional studies, and studies characterizing similar populations and/or 
geography, over default values. EPA also received support for 
encouraging decisions on intake rates to be made at the State or Tribal 
level. EPA generally received support for its default fish consumption 
rates, including the national 304(a) criteria value of 17.8 g/day (now 
17.5 g/day based on the 1994-96 CSFII data). There was support for the 
new default rates as more accurately representing current levels of 
fish consumption among the general population than the old assumption 
of 6.5 g/day. Support was also received for providing the variety of 
default values to protect highly sensitive or highly exposed population 
groups. One commenter advocated that EPA clearly state that using the 
90th percentile value is a risk management decision. However, others 
stated that EPA has overestimated fish consumption for the population 
at large. A commenter stated that EPA should use the intake value that 
its Superfund program utilizes (i.e., 54 g/day). EPA also received 
support for the default of 86.3 g/day for subsistence fishers (now 
142.4 g/day based on the most recent USDA survey data). Some commenters 
disagreed with the use of a subsistence default as contrary to the 
purpose of AWQC (while conceding its use for site- or region-specific 
criteria) or recommended that EPA caution against the use of 
subsistence values without risk management decisions balancing risk 
benefits and costs. One commenter stated that subsistence populations 
are very rare and cannot generally be defined by socioeconomic factors 
and, thus, EPA's assumption of 86.3 g/day may be over-or 
underprotective. Several commenters stated their support for the 
subsistence default but also advocated that EPA should require States 
to consult with Tribes in order to select an adequate fish consumption 
rate. Other comments expressed the opinion that a Tribe would be 
obligated to use EPA's default value if the Tribe could not conduct its 
own survey or expressed concern over the extrapolation of data from the 
general population to subsistence populations. Several commenters 
questioned EPA's choice in selecting a value to represent the 90th 
percentile of the general population, in contrast to selecting average 
values for sportfishers and subsistence fishers. A commenter stated 
that the assumption of 17.8 g/day as a default for sport anglers was 
not supported by peer-reviewed studies and contradicts the EPA's 
Exposure Factors Handbook. Another commented that because 17.8 g/day is 
recommended to represent the general population, it should not be used 
to represent sportfishers and indicated that 39 g/day may be more 
appropriate. Other comments advocated the use of actual sportfisher/
subsistence population data or making sure that the defaults chosen 
appropriately correspond to these groups.
    Two commenters stated that the recommended values for children and 
women of childbearing age were overly conservative and inappropriate 
because developmental effects would not result from short-term 
exposures. However, another commenter stated that evidence on 
reproductive/developmental effects should make EPA take the most 
conservative approach to protect pregnant women, fetuses, and young 
children. Other commenters found these values acceptable and believed 
that the approach is consistent with EPA developmental toxicity 
guidelines. One commenter noted that single meal or short-term 
consumption for these groups could easily exceed the EPA defaults. 
Other comments cautioned EPA to make sure that the exposure assumptions 
to protect against developmental health effects be used only with 
chemicals causing acute toxicity, or believed the defaults are 
unrealistically high and favored an averaged daily equivalent (mean or 
median value). Two commenters believed that basing both national and 
regional criteria on a fish consumption rate in the 90th to 95th 
percentile would be most appropriate, and one stated that the high-end 
percentile should be used with rates for children and women of 
childbearing age to protect against reproductive or developmental 
effects. Another commented that criteria to protect subsistence fishers 
or pregnant women should be left to the States and Tribes to consider. 
Still another suggested that EPA develop special fish consumption rates 
for populations that consume much higher amounts than average and, 
thus, not be overly conservative in its default assumptions. Two 
commenters questioned EPA's assumption that children consume more fish 
on a body weight basis than adults, and one commenter advocated use of 
childhood fish consumption rates. Concern was also expressed that all 
of

[[Page 66468]]

the default rates assume that consumers eat from a single source only, 
and that the RSC factor results in a double-counting of fish intake 
rates. One commenter said that EPA should not establish default values. 
Finally, one commenter advocated using mean consumption rates (not the 
90th percentile) if the Agency intends on retaining its RSC factor.
    Response--EPA acknowledges the support for the default fish intake 
rates. Our national 304(a) water quality criteria serve as guidance to 
States and authorized Tribes, who must in turn adopt legally 
enforceable water quality criteria into water quality standards. States 
and authorized Tribes have the option to develop their own criteria and 
the flexibility to base those criteria on population groups that they 
determine to be at potentially greater risk because of higher 
exposures, yet, EPA cannot oblige the States to specific consulting 
agreements because, again, criteria are guidance, not enforceable 
regulations, and do not impose legally binding requirements. Therefore, 
we recommend that States and Tribes give priority to identifying and 
adequately protecting their most highly exposed population by adopting 
more stringent criteria, if the State or Tribe determines that the 
highly exposed populations would not be adequately protected by 
criteria based on the general population. In all cases, States and 
authorized Tribes have the flexibility to use local or regional data 
that they believe to be more indicative of the population's fish 
consumption--instead of EPA's default rates--and we strongly encourage 
the use of these data. In most instances, using alternate fish intake 
rates should not be difficult, once the value has been determined, in 
that the criteria calculation is performed by substituting the State/
Tribal intake rate in place of EPA's default rate. We believe that the 
assumption of 17.5 g/day (again, based on the recent 1994-96 CSFII 
data) will protect a majority of the population of consumers of fresh/
estuarine finfish and shellfish, especially population groups who rely 
on a particular waterbody for most or all of their fresh/estuarine 
intake. It is our goal to utilize an intake rate that represents more 
of the population than would a central tendency value. Thus, we intend 
to derive our national 304(a) criteria using this 90th percentile 
assumption, based on the updated analysis of the 1994-96 CSFII data. 
EPA also acknowledges that other Agency programs may utilize different 
default assumptions. In the case of the Superfund program, the value 
used (54 g/day) represents a default used for recreational fishers. It 
reflects total fish consumption from both marine and fresh/estuarine 
sources; however, it includes only finfish, not shellfish. As such, it 
cannot be directly compared to our default based on the general 
population for finfish and shellfish from fresh/estuarine sources only. 
[Note: The comparable 90th percentile CSFII value from the 1994-96 
data, if marine species were included, would be 74.87 g/day.] For the 
AWQC program, EPA believes it has selected an appropriate, not overly 
conservative default value, given the goals of the CWA and the criteria 
program.
    For the rationale stated above, we strongly believe that providing 
a default rate for subsistence fishers is important for States and 
Tribes, if they choose to use it in lieu of their own study data. We 
disagree with the commenter that the concept is contrary to the purpose 
of AWQC. Moreover, the commenter appears to have incorrectly assumed 
that EPA would base its national 304(a) water quality criteria on the 
subsistence fishers intake value. We intend to base our national 
criteria on the recommended value for the general population. We 
emphasized in our 1998 draft Methodology revisions that States and 
Tribes should consider developing criteria based on highly exposed 
populations when those populations would not be adequately protected by 
criteria based on the general population. This is, in fact, consistent 
with the purpose of AWQC. We also acknowledge that there is variation 
in fish consumption patterns, especially among subsistence fishers. For 
the purpose of providing one national intake rate for subsistence 
fishers, we believe that the value of 142.4 g/day (an estimated 
national average value based on comparing the CSFII 1994-96 data with 
subsistence fisher studies) is appropriate. Although the exact 
percentile represented by the arithmetic mean varies from survey to 
survey, we believe this value is more appropriate and protective than a 
median or central tendency value--which we cautioned against using in 
the 1998 draft Methodology revisions, because median values in the 
available short-duration surveys may be zero. However, as indicated 
above, EPA strongly encourages the use of site or regional-specific 
studies instead of this default value, and the State's/Tribe's 
discretion in considering higher intake rates than an arithmetic mean. 
We reemphasize here our four-preference hierarchy, which is designed to 
give States and Tribes more options than simply conducting a survey or 
using our default. EPA's national 304(a) criteria are health-based 
values only and are not intended to account for cost/benefit analyses. 
As indicated in our 1998 draft Methodology revisions, risk management 
decisions regarding balancing risk benefits should be made at the State 
or Tribal level.
    EPA believes it is appropriate to offer default fish intake rates 
for children and women of childbearing age for States and authorized 
Tribes to consider if exposures resulting in health effects in children 
or developmental effects in fetuses are of primary concern. We have 
recommended a 90th percentile from the 1994-96 CSFII for this potential 
situation, in order to protect a majority of these population groups. 
As stated in the 1998 draft Methodology revisions, EPA is not 
recommending the development of additional water quality criteria, 
similar to the drinking water health advisories, which focus on acute 
or short-term effects because these are not seen routinely as having a 
meaningful role in the water quality standards program. However, we 
disagree with the commenter that developmental effects cannot result 
from short-term exposures. To the contrary, we believe there may be 
instances where the consideration of acute or subchronic toxicity and 
exposure in the derivation of AWQC is warranted--specifically when such 
toxicity and exposure are the basis of an RfD, not a chronic effect. 
Only in this situation would EPA consider such a basis for its national 
304(a) criteria. Using long-term consumption rates to evaluate 
potential developmental effects would not accurately reflect meal size 
and would be inappropriate for use in such assessments. The separate 
distribution of short-term (i.e., consumers-only) consumption estimates 
represents the amount of fish an individual consumes in a day, or 
multiple days in a short time period, if the person eats fish on that 
day. The consumers-only consumption estimate approximates a serving 
size for women of childbearing age or for children. The intent is to 
characterize consumption over a very short period of time, not as an 
average or per capita value over a longer period of time. We recommend 
the use of the short-term (consumers-only) consumption values in 
assessing developmental risks to children or women of childbearing age. 
However, we intend to routinely base our national 304(a) criteria on 
the recommended fish intake rate for the general population. One 
commenter appears to have incorrectly assumed that EPA would normally 
base its national criteria on

[[Page 66469]]

acute toxicity scenarios. EPA acknowledges that it may have overstated 
the likelihood that children are more highly exposed in terms of the 
frequency of their consumption of freshwater and estuarine fish, 
although this may certainly be true for various subpopulation groups. 
However, the CSFII data clearly show that children do consume more fish 
per unit body weight than do adults. Therefore, as stated above, we 
believe it is useful to provide intake defaults to States and 
authorized Tribes for children, and we have specifically used childhood 
fish consumption rates (to the extent allowable by the CSFII data) as 
advocated by the commenter.
    EPA disagrees with the comment that the sportfisher default 
assumption (i.e., that 17.5 g/day based on the 1994-96 CSFII data 
represents average consumption rates for this population group) is not 
supported by available studies or by the Exposure Factors Handbook. The 
value of 17.5 g/day falls within the range of mean values from 
sportfisher/angler studies reviewed by EPA. The Exposure Factors 
Handbook indicates that mean intakes from recreational freshwater 
studies ranged from 5 to 17 g/day, with mean values from the key West 
et al. studies used in the GLI between 12.1 and 16.7 g/day (USEPA, 
1997a). Furthermore, the default rate recommended here for the AWQC is 
representative of consumption of both freshwater and estuarine fish 
species, not freshwater species only. We are also aware that some of 
the sportfisher studies that support higher estimates (e.g., 39 g/day) 
include marine species.
    EPA's fish intake assumption is that all of the consumed fish is 
taken from one particular waterbody. This is to ensure that any 
population can safely eat fish from waters designated for fishing, 
including those who may rely on a single source for their fish (for 
additional discussion on this issue, see response to Comment E.2, 
Assumption That All Fish Consumed Is Contaminated at the Criteria 
Level).
    EPA disagrees with the idea that using a 90th percentile value as a 
default is inappropriate because of the RSC factor. The RSC is used to 
account for other sources of exposure and, thus, is independent of 
potential exposures from fresh/estuarine fish. The fresh/estuarine 
species are not double-counted, as the commenter suggests. (For 
additional discussion on RSC, refer to the responses in the RSC section 
below.)
8. Effect of Cooking on the Contaminant Concentration
    Comments--Commenters stated that the concept of changes in 
contaminant level caused by cooking is important to recognize. They 
recommended that a loss from cooking should be accounted for and that 
EPA should provide factors in order to calculate this loss into 
criteria. However, one commenter did not believe that increases caused 
by cooking should be factored into criteria. One commenter stated that 
it is not appropriate to assume no loss as a default when no data exist 
to account for it. Another recommended that the chemical structure be 
assumed as constant before and after cooking. One commenter stated that 
the relevance of cooking methods is not clear.
    Response--EPA has stated its intention to assume no loss from 
cooking unless there are adequate data to characterize such a loss. We 
are aware of some studies on cooking loss and provide reference to 
quantified information in the 2000 Human Health Methodology. However, 
we believe it is important to consider both losses and gains in the 
chemical contaminant from cooking. EPA has also received input from 
several States regarding the difficulty in making such adjustments on a 
routine basis. We continue to evaluate this issue in the context of the 
national 304(a) criteria. We believe that providing guidance on making 
such adjustments may be useful in the Exposure Assessment TSD volume 
for States or Tribes that wish to modify their criteria accordingly. 
However, EPA does not intend to provide specific cooking loss default 
factors.
9. Inclusion of Marine Species in the Default Rate
    Comments--A commenter stated that coastal States have a need to 
derive water quality criteria for saline waters under their 
jurisdiction and, therefore, requested additional consideration of 
marine fish consumption. Another commenter requested that EPA provide 
greater clarification on its policy not to include marine species, 
again believing that States and Tribes need to include this in their 
criteria development.
    Response--In the 1998 draft Methodology revisions, EPA recommended 
inclusion of fresh/estuarine species only for the intake parameter, and 
accounting for the intake of marine species as part of the RSC. We 
consider this appropriate because the 304(a) water quality criteria are 
applicable to discharges from fresh and estuarine waters, not deep 
marine waters. EPA's 304(a) water quality criteria apply to navigable 
waters of the United States up the three miles off-shore. However, EPA 
also says that coastal States and authorized Tribes could consider 
total fish consumption (fresh/estuarine and marine species) when 
appropriate for protecting the population of concern. It is important 
that the marine intake component not be double-counted with the RSC 
estimate. We maintain our default policy decision and the flexibility 
afforded to a State or authorized Tribe to base its criteria on 
alternative assumptions.
10. Precision of the Drinking Water Parameter
    Comments--A commenter interpreted EPA's discussion on significant 
figures as indicating that the drinking water intake should not be 
factored into that determination because the number represents a 
science policy value. The commenter also requested that EPA specify a 
level of protection represented by the AWQC.
    Response--The commenter has misunderstood EPA's discussion in the 
1998 draft Methodology revisions on significant figures; they have 
extended the discussion to an evaluation of overall criteria 
conservativeness via statistical analysis. We stated that the AWQC 
should not necessarily always be limited to one significant figure 
because the 2 L/day drinking water value, although supported by data, 
represents a science policy decision. The discussion only addresses the 
issue of significant figures, not characterization of criteria 
protectiveness. For discussion of the issue regarding the population 
protected by the criteria level, refer to the response for Comment B.3, 
Protectiveness of the Methodology.
11. Redesignation of Salmon as a Marine Species
    Comments--Some commenters disagreed with EPA's reclassification of 
salmon to the marine category. They stated that EPA has ignored salmon 
biology and life history, that salmon is an anadromous species, and 
that salmon eggs, fry, and juveniles take up chemicals. Commenters 
specifically criticized EPA for ignoring steelhead salmon's life 
history. Three commenters thought the redesignation is reasonable. One 
had no objection to the redesignation for threshold toxicants but did 
object for carcinogenic effects based on a linear low-dose 
extrapolation, because it would not account for exposures of salmon to 
ubiquitous chemicals (e.g., PCBs) contributing a substantial portion to 
total exposure. Another commenter who supported the redesignation 
advocated flexibility

[[Page 66470]]

regarding coastal sportfisher consumption.
    Response--EPA has not ignored the life history of salmon. We 
provided information on the known biology and life history of the 
species consumed that were included in the CSFII survey, the basis of 
the default values, in our 1998 draft Methodology revisions. The term 
anadromous generally refers to a species that spawns in fresh water or 
near-fresh water and then migrates into the ocean to grow to maturity. 
It can also refer to an ocean species that spawns in fresh/near-fresh 
waters. The life cycles of anadromous species vary as to whether they 
remain in fresh/near-fresh waters until they die or whether they return 
to ocean waters after spawning. As such, the description provided by 
EPA in the 1998 draft Methodology revisions is correct and does not 
conflict with the term anadromous. The CSFII food codes for salmon do 
not indicate the source of the salmon (e.g., land-locked freshwater, 
farm-raised, or wild). We based our allocation of salmon between 
freshwater and marine habitats on commercial landings data provided by 
the National Marine Fisheries Service for the period 1989-1991. All 
landings of Pacific salmon, including chum, coho, king, pink, or 
sockeye, were assigned to the marine habitat. All land-locked Great 
Lakes salmon and farmed salmon received the classification of fresh 
water. The resulting apportionment for salmon was 1.18% to the fresh-
water habitat and 98.82% to the marine habitat. We believe this is 
appropriate for our national default intake rates.
    EPA understands that steelhead salmon, also known as steelhead 
trout (Oncorhynchus mykiss), is an oceangoing version of rainbow trout 
with a complicated life history, and may spend a significant portion of 
its lifetime in fresh waters. States and authorized Tribes have the 
flexibility to use different assumptions in deriving their water 
quality criteria, as we stated in the 1998 draft Methodology revisions. 
That is, States and authorized Tribes could make alternative 
assumptions to specifically account for steelhead salmon intake. We 
strongly encourage States and authorized Tribes to do so, as reflected 
by the recommended fish intake hierarchy of preferences. However, we do 
not intend to ignore the contribution from salmon in the calculation of 
our 304(a) criteria. We recommended accounting for this as part of the 
RSC, thereby ensuring that the criteria would account for the 
contribution of a contaminant from marine salmon.
12. Studies on Sportfishers and Subsistence Fishers
    Comments--Two commenters stated that in summarizing various 
sportfisher and subsistence fisher studies, EPA failed to provide 
direction on how States or Tribes can use and interpret the 
information. One commenter requested additional guidance on the use of 
local data, while cautioning about such data's reliability. Commenters 
also listed errors, discrepancies, or missing information from numerous 
studies that appear in the 1998 draft TSD. One commenter recommended 
separating studies by type, population, and basis for consumption rate 
(presumably referring to habitat designations of fish), along with 
providing comments on the studies. Another stated that many angler 
studies are biased because the respondents are more ``avid'' in their 
fishing habits, and a study of fresh-water anglers from Maine might 
serve better as the basis of EPA's default for sportfishers.
    Response--It is EPA's intention to provide summaries of various 
studies for States and Tribes to consider using and, as such, the 
Agency is merely providing information, not critiquing or endorsing 
particular studies. We do not intend to rank the studies because there 
are significant differences in the purposes and limitations of each 
study, in addition to the fact that consumption rates vary 
significantly throughout the country. Therefore, any particular study 
may be most appropriate to the State or Tribe's particular 
circumstances. However, we are committed to providing accurate 
information and intend to correct errors or missing information that 
would make the summaries of greater use to States and Tribes. We have 
reviewed the commenters' listed errors or omissions and made 
appropriate changes. EPA disagrees that any of the sportfisher studies 
are biased from ``avidity'' among recreational anglers. Although the 
rates may vary significantly from study to study, the studies 
specifically sample fishing patterns of these groups and are the most 
appropriate data for prospective use by States and Tribes. We 
considered the Maine angler study along with the others presented in 
the 1998 draft TSD to evaluate the range of mean values before 
recommending the default value. However, we do not believe this 
particular study is necessarily best suited for deriving a national 
default value. Just as with EPA's national 304(a) criteria, States and 
Tribes always have the flexibility to use other local- or regional-
specific studies. We have provided additional guidance on how to 
consider the studies included in the Exposure Assessment TSD.
13. USDA Continuing Survey of Food Intake by Individuals (CSFII)
    Comments--Some commenters believed that the CSFII data are 
appropriate for deriving AWQC and supported their use in the hierarchy 
of choices. Others stated that the CSFII data are not appropriate 
because they include marine species, and combine recreationally and 
commercially acquired species. One commenter suggested that a 
significant fraction of the default rate would include farm-raised 
fish, which would not bioaccumulate the same as wild fish. One 
commenter stated that the default inappropriately assumes consumption 
from a single waterbody. Two commenters stated that the CSFII data are 
biased toward individuals consuming large quantities of fish (assuming 
constant consumption every day and failing to consider those people who 
consume less frequently). One of these stated that the CSFII assumes 
that participants who did not eat fish during the study period are not 
fish eaters. Several commenters recommended that longer term studies be 
used, one specifically stating the difficulty in estimating the upper 
end of the distribution. Comments also referred to or recommended data 
from NPD Research Inc. or the Tuna Research Institute, presumably 
referring to the National Purchase Diary (NPD). One commenter assumed 
that the CSFII default estimates exclude individuals who consume fish 
but did not report consumption during the sampling period. Another 
questioned dividing reported consumption by the days of the survey and 
incorporating nonconsumption. Instead, this commenter recommended using 
the positive values only (``acute consumers'') for determining default 
intake rates, which it believed to be consistent with the concept of 
identifying the population to be protected. One commenter also 
indicated that intake rates do not vary significantly for fish obtained 
from different sources--that is, fresh or marine waters. Another stated 
that the CSFII data assume short-term consumption is representative of 
long-term consumption. One commenter advocated that EPA use 
probabilistic methods to derive AWQC.
    Response--The comments are incorrect about the exclusion of 
respondents who did not report fish consumption during the CSFII 
sampling period. The general population,

[[Page 66471]]

recreational fisher, and subsistence fisher default values all include 
both CSFII respondents who reported eating fish during the sampling 
period and respondents who reported zero consumption (what the 
commenter referred to as ``non-consumers''). The CSFII mean values are 
not biased. Specifically, the intraindividual variation does not bias 
estimates of the mean intake of the population. The estimates of the 
upper percentiles of per capita fish consumption based on the short 
sampling period data may be biased upward, thereby resulting in a 
conservative estimate of risk. However, the extent to which this is 
overestimated is not knowable. We note that we did not rely exclusively 
on the CSFII data; rather, the data were analyzed with those from other 
studies (especially for recreational fisher and subsistence fisher 
estimates) to evaluate and corroborate our decision. We believe the 
CSFII data are representative of fish intake rates among the general 
population. As part of the CSFII analysis, sampling weights were 
adjusted to account for nonresponse and were subsequently reweighted 
using regression techniques that calibrated the sample to match 
characteristics correlated with eating behavior.
    EPA generated mean and percentile estimates of daily average per 
capita fish consumption based on the USDA 1994-96 CSFII. The strengths 
of this survey for supporting estimates of per capita food consumption 
are twofold. First, the survey design is structured to obtain a 
statistically representative sample of the U.S. population. Second, the 
survey is designed to record daily intakes of foods and nutrients and 
to support estimation of food consumption. These features are in direct 
alignment with the objective of producing current, per capita fish 
consumption estimates for the U.S. population. The 1994-96 CSFII 
collected two non-consecutive days of food consumption data from a 
sample of 11,912 individuals in the 50 states and the District of 
Columbia. The method employed to collect dietary intake data also 
strengthened the CSFII design for supporting per capita consumption 
estimates. For example, the survey was administered by an interviewer 
on both days of data collection. For these reasons, we believe that the 
1994-96 CSFII is the best source of data on a nationwide basis for 
estimating fish consumption by the U.S. population.
    The NPD study was conducted over 25 years ago. The NPD is the basis 
of the 6.5 g/day default value that EPA has historically used for 
fresh/estuarine fish consumption. We have received consistently strong 
input from many of our stakeholders (including States and Tribes) who 
consider the 6.5 g/day value inadequate and advocate the use of much 
more recent data. The Agency also believes that such an update is 
needed. We are not aware of any subsequent major survey conducted 
during a 30-day period as was done by the NPD. The Agency does not 
believe that the year-long study of 29 people mentioned by one 
commenter is appropriate to use for a national default value. The use 
of probabilistic methods was discussed earlier in our response to 
Comment B.3, Protectiveness of the Methodology.
    EPA also believes that its discussion of identifying population 
groups to protect is not contradicted by its combining positive and 
zero values to estimate long-term or average consumption. We reiterate 
here that we believe the summation of the amounts of fish consumed by 
each individual across the 2-day reporting period for the CSFII 1994-96 
data (formerly a 3-day reporting period), followed by dividing that 
total individual consumption by 2, is a reasonable approach to 
estimating average consumption. The CSFII did not specifically ask 
questions on whether respondents consume fish or how often and, 
therefore, it is not possible to distinguish fish consumers from fish 
nonconsumers. EPA is aware from other major surveys that most people 
consume fish--at least episodically--and, therefore, believes that 
using the positive and zero values from the CSFII is a reasonable 
method of estimating average intake. We contrast this to using only the 
subset of survey responses where fish was actually consumed as a method 
to estimate an ``acute consumer,'' that is, to provide an estimate of 
the amount of fish consumed in the context of acute or short-term 
exposures (not in the context of average or long-term exposures).
    The commenters are also incorrect about the inclusion of marine 
species. The proposed default rates for the general population, as well 
as for children and women of childbearing age, are based on freshwater 
and estuarine species only. The CSFII study does include marine species 
and EPA has additionally provided States and Tribes with these data in 
the Exposure Assessment TSD; however, they are not included in the 
default estimates of national freshwater and estuarine fish 
consumption. According to the CSFII data, most persons in the general 
population appear to consume more marine species than fresh/estuarine 
species. However, EPA supports State/Tribal use of local or regional 
data that indicate otherwise. We have not made any specific assumptions 
regarding farm-raised fish and their contribution to the default intake 
rate, nor have we received any information that would allow us to 
characterize (or discount) the amount that farm-raised fish contributes 
to the national default value or to differentiate bioaccumulation 
levels.
14. Use of Uncooked or As Consumed Fish Weight for Default Intake Rates
    Comments--One commenter stated that either raw weight or cooked 
weight can be appropriate as long as the effect of cooking on the 
contaminant is accounted for. Some commenters stated that the cooked 
weights are the most technically defensible, because they are the basis 
for the consumption estimates. However, others believed the default 
intakes should be adjusted to reflect uncooked weights, with one 
commenter concerned that a cooked weight would result in incomplete 
accounting of exposure to threshold toxicants. One commenter also 
pointed out the difficulty of making appropriate adjustments to the BAF 
because of uncertainties in concentration levels of contaminant due to 
cooking and that many cooking techniques result in retention of fish 
fluids. Another commenter stressed the need to use uncooked weights in 
order to be consistent with fish tissue studies and BAF values. One 
commenter expressed concern that use of cooked weights would produce an 
inadequately protective criterion for mercury, while another believed 
that cooked values introduce a source of uncontrolled variability.
    Response--We have considered the pros and cons of using uncooked/as 
consumed weights on several levels. First, the intake parameters of the 
criteria derivation equation are intended to capture ingestion--that 
is, what people actually consume and are exposed to. By and large, 
people consume cooked fish, and if raw shellfish or sushi was consumed 
by the CSFII respondents, those intakes were included in the as 
consumed weights. This assumption is also consistent with the dietary 
estimates based on prepared foods (not raw commodities) that are made 
by both EPA's pesticide program and the Food and Drug Administration 
(FDA) Total Diet Study program. We also considered the ``consistency'' 
issue in the context of the fact that the CSFII survey respondents 
estimated the weight of fish that they consumed. Similar to the CSFII, 
EPA's GLI was based on a consumption survey of fish intakes for 
prepared meals. EPA additionally considered the effect of the

[[Page 66472]]

cooking process. There are comparatively few chemicals for which 
measurements are available, and the process is complicated further by 
the different parts of a fish where the chemical may accumulate, the 
method of preparation, and how the cooking process may transform the 
chemical. What is certain is that the mass of the contaminant will 
either remain constant or be reduced. The resulting concentration is 
harder to predict. In the 1998 draft Methodology revisions, we 
recommended the use of as consumed weights and an adjustment of the 
bioaccumulation factor for cooking loss, if information was available. 
Otherwise, we recommended using the as consumed weight along with the 
full bioaccumulation factor (unadjusted for cooking loss), which would 
produce slightly more stringent AWQC. We have also received input from 
stakeholders regarding potential confusion over the fact that uncooked 
weights are used in the Agency's fish advisory program and that having 
two sets of values may prove confusing to States and Tribes, as well as 
the general public. Furthermore, the measures of a contaminant in fish 
tissue samples that would be applicable to either compliance monitoring 
or the permitting program are related to the uncooked fish weights.
    Therefore, EPA has reconsidered its position based on these facts 
and despite the fact that the as consumed values more accurately 
represent actual intake, we will derive our national 304(a) criteria on 
the uncooked weight fish intakes. The approach of using an uncooked 
weight in the calculation will result in somewhat more stringent AWQC 
(studies indicate that, typically, the weight loss in cooking is about 
20%). We will also provide guidance on site-specific modifications in 
the Exposure Assessment TSD. Specifically, we will describe an 
alternative approach for calculating the AWQC using the as consumed 
weight (again, more directly associated with exposure and risk) which 
is subsequently adjusted by the approximate 20% cooking loss to a 
resultant uncooked equivalent. Thus, the AWQC conversion to an uncooked 
equivalent can be consistently used between State/Tribal standards 
programs and still represent the same relative risk as the as consumed 
value. It is important to understand that the two approaches will not 
result in the same AWQC value. Whereas the as consumed approach is more 
scientifically rigorous and represents a more direct translation of the 
as consumed risk to the uncooked equivalent, it may be too intensive a 
process to expect of State and Tribal organizations whose resources are 
already constrained.

Relative Source Contribution (RSC)

15. Default Percentages and RSC Floor of 20% and Ceiling of 80%
    Comments--A commenter criticized EPA's recommended RSC default rate 
in the face of uncertainty about other routes of exposure. Another 
commenter considered the ceiling of 80% to be a redundant uncertainty 
factor. Other comments suggested the use of an 80% RSC for 
bioaccumulative chemicals so that the contribution from fish 
consumption would not be underestimated, did not support the range of 
20% to 80%, or requested additional justification for the assignments 
of 20%, 50%, or 80%.
    Response--EPA has recommended using the 20% RSC default when routes 
of water exposure other than oral or sources of exposure other than 
fish and water are anticipated, but adequate data are lacking to 
quantify those exposures. When data are adequate, they should be used 
instead of the default. If it can be demonstrated that other sources 
and routes of exposure are not anticipated for the chemical in question 
(based on information about its known/anticipated uses and chemical/
physical properties), then the 80% ceiling is recommended. The ceiling 
is intended to provide adequate protection for those who experience 
exposures (from any or several sources) higher than available data 
indicate. For many of the chemical contaminants that EPA evaluates, 
data are not available on multipathway exposures. It is possible that 
as we progress with our development of a cumulative risk policy, we may 
find an 80% RSC to be underprotective. This concern was expressed 
during the scientific peer review workshop on the Methodology. One 
commenter misunderstood the application of lower ceilings (i.e., 50%, 
20%) when existing information indicates no other media-specific uses 
or sources. Also, some chemicals that bioaccumulate in fish also 
bioaccumulate in other meat and dairy products (e.g., dioxins). 
Therefore, to simply assume an 80% default in all cases would not be 
appropriate. The RSC approach allows for an apportionment of 80% when 
information indicates that other exposures are not relevant for the 
chemical being evaluated. EPA has added discussion in the final 
Methodology to address these situations and to better explain the 
application of the lower ceilings.
16. Duplication of Fish Intake Assumptions
    Comments--Commenters stated that applying an RSC factor results in 
a double-counting of fish from other sources.
    Response--The commenters are incorrect. The fish intake default 
used in the equation accounts for fresh and estuarine species only. The 
RSC factor potentially applies to nonfish dietary intake, air 
exposures, and marine fish species. To protect humans who additionally 
consume marine species of fish, the marine portion should be considered 
as part of the ``other sources of exposure,'' that is, part of the RSC 
or dietary value. EPA specifically emphasized in the 1998 draft 
Methodology revisions that States and authorized Tribes need to ensure, 
when evaluating overall exposure to a contaminant, that the marine fish 
intake is not double-counted with the dietary intake estimate used. 
This applies if the State or authorized Tribe chooses to account for 
total fish consumption (i.e., fresh/estuarine and marine species) in 
the fish intake parameter used in the AWQC equation.
17. Exposure Route Differences
    Comments--EPA received support for its rationale on accounting for 
differences in bioavailability and absorption between exposure routes 
when data are available, and assuming equal rates when data are absent.
    Response--We acknowledge this support.
18. Need for an RSC Factor/Considering Multiple Routes of Exposure
    Comments--Commenters supported the greater emphasis on RSC, 
including the use of empirical data. Some stated that EPA should give 
full consideration to multiple routes of exposure (i.e., ingestion, 
inhalation, dermal), with emphasis on the variety of water-related 
activities, cultural practices, and lifestyles. Several commenters 
pointed to published studies on assessing inhalation and dermal 
exposures, and two commenters advocated that EPA determine when there 
is a need to factor in these exposures, based on available information 
on the chemical. One commenter stated that there are circumstances 
where inhalation exposures can be a significant portion of total 
exposure (e.g., for some chemicals during showering). However, another 
suggested that consideration of inhalation and dermal exposures is 
premature. Two commenters stated that uncertainty factors, severity of 
effects, essentiality, and additive/synergistic

[[Page 66473]]

effects should be factored into the RfD apportionment, with one 
believing that this should also include the option of developing less 
stringent criteria when there is great uncertainty in the data. Five 
commenters stated that they believe the RSC/Exposure Decision Tree 
concepts represent an unnecessary safety factor or should not be 
considered. One suggested that the water quality criterion should 
relate only to water exposures. Two commenters suggested that factoring 
in other exposures is ``penalizing'' the AWQC and makes them overall 
environmental exposure criteria. Another questioned the need to 
apportion the RfD, but focused on drinking water regulations, stating 
that accounting for other sources of exposure would likely have no 
benefit, presumably due to conservatism in the RfD derivation (yet 
acknowledging that those uncertainty factors are independent of the 
exposure assessment). Several commenters recommended that EPA 
reconsider the SAB's advice not to routinely apportion the RfD. Others 
believed that the RSC should be used only for site-specific criteria, 
or that States should have the flexibility to make adjustments for 
local conditions. Two commenters also stated that the Exposure Decision 
Tree is unclear, is overly complicated, or has unrealistic data 
requirements. Another stated that the approach is generally desirable 
but that EPA needs to provide a greater and more easy-to-follow 
explanation of the rationale, indicating policy judgments where they 
occur. However, other commenters supported the Decision Tree approach 
for its facilitation of identifying the decisions necessary to select 
the most appropriate RSC value and considered it scientifically valid. 
One commenter cautioned that if probabilistic analysis techniques are 
used, their application must be valid and underlying assumptions 
clearly indicated. Commenters expressed the need for data to avoid the 
20% default, others stated that defaults should be avoided altogether, 
and one recommended a 100% RSC for highly bioaccumulative chemicals. 
One of the supporters believed that the approach is a reasonable 
compromise between avoiding problematic increases in exposures to 
substances and not setting unduly restrictive requirements. A commenter 
questioned how new data would be considered in the context of RSCs 
based on older data. Another recommended that non-zero values for other 
exposure sources not be assumed unless a significant number of samples 
are positive. It was also recommended that EPA coordinate the RSC 
policy with other Agency programs.
    Response--EPA disagrees that the RSC represents an excessive or 
unnecessary safety factor. The purpose of the RSC is to ensure that the 
level of a chemical allowed by a criterion or multiple criteria, when 
combined with other identified sources of exposure common to the 
population of concern, will not result in exposures that exceed the RfD 
or POD/UF. The policy of considering multiple sources of exposure when 
deriving health-based criteria has become common in EPA's program 
office risk characterizations and criteria and standard-setting 
actions. Since the SAB expressed concerns in 1993, numerous Agency 
workgroups have evaluated the appropriateness of factoring in such 
exposures and concluded that it is important for adequately protecting 
human health. Consequently, Agency policy has evolved significantly 
over the last 6 years. Various EPA program initiatives and policy 
documents regarding aggregate exposure and cumulative risk have been 
developed, and include consideration of inhalation and dermal 
exposures. Additionally, accounting for other exposures has been 
discussed in recent mandates (e.g., the Food Quality Protection Act) 
and, thus, is becoming a requirement for the Agency. The RSC approach 
has been shared with other EPA offices, and efforts to coordinate 
policies on aggregate exposure, where appropriate, have begun. EPA 
intends to continue developing guidance on the RSC issue and guidance 
to address the concern that human health may not be adequately 
protected if criteria allow for higher levels of exposure that, 
combined, may exceed the RfD or POD/UF. We also intend to refine the 
2000 Human Health Methodology in the near future to incorporate 
guidance on inhalation and dermal exposures. As stated previously, we 
are required to derive water quality criteria under Section 304(a) of 
the CWA and do not intend to derive site-specific criteria for 
individual waterbodies. However, States and authorized Tribes do have 
the flexibility to make different exposure and RSC estimates based on 
local data.
    Uncertainty factors used in the derivation of the RfD to account 
for intra-and interspecies variability and the incompleteness of the 
toxicity dataset(s)/animal studies are specifically relevant to the 
chemical's internal toxicological action, irrespective of the sources 
of exposure to humans. The Agency's policy is to consider and account 
for other sources of exposure in order to set protective health 
criteria. We disagree that uncertainty in the data should result in 
less stringent criteria. However, we have provided additional 
clarification on the guidance allowing less stringent assumptions when 
multiple sources of exposure are not anticipated.
    The adequacy requirements for the Exposure Decision Tree are not 
unduly restrictive. The ideas of representativeness, quality assurance, 
and sampling size are fundamental to properly conducted monitoring 
studies. Furthermore, the minimal requirement of samples to make an (at 
least, nominally) acceptable estimate of average and high-end exposure 
from that relative source (i.e., 45 samples) is not unreasonable 
guidance. EPA also believes that the number of decision points in the 
Decision Tree for any particular chemical are not excessive. We have 
provided additional discussion in the 2000 Human Health Methodology in 
order to clarify numerous issues on the Decision Tree approach, 
including the discussion on the use of defaults. We believe that 
probabilistic techniques are potentially appropriate for use and agree 
that they must be valid, appropriately applied, and clearly presented.
    Regarding changes in ambient chemical concentrations that would 
affect the RSC calculation, States and authorized Tribes have the 
opportunity to make changes in their water quality standards during 
triennial reviews, and EPA would evaluate those changes based on 
information submitted with the proposed changes. Similarly, EPA would 
consider changes to AWQC when significant changes in sources of 
exposure occur that affect the default values.
19. Use of RSC With Carcinogenic Effects Based on Linear Low-Dose 
Extrapolation
    Comments--A commenter advocated the use of an RSC factor with 
carcinogenic effects based on linear low-dose extrapolation in order to 
account for other sources of exposure.
    Response--EPA does not apply the RSC to carcinogenic effects based 
on linear low-dose extrapolation because the AWQC are being determined 
with respect to the incremental lifetime cancer risk posed by a 
substance's presence in the exposure sources relevant to the specific 
criterion, not in terms of an individual's total cancer risk from all 
sources of exposure. In the case of carcinogens based on nonlinear low-
dose response extrapolation or a noncancer endpoint where a threshold 
is assumed to exist, non-water

[[Page 66474]]

exposures (i.e., non-drinking water and non-fish ingestion exposures, 
and inhalation or dermal exposures) are considered when deriving the 
AWQC. The rationale for this approach has been that for pollutants with 
effect thresholds, the objective of the AWQC is to ensure that an 
individual's total exposure does not exceed that threshold level. 
Health-based and medium-specific criteria values for carcinogens based 
on a linear low-dose extrapolation typically vary from other medium-
specific criteria values in terms of the concentration value, and often 
the associated risk level. Therefore, the RSC concept could not apply 
unless all risk assessments for a particular carcinogen based on a 
linear low-dose extrapolation used the same concentration value and 
same risk level; that is, an apportionment would need to be based on a 
single risk concentration value and level.
20. Use of Subtraction or Percentage Methods in RSC Apportionment
    Comments--One commenter advocated the subtraction method instead of 
the percentage method for RfD apportionment, and advocated the use of 
central tendency values. This commenter criticized the percentage 
method as irrational and likely to produce overly stringent criteria. 
In addition, it was stated that the percentage method would allow 
criteria that could result in exposure levels that exceed the RfD when 
combined exposures are high. Other commenters expressed concern over 
basing the RSC on current levels of contamination. However, one 
believed that the percentage apportionment was reasonable given the 
difficulty in alternative apportionment methods (for example, an 
apportionment that would minimize the costs of reducing total exposure 
to/below a certain amount). One commenter suggested using a multiple 
default system.
    Response--The first commenter has significantly misunderstood EPA's 
policy goals. The argument against use of the percentage approach is 
based on the idea that the maximum possible amount of chemical 
concentration, after subtracting other sources, should be allocated to 
drinking water criteria or standards. This is not EPA's goal nor is it 
stated in any relevant mandate. The rationale of deliberately removing 
the entire cushion between precriteria levels (i.e., actual levels) and 
the RfD, and thereby setting criteria at the highest levels short of 
exceeding the RfD, is counter to the goals of the CWA for maintaining 
and restoring the nation's waters. It is also directly counter to 
Agency policies, explicitly stated in numerous programs, regarding 
pollution prevention. EPA has advocated that it is good health policy 
to set criteria such that exposures are kept low when current levels 
are already low. The subtraction method generally results in 
prospective criteria values for a contaminant in a particular medium at 
significantly higher levels than the percentage method and, in this 
respect, is contradictory to these Agency goals. In fact, many 
chemicals have existing levels in environmental media, based on 
available monitoring data, substantially lower (compared with the RfD) 
than the resulting criteria allow. This is the case with most of the 
theoretical examples that one commenter provided to refute the method.
    The Agency has modified its policy with the Exposure Decision Tree 
approach to allow use of the subtraction method when multiple media 
criteria are not relevant. The Agency RSC Workgroup recommended that, 
although combined exposures above the RfD may or may not present an 
actual health risk, a combination of health standards exceeding the RfD 
may not be sufficiently protective. Therefore: (1) Maintaining total 
exposure below the RfD is a reasonable health goal; (2) there are 
circumstances where health-based criteria for a chemical should not 
exceed the RfD (either alone or in combination); and (3) the best way 
to prevent exceedance of the RfD is to apportion it when multiple 
health criteria are relevant to a given chemical. We believe that the 
percentage method is rational in the context of the above goals when 
multiple media criteria are at issue. However, as a commenter 
suggested, the percentage method does not simply depend only on the 
amount of the contaminant in the prospective criterion source. It is 
not a set amount. It is intended to reflect health considerations, the 
relative contribution of other sources, and the likelihood for ever-
changing levels in each of those multiple sources (due to ever-changing 
sources of emissions and discharges). The percentage method does not 
break any ``logical link,'' as a commenter suggested (the commenter 
referenced an unpublished report from discussions prior to the 
development of the Exposure Decision Tree approach). EPA is interested 
in knowing the amounts of current exposures, including water, and is 
always cognizant of their relationship to the RfD (one commenter 
suggested that EPA does not compare actual exposures to the RfD; this 
comparison is always known). We have historically evaluated chemicals 
in the context of their current levels (i.e., ambient levels prior to 
either criteria development or regulatory activity). Evaluating these 
levels, along with the hazard identification, has historically formed 
the basis for prioritization and whether the Agency would pursue 
criteria or standards development. We disagree with the comment that 
criteria should be set without regard to the actual level of the 
contaminant. Actual levels are advocated by a commenter for use with 
the subtraction method. In the case of multiple criteria for a given 
chemical, the commenter's claim that the subtraction method will ensure 
that ``an individual's exposure to a chemical does not exceed the RfD'' 
is not necessarily guaranteed if criteria for other media allow for 
concentrations in environmental media that, combined, may result in 
exposures greater than the RfD. EPA acknowledges that the percentage 
approach outcome varies depending on the magnitude of current 
exposures, and we have sought to provide greater clarification on this 
policy issue in the 2000 Human Health Methodology. Of course, depending 
on the levels from each source, the subtraction method can also produce 
unstable values--that is, they could vary from very high, to moderate, 
to very low, even to a negative number.
    As previously indicated, probabilistic analyses are appropriate 
when they are validated techniques that are applied correctly and 
supported by adequate data. However, much of the time, the amount of 
data available to describe distributions of exposure from various known 
sources to the U.S. population--for use in setting nationwide 
criteria--is inadequate to support meaningful probabilistic analyses. 
Nevertheless, rather than simply using a default value in every 
instance, the Agency attempts to compare exposure intakes based on 
available data to estimate their relative contribution to the total--
given that understanding the degree to which their concentrations vary, 
or making any distributional analysis, is not possible. When multiple 
criteria are at issue, the criteria values are based on the best 
available information, with an assumption that there may be enough 
relative variability such that an apportionment (relating that 
percentage to the RfD) is a reasonable way of accounting for the 
uncertainty regarding that variability. Again, in the context of making 
an estimate of potential national exposures, there is great uncertainty 
in the range of exposures, and as previously stated, the goal is not to 
allow a water criterion to use up the

[[Page 66475]]

``space'' between the total exposure and the RfD. An example of the 
percentage apportionment's potential use is when pesticides are at 
issue. It does not make sense to allow the water criterion to use up 
that space when (in terms of the chemical's intended uses) the dietary 
route is obviously the ``direct'' source of exposure. When the course 
of pesticide tolerance-setting activities may, over time, vary the 
exact amount of the RfD taken up, an apportionment may also be best for 
pesticide program planning. The Exposure Decision Tree has allowed for 
the use of the subtraction approach when only one criterion is 
relevant. Also, given the future need to develop cumulative risk 
policies, the subtraction method in these cases could be a short-lived 
option.
    Finally, one commenter incorrectly assumed that the percentage 
method would allow criteria that could result in exposure levels that 
exceed the RfD when combined exposures are high. Again, this commenter 
incorrectly assumed that EPA is not aware of the relationship of the 
estimated exposures to the RfD. The Exposure Decision Tree approach 
states that, in these situations, a risk management decision would be 
made in order to reduce exposures to levels that would prevent 
exceedance of the RfD. We have provided greater clarification on this 
issue in the 2000 Human Health Methodology. We have also provided 
clarification on the use of central tendency values when estimating 
exposures, which we do not believe to be fully adequate for protection 
of human health when setting national 304(a) criteria.

F. Bioaccumulation

1. Use of Bioaccumulation Factors (BAFs) in General
    Comments--Overall, commenters were not adverse to incorporating 
bioaccumulation into criteria derivation, but were concerned with the 
methodology EPA proposed to use. Most comments received were focused on 
the general use of BAFs. Because of the site-specific nature that BAFs 
can take, several commenters are concerned with applying national BAFs 
developed from a limited set of data and array of aquatic systems, or 
from a model, to all waterbodies in the United States. Some commenters 
did not agree with EPA's proposed BAF tiered hierarchy. These 
commenters stated that EPA should not derive single national BAFs 
because there is substantial variation among waterbodies in factors 
that influence bioaccumulation (e.g., food chain, metabolism, 
bioavailability, loading history). They recommended that BAFs be 
calculated on a site-specific basis, or that field-derived BAFs be used 
in conjunction with modeled BAFs in a weight-of-evidence approach to 
select a final BAF. Some commenters also wanted the BAF guidance to 
more clearly state how it applies to different groups of compounds 
(e.g., nonionic organics, ionic organics, metals, organometallics). 
Several commenters did agree with EPA that field-derived BAFs better 
reflect potential exposure to chemicals from all sources than BCFs and 
incorporate factors in the field (e.g., food chain, metabolism, 
chemical loading history, temperature) that can affect bioaccumulation.
    Response--Although EPA acknowledges there are site-specific factors 
that affect bioaccumulation, we disagree that national BAFs should not 
be derived. For some pollutants (e.g., PCBs, methylmercury), 
biomagnification through the food chain can be substantial. Using a 
BCF, which only accounts for exposure from the ambient water, could 
substantially underestimate the potential exposure to humans for some 
chemicals and result in criteria that are underprotective of the 
designated uses. Since publishing the 1980 Methodology, there has been 
a growing body of scientific knowledge that clearly supports the 
observation that bioaccumulation and biomagnification occur and are 
important exposure issues to consider for many highly hydrophobic 
organic compounds and certain organometallics (Russell et al., 1999; 
Fisk et al., 1998; USEPA, 1998d; Watras and Bloom, 1992; Oliver and 
Niimi, 1988; Swackhammer and Hites, 1988; Niimi, 1985; Oliver and 
Niimi, 1983). For highly persistent and bioaccumulative chemicals that 
are not easily metabolized, BCFs do not reflect what the science 
indicates. For this group of chemicals, bioaccumulation (i.e., 
accumulation of a chemical in aquatic biota from all routes of 
exposure) should be accounted for in the derivation of water quality 
criteria in order to protect against unacceptable risks from 
contaminated biota. The use of properly derived BAFs will enable 
chemical exposure from all sources to be accounted for in water quality 
criteria. The lack of national BAFs would greatly hinder the 
development of water quality criteria because many States and 
authorized Tribes may not have the resources to develop site-specific 
BAFs. We continue to believe that using national BAFs is the most 
scientifically valid approach to deriving national AWQC.
    EPA acknowledges that data available to derive national BAFs and to 
validate the overall bioaccumulation methodology are primarily limited 
to persistent, hydrophobic chemicals from selected locations (e.g., 
Lake Ontario, Green Bay, Bayou d'Inde, Hudson River). However, we 
believe these chemicals and sites encompass a reasonable range of 
chemicals, locations, and ecosystems from which to evaluate the 
appropriateness of the bioaccumulation methodology. To obtain better 
representation of lotic (e.g., river) systems, we also performed 
evaluation of the predictive BAF methods with PCB, pesticide, and 
chlorinated benzene data from the Hudson River and Fox River/Green Bay. 
In the vast majority of comparisons between the predicted BAFs and 
field-measured BAFs using all four methods, the predicted BAFs were in 
very good agreement with the field-measured BAFs. We further 
acknowledge commenters' concerns that certain portions of the 
methodology may not be applicable to some types of chemicals. As a 
result, we have developed additional guidance that restricts some 
aspects of the methodology to certain types of chemicals. For example, 
we have revised the 1998 draft Methodology revisions to remove the use 
of Kow x FCM to estimate BAFs for chemicals that have been 
consistently shown to be metabolized substantially in aquatic biota 
(e.g., certain PAHs) and have clearly differentiated which methods 
apply to ionizable chemicals and which do not.
    We also recognize that there were some uncertainties in the 1998 
draft Methodology revisions on how the BAF methodology would be applied 
both nationally and on a site-specific basis. In response to this, we 
made substantial revisions to the 1998 draft bioaccumulation 
methodology which we believe makes the revised methodology applicable 
on a national basis. First, we improved the readability and guidance 
presented in the bioaccumulation methodology based on public and peer 
reviewers' comments. Specifically, we separated guidance for developing 
national BAFs from guidance for developing site- or region-specific 
BAFs and revised the Methodology document to make it more clear to the 
reader on how EPA will derive national BAFs. Second, EPA expanded the 
guidance for deriving site-or region-specific BAFs to better enable 
such adjustments to be made by States and authorized Tribes. For 
example, we updated, expanded, and made more accessible the databases 
used to develop national values for lipid content in aquatic biota and 
organic carbon content

[[Page 66476]]

in water. Third, we plan to develop detailed guidance to assist States 
and authorized Tribes in designing and conducting field studies to 
measure site-specific BAFs and BSAFs (biota-sediment accumulation 
factors). This guidance will specify our recommendations for how, when, 
where, and how often one should sample water, biota, and sediment for 
producing reliable measurements of BAFs and BSAFs.
    In addition to improved clarity and expanded guidance, EPA believes 
the changes we made to the national BAF methodology address concern 
indicated by some public commenters about uncertainty in various 
aspects of the methodology. We believe the changes we have made reduce 
the uncertainty in several components of the national BAF methodology. 
For example, development of separate procedures for deriving BAFs for 
different chemical classes (e.g., high vs. low hydrophobicity, high vs. 
low metabolism in biota, ionic vs. nonionic organics) will reduce 
uncertainty in national BAFs and simplify procedures. As part of these 
revisions, we recommended that Kow-based estimates of BAFs 
and food chain multipliers (FCMs) not be used for nonionic organics 
that are known to be metabolized substantially in targeted biota (e.g., 
some PAHs). Restrictions have also been placed on the use of the BSAF 
methodology so that the method is used for the chemicals for which it 
is most appropriate.
    We clearly recognize that even with these revisions incorporated 
into the national BAF methodology, significant uncertainty might exist 
in the assessment and application of national BAFs at some sites 
throughout the United States because of the influence of site-specific 
factors. Therefore, we have more clearly indicated that development of 
site-specific BAFs is encouraged and supported when it can be shown 
that a national BAF is inappropriate, or when a State or authorized 
Tribe prefers to derive a site-specific BAF.
    EPA agrees with commenters that in some cases it may be appropriate 
to derive a BAF using several of the recommended methods (Methods 1-4), 
with the final BAF chosen using a weight-of-evidence approach. We have 
provided general guidance on the assessment of uncertainty in using 
field-measured BAFs (and BAFs derived using the other methods) when 
deriving national BAFs. However, we do not believe that the mere 
existence of uncertainty means that national BAFs (and resulting 
national 304(a) water quality criteria) cannot be implemented 
effectively throughout the United States. For more than two decades, we 
have developed and implemented our national 304(a) water quality 
criteria (aquatic life and human health) through State, Tribal, and on 
occasion, Federal water quality standards programs. Implementation of 
this program has relied on the use of national 304(a) criteria as a 
cornerstone but has evolved to allow the use of procedures to modify 
national criteria by States and authorized Tribes where appropriate. 
EPA's national bioaccumulation methodology is consistent with this 
programmatic practice, by enabling States and authorized Tribes to 
readily adopt national 304(a) water quality criteria into standards 
(based on National BAFs) that achieve the CWA goals of protecting 
public health while also allowing site- or State-specific adjustments 
in situations where national AWQC may be considered overprotective or 
in some cases, underprotective.
    Comments--Some commenters questioned the application of the BAF 
prediction approaches (Tiers 2-4; referred to as Methods 2-4 in the 
revised Methodology) on a national scale because the data used to 
validate the approaches and develop predicted BAFs come primarily from 
chemical partitioning relationships observed from a limited set of 
studies (e.g., Great Lakes region).
    Response--EPA agrees that the locations for which the BAF 
methodology has been fully applied are limited in number (e.g., Lake 
Ontario, Green Bay). To address this concern, we have conducted 
additional assessments and comparisons among the bioaccumulation 
approaches (Methods 1-4) to further validate their usefulness and have 
validated the methods using other locations (e.g. Bayou d'Inde, LA, Fox 
River/Green Bay, Hudson River, NY). We acknowledge that a model 
prediction is not a perfect simulation of what occurs in a natural 
aquatic ecosystem and that uncertainty exists in the BAFs. However, 
this does not invalidate the usefulness of models validated using data 
from the Great Lakes and Hudson River in predicting bioaccumulation in 
other ecosystems. Results of analyses that support using a predictive 
bioaccumulation approach for a variety of chemicals and aquatic 
ecosystems can be found in Burkhard et al. (1997), Burkhard (1998), 
Oliver and Niimi (1988), Swackhammer and Hites (1988), and Oliver and 
Niimi (1983). Data from these studies clearly indicate that the food 
web is a dominate exposure route for many highly hydrophobic chemicals 
and that use of BCFs only underestimates exposure. EPA's proposed BAF 
methodology does account for some site-specific differences in 
bioaccumulation (an issue expressed by commenters) by considering 
factors such as percent lipid in the fish consumed and the freely 
dissolved concentration of the chemical in the ambient water (i.e., a 
baseline BAF). This allows a BAF developed from one set of data and 
location(s) to be ``normalized'' and applied to another location. We 
believe the approach in the 2000 Human Health Methodology appropriately 
balances protectiveness with the uncertainties surrounding the science 
currently available to predict bioaccumulation. Comparisons of field-
measured and predicted BAFs demonstrate agreement within an order of 
magnitude in the vast majority of cases, and often within a factor of 
two to five. Burkhard (1998) observed good agreement between measured 
and predicted BAFs for the Lake Ontario food web using the Gobas and 
Thomann food web models. For individual commonly detected PCBs and 
chlorinated pesticides, the BAFs estimated using the two Gobas and 
Thomann models were on average within a factor of 1.2 and 2.5 of the 
observed (i.e. field-measured) BAFs, respectively (Burkhard 1998). The 
overall uncertainties in each of these two bioaccumulation models 
(expressed as the ratio of the 90th to 10th percentile predicted BAF 
for each model) were a factor 3.6 and 4.0 for the Gobas and Thomann 
models, respectively (Burkhard 1998). Furthermore, Burkhard et al. 
(1997) reported that predicted BAFs (using EPA's national BAF 
methodology) were within a factor of 5 for 94% (n=32, using laboratory 
measured BCFs and FCMs) and 90% (n=48, using predicted Kows 
and FCMs) in Bayou d'Inde (Lake Charles, LA). These data comparisons 
show the good predictability of the methods used in the national BAF 
methodology. Should States or authorized Tribes have information to 
suggest that a national BAF is inappropriate for their situation, the 
2000 Human Health Methodology specifically allows and encourages 
development of site-specific BAFs. With this in mind, we will be 
developing guidance on how to collect and interpret field data for the 
purpose of deriving site-specific field BAFs. This guidance will 
specifically address major sources of variability, including spacial 
and temporal factors and species life history.
    Finally, to further address concerns that the predictive approaches 
used to derive BAFs may not be applicable at a

[[Page 66477]]

national scale, we revised the 1998 draft Methodology to clarify and 
limit for which chemicals and under what conditions BAFs based on 
Methods 2 to 4 are most applicable. For example, chemicals were grouped 
into broad categories based on their persistence and bioaccumulation 
potential (e.g., high vs. low hydrophobicity, high vs. low biota 
metabolism, ionic vs. nonionic), and we have limited the use of 
predicted BAF approaches to selected groups of chemicals for which the 
data reasonably support their use (i.e., highly hydrophobic chemicals 
that are not expected to be metabolized appreciably). The national BAF 
methodology was also changed to indicate that for those chemicals with 
sufficient data to indicate they are metabolized, model-predicted BAFs 
are not recommended; rather, field BAFs or laboratory BCFs are 
recommended. The use of the BSAF methodology has been restricted to 
chemicals that are highly hydrophobic (e.g., log 
Kow4).
    EPA believes these revisions to the 1998 draft Methodology have 
improved the Methodology and have addressed many of the commenters' 
concerns and questions about uncertainty in applying the various 
approaches and BAFs on a national scale.
    Comments--One commenter suggested that it is ``scientifically 
indefensible to use the field-measured BAF procedure to derive BAFs for 
benthic systems.'' They commented that in a benthic-based aquatic food 
web, the water column concentration of a chemical is not directly 
related to aquatic organism exposure potential for that chemical. 
Therefore, their view is that a field-measured BAF may over- or 
underestimate bioaccumulation in benthic-based systems.
    Response--EPA acknowledges that the concentration of a chemical in 
the water column is not directly related to what pelagic organisms 
(i.e., fish) are exposed to in a benthic-based system. However, the 
concentrations of a chemical in water, sediment, and fish are 
interconnected, although they may not be equally partitioned into each 
compartment, and residues in fish can be predicted equally well using 
either a sediment or water concentration as the starting basis. In the 
revised TSD on Bioaccumulation, the relationships between BAFs and 
BSAFs have been shown more clearly in order to demonstrate this 
interconnectedness. In the BAF methodology, we are assessing exposure 
through all routes (i.e., from water, sediment, and contaminated food) 
in the aquatic ecosystem. By including all routes of exposure, the BAFs 
do not assume simple water-fish partitioning; rather they are an 
overall expression of the total bioaccumulation using the concentration 
of the chemical in water column as a reference point. Thus, a field-
measured BAF or BASF at any given time is reflective of historic 
chemical loadings and bioaccumulation that has occurred. EPA does agree 
that a BAF may change over time because of differential chemical 
loadings; however, some frame of reference has to be chosen as the 
starting point to assess bioaccumulation. EPA has chosen to use the 
water concentration as that reference point. Science has shown that 
bioaccumulation occurs and is an important exposure pathway to humans 
for many chemicals, and EPA cannot ignore bioaccumulation in 
development of its AWQC simply because variability and uncertainty 
exist. In situations where chemical loadings are highly variable or are 
reduced substantially, EPA believes that a field-measured BAF will 
still be predictive of what will bioaccumulate in fish until the 
concentrations in sediments and benthic organisms are reduced enough to 
lead to reduced bioaccumulation. In situations such as this, a revised 
site-specific field BAF can be developed to reflect the change in 
chemical loading and partitioning.
    This issue of field-measured BAFs and benthic-based food webs was 
also brought up in public comments made at the stakeholders meeting 
held in May 1999. At that time, we asked commenters if they could 
recommend another approach to assess bioaccumulation in benthic-based 
systems. No other approaches were suggested. We have concluded that in 
the absence of any other approaches, field-derived BAFs are good 
predictors of bioaccumulation because they integrate biological, 
chemical, and physical factors that influence bioaccumulation.
2. Guidance for Deriving Field Bioaccumulation Factors (BAFs)
    Comments--Several commenters agreed with EPA that field-derived 
BAFs should take precedence over modeled BAFs. However, many commenters 
discussed the need for guidance on how to collect and review field data 
so that high-quality, field-based BAFs can be derived. Commenters noted 
that there are numerous site-specific biological, chemical, and 
physical factors that affect bioaccumulation, which should be 
considered during design of field sampling programs.
    Response--We agree that properly derived field BAFs should take 
precedence over modeled BAFs; we have clearly indicated in the 2000 
Human Health Methodology that this is our preferred approach for 
deriving a BAF. We also acknowledge that, as with any field 
measurement, there can be errors in determining field-measured BAFs. In 
the development of national BAFs, EPA will attempt to minimize 
potential errors or uncertainties by carefully screening the data based 
on the criteria outlined in the Bioaccumulation TSD. Furthermore, an 
additional validation of national BAFs will be conducted as part of the 
external peer review process that occurs for all published 304(a) water 
quality criteria. We continue to assert that for many chemicals, a 
field-measured BAF is a better gauge of what is occurring in nature 
than a laboratory-measured or predicted BCF; the BAF measures the 
actual effects of bioavailability, concentration in the water or 
sediment, growth dilution, metabolism, and biomagnification rather than 
predicting them through use of a model. We do agree with commenters 
concerned about the difficulty of collecting and interpreting field-
measured BAFs; however, we believe that States and Tribes can 
adequately design and interpret field studies. To assist them in this 
task, we will be developing guidance concerning field data collection 
and interpretation for site-specific field-measured BAFs and BSAFs.
3. Use of Biota-Sediment Accumulation Factors (BSAFs)
    Comments--Several commenters stated that the use of the BSAF 
approach for deriving a BAF is inappropriate. Some comments centered 
around the perceived lack of validation and peer review of the BSAF 
approach, and others focused on the relationship between the water 
column concentration of a chemical and its sediment concentration, 
represented by the factor socw. One commenter noted 
that the BSAF method is simply a means to predict a water concentration 
of a chemical of interest from the sediment concentration of that 
chemical, the water and sediment concentration of a reference 
chemical(s), and the ratio of Kow for the chemical of 
interest and the reference chemical(s). A commenter indicated that 
loading history of a given chemical directly affects what the value of 
socw would be at any given time, and that 
socw/Kow (disequilibrium ratio) for the 
chemical in question and the reference chemical has to be constant 
under the assumptions of the BSAF approach. The commenter stated, 
however, that socw/Kow will not be 
constant because of

[[Page 66478]]

differential loading histories, and that because the concentration of 
the chemical of interest cannot be measured in water, the assumptions 
about socw/Kow cannot be verified. In 
their view this made the use of BSAFs invalid.
    Response--The method of predicting BAFs from BSAFs has been 
evaluated for certain pesticides, PCBs, chlorinated benzenes, and 
dioxins using two data sets from Lake Ontario (Oliver and 
Niimi,1988;USEPA, 1990) and one from Green Bay (USEPA, 1992b). EPA has 
also recently completed further evaluation of this method for certain 
PCB congeners, pesticides, and chlorinated benzenes in Lakes Ontario, 
Green Bay, and the Hudson River. This additional evaluation and 
validation work is included in the Bioaccumulation TSD. The evaluations 
show that in the vast majority of situations, the BSAFs predict field-
measured BAFs very well.
    EPA agrees with the commenter who noted that the BSAF method is 
structured to predict water concentrations for chemicals that cannot be 
measured for the purpose of directly measuring a field BAF. However, 
the BSAF method is more important for its ability to capture the net 
effect of biomagnification, food web structure, hydrophobicity, 
bioavailability factors, and metabolism on a specific chemical's net 
potential for bioaccumulation. The BSAF method is needed to predict 
BAFs for chemicals with nondetectable and difficult-to-predict 
concentrations in water (e.g., dioxins). No alternative methods to 
predict BAFs for such chemicals were identified by either public 
commenters or peer reviewers. The BSAF method equation has been 
modified (see below) in the Bioaccumulation TSD to clarify the 
essential data components of the method. The revised BSAF equation 
shows that measured concentrations in water and surface sediment, not a 
complete BSAF, are needed for the reference chemical. The equation also 
shows that a measured BSAF for the chemical of interest is the most 
important component for determination of a BAF when the concentration 
in water cannot be measured.
    EPA agrees with commenters that the BSAF method should not be used 
for all organic chemicals that may be addressed through the 2000 Human 
Health Methodology, and accordingly have restricted application of the 
method to nonionic organic chemicals with log Kows 
 4.0. We have also provided more specific guidance on 
selection of reference chemicals and use of multiple reference 
chemicals to secure the most accurate estimate of a chemical's BAF.
    One commenter contended that the BSAF approach for deriving BAFs is 
seriously flawed. The concern is that the approach is valid only if a 
reference chemical (chemical r) can be found with a sediment-water 
fugacity ratio (which represents the differential partitioning of a 
chemical between water and sediment) equal to that of the chemical for 
which the BAF is being determined (chemical of interest). The commenter 
contends that the BSAF approach could validly be used only if it could 
be shown that the fugacity ratio is a constant for the chemical of 
interest and the reference chemical. The commenter submitted figures to 
demonstrate conceptually that two chemicals with radically different 
loading histories will have dissimilar fugacity ratios. EPA disagrees 
that in order for the BSAF to work, the fugacity ratio has to be 
constant, but does agree that in order to best use the BSAF approach, a 
general knowledge of chemical loading histories to an ecosystem is 
needed to help provide a basis for choosing appropriate reference 
chemicals. Such information may be obtained from chemical production 
records, historical fish residue monitoring data, or dated sediment 
core analysis. We recognize that due to various factors (loading 
histories, microbial degradation, etc.) fugacity ratios for both 
chemical (i) and (r) may shift over time, leading to the potential for 
temporal variability of sediment-water distributions of nonpolar 
organic chemicals. Although it was not shown explicitly in the 1998 
draft TSD, an important benefit of the BSAF approach is that it can 
account precisely for such differences in sediment-water distributions 
of nonpolar organic chemicals. The BSAF method is robust to the extent 
that the choice of reference chemicals is based on meeting the 
sediment-to-water fugacity ratio condition: That the ratios be 
similar--they do not have to be constant. The extent that these ratios 
for chemicals with log Kows  4 may change with 
chemical loading over long periods of time after sediments become 
contaminated, and thereby contribute to small shifts in BSAFs and 
larger shifts in BAFs, is an issue of possible concern that EPA 
recognized in the 1998 draft TSD. EPA noted on page 188 of the TSD 
(USEPA, 1998d) that ``BSAFs measured for systems with new chemical 
loadings or rapid increases in loadings may be unreliable due to 
underestimation of steady-state Csocs.''
    To better address the water-to-sediment relationship issue, EPA has 
revised the equations that serve as the basis for deriving a BSAF. In 
the revised equations, a factor Di/rhas been added, which is 
defined as the ratio of the fugacity gradient (modeled as 
socw/Kow) between sediment and water for 
chemical (i) in comparison to that of a reference chemical (r). The 
revised equations are as follows:
[GRAPHIC] [TIFF OMITTED] TN03NO00.022

    By definition, socw can be used to relate 
chemical i's BSAF to its BAFfd:

[[Page 66479]]

[GRAPHIC] [TIFF OMITTED] TN03NO00.023

    By substituting rearranged Equation 1 into rearranged Equation 2:
    [GRAPHIC] [TIFF OMITTED] TN03NO00.024
    
where:
(BAFfd)i = BAF expressed on a freely dissolved 
and lipid-normalized basis for chemical of interest ``i''.
(BSAF)i = Biota-sediment accumulation factor for chemical of 
interest ``i''.
(Csoc)i = Concentration of chemical of interest 
``i'' in sediment normalized to sediment organic carbon.
(Csoc)r = Concentration of a reference chemical 
in sediment normalized to sediment organic carbon.
(Cwfd)i = Concentration of chemical of 
interest ``i'' freely dissolved in water.
(Cwfd)r = Concentration of the 
reference chemical freely dissolved in water.
Di/r = ratio between socw/Kow 
for chemicals ``i'' and ``r'' (normally chosen so Di/r = 1).
(Kow)i = octanol-water partition coefficient for 
chemical of interest ``i''.
(Kow)r = octanol-water partition coefficient for 
the reference chemical ``r''.
(socw)i = sediment organic carbon to 
water freely dissolved concentration ratio of chemical of interest 
``i''.
(socw)r = sediment organic carbon to 
water freely dissolved concentration ratio of reference chemical ``r''.

    Equation 3 is intended to provide an improved representation of how 
the BSAF method/model works. By using Di/r, the new equation 
accounts for differences in sediment to water column concentrations 
that might exist between the chemical of interest and the reference 
chemical because of factors such as loading histories or degradation. 
Unlike one commenter's analysis, in which an equation was derived 
without the BAF or BSAF, equation 3 shows these quantities as central 
to the model; that is, the BSAF is measured and then transformed into a 
BAF by estimating the chemical's socw/
Kow. This model could alternatively be described as a 
determination of (Cwfd)i from a 
measured value of (Csoc)i combined with a 
measured value of (C)i to give an accurate measure of 
(BAFfd)i. However, we believe that equation 3 
best describes the BSAF method as allowing measured BSAFs to be 
transformed into BAFfds for the specific purpose of 
developing either national or a site-specific water quality criteria 
when directly measured BAFfds cannot be obtained.
    When good-quality data are available for reference chemicals (r) 
that should have equal or similar sediment-water fugacity ratios as a 
chemical (i) whose (BAFfd)S cannot be measured 
directly, then Di/r = 1. When Di/r  1, 
it may be estimated based on properties of the chemicals and knowledge 
of their loading histories to the ecosystem. Equation 3 provides a 
greater degree of flexibility for use of the BSAF method than the 
original equation. This flexibility highlights a logical stepwise 
transition from measured to fully modeled site-specific BAFs that can 
incorporate estimates of Di/r through fate modeling, should 
interested parties choose to do so. In such a situation, if the 
uncertainty associated with choice of Di/r is perceived to 
be too great, a determination of a site-specific 
(BAFfd)i, which still takes advantage of 
measured values of (C)i and (Csoc)i, 
could be accomplished if a mass balance model, specifically calibrated 
with (C)i and (Csoc)i, is used to 
predict (Cwfd)i. Such an approach 
would be time consuming and expensive but would allow prediction of 
(BAFfd)i over time as a function of changes in 
(socw)i associated with anticipated 
changes in mass loading of the chemical into an ecosystem. In cases 
where the intended use of the site-specific criterion is to determine 
permit conditions or establish a TMDL, a mass balance model presumably 
would have to be developed, and thus use of the model for providing a 
(BAFfd)i would not require an extraordinary 
effort. However, as with the BSAF method, it should be noted that mass 
balance model predictions of Cwfdi 
also cannot be directly validated through measurements. EPA's 
appreciation for the value of hybrid models comes from recognition that 
incorporation of measured bioaccumulation potentials, including those 
provided by the BSAF method, are especially advantageous for those 
chemicals with transformation rates, such as metabolism throughout the 
food chain, that are presently not accurately known or incorporated 
into mechanistic bioaccumulation models.
    Finally, we disagree with the circular argument that the BSAF 
approach has ``extremely limited utility'' because ``it will not be 
possible to demonstrate that socw/Kow is 
a constant'' because socw/Kow cannot be 
measured directly for one chemical. The inherent limitation for 
validation of a predicted BAF because of the inability to measure the 
concentration of freely dissolved chemical in water 
(Cwfd) applies to any approach/model available 
and is not a just criterion for rejection of a BAF method. Validation 
may be based on the ability of the BAF to predict concentrations in 
fish from predicted values of Cwfd. Data from the 
Great Lakes clearly show that such predictions are possible, and 
accurate (USEPA, 1998d). It should also be noted that during the 
external peer review of the BSAF approach, the peer reviewers stated 
``for the chemicals examined (persistent and bioaccumulative), 
extrapolation to other circumstances may be reasonable,'' thereby 
disagreeing with public commenters. EPA believes that restricting the 
use of the BSAF method to highly hydrophobic chemicals, clarifying the 
use of reference chemicals, elaborating on the primacy of the sediment-
water fugacity equivalence

[[Page 66480]]

condition for use of the method, and validation with additional data 
sets alleviates concerns about using this new method.
4. Dissolved Organic Carbon (DOC) and Particulate Organic Carbon (POC)
    Comments--Two comments were received on the DOC/POC approach used 
to determine the bioavailable fraction of organic chemicals in surface 
water and sediments. Rather than solely use default organic carbon 
values, commenters wanted to the ability to select DOC/POC values they 
believe are more representative of their waterbody type or site-
specific conditions.
    Response--In the 2000 Human Health Methodology, EPA allows use of 
site-specific DOC and POC data when normalizing the BAF to organic 
carbon content. One can either conduct studies to generate the 
necessary site-specific data or modify the national organic carbon 
database to their particular site and conditions. To facilitate the 
latter, we have updated and expanded the organic carbon database used 
to develop the national default POC/DOC values to enable the regulated 
community to choose which values best represent their site conditions 
and will provide defensible site-specific DOC and POC estimates. The 
national DOC/POC database will be made available for use by all States, 
Tribes, and other members of the regulated community.
5. Fish Lipid Content
    Comments--A commenter stated that lipid content can affect the 
results of the Gobas model used to derive national default FCMs. The 
commenter noted that the model is relatively insensitive to fish lipid 
content but more sensitive to benthic invertebrate lipid content. They 
believed this should be considered in the development of FCMs.
    Response--EPA agrees that lipid content can affect the results of 
the Gobas model and is only using the Gobas model with default lipid 
values to derive national BAFs when there are no data to derive a 
field-measured BAF. In cases where a State or authorized Tribe has 
site-specific data on fish lipid content, the revised methodology 
allows input of those site-specific data to estimate bioaccumulation. 
Furthermore, to facilitate the generation of site-specific lipid 
values, we have updated and expanded the lipid database used to develop 
the national default values based on a whole range of organisms 
commonly consumed by persons in the United States. We will include 
additional guidance for States and authorized Tribes on how to adapt 
the national default lipid values to reflect State and local 
consumption patterns. To enable such adaptions, EPA will make the raw 
data available to States and authorized Tribes.
6. Use of Food Chain Multipliers (FCMs)
    Comments--Several commenters stated that the use of model-derived 
FCMs (Gobas 1993) to calculate a BAF from either a BCF or a 
Kow (Methods 3 and 4) is inappropriate. The commenters noted 
issues with several of the default input parameters (e.g., food web, 
lipid, socw, temperature). The primary concern of 
commentors is that Gobas model-based national default FCMs do not 
account for site-specific factors that influence bioaccumulation, such 
as food web structure, nor does the current use of the model account 
for metabolism. Commenters expressed concern that use of default FCMs 
in predictive approaches may lead to overestimates of bioaccumulation. 
Some commenters preferred the use of field-based FCMs or direct use of 
the Gobas model, which allows for input of site-specific data and 
metabolism rates if available, rather than uses of model-derived 
default FCMs.
    Response--EPA is using a state-of-the-art food web model for 
deriving FCMs, which incorporates the latest thinking and knowledge on 
the processes occurring in aquatic food webs. Commenters suggested that 
the assumptions used in constructing these models are not appropriate. 
We recognize that any modeling formulation of contaminant behavior in 
aquatic food webs requires simplification of a very complex biological 
system in order to assemble a tractable model. These simplifications do 
not imply or mean that our scientific understanding of all processes 
occurring in food webs is complete. As documented in the scientific 
literature, these simplifications provide reasonable model formulations 
with good predictive power. The suggestion that every modeling 
assumption has to be completely understood and validated under all 
circumstances before using or constructing a useful modeling tool is 
unreasonable. EPA has performed a detailed analysis of the importance 
and sensitivities of individual input parameters for food web models 
and of the overall uncertainties associated with predictions from food 
web models (Burkhard 1998). We have provided a discussion in the 
Bioaccumulation TSD of the Gobas model and implications that 
uncertainties in their respective input parameters have on derived 
FCMs. EPA has retained the use of Gobas model to derive default FCMs.
    To address national versus site-specific concerns expressed by some 
commenters, the methodology has been revised to separate the BAF 
methodology into national and site-specific guidance. The national 
methodology for deriving national BAFs retains the use of default FCMs 
based on a mixed benthic/pelagic food web and national averages of 
various model input values. We believe this food web is the most 
broadly applicable food web encountered in nature; its use results in 
FCMs that are midway between pure benthic and pure pelagic structures. 
The revised guidance includes a brief discussion of the uncertainties 
associated with our selection of the mixed benthic/pelagic food web. In 
the site-specific guidance, the 2000 Human Health Methodology provides 
guidance on which of EPA's recommended FCMs to use depending on the 
situation. In addition, we encourage direct use of the Gobas model by 
stakeholders so that changes could be made to the default food web 
inputs to reflect site-specific factors that influence bioaccumulation, 
and also encourage derivation of field-based FCMs. States and 
authorized Tribes have the option to generate site-specific FCMs by 
conducting site-specific field studies, reviewing published literature, 
or using other scientifically defensible models.
    Although several commenters criticized the national application of 
the Gobas model because metabolism rate is set equal to zero, the peer 
review panel acknowledged EPA's position that there are currently no 
acceptable methods available to adequately determine species and 
chemical-specific metabolism rates for use in the Gobas model. Because 
EPA agrees that for certain chemicals metabolism can be an important 
factor in bioaccumulation, the revised methodology does not use FCM-
based predictions for chemicals that are expected to be metabolized 
substantially. To assist users of the 2000 Human Health Methodology in 
determining for which chemicals or groups of chemicals metabolism 
should be of little concern, we have developed a table of chemicals 
that are not substantially metabolized or are likely very slowly 
metabolized. This table has been put in the Bioaccumulation TSD. The 
table is not all inclusive because there are numerous chemicals (e.g., 
hundreds of thousands in use commercially today) for which few or no 
metabolism data exist, but is representative of chemicals or groups of 
chemicals that are likely to be commonly encountered in aquatic 
systems. When metabolism is suspected,

[[Page 66481]]

users of the 2000 Human Health Methodology might be more inclined to 
use or develop field data and/or measure a BCF in the laboratory in 
these situations. It should also be noted that in the future, should 
appropriate chemical and species-specific metabolism data become 
available, the Gobas model can incorporate it with little effort.
    Finally, EPA partially agrees with commenters that certain 
procedures of the 1998 draft Methodology revisions (e.g., 
Kow and FCM-predicted BAFs) might lead to overestimates of 
BAFs for certain types of pollutants, such as those that are 
metabolized substantially to chemical forms not addressed by the AWQC. 
In response to this issue, and as discussed previously, additional 
guidance and limitations have been placed on several of the procedures 
in the revised methodology. However, EPA does not agree with the notion 
that our methodology would lead to a general over prediction for all 
BAFs. We use central tendencies where possible for all inputs in the 
Gobas model, and a geometric mean BCF for chemicals that have more than 
one BCF for a given trophic level. Thus, we know of no reason why 
laboratory-measured BCFs multiplied by a FCM would always result in 
overestimates of BAFs, or why the BSAF and Kow * FCM-
predicted BAFs applied to highly hydrophobic contaminants that do not 
metabolize substantially would be biased a priori toward overestimating 
BAFs. These views are supported by information in the 1998 TSD 
(Exhibits 2.4.1, 2.4.3, and 2.4.6 for BSAFs), Burkhard et al. (1997) 
for the Kow*FCM method, and information presented in the 
Bioaccumulation TSD.
7. Fish Tissue Criteria
    Comments--A few commenters suggested that for selected highly 
bioaccumulative chemicals that are difficult to measure in water, 
criteria based on fish tissue concentration may be more appropriate 
than ambient water column concentration criteria.
    Response--Regarding fish tissue criteria, EPA agrees that the 
development of human health criteria for highly bioaccumulative 
chemicals which are expressed in terms of tissue residues in aquatic 
organisms is worthy of consideration. However, such tissue residue 
criteria would still require a mechanism to relate chemical loads and 
concentrations in water and sediments to concentrations in tissues of 
appropriate aquatic organisms (i.e., bioaccumulation factors or 
bioaccumulation models). EPA is presently exploring the feasibility of 
developing tissue-based criteria and is evaluating numerous issues 
associated with implementation of tissue-based criteria. At an 
appropriate in the future, EPA will consider development of additional 
guidance on tissue residue criteria pending the outcome of this 
evaluation.

G. Literature Cited

Burkhard LP. 1998. Comparison of two models for predicting 
bioaccumulation of hydrophobic organic chemicals in a Great Lakes 
food web. Environ. Toxicol. Chem. 17(3):383-393.
Burkhard LP, Sheedy BR, McCauley DJ, DeGraeve GM. 1997. 
Bioaccumulation factors for chlorinated benzene, chlorinated 
butadienes and hexachloroethane. Environ. Toxicol. Chem. 16(8):1677-
1686.
Fisk AT, Norstrom RJ, Cymbalisty CC, Muir DCB. 1998. Dietary 
accumulation and depuration of hydrophobic organochlorines: 
bioaccumulation parameters and their relationship with the octanol/
water partition coefficient. Environ. Toxicol. Chem. 17(5):951-961.
Gobas FAPC. 1993. A model for predicting the bioaccumulation of 
hydrophobic organic chemicals in aquatic food-webs: application to 
Lake Ontario. Ecol. Model. 69:1-17.
Niimi AJ. 1985. Use of laboratory studies in assessing the behavior 
of contaminants in fish inhabiting natural ecosystems. Water Poll. 
Res. J. Can. 20:79-88.
Oliver BG, Niimi AJ. 1983. Bioconcentration of chlorobenzenes from 
water by rainbow trout: correlations with partition coefficients and 
environmental residues. Environ. Sci. Technol. 17:287-291.
Oliver BG, Niimi AJ. 1988. Trophodynamic analysis of polychlorinated 
biphenyl congeners and other chlorinated hydrocarbons in the Lake 
Ontario ecosystem. Environ. Sci. Technol. 22:388-397.
Russell RW, Gobas FAPC, Haffner GD. 1999. Role of chemical and 
ecological factors in trophic transfer of organic chemicals in 
aquatic food webs. Environ Toxicol Chem. 18:1250-1257.
Swackhamer, DL, Hites RA. 1988. Occurrence and bioaccumulation of 
organochlorine compounds in fishes from Siskiwit Lake, Isle Royale, 
Lake Superior. Environ Sci. Technol. 22:543-548.
USEPA (U.S. Environmental Protection Agency). 1980. Guidelines and 
methodology used in the preparation of health effect assessment 
chapters of the consent decree water criteria documents. Federal 
Register 45:79347, Appendix 3.
USEPA (U.S. Environmental Protection Agency). 1984. Proposed 
guidelines for carcinogen risk assessment. Federal Register 
49:46294.
USEPA (U.S. Environmental Protection Agency). 1986a. Guidelines for 
carcinogen risk assessment. Federal Register 51:33992-34003.
USEPA (U.S. Environmental Protection Agency). 1986b. Guidelines for 
mutagenicity risk assessment. Federal Register 51:34006-34012.
USEPA (U.S. Environmental Protection Agency). 1986c. Total Exposure 
Assessment Model (TEAM) Study: Summary and Analysis, Volume I, Final 
Report. EPA/600/6-87/002a.
USEPA (U.S. Environmental Protection Agency). 1986d. Quality 
Criteria for Water--1986. Office of Water Regulations and Standards, 
Office of Water. Washington, DC. EPA/440/5-86/001.
USEPA (U.S. Environmental Protection Agency). 1990. Lake Ontario 
TCDD Bioaccumulation Study--Final Report. USEPA, Region II. New 
York, NY. EPA/822/R-94/002.
USEPA (U.S. Environmental Protection Agency). 1991a. Guidelines for 
developmental toxicity risk assessment. Federal Register 56:63798-
63826.
USEPA (U.S. Environmental Protection Agency). 1991b. Technical 
Support Document for Water Quality-Based Toxics Control. Office of 
Water. Washington, DC. EPA/505/2-90/001.
USEPA (U.S. Environmental Protection Agency). 1992a. Guidelines for 
exposure assessment. Federal Register 57:22888-22938.
USEPA (U.S. Environmental Protection Agency). 1992b. Development and 
Application of a Model of PCBs in the Green Bay, Lake Michigan 
Walleye and Brown Trout and Their Food Webs. Prepared by Manhattan 
College, Riverdale, NY.
USEPA (U.S. Environmental Protection Agency). 1994. Water Quality 
Standards Handbook and Appendices. Second edition. Office of Water. 
Washington, DC. EPA/823/B-94/005a.
USEPA (U.S. Environmental Protection Agency). 1996a. Proposed 
guidelines for carcinogen risk assessment. Federal Register 
61:17960.
USEPA (U.S. Environmental Protection Agency). 1996b. Guidelines for 
reproductive toxicity risk assessment. Federal Register 61:56274-
56322.
USEPA (U.S. Environmental Protection Agency). 1996c. Report on the 
Benchmark Dose Peer Consultation Workshop. Office of Research and 
Development. Washington, DC. EPA/630/R-96/011. November.
USEPA (U.S. Environmental Protection Agency). 1997a. Exposure 
Factors Handbook. Office of Research and Development. Washington, 
DC. EPA/600/P-95/002Fa.
USEPA (U.S. Environmental Protection Agency). 1997b. Guiding 
Principles for Monte Carlo Analysis. Risk Assessment Forum. 
Washington, DC. EPA/630/R-97/001.
USEPA (U.S. Environmental Protection Agency). 1997c. Policy for Use 
of Probabilistic Analysis in Risk Assessment at the U.S. 
Environmental Protection Agency. Risk Assessment Forum. Washington, 
DC. Web site: http://www.epa.gov/ncea/mcpolicy.htm.
USEPA (U.S. Environmental Protection Agency). 1998a. Guidelines for

[[Page 66482]]

neurotoxicity risk assessment. Federal Register 63:26926.
USEPA (U.S. Environmental Protection Agency). 1998b. National 
recommended water quality criteria. Federal Register 64:19781.
USEPA (U.S. Environmental Protection Agency). 1998c. Draft water 
quality criteria methodology: Human health. Federal Register 
63:43756.
USEPA (U.S. Environmental Protection Agency). 1998d. Ambient Water 
Quality Criteria Derivation Methodology: Human Health. Technical 
Support Document (TSD). Office of Water. Washington, DC. EPA-822-B-
98-005.
USEPA (U.S. Environmental Protection Agency). 1998e. National 
recommended water quality criteria; republication. Federal Register 
63:68354-68364.
USEPA (U.S. Environmental Protection Agency). 1999a. 1999 Guidelines 
for Carcinogen Risk Assessment--Review Draft. Office of Research and 
Development. Washington, DC. NCEA-F-0644. July.
USEPA (U.S. Environmental Protection Agency). 1999b. Guidance for 
Conducting Health Risk Assessment of Chemical Mixtures--External 
Peer Review Draft. Risk Assessment Forum. Washington, DC. NCEA-C-
0148. April.
USEPA (U.S. Environmental Protection Agency). 1999c. National 
Recommended Water Quality Criteria--Correction. Office of Water. 
Washington, DC. EPA-822-Z-99-001. April.
USEPA (U.S. Environmental Protection Agency). 1999d. Review of 
Revised Sections of the Proposed Guidelines for Carcinogen Risk 
Assessment. Science Advisory Board (1400). Washington, DC. EPA-SAB-
EC-99-015. July.
USEPA (U.S. Environmental Protection Agency). 2000a. Review and 
approval of state and tribal water quality standards. Federal 
Register 65:24641.
USEPA (U.S. Environmental Protection Agency). 2000b. Health 
Assessment for 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and 
Related Compounds, Internal Review Draft. Office of Research and 
Development. Washington, DC. EPA-600-P-001Af. February 14.
USEPA (U.S. Environmental Protection Agency). 2000c. Estimated Per 
Capita Water Ingestion in the United States: Based on Data Collected 
by the United States Department of Agriculture's 1994-96 Continuing 
Survey of Food Intakes by Individuals. Office of Science and 
Technology, Office of Water. Washington, DC. EPA-822-00-008. April.
USEPA (U.S. Environmental Protection Agency). 2000d. Estimated Per 
Capita Fish Consumption in the United States: Based on Data 
Collected by the United States Department of Agriculture's 1994-1996 
Continuing Survey of Food Intake by Individuals. Office of Science 
and Technology, Office of Water. Washington, DC. March.
Watras CJ, Bloom NS. 1992. Mercury and methylmercury in individual 
zooplankton: Implications for bioaccumulation. Limnol. Oceanogr. 
37(6):1313-1318.

    This Notice finalizes revisions to EPA's 1980 Methodology for the 
development of water quality criteria to protect human health. The 
revisions reflect scientific advancements since 1980 in a number of 
areas, including cancer and noncancer risk assessments, exposure 
assessments and bioaccumulation. The revised Methodology provides 
guidance to States, Tribes, and the public on the approach that EPA 
expects to take in developing recommended human health criteria. The 
revised Methodology also provides guidance to States and Tribes that 
they may use in developing human health criteria as part of their water 
quality standards; States and Tribes use such standards in implementing 
a number of environmental programs, including setting discharge limits 
in NPDES permits. The revised Methodology does not substitute for the 
Clean Water Act or EPA's regulations; nor is it a regulation itself. 
Thus, the revised Methodology cannot impose legally-binding 
requirements on EPA, States, Tribes or the regulated community, and may 
not apply to a particular situation based upon the circumstances. EPA 
and State/Tribal decision-makers retain the discretion to use 
different, scientifically defensible, methodologies to develop human 
health criteria on a case-by-case basis that differ from this guidance 
where appropriate. EPA may change the Methodology in the future through 
intermittent refinements as advances in science or changes in Agency 
policy occur.
    This criteria Methodology incorporates scientific advancements made 
over the past two decades. The use of this Methodology is an important 
component of the Agency's efforts to improve the quality of the 
Nation's waters. EPA believes the Methodology will enhance the overall 
scientific basis of water quality criteria. Further, the Methodology 
should help States and Tribes address their unique water quality issues 
and risk management decisions, and afford them greater flexibility in 
developing their water quality programs.

    Dated: October 24, 2000.
J. Charles Fox,
Assistant Administrator for Water.
[FR Doc. 00-27924 Filed 11-2-00; 8:45 am]
BILLING CODE 6560-50-P