[Federal Register Volume 67, Number 113 (Wednesday, June 12, 2002)]
[Notices]
[Pages 40554-40576]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 02-14761]



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Part VI





Environmental Protection Agency





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Standards for the Use or Disposal of Sewage Sludge; Notice

  Federal Register / Vol. 67 , No. 113 / Wednesday, June 12, 2002 / 
Notices  

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ENVIRONMENTAL PROTECTION AGENCY

[FRL -7228-9]


STANDARDS FOR THE USE OR DISPOSAL OF SEWAGE SLUDGE

AGENCY: Environmental Protection Agency.

ACTION: Notice of data availability.

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SUMMARY: The Environmental Protection Agency (EPA) proposed to amend 
the Standards for the Use or Disposal of Sewage Sludge to limit dioxin 
and dioxin-like compounds (``dioxins'') in sewage sludge that is 
applied to the land on December 23, 1999. Since that time, EPA 
collected new data on the levels of dioxins in sewage sludge. EPA also 
has extensively revised the risk assessment which estimates the risks 
from dioxin and dioxin-like compounds associated with land application 
of sewage sludge. This document summarizes the new sewage sludge data 
and risk assessment. In addition, EPA is inviting comment on the effect 
of applying approaches in EPA's current Draft Dioxin Reassessment 
concerning non-cancer health effects of exposure to dioxins as they 
relate to land application of sewage sludge. EPA also conducted a 
screening analysis of the effects of dioxins in land-applied sewage 
sludge on ecological species, which is addressed in this notice. EPA is 
requesting comments on the new data and risk analysis, as well as 
dioxin exposure information, and any impact that this may have on the 
proposed rule with respect to land application of sewage sludge.
    EPA is under a court-ordered deadline to take final action on the 
proposed land application rule. The deadline was recently extended to 
October 17, 2003 with respect to land application; EPA met the previous 
court-ordered deadline of December 15, 2001 for taking final action on 
the Round Two proposal concerning surface disposal and incineration in 
a sewage sludge incinerator. EPA gave final notice of its determination 
that numeric standards or management practices are not warranted for 
dioxin and dioxin-like compounds in sewage sludge that is disposed of 
in a surface disposal site or incinerated in a sewage sludge 
incinerator (66 FR 66228, Dec. 21, 2001).

DATES: Your comments on this document must be submitted to EPA in 
writing and must be received or postmarked on or before midnight 
September 10, 2002.

ADDRESSES: Written comments and enclosures should be mailed or hand-
delivered to: W-99-18 NODA Comment Clerk, Water Docket (MC-4101), 
USEPA, 1200 Pennsylvania Ave., NW., Washington, DC 20460. Hand 
deliveries should be delivered to: EPA's Water Docket (MC 4101) at 401 
M St., SW., Room EB57, Washington, DC 20460. Comments may also be 
submitted electronically to [email protected]. Electronic 
submission of comments is recommended to avoid possible delays in mail 
delivery. Comments must be received or post-marked by midnight 
September 10, 2002. For additional information see Additional Docket 
Information section below.

FOR FURTHER INFORMATION CONTACT: Arleen Plunkett, U.S. Environmental 
Protection Agency, Office of Water, Health and Ecological Criteria 
Division (4304T), 1200 Pennsylvania Avenue, NW., Washington, DC 20460. 
(202) 566-1119. [email protected]

SUPPLEMENTARY INFORMATION:  

I. Additional Docket Information
II. Abbreviations Used
III. How Does This Document Relate to the Proposed Rule?
    A. What EPA Proposed
    B. Developments Since Proposal
    C. Proposed Definition of Dioxins
IV. Why Did EPA Collect New Data and Revise the Land Application 
Risk Assessment?
V. What Information Concerning Dioxins in Sewage Sludge Does the New 
Data Provide?
    A. What Data were Collected in the 2001 National Sewage Sludge 
Survey?
    B. What Techniques were Used to Collect Samples?
    C. What Analytical Methods were Used?
    D. How were the Concentrations of Dioxin Measured?
    E. How were the Concentrations Reported?
    F. How were the Non-Detect Measurements Handled in Developing 
National Summary Statistics?
    G. What were the Results of the EPA 2001 Dioxin Update of the 
National Sewage Sludge Survey?
    H. How do the Results of the EPA 1988 National Sewage Sludge 
Survey Compare with the EPA 2001 Dioxin Update Survey?
    I. Why is Temporal Variability of Dioxin in Sewage Sludge 
Important?
    J. What does the Variability of the Dioxin Levels Show?
    K. What does Month to Month Variability in the Concentration of 
Dioxins Show?
    L. What Other Data did EPA Evaluate?
VI. What are the Principal Features and Assumptions of the Revised 
Land Application Human Health Risk Assessment?
    A. What did the Hazard Identification Analysis Conclude?
    B. What did the Dose-Response Assessment Conclude?
    C. How was the Exposure Analysis and Risk Assessment Conducted?
    D. How did the Framework Change?
    E. What are the Factors in Estimating How Much Dioxin is 
Released to the Environment?
    F. What are the Factors in Estimating How Much Dioxin is being 
Transported in the Environment to the Individual in the Farm Family?
    G. What Additional Factors are Applied to Dioxin Concentrations 
to Determine How Much of the Congeners are Being Ingested or Inhaled 
by a Farm Family Member?
    H. How did EPA Calculate the Final Exposure Level?
    I. How was Childhood and Infant Exposure Evaluated in the 
Exposure Analysis?
    J. How is the Risk Estimate Calculated?
    K. How did EPA Analyze the Relative Importance of Inputs to the 
Risk Model?
    L. How does EPA Characterize the Risk?
VII. What Are the Implications of EPA's Dioxin Reassessment Process 
for This Rulemaking?
    A. How Would the Dioxin Cancer Risk from Land Application 
Compare to Background Dioxin Cancer Risk?
    B. How Would the Non-Cancer Dioxin Risk from Land Application 
Compare to Background Non-Cancer Dioxin Risk?
VIII. What is EPA's Assessment of Effects on Ecological Species?
    A. What Approach did EPA Use for the Screening Ecological Risk 
Analysis of Dioxins in Land-Applied Sewage Sludge?
    B. How did EPA Conduct the Screening Ecological Risk Analysis?
    C. What are the Results of the Screening Ecological Risk 
Analysis?
IX. How Might the New Data and Revised Risk Assessment Affect EPA's 
Proposed Dioxin Concentration Limit for Land-Applied Sewage Sludge 
and the Proposed Monitoring Requirements?
X. How Might the New Data and Revised Risk Assessment Affect EPA's 
Proposal for Small Entities?
XI. How Does the New Data and Revised Risk Assessment Affect EPA's 
Cost Estimates?
XII. Identification and Control of Dioxin Sources that Contribute to 
Elevated Dioxin Levels in Sewage Sludge.
XIII. Request for Public Comments
XIV. List of References

I. Additional Docket Information

    The record for this Notice has been established under docket number 
W-99-18 and includes supporting documentation as well as the printed 
paper versions of electronic materials. The record is available for 
inspection from 9 a.m. to 4 p.m. Eastern Standard or Daylight time, 
Monday through Friday, excluding legal holidays, at the Water Docket, 
Room EB57, USEPA Headquarters, 401 M Street, SW., Washington, DC 20460. 
For access to the docket materials, please call 202-260-3027 to 
schedule an appointment.
    For information on the existing rule in 40 CFR Part 503, you may 
obtain a copy of A Plain English Guide to the EPA Part 503 Biosolids 
Rule on the Internet at

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http://www.epa.gov/owm/bio.htm or request the document (EPA publication 
number EPA/832/R-93/003) from: Municipal Technology Branch, Office of 
Wastewater Management (4204M), Office of Water, U.S. Environmental 
Protection Agency, 1200 Pennsylvania Avenue, NW., Washington, DC 20460-
0001.

II. Abbreviations Used

AMSA--Association of Metropolitan Sewerage Agencies
CFR--Code of Federal Regulations
DL--detection limit
ED01--dose corresponding to a one percent increase in an adverse effect 
relative to the control response
EPA--Environmental Protection Agency
HQ--hazard quotient
kg/m\3\--kilograms per cubic meter
LADD--lifetime average daily dose
Ln--natural logarithm
LOEL--lowest-observed-effect level
Max.--maximum
MGD--million gallons per day
mg/kg/day--milligrams per kilogram per day
MOE--margin of exposure
ng/kg--nanograms per kilogram
NOEL--no-observed-effect level
NSSS--National Sewage Sludge Survey
PCBs--polychlorinated biphenyls
PCDFs--polychlorinated dibenzofurans
PCDDs--polychlorinated dibenzo-p-dioxins
pg/kg/day--picograms per kilogram per day
pg TEQ/day--picograms toxic equivalents per day
pg TEQ/kg-d--picograms toxic equivalents per kilogram body weight per 
day
POTWs--Publicly Owned Treatment Works
ppt--parts per trillion
Q1*--cancer slope factor
RfD--reference dose
SAB--Science Advisory Board
SERA--screening ecological risk analysis
Std. Dev.--standard deviation
TCDD--tetrachlorodibenzo-p-dioxin
TEF--toxicity equivalent factor
TEQ--toxic equivalent
WHO--World Health Organization

III. How Does This Document Relate to the Proposed Rule?

A. What EPA Proposed

    In December 1999, EPA proposed to amend management standards for 
sewage sludge by adding a numeric concentration limit for dioxins in 
sewage sludge that is applied to the land (64 Fed. Reg. 72045, Dec. 23, 
1999) (``Round Two proposal'').\1\ The proposed numeric limit would 
prohibit land application of sewage sludge that contains greater than 
300 parts per trillion (ppt) toxic equivalents (TEQ) of dioxins. EPA 
based this proposed numeric limit on the results of a risk assessment 
for dioxins in sewage sludge that is applied to the land.
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    \1\ Section 405(d)(2)(A) of the Clean Water Act (CWA), 33 U.S.C. 
Sec. 1345(d)(2)(A) required EPA to establish numeric limits and 
management practices for toxic pollutants in sewage sludge 
identified on the basis of available information. In 1993, EPA 
promulgated the ``Round One'' rule for such toxic pollutants in 
sewage sludge that is applied to the land, disposed of in surface 
disposal units, and incinerated in sewage sludge incinerators. 58 
Fed. Reg. 9248 (Feb. 19, 1993). Under section 405(d)(2)(B), EPA was 
directed to propose and promulgate regulations for other toxic 
pollutants not regulated in Round One, i.e., ``Round Two.'' The 
Round Two proposal identified dioxins, and included proposed 
standards for land-applied sewage sludge, but did not propose 
further regulation of sewage sludge disposed of by surface disposal 
or incineration.
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    EPA proposed a standard for dioxins in sewage sludge that is 
applied to the land in order to protect public health and the 
environment from unreasonable risks of exposure to dioxins. The purpose 
of this standard would be to prohibit land application of sewage sludge 
containing concentrations of dioxins above the limit, and thereby 
protect the health of highly exposed individuals as well as the health 
of the general population.
    EPA also proposed to exclude from the proposed numeric limit and 
monitoring requirements treatment works with a flow rate equal to or 
less than one million gallons per day (MGD) and certain sewage sludge-
only entities that receive sewage sludge for further processing prior 
to land application. This exclusion was based on the relatively small 
amount of sewage sludge that is prepared by these facilities and 
entities and, therefore, the low probability that land application of 
these materials could significantly increase risk from dioxins to human 
health or the environment.
    Finally, EPA proposed technical amendments to the frequency of 
monitoring requirements for pollutants other than dioxin. These 
amendments were intended to clarify but, with one exception, not alter 
the monitoring schedule in the existing sewage sludge rule. The one 
exception would require preparers of material derived from sewage 
sludge to determine the appropriate monitoring schedule based on 
quantity of material derived rather than quantity of sewage sludge 
received for processing.

B. Developments Since Proposal

    The Agency's risk assessment for land application of sewage sludge 
used for the proposal estimated that sewage sludge with concentrations 
of dioxins above the proposed limit may present an unreasonable cancer 
risk to specific highly exposed individuals. Subsequently, for reasons 
discussed below, the Agency extensively revised the land application 
risk assessment. EPA also gathered new data on dioxins in sewage sludge 
that was used in the revised risk assessment. This information, 
however, does not change the overall technical approach for the 
proposal.
    The new data and the methodology of the revised risk assessment are 
summarized in this notice. In addition, the results of the revised risk 
assessment are described in today's notice. Also discussed in today's 
notice are the possible implications of the new data and revised risk 
assessment on the proposed limit, the monitoring requirements, the 
small entity exclusion, and the projected cost of the proposed 
regulation.
    Another development since the proposal in December 1999 concerns 
EPA's Dioxin Reassessment, which began in 1991. In September 2000, EPA 
provided Draft Dioxin Reassessment documents to the Science Advisory 
Board (SAB) for their review, and in May 2001, the SAB issued its 
report. The current Draft Dioxin Reassessment (USEPA, 2000a), 
``Exposure and Human Health Reassessment of 2,3,7,8-Tetrachlorodibenzo-
p-Dioxin (TCDD) and Related Compounds,'' consists of three parts. Part 
I. Estimating Exposure to Dioxin-Like Compounds focuses on sources, 
levels of dioxin-like compounds in environmental media, and human 
exposures. Part II. Health Assessment for 2,3,7,8-Tetrachlorodibenzo-p-
Dioxin (TCDD) and Related Compounds includes information on critical 
human health end points, mechanisms of toxicity, pharmacokinetics, 
dose-response, and toxic equivalent factors (TEFs). Part III. 
Integrated Summary and Risk Characterization for 2,3,7,8-
Tetrachlorodibenzo-p-Dioxin (TCDD) and Related Compounds describes key 
findings pertinent to understanding the potential hazards and risks of 
dioxins, including a discussion of important assumptions and 
uncertainties.
    The Draft Dioxin Reassessment documents do not represent Agency 
policy or factual conclusions, and EPA has not yet issued final 
findings or conclusions as a result of the Dioxin Reassessment process. 
However, much of the information incorporated into the Draft Dioxin 
Reassessment documents reflects the state of knowledge with respect to 
dioxin, and scientific updates resulting from or reflected in these

[[Page 40556]]

documents are relevant to the assessment of risk from dioxins in sewage 
sludge that is applied to the land. For example, the revised sewage 
sludge land application risk assessment incorporates the latest science 
and state of knowledge concerning characteristics of dioxin and 
exposure pathways which are described in the Draft Dioxin Reassessment.
    The Draft Dioxin Reassessment also presents conclusions and 
findings which are still under review and which EPA has not applied to 
the analysis of dioxins in sewage sludge. These aspects of the Draft 
Dioxin Reassessment include, for example, a revised cancer slope factor 
for calculating cancer risk from exposure to dioxins, and discussions 
of various approaches to evaluating risks of non-cancer health effects 
from exposure to dioxins. Although not incorporated into the revised 
risk assessment, today's Notice also discusses potential implications 
that these aspects of the Draft Dioxin Reassessment could have for this 
rulemaking, when and if the Dioxin Reassessment is issued by EPA in 
final form, and if the final version takes the same approaches and 
reaches the same conclusions as the current draft.
    Finally, EPA was under a consent decree deadline of December 15, 
2001 to take final action on the proposed rule. Gearhart v. Whitman, 
Civil No. 89-6266-HO (D. Ore.). In accordance with the consent decree, 
EPA took final action on the proposal not to establish numeric limits 
or management practices for dioxins in sewage sludge that is disposed 
of in surface disposal units or incinerated in sewage sludge 
incinerators. 66 Fed. Reg. 66228 (Dec. 21, 2001). The consent decree 
deadline was extended to October 17, 2003, for EPA to take final action 
on the land application portion of the proposed Round Two rule.

C. Proposed Definition of Dioxins

    The proposed rule included a definition of ``dioxins'' to specify 
the seven 2,3,7,8,-substituted congeners of polychlorinated dibenzo-p-
dioxins (PCDDs), the ten 2,3,7,8-substituted congeners of 
polychlorinated dibenzofurans (PCDFs), and the twelve coplanar 
polychlorinated biphenyl (PCB) congeners to which the numeric standard 
applies. The vast majority of information on the toxicity of dioxins 
relates to the congener 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). 
Animals exposed to 2,3,7,8-TCDD exhibit a variety of biological 
responses and adverse effects. These include both carcinogenic and non-
carcinogenic effects. These effects are primarily classified as chronic 
effects and consequently they are generally associated with long term 
exposure over years and decades. Relatively speaking, these exposures 
and effects are observable at very low levels in the laboratory and in 
the environment when compared with other environmental toxicants 
(USEPA, 1994a).
    Studies to elucidate the mechanism of toxicity for 2,3,7,8-TCDD in 
mammalian and other species have indicated that the overall shape and 
chlorine substitution of this congener are keys to its biological 
potency. The fact that all of the lateral positions (the 2,3,7,8 
positions) on the multi-ring system are substituted with chlorine and 
that the overall molecule assumes a flat or planar configuration 
apparently are essential factors that make this congener biologically 
active. Other congeners with a similar structure and chlorine 
substitution pattern are assumed to exhibit similar biological 
properties. These include the other six 2,3,7,8-chlorinated substituted 
dibenzo-p-dioxin congeners, the ten 2,3,7,8-chlorinated substituted 
dibenzofuran congeners and the 12 coplanar PCB congeners. Coplanar PCB 
congeners are those congeners with no more than one ortho position and 
both para positions substituted with chlorine in the biphenyl ring 
system. Additionally, the coplanar PCB molecule assumes a relatively 
planar (i.e., flat) configuration.
    The proposed TEQ numeric limit would apply to these 29 congeners in 
ppt TEQ or nanograms TEQ per kilogram of dry sewage sludge. The TEQ 
concentration is calculated by multiplying the concentration of each 
congener in the sewage sludge by its corresponding ``toxicity 
equivalent factor,'' or TEF, and then summing the resulting products 
from this calculation for all 29 congeners. The TEFs (relative 
potencies) are based on expert judgment about toxicity and other 
biological effects for the individual compounds. The TEQs of these 
compounds are summed because they are believed to act by the same 
mechanism of toxicity. The December 1999 proposal specified that the 
International TEF scheme described in USEPA, 1989, would be used for 
the 17 2,3,7,8-substituted PCDDs and PCDFs, and the World Health 
Organization's TEF scheme (Van den Berg M, et al., 1998) would be used 
for the 12 coplanar PCBs, because the sewage sludge data EPA had at 
that time used these TEF schemes. The World Health Organization (WHO) 
has subsequently recommended and developed a single TEF scheme which 
includes all relevant information on dioxins, furans and dioxin-like 
(coplanar) PCBs. As part of this process, various terminologies or 
definitions applicable to TEFs were reviewed and standardized.
    The 2001 sewage sludge data and the revised risk assessment use the 
WHO's 1998 TEF scheme (Van den Berg M, et al., 1998) for all 29 dioxin, 
furan and coplanar PCB congeners. EPA intends to use the 1998 WHO TEF 
scheme (or later, if the WHO adopts a revised scheme) for any final 
Part 503 TEQ numeric limit.
    A 1997 WHO meeting of experts concluded that an additive TEF model 
remained the most feasible risk assessment method for complex mixtures 
of dioxin-like compounds. The WHO panel indicated that although 
uncertainties in the TEF methodology have been identified, one must 
examine this method in the broader context of the need to evaluate the 
public health impact of complex mixtures of persistent bioaccumulative 
chemicals. On this basis, EPA has used the 1998 WHO TEF methodology for 
the Agency's Draft Dioxin Reassessment, noting that it decreases the 
overall uncertainties in the risk assessment process.
    A Panel of EPA's Science Advisory Board has reviewed the Agency's 
use of the 1998 WHO TEF scheme. The consensus of the Panel was that 
this is a reasonable and widely accepted way of dealing with the joint 
effects of dioxin-like compounds on human health. The majority of the 
Panel noted that the TEF approach is well accepted internationally.

IV. Why Did EPA Collect New Data and Revise the Land Application Risk 
Assessment?

    The proposal to amend the Standards for the Use or Disposal of 
Sewage Sludge to limit dioxins in sewage sludge that is applied to the 
land was followed by a 90 day public comment period. During this time 
the risk assessment which supported the proposed rulemaking also was 
peer reviewed in accordance with EPA peer review procedures. Both the 
public comments and the peer review comments raised significant issues 
concerning the methodology and assumptions used for the land 
application risk assessment. The public and peer review comments also 
emphasized the need to collect new data on dioxins in sewage sludge. 
This data is used in the risk assessment, economic analysis, and other 
aspects of the rulemaking.
    The data on dioxins in sewage sludge used for the proposal came 
from two separate sources. The data on dioxin and furan congeners was 
from the 1988 EPA National Sewage Sludge Survey

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(USEPA, 1990). Since the National Sewage Sludge Survey (NSSS) did not 
include specific information on coplanar PCBs, EPA used a separate 
database to estimate the amount of coplanar PCBs found in sewage sludge 
(Green, et al., 1995). In addition to developing a single database 
which includes information on all 29 dioxin-like congeners, EPA 
developed new data on dioxins in sewage sludge to test the Agency's 
assumption that dioxin levels in sewage sludge have changed over time, 
and to more accurately determine dioxin levels in sewage sludge using 
analytical methods with lower limits of detection. The Agency is also 
using this more recent data to more reliably estimate the risk, 
impacts, and costs associated with dioxins in land applied sewage 
sludge. A discussion of the sewage sludge sampling and data analysis is 
presented in Section V. of this Notice.
    The principal comment concerning the risk assessment methodology 
was that the Agency should use a probabilistic approach instead of the 
deterministic approach that was used for the proposal. A probabilistic 
approach uses values for certain input variables over the range of 
available data, instead of the deterministic approach of determining, 
or setting, certain input variables at particular values. Conducting a 
risk analysis with a probabilistic approach can yield better 
information about sources of variability and uncertainty in the final 
risk estimates, compared to conducting a risk analysis with a 
deterministic approach.
    Other comments on the risk assessment recommended that the Agency 
use an exposure analysis more consistent with that used in the Agency's 
current Draft Dioxin Reassessment (USEPA, 2000a); that the Agency use 
data from the current EPA Exposure Factors Handbook (USEPA, 1997); and 
that the risk assessment include a sensitivity analysis of the critical 
input variables.
    The revised risk assessment is described in Section VI. of this 
Notice. The revised risk assessment was submitted for peer review. The 
consensus view of the peer reviewers agreed with the revised risk 
assessment methodology and assumptions on input parameters. The revised 
risk assessment, described below and available in the docket, 
incorporates revisions made in response to the peer review.

V. What Information Concerning Dioxins in Sewage Sludge Does the New 
Data Provide?

A. What Data Were Collected in the EPA 2001 Dioxin Update of the 
National Sewage Sludge Survey?

    The EPA 2001 dioxin update of the NSSS provides data that support 
the calculation of unbiased national estimates (i.e., based on a random 
selection of publicly owned treatment works) for dioxin and dioxin-like 
compounds in sewage sludge (USEPA, 2002a). The publicly owned treatment 
works (POTWs) sampled in the EPA 2001 dioxin update survey were 
randomly selected from all POTWs in four size categories: <1 MGD, 1 
MGD-10 MGD, 10 MGD-100 MGD and 100 MGD. This survey updates 
the 1988 NSSS. The updated survey includes coplanar PCBs, which had not 
been included in the 1988 NSSS because approved analytical methods for 
these analytes were not available at that time. The updated survey also 
uses the current TEFs, which have been revised since the 1988 NSSS. For 
the EPA 2001 dioxin update survey, EPA collected sewage sludge samples 
from 94 POTWs selected from the 174 POTWs which had been surveyed in 
the 1988 NSSS. The sample of 174 POTWs included in the 1988 NSSS were 
selected from the national population (as of 1988) of approximately 
10,000 POTWs with secondary treatment. EPA used a survey design which 
accounted for the different numbers of POTWs in different size 
categories for both the 1988 NSSS and the EPA 2001 dioxin update 
survey. EPA conducted the sampling at the 94 POTWs in the first 
calendar quarter of 2001 and completed the laboratory analysis, data 
review, and database development by mid-2001.

B. What Techniques Were Used To Collect Samples?

    Sewage sludge samples were collected, documented, preserved, and 
shipped to the laboratory where the analyses for dioxins were conducted 
using the protocol entitled ``Sampling Procedures for the 2001 National 
Sewage Sludge Survey'' (USEPA, 2001a). This document specifies the 
sampling procedures used for the sewage sludge samples obtained from 
the 94 POTWs that participated in the EPA 2001 dioxin update survey. 
The procedures were used on a number of different types of sewage 
sludge samples including liquids, samples with low solids content, 
dewatered sewage sludges from filter presses and centrifuges, composted 
products, and pellets. The sampling protocol specifies sample 
preservation methods, collection devices and apparatus, containers, 
types of labels, and label information. In accordance with the sampling 
protocol used for the EPA 2001 dioxin update survey, duplicate samples 
were collected for 15 percent of the samples collected for subsequent 
analysis to determine the precision of the analyses. At each treatment 
works sampled, a second sample aliquot was collected and archived for 
potential future analyses. Chain of custody forms were completed for 
the samples collected at each sampling site to ensure the integrity of 
the results of the survey.

C. What Analytical Methods Were Used?

    EPA used analytical methods that are considered state of the art 
for the sewage sludge matrix. Dioxin and dibenzofuran congener 
concentrations were determined by EPA Method 1613B (USEPA, 1994b) using 
high resolution gas chromatography-mass spectrometry as the end point 
system of measurement. The coplanar PCB analyte concentrations were 
determined by EPA Method 1668A (USEPA, 1999a) which employs the same 
type of measuring instrumentation. Method 1613B is an official EPA 
analytical methodology codified at 40 CFR Part 136. EPA anticipates 
that Method 1668A will be codified in Part 136 within the next two 
years.

D. How Were the Concentrations of Dioxin Measured?

    The sewage sludge samples were analyzed for 29 dioxin congeners 
consisting of the 7 dioxin congeners, 10 dibenzofuran congeners, and 12 
coplaner PCB congeners that EPA proposed for the definition of 
``dioxins'' (see Section III.B. above). For the EPA 2001 dioxin update 
survey, whole (wet) weight sample sizes were individually determined 
for each sewage sludge sample by considering the percent solids in each 
sample. Smaller whole weight sample sizes were used for the analyses 
when the percent solids content of the sewage sludge sample was 
greater, and vice versa. This approach led to lower and more consistent 
detection limits for concentrations of target analytes for all of the 
sewage sludge samples in the EPA 2001 dioxin update survey. This 
procedure was a significant improvement compared to the method used for 
handling the sewage sludge samples in the 1988 NSSS. For the 1988 NSSS, 
equal whole weight sample sizes were used regardless of the percent 
solids content of the samples. This led to higher and less consistent 
detection limits for the sewage sludge samples in

[[Page 40558]]

the 1988 NSSS. In addition, other improvements in the analytical 
methodology and the analytical instrumentation also contributed to 
lower and more consistent detection limits than those obtained in the 
1988 NSSS.

E. How Were the Concentrations Reported?

    All of the individual 29 congener concentrations were converted to 
TEQ concentrations by multiplying the congener concentrations by the 
1998 WHO TEFs. For comparison purposes, TEQs for total dioxin and 
dioxin-like compounds in the 1988 NSSS samples and the EPA 2001 dioxin 
update survey samples are reported in Table 1, Table 2 and Table 3 in 
nanograms per kilogram (ng/kg) dry weight basis.

F. How Were the Non-Detect Measurements Handled in Developing National 
Summary Statistics?

    Where congeners were not detected in sample measurements, three 
different substitution methods were used in calculating national 
estimates of dioxin concentrations in sewage sludge: (1) Zero was 
substituted for a non-detect; (2) one-half the detection limit for the 
congener was substituted for a non-detect; (3) the detection limit for 
the congener was substituted for a non-detect. As a result of the small 
detection limits achieved in the EPA 2001 dioxin update survey, there 
were only small differences in the national summary statistics among 
the three substitution methods for the EPA update survey.

G. What Were the Results of the EPA 2001 Dioxin Update of the National 
Sewage Sludge Survey?

    Table 1 presents the mean, standard deviation, maximum and 99th, 
98th, 95th, 90th and 50th percentiles dioxin TEQ values for the sewage 
sludges from the 94 POTWs in the EPA 2001 dioxin update survey. Table 1 
reports summary results separately for dioxins and furans, coplanar 
PCBs, and total dioxin-like compounds (i.e., 29 dioxin, furan and 
coplanar PCB congeners) using the three alternative substitution values 
for non-detects (i.e., zero, one-half the detection limit, and equal to 
the detection limit). In Table 1, the results obtained using zero, one-
half the detection limit and the detection limit are shown in the rows 
denoted by ``0'', ``\1/2\ DL'' and ``DL'', respectively. The complete 
statistical analysis of the data from the EPA 2001 dioxin update survey 
is presented in Statistical Support Document for the Development of 
Round Two Sewage Sludge Use or Disposal Regulations (USEPA, 2002a).

  Table 1.--EPA 2001 Dioxin Update Survey--National Toxic Equivalent Estimates (nanograms/kilogram dry matter basis)--Total Toxic Equivalents for POTWs
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                           Method                                Mean    Std. Dev.     Max.        99th %       98th %     95th %     90th %     50th %
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                                            Total Dioxin and Furan TEQs (nanograms/kilogram dry matter basis)
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0...........................................................      21.70       47.5     682.00        100.00       54.40      33.30      31.40      15.50
\1/2\ DL....................................................      21.70       47.5     682.00        100.00       54.40      33.30      31.60      15.50
DL..........................................................      21.80       47.5     682.00        100.00       54.40      33.30      31.70      15.50
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                              Total Coplanar PCB TEQs (nanograms/kilogram dry matter basis)
--------------------------------------------------------------------------------------------------------------------------------------------------------
0...........................................................       5.22       10.3      58.30         50.60       44.80      13.10       9.66       2.05
\1/2\ DL....................................................       9.87       14.0      58.30         55.10       54.50      49.40      19.20       6.04
DL..........................................................      14.50       22.4     103.00         97.2        91.60      78.00      35.00       8.11
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                         Total Dioxin and Dioxin-Like TEQs (nanograms/kilogram dry matter basis)
--------------------------------------------------------------------------------------------------------------------------------------------------------
0...........................................................      26.90       49.6     718.00        114.00       76.60      59.30      42.80      19.70
\1/2\ DL....................................................      31.60       50.0     718.00        115.00       80.10      73.50      55.10      23.40
DL..........................................................      36.30       52.7     718.00        138.00       96.00     113.00      69.10      24.00
--------------------------------------------------------------------------------------------------------------------------------------------------------

    Under the proposed rule, treatment works with a flow rate equal to 
or less than one MGD and certain sewage sludge-only entities that 
receive sewage sludge for further processing prior to land application 
would be excluded from the proposed numeric limit and monitoring 
requirements. The EPA 2001 dioxin update survey provides additional 
data with respect to dioxin concentrations from POTWs that would be 
excluded under the proposal. Table 2 below shows the results for dioxin 
concentrations in sewage sludge for POTWs with flows of less than and 
greater than one MGD. Results shown in Table 2 indicate very small 
differences in the median dioxin concentrations between small and large 
POTWs. At the upper percentiles, the differences between the small and 
large POTW values are substantial. However, the significance of these 
differences is difficult to assess due to the relatively small sample 
sizes, the sensitivity of the results to the treatment of non-detect 
measurements and the low precision typically associated with estimates 
of upper percentiles based on small sample sizes. An additional 
discussion of the proposed exclusion for small entities is presented in 
Section X. of this Notice. EPA requests comments on the significance of 
the differences in dioxin concentrations in sewage sludge measured at 
facilities with wastewater flows greater than one MGD compared to 
dioxin concentrations in sewage sludge at facilities with wastewater 
flows less than one MGD.

[[Page 40559]]



  Table 2.--EPA 2001 Dioxin Update Survey--Total Dioxin and Furan and Dioxin-Like PCB National TEQ (nanograms/
                           kilogram dry weight basis) Estimates--POTWs by Flow Groups
----------------------------------------------------------------------------------------------------------------
                  Method                      Zero for Nondetects        \1/2\ DL for        DL for Nondetects
-------------------------------------------------------------------       Nondetects      ----------------------
                                                                   -----------------------
                 Estimate                     1 MGD   1                1 MGD   
                                                           MGD        1 MGD      1 MGD                  1 MGD
----------------------------------------------------------------------------------------------------------------
Mean......................................     22.10       38.50       26.50       44.10      30.80       49.60
Std. dev..................................      16.8        86.7        18.3        86.8       24.6   88.2 dev.
Maximum...................................     78.60      718.00        78.6      718.00     118.00      718.00
99th %....................................     71.80      401.00       76.40      403.00     109.00      406.00
98th %....................................     65.10      265.00       74.20      269.00     101.00      276.00
95th %....................................     46.00       62.60       67.10       94.80      77.00      134.00
90th %....................................     37.20       54.00       46.10       64.20      46.60       86.90
50th %....................................     19.90       18.90       22.90       22.60      23.80       25.80
----------------------------------------------------------------------------------------------------------------

H. How Do the Results of the EPA 1988 National Sewage Sludge Survey 
Compare with the EPA 2001 Dioxin Update Survey?

    A comparison of results for dioxin and furan congeners obtained in 
the 1988 and 2001 surveys is presented in Table 3.

  Table 3.--National Estimates (nanograms/kilogram dry matter basis) for Dioxin and Furan Congeners in the EPA
                                     2001 Dioxin Update Survey and NSSS 1988
----------------------------------------------------------------------------------------------------------------
                    Method                       Zero for nondetects      \1/2\ DL for        DL for nondetects
---------------------------------------------------------------------      nondetects      ---------------------
                                                                     ----------------------
                   Estimate                        2001       1988       2001       1988       2001       1988
----------------------------------------------------------------------------------------------------------------
Mean..........................................      21.70      46.50      21.70      67.30      21.80      88.20
Std. dev......................................       47.5      153.0       47.5      153.0       47.5     157.00
Maximum.......................................     682.00    1870.00     682.00    1870.00     682.00    1870.00
99th %........................................     100.00     450.00     100.00     453.00     100.00     466.00
98th %........................................      54.40     402.00      54.40     404.00      54.40     455.00
95th %........................................      33.30     301.00      33.30     303.00      33.30     340.00
90th %........................................      31.40      56.70      31.60     152.00      31.70     226.00
50th %........................................      15.50       5.68      15.50      34.20      15.50      52.40
----------------------------------------------------------------------------------------------------------------

    The values obtained in the EPA 2001 dioxin update survey for the 
upper percentiles are lower than those obtained in the 1988 NSSS. On 
this basis, the concentrations of dioxins in sewage sludge appear to 
have declined since 1988. However, the significance of these 
differences between the two surveys is not certain due to changes in 
the sampling procedures and analytic methods . These comparisons do not 
include coplanar PCB congeners because the 1988 NSSS did not collect 
coplanar PCB congener data. For the purposes of the December 1999 
proposed rule, data on coplanar PCB levels in sewage sludge from a 1995 
Association of Metropolitan Sewerage Agencies Survey (Green, et al., 
1995) were combined with the 1988 NSSS dioxin and furan results to 
provide an estimate of total dioxin levels in sewage sludge. EPA 
requests comments on the significance of the differences in dioxin 
concentrations in sewage sludge measured in the EPA 2001 dioxin update 
survey compared to dioxin concentrations in sewage sludge measured in 
the 1988 NSSS.

VIII.Why Is Temporal Variability of Dioxin in Sewage Sludge Important?

    The variability of dioxins in sewage sludge over time is important 
for a number of reasons. First, understanding the temporal variability 
of dioxin concentrations in sewage sludge is important for establishing 
numerical limits for dioxins in sewage sludge which protect public 
health and the environment with an adequate margin of safety. 
Specifically, this information helps in assessing the likelihood that 
individuals will be exposed to higher levels of dioxins from land 
application of sewage sludge over time. A more complete discussion of 
this issue is presented in the risk characterization in Section VI.L. 
of this Notice. Second, information on the variability of dioxin 
concentration in sewage sludge is important for determining the 
appropriate frequency of monitoring for concentrations of dioxins in 
sewage sludge that will ensure that any numerical limit that is 
established will not be exceeded.

J. What Does the Variability of the Dioxin Levels Show?

    It is not possible to draw general inferences with regard to the 
variability or differences in dioxin levels observed in the two 
surveys. This is due to a number of factors that include the large time 
interval between the surveys (i.e., 13 years), changes that may have 
occurred at the POTWs, and changes and improvements in analytical 
methods. It is possible, however, to make a number of observations with 
regard to changes in dioxin levels based on the data. Of the 94 POTWs 
participating in both the 1988 NSSS and the EPA 2001 dioxin update 
survey, a total of 14 POTWs have sewage sludge dioxin concentrations 
(dioxins and furans only) equal to or greater than 93 ppt TEQ from at 
least one of the surveys. These same 14 POTWs exhibited the greatest 
differences in the dioxins and furans concentrations when comparing the 
results of the 1988 and 2001 EPA surveys. The other 80 POTWs

[[Page 40560]]

participating in both surveys have substantially smaller differences, 
as well as lower dioxin levels measured in both surveys. Of the 14 
POTWs with the greatest differences between the two surveys, four had 
large increases in sewage sludge dioxin concentrations and ten had 
large decreases in sewage sludge dioxin concentrations from 1988 to 
2001.
    Based on these data, no POTWs had consistently high levels of 
dioxins in sewage sludge. It appears that sewage sludge samples with 
higher concentrations of dioxins may experience a greater variability 
in dioxin concentrations over time and that higher dioxin levels may 
not remain high for a significant period of time. Likewise, POTWs with 
moderate or low levels of dioxins in their sewage sludge may experience 
much less variability in dioxin concentrations over time. It is 
possible that in the group of POTWs where higher concentrations of 
dioxins were measured in their sewage sludge, there are unidentified 
sources with relatively high levels of dioxins entering the sewers 
intermittently. The second group of POTWs where lower concentrations of 
dioxins were measured in both surveys appear to be experiencing typical 
environmental background variation of dioxin levels. The possible 
sources of dioxins which contribute to higher levels of dioxins in 
sewage sludge are discussed in greater detail later in this Section and 
Section XII of the Notice. EPA's assessment of the variability in 
higher levels of dioxins in sewage sludge is discussed further as part 
of the risk characterization in Section VI.L. of this Notice.

K. What Does Month-to-Month Variability in the Concentration of Dioxins 
Show?

    EPA also examined both long and short term variability in sewage 
sludge dioxin concentrations in three wastewater treatment plants that 
have routinely monitored for dioxins in their sewage sludge over 
relatively long periods of time and voluntarily submitted their data to 
EPA (USEPA, 2001b). EPA did this to better understand the extent of 
variability using data collected on a relatively frequent basis.
    Of the three POTWs which provided their data to EPA, one of the 
POTWs provided data on two different sewage sludge products that they 
produce. These data were standardized using the WHO98 
standard for TEQs to provide consistency.
    The December 1999 proposal specified annual monitoring for land 
applied sewage sludges with dioxin concentrations between 30 ppt TEQ 
and the proposed limit of 300 ppt TEQ. Sewage sludges with two 
consecutive annual dioxin measurements less than 30 ppt TEQ would be 
required to monitor once every five years. These less frequent 
monitoring requirements were based on EPA's assumption that dioxin 
concentrations in sewage sludge remained relatively constant over time.
    The data for the facilities where monthly data were available 
indicate that the dioxin concentrations are relatively consistent over 
time on a month-to-month basis. The maximum monthly concentration was 
within a factor of two to four times the average (mean) concentration 
for the same facility. Similar to the comparison data from the 1988 
NSSS and the 2001 update, the variability appeared the greatest for the 
facility with the highest dioxin concentrations measured in its sewage 
sludge. A complete analysis of the month-to-month data is presented in 
the Statistical Support Document for the Development of Round Two 
Sewage Sludge Use or Disposal Regulations (USEPA, 2002a).
    The month-to-month variability in the dioxins concentration 
observed in the sewage sludge for which the Agency had data, as well as 
the longer term variability observed in the small percentage of sewage 
sludge with higher concentrations of dioxins (discussed above), has led 
us to re-evaluate the proposed monitoring frequency. A more complete 
discussion of monitoring frequency is presented in Section IX. of this 
Notice.

L. What Other Data Did EPA Evaluate?

    The Association of Metropolitan Sewerage Agencies (AMSA) 
voluntarily collected sewage sludge samples from 171 POTWs and analyzed 
these samples for dioxins using the same methods used for the 2001 EPA 
dioxin update survey. AMSA submitted the results of their survey to EPA 
in a report entitled ``AMSA 2000/2001 Survey of Dioxin-Like Compounds 
in Biosolids: Statistical Analyses (Final Report)'' (AMSA, 2001). The 
AMSA survey began in October 2000 and was completed in July 2001. The 
AMSA survey was designed to measure levels for the same 29 dioxin and 
dioxin-like congeners measured in the EPA 2001 dioxin update survey. 
AMSA also compared the results of their 2001 survey with the results of 
their 1994/1995 survey of dioxins in sewage sludge. Participation in 
AMSA's survey was on a voluntary basis.
    Most participants in the AMSA survey were larger POTWs which make 
up the bulk of the AMSA membership. Some non-AMSA members also 
participated in the AMSA survey, including some smaller POTWs. Overall, 
111 separate wastewater treatment agencies participated in the 2001 
AMSA survey, providing 200 samples from 171 POTWs, located in 31 
states. The sewage sludge dioxin concentrations measured in the AMSA 
survey generally ranged from 7.1 ppt TEQ to 256 ppt TEQ, with one 
sample measured at 3,590 ppt TEQ. The mean (average) concentration and 
the median dioxin concentrations in sewage sludge from the AMSA survey 
were 48.5 ppt TEQ and 21.7 ppt TEQ, respectively.
    EPA has found the data from the AMSA survey to be useful in 
describing dioxins in sewage sludge from larger POTWs. The results of 
the AMSA survey tend to corroborate the results obtained from the EPA 
2001 dioxin update survey. However, the AMSA results were not used by 
EPA to establish national estimates of dioxin concentrations in sewage 
sludges or for purposes of estimating risks from dioxins in land-
applied sewage sludge. EPA did not use these results because the POTWs 
participating in the AMSA survey volunteered for this survey and were, 
therefore, not randomly selected, as were the POTWs in the EPA 2001 
dioxin update survey. The final report from the AMSA survey and 
associated appendices are in the docket and can also be found on AMSA's 
web site at: http://www.amsa-cleanwater.org/advocacy/dioxin/dioxin.cfm.

VI. What Are the Principal Features and Assumptions of the Revised Land 
Application Human Health Risk Assessment?

    The revised risk assessment is entitled ``Exposure Analysis for 
Dioxins, Dibenzofurans, and CoPlanar Polychlorinated Biphenyls in 
Sewage Sludge--Technical Background Document'' (USEPA, 2002b). The risk 
assessment methodology, assumptions, results and characterization are 
summarized below.
    The revised risk assessment contains the following standard 
elements used in EPA human health risk assessments: hazard 
identification, dose-response assessment, exposure assessment, and risk 
characterization. The revised risk assessment includes a probabilistic 
methodology to determine the adult and child exposure to the 29 dioxin 
and dioxin-like congeners. For the proposed rule, the risk assessment 
depended on a deterministic analysis based on single value inputs and 
outputs. A probabilistic analysis was well-suited for this risk 
assessment because sewage

[[Page 40561]]

sludge is generated nationwide and, therefore, may be used on 
agricultural fields anywhere in the United States. The probabilistic 
analysis not only captures the variability in sewage sludge application 
practices, it also captures the differences in the environmental 
settings (e.g., soils, meteorology and agricultural practices) in which 
sewage sludge may be land-applied.
    In addition to a new methodology of analysis, the revised risk 
assessment uses new inputs which include a redefined ``highly exposed 
individual,'' new pathways and mechanisms of exposure consistent with 
EPA's Draft Dioxin Reassessment (USEPA, 2000a. See Part I, Vol. 3, 
Chap. 2.), a number of new exposure factors adopted from the latest EPA 
Exposure Factors Handbook (USEPA, 1997), and a sensitivity analysis to 
determine the relative importance of the input variables. In this 
Section, EPA describes the features of the revised risk assessment with 
emphasis on the new inputs used in the probabilistic analysis.

A. What Did the Hazard Identification Analysis Conclude?

    The risk assessment that EPA used for the December 1999 proposal 
identified cancer as the human health endpoint, i.e., as the ``hazard'' 
(64 FR 72051). The revised risk assessment does not change this hazard 
identification and continues to assess the risk of cancer as the human 
health endpoint.

B. What Did the Dose-Response Assessment Conclude?

    EPA's dose-response assessment evaluated the risk of the dioxin, 
dibenzofuran, and PCB congeners using cancer slope factors that are 
based on the toxicity of the most highly characterized of the dioxin 
congeners, 2,3,7,8-TCDD (USEPA, 2000a. See Part II, Chap. 7, Part A.). 
The cancer slope factor for TCDD used by EPA in recent assessments, 
including the revised sewage sludge land application risk assessment, 
is 1.56 x 10-4/picograms toxic equivalents/kilogram body 
weight/day (pg TEQ/kg-d) (USEPA, 1994a). The cancer slope factor (also 
referred to as Q* or ``cancer potency'') is a numeric value which 
relates the incremental probability of developing a cancer from 
exposure to a particular substance. This cancer slope factor value is 
expressed as a lifetime excess cancer risk per unit exposure, and is 
usually quantified in terms of (milligrams of substance per kilogram of 
body weight per day)-1. The greater the numeric value of the 
cancer slope is, the greater the carcinogenic potency of the substance. 
The same slope factor is used to estimate cancer risks for both 
children and adults. For this analysis, only the cancer endpoint was 
evaluated and a linear dose response relationship was used in the 
analysis.
    An extensive discussion of the dose response mechanism for TCDD is 
provided in the Draft Dioxin Reassessment document (USEPA, 2000a. See 
Part II, Chap. 8.). The Draft Dioxin Reassessment also includes a 
revised cancer slope factor. Because the Draft Dioxin Reassessment is 
preliminary and does not state EPA policy conclusions or factual 
findings, the draft cancer slope factor was not used in the revised 
risk assessment. However, for purposes of discussion and public 
comment, this Notice includes a discussion of how the EPA Draft Dioxin 
Reassessment could apply to the analysis of impacts from dioxins in 
land-applied sewage sludge, including use of the revised cancer slope 
factor, in Section VII.A. of this Notice. EPA is seeking comment on the 
implications of this information in the event that, prior to taking 
final action on the Round Two rule, EPA finalizes a cancer slope factor 
or other policies or approaches currently reflected in the current 
Draft Dioxin Reassessment and discussed in this Notice.

C. How Was the Exposure Analysis and Risk Assessment Conducted?

    The primary methodology for the exposure analysis was to estimate 
exposure to dioxins in land-applied sewage sludge using a probabilistic 
approach. A probabilistic exposure analysis produces a distribution of 
exposures which is then used to estimate the range of risks for the 
highly exposed population being modeled. The distribution of exposure 
is determined by varying parameter values where data is available over 
multiple iterations of the exposure model. Values were varied for such 
parameters as dioxin concentrations in sewage sludge, number of years 
on the farm, and number of applications. While ranges of data were 
available for the majority of input parameters, ``single point'' values 
were used for some key input parameters for the exposure analysis, 
including values for parameters used to define the highly exposed 
population, soil ingestion rates, and number of days per year of 
exposure. These assumptions are discussed in greater detail elsewhere 
in this Notice.
    A receptor is the entity exposed to a physical, chemical or 
biological source which can cause an adverse effect. In this case the 
receptors are infants, children, and adults in highly exposed farm 
families living on farms where sewage sludge is applied. ``Highly 
exposed'' farm families are defined as farm families whose diets 
consist of 50 percent of products produced on their own farm. EPA 
estimates that the maximum number of individuals in this highly exposed 
population would be less than 11,000 even if all of the Nation's sewage 
sludge were applied to family farms (see Section VI.L.). Since the 
general population consumes only a small fraction of their diets from 
products grown on farms with land-applied sewage sludge, EPA assumed 
that a regulatory decision that is protective of this highly exposed 
family is also protective of the general population.
    The probabilistic analysis was performed using a Monte Carlo 
simulation. In a Monte Carlo simulation, the model is run for a number 
of iterations, each producing a single result (e.g., a single estimate 
of cancer risk). For this assessment, 3,000 iterations were run in the 
Monte Carlo simulation; therefore, the output of the probabilistic 
analysis was a distribution of 3,000 values. This distribution 
represents the distribution of possible outcomes, which reflects the 
underlying variability in the data used in the analysis. These results 
were then used to identify risk to the highly exposed population at 
various percentile levels (e.g., 90th percentile risk value). As noted 
above, the corresponding percentile risk values to the general 
population would be significantly lower.
    Some model input parameters used in the Monte Carlo simulation, 
such as the concentrations of dioxin congeners in sewage sludge 
samples, were drawn from statistical distributions. For others, 
variability was associated with variable locations; thus, location 
variability was explicitly considered in the setup of the data used for 
the probabilistic analysis. For location-dependent parameters, 
locations were first selected at random with equal probability of 
occurrence \2\ based on the 41 climate regions. These regions defined a 
set of related environmental conditions (e.g., soil type, hydrogeologic 
environment) that characterized the environmental setting. All 
location-specific parameters (e.g., rainfall) thus remained correlated, 
while non-location-specific parameters were varied both within and 
among locations.
---------------------------------------------------------------------------

    \2\ Information was not available to allow the weighting of 
these 41 climate regions based on the number of farm families in 
each region.
---------------------------------------------------------------------------

D. How Did the Framework Change?

    In the exposure analysis, the risk assessment evaluated a revised 
scenario for exposure to sewage sludge: exposure

[[Page 40562]]

of a farm family that consumes 50% of its diet from home-produced crops 
and animal products grown on their own sewage sludge-amended land. For 
the December 1999 proposal, a rural family consuming a smaller 
proportion of home-grown products derived from sewage sludge-amended 
soil was modeled in the original risk assessment. EPA selected the new 
scenario specifically to address groups of individuals who may have 
high levels of exposure to dioxins in sewage sludge. EPA assumed that 
the farm family lives immediately adjacent to the sewage sludge-amended 
field and is exposed to a combination of agricultural products produced 
on the farm, including beef and dairy products. The farm family also is 
assumed to raise free-range chickens near their house (in the buffer 
area). On the opposite side of the house from the field and pasture is 
a fishable stream where a recreational fisher is assumed to catch fish 
for personal consumption. There are four types of people who were 
assumed to be representative of the individuals who would be exposed to 
dioxin from sewage sludge: an infant of a farmer, a child of a farmer, 
an adult farmer, and an adult recreational fisher. The exposure to the 
adult fisher was combined with that of the adult farmer, when the total 
exposure to the adult was calculated. Therefore, the fisher and farm 
adult can be considered as the same adult. Table 4 summarizes the 
exposure pathways for each type of individual.

                                    Table 4.--Receptors and Exposure Pathways
----------------------------------------------------------------------------------------------------------------
                                                          Ingestion              Ingestion
                                  Inhalation              of above-   Ingestion      of                Ingestion
            Receptor              of ambient  Ingestion      and       of beef    poultry   Ingestion  of breast
                                      air      of soil   belowground  and dairy   and egg    of fish      milk
                                                           produce     products   products
----------------------------------------------------------------------------------------------------------------
Adult...........................
Child...........................
Infant..........................
----------------------------------------------------------------------------------------------------------------

    The new scenario includes new exposure pathways and exposure 
mechanisms, incorporating updated scientific analysis for dioxin, which 
is also reflected in EPA's Draft Dioxin Reassessment (USEPA, 2000a. See 
Part I, Vol. 3, Chap. 2.). For the proposed rule, the risk assessment 
evaluated pastured animals eating sewage sludge containing dioxins 
after sewage sludge land application. The revised risk assessment 
assumes tilled soil only for production of vegetables, fruits, and root 
crops and untilled soil for pasturage to which sewage sludge is 
applied. Half the acreage on the modeled farm is assumed to be used for 
crop production (tilled) and half permanently used for pasturage 
(untilled). Rather than assuming that cattle are exposed to dioxins 
only by eating sewage sludge-containing soil, the Agency now assumes 
that cattle are exposed to dioxins in sewage sludge by three 
mechanisms: ingesting dioxins from the leaf surfaces of plants 
containing dioxins which have volatilized from the top two centimeters 
of the soil to which sewage sludge has been applied; ingesting dioxins 
from sewage sludge particles which remain on the leaf surfaces of 
plants after land application; and direct ingestion of sewage sludge-
containing soil by the grazing cattle. Of these three mechanisms of 
dioxin transfer to cattle from the sewage sludge, the predominant 
mechanism is ingestion of dioxins from leaf surfaces containing dioxins 
which have volatilized from the sewage sludge-soil mixture. The dioxins 
from land-applied sewage sludge that does not erode away from the land 
application site are assumed to reside permanently in the top two 
centimeters of the soil. Another new assumption reflecting the latest 
science on dioxin and consistent with EPA's Draft Dioxin Reassessment 
documents is that chickens will be ingesting dioxins from the buffer 
area which receives dioxins from the pasture and crop fields through 
erosion. EPA requests comments on the Agency's use of the farm family 
scenario described for the revised risk assessment. EPA also requests 
comments on the specific assumptions outlined above.

E. What Are the Factors in Estimating How Much Dioxin is Released to 
the Environment?

    Various inputs for sewage sludge characteristics were used in the 
exposure analysis to determine how much dioxin is available for 
volatilization, erosion or leaching. These included: concentrations of 
each of the 29 congeners in sewage sludge (empirical distribution of 
concentrations for each dioxin congener varied by sample), bulk density 
of sewage sludge (single value), porosity of sewage sludge (single 
value), percent moisture of sewage sludge when applied to agricultural 
fields (single value), and fraction of organic carbon of sewage sludge 
(single value). The use of the congener concentrations was different in 
the revised exposure analysis. Rather than using point estimates for 
the 29 congeners for the probabilistic analysis, all of the congener 
concentrations measured in the 94 samples from the EPA 2001 dioxin 
update survey were used. Specifically, for each iteration of the Monte 
Carlo analysis, one of the 94 sewage sludge samples from the EPA 2001 
dioxin update survey was randomly selected and the concentrations of 
all congeners from that sample were considered in that iteration of the 
analysis. For each iteration, the concentration of dioxins in the 
sludge was assumed to remain constant for the entire period of 
application since family farms would likely receive sewage sludge from 
a single POTW.
    When the chemical content of a substance is analyzed, the 
assumption used to address non-detected chemicals can have a 
significant impact on the reported results if the detection limits are 
relatively large. Non-detects can be reported as zero, one-half the 
detection limit, or the detection limit. Because of the excellent 
sensitivity and limits of detection achieved by the analytical 
procedures used in the EPA 2001 dioxin update survey, the reported 
values for dioxin congeners in the samples of sewage sludge are 
relatively unchanged whether non-detects are treated as zero, one-half 
of the detection limit, or at full detection limit. For this risk 
assessment, EPA assumed that non-detects are equal to one-half of the 
detection limit. This assumption is prevalently used by EPA for risk 
assessments based on data sets for non-detects, including the Draft

[[Page 40563]]

Dioxin Reassessment for calculating TEQ concentrations for dioxins in 
environmental media (i.e., air, soil, water) and in exposure media 
(i.e., food). Furthermore, it appears that there would be no 
quantifiable difference in the estimated risk regardless of the 
assumption made for non-detects for the reasons discussed above. EPA 
requests comment on the treatment of non-detects in the revised risk 
assessment and the effect on estimating risk.
    Another sewage sludge characteristic, bulk density of sewage sludge 
as it is applied to the agricultural field, was used to estimate the 
loading of constituents to the soil in the model. Sewage sludge is 
assumed not only to add constituents to the soil, but also to add 
volume when mixed with the existing soil. Thus, bulk density is a 
required parameter for the modeling scenario used in the exposure 
analysis. Bulk density of the land-applied sewage sludge may be a 
direct measurement or may be estimated using the dry bulk density, the 
percent moisture, and the porosity of the sewage sludge.

F. What Are the Factors in Estimating How Much Dioxin Is Being 
Transported in the Environment to the Individual in the Farm Family?

    A conceptual site model was used to represent exposures to the 
highly exposed modeled population from land application of sewage 
sludge. To capture some of the variability in environmental settings 
across the United States, the conceptual site model was placed in 
different regions throughout the continental United States.
    The risk assessment was intended to be representative of a national 
distribution of environmental conditions. The 48 contiguous states 
(excluding Hawaii, Alaska, and the off-shore possessions) were divided 
into 41 meteorologic regions. These regions were selected to represent 
the national variation of location-specific variables. Each area is 
assumed to represent a single climate region (i.e., conditions within 
that area can be modeled using the meteorologic data from a single 
meteorologic observation station). Meteorologic and climate data were 
used in air modeling, partitioning in the source model, and surface and 
subsurface fate and transport modeling.
    In addition, farm areas were assumed to be linked to geographic 
area. Large farms are more common in the Midwest and western parts of 
the United States, and smaller farms are more common in the eastern and 
southern parts of the United States. Thus, a regional estimate for a 
median farm size was developed and was used in this risk assessment. 
The U.S. agricultural census contains estimates for the distribution of 
farms within each county. These data were used to develop a median farm 
size for each county. These county-wide median farm sizes were 
classified according to the 41 geographic areas and the median of the 
median farm sizes was estimated for each of the 41 regions. The median 
area was then used in the air modeling and the erosion to surface water 
modeling. This methodology was used to account for the regional 
variation in agricultural practices throughout the nation, but it did 
not consider variation in size within a single region.
    A series of models was used to estimate concentrations of the 
congeners in the environment with which a farm family may come into 
contact. The revised risk assessment assumes that there are six direct 
and indirect exposure pathways that the models describe:
     Inhalation of ambient air;
     Incidental ingestion of soil in the buffer area;
     Ingestion of above- and below-ground produce grown on the 
crop land;
     Ingestion of beef and dairy products from the pasture;
     Ingestion of home-produced poultry and eggs from the 
buffer area; and
     Ingestion of fish from the nearby water body.
    As indicated above, a regional approach was used to define the area 
surrounding the agricultural application site. A source partition model 
was then used to estimate environmental releases of each constituent. 
These estimated environmental releases in turn provided input to the 
fate and transport models to estimate media concentrations in air, 
soil, and surface water. A food chain model was used to estimate 
constituent concentrations in produce, beef, dairy products, poultry, 
eggs, and fish.
    The source partition model determines the initial release of 
congeners into the environment. Sewage sludge application to pastures 
or crop land is assumed to be different and these differences affect 
the behavior of constituents in the environment. The model uses 
information described above on sewage sludge characteristics (e.g., 
moisture content and congener concentrations), and environmental 
setting (e.g., precipitation, temperature, and soil characteristics) to 
estimate environmental releases.
    Fate and transport modeling procedures describe the mechanism by 
which the congeners move from the source through the environment. As 
described above, a source partition model was used to determine the 
amount and nature of congener released from the agricultural field. A 
multimedia approach was used to characterize the movement of the 
dioxins through the environment. This approach considered atmospheric 
concentrations, atmospheric deposition, soil concentrations, and 
sediment concentrations in potentially impacted water bodies.
    Air modeling procedures estimated air concentrations and deposition 
of vapors and particles on the agricultural farm, onto the buffer area, 
directly into the surrounding water bodies, and onto the regional 
watershed. Air dispersion and deposition of vapors and particles were 
modeled using the Industrial Source Complex Short Term Model. Soil 
erosion comes from the crop fields and pastures, the buffer area 
containing the house and chicken yard, and the remaining portion of the 
watershed. Erosion was modeled using the Universal Soil Loss Equation. 
All impacts in the same period of time were summed to estimate the 
concentration in the stream sediment and water column.
    The exposure pathways included inhalation of dioxins in ambient air 
during tilling of agricultural fields, incidental ingestion of soil, 
ingestion of aboveground and belowground produce (i.e., root crops), 
ingestion of beef and dairy products, ingestion of eggs and poultry 
products, and ingestion of fish. EPA's preliminary analysis indicated 
that exposure to dioxins from the consumption of ground water was 
insignificant due to the extremely low solubility of dioxins in water 
and negligible leaching of dioxins to ground water (USEPA, 1999b).
    With concentrations of the congeners determined for water and air, 
the concentrations being delivered to humans from aboveground produce, 
belowground produce, poultry, eggs, beef, dairy products, and fish were 
then calculated. This was accomplished using food chain models. The 
food crops (vegetables, fruits, and root vegetables) were assumed to be 
grown on the sewage sludge-amended fields, and cattle (beef and dairy) 
were assumed to be raised on pastures receiving sewage sludge. These 
processes were modeled using a multi-pathway exposure model and the 
fate and transport parameters and modeling procedures reflecting the 
latest scientific knowledge on the fate and transport of dioxin. The 
exposure pathways considered the transport of constituents from the 
soil to plants (vegetables, fruits, roots, and pasture grass) and 
ingestion of these materials by humans and animals. The transport to 
plants

[[Page 40564]]

may occur through the root system, but most occurs through air-to-plant 
transfer mechanisms. The contaminated plants are in turn consumed by 
cattle and humans.
    The latest scientific knowledge with respect to the methodology of 
estimating concentration of congeners in beef and/or dairy products is 
also described in the Draft Dioxin Reassessment document. This 
methodology has been developed based on the transfer of congeners from 
the total diet of the cattle into the fat. The method described in the 
Draft Dioxin Reassessment emphasizes the importance of the differences 
in diet between beef and dairy cattle in explaining different food 
concentrations. While the same equation was used for all cattle, 
whether they are beef cattle or dairy cattle, the differences were in 
the dietary fraction assumptions. These assumptions were based on how 
much of the time the cattle are pastured and how much of the time they 
are confined with supplemental feed. Forage was assumed to be raised on 
the sewage sludge-amended pasture where the sewage sludge was assumed 
to remain on the top two centimeters of the soil and to volatilize onto 
the forage. The soil was assumed to be the soil in the sewage sludge-
amended pasture. The supplemental feed for the cattle was assumed to be 
grown on sewage sludge-amended crop land where the sewage sludge was 
tilled into the soil. Half of the supplemental feed was assumed to be 
vegetation and half was assumed to be grains. Supplemental feed was 
assumed to contain a lower dioxin concentration than forage because it 
was assumed to contain less volatilized dioxins (due to tilling), and 
the grain portion was assumed to be free of contamination due to 
stripping of the outer leaves where dioxins accumulate.
    To determine the dioxin concentrations in poultry and eggs, the 
risk assessment starts with the assumption that sewage sludge is not to 
be applied directly to the chicken yard. The chickens are assumed to be 
free range within a confined area of the buffer near the farm 
residence. The chicken diet is assumed to consist of 90 percent store 
bought chicken feed (uncontaminated by dioxins in sewage sludge applied 
on the farm land) and 10 percent buffer soil.
    As already indicated, the receptors included in the modeling are 
adults and children living and working on farms where fruits, 
vegetables, root crops, and farm animals are raised, and half of these 
food items consumed by the adults and children living on the farm are 
produced on the farm. The farm family also is assumed to be exposed to 
inhalation risks from windblown and tilling emissions from the 
agricultural field. Soil ingestion risks are also assessed for both 
adults and children. Children are assumed to ingest soil from the 
buffer area, and the adult farmer is assumed to ingest soil from the 
tilled field. In addition, risks to recreational fishers who catch and 
consume fish from the stream adjacent to the agricultural field is 
considered and summed with the other exposure pathways on the 
assumption that farmers are also recreational fishers.
    EPA requests comment on the assumptions and values used in this 
Section to estimate how much dioxins are being transported to 
individuals in the modeled farm family (e.g., the sources (store-bought 
versus farm-produced) and dioxin contamination levels of poultry 
feeds).

G. What Additional Factors Are Applied to Dioxin Concentrations To 
Determine How Much of the Congeners are Being Ingested or Inhaled by a 
Farm Family Member?

    To determine how much of the congeners adults and children are 
inhaling and ingesting, exposure factors were applied to the 
concentrations of the contaminants from air, produce, cattle, dairy, 
poultry, eggs, and fish. The exposure factors used in this analysis 
were taken from the Exposure Factors Handbook (USEPA, 1997). The 
Exposure Factors Handbook summarizes data on human behaviors and 
characteristics related to human exposure from relevant key studies and 
provides recommendations and associated confidence estimates on the 
values of exposure factors.\3\
---------------------------------------------------------------------------

    \3\ EPA carefully reviewed and evaluated the quality of the data 
before their inclusion in the Exposure Factors Handbook. EPA's 
evaluation criteria included peer review, reproducibility, 
pertinence to the United States, currency, adequacy of the data 
collection period, validity of the approach, representativeness of 
the population being modeled (in this case, farm families), 
characterization of the variability, lack of bias in study design, 
and measurement error (USEPA, 1997).
---------------------------------------------------------------------------

    The proportion of home produced food commodities eaten by highly 
exposed farm families was assumed to be 50% of their diet for all 
iterations. This assumption defined the modeled population. Specific 
distributions of other exposure factors for the general population of 
farm residents were compiled from the Exposure Factors Handbook. These 
include ingestion rates for adults and children for aboveground 
vegetables, root vegetables, fruits, beef, dairy products, poultry, and 
eggs. Distributions have been developed for adults and for three age 
groups of children for these dietary categories.
    Exposure factors are related to the pathways in that they describe 
the rates at which dioxin doses are ingested or inhaled from the 
various sources noted above (e.g., air, soil, beef, and diary, by the 
highly exposed farm family adults and children). The exposure factors 
used in this risk assessment are represented by a distribution or a 
fixed value in the Monte Carlo probabilistic analysis.
    For the probabilistic exposure analysis, probability distribution 
functions were developed from the values in the Exposure Factors 
Handbook. The intake factors, for which either single values or 
distributions were used from the Exposure Factors Handbook, are: soil 
ingestion (one value for children aged 1 to 6 and another value for all 
other receptors); and fruits and vegetables ingestion, beef and dairy 
ingestion, fish ingestion, and inhalation rates (all of which are 
distributions of values.)

H. How Did EPA Calculate the Range of Exposure Levels?

    For cancer effects, where the biological response is described in 
terms of lifetime probabilities, dose is presented as a ``lifetime 
average daily dose'' (LADD). Because exposure duration varies from 
person to person (i.e., may not occur over the entire lifetime), 
calculation of exposure produces a distribution of exposure levels (or 
doses). In addition to exposure duration, the LADD takes a number of 
variable factors into account, including when exposure begins, how 
often and in what amounts sewage sludge is applied to the land, and the 
length of time over which land application occurs. For this risk 
assessment, the LADD takes into account: (1) A distribution of randomly 
selected times when land application begins, i.e., either when the 
highly exposed farm family begins applying sewage sludge to their land 
or moves onto a farm where sewage is being or has been applied; (2) a 
distribution of exposure durations ranging from one year to 70 years; 
\4\ (3) a distribution of sewage sludge application duration, ranging 
from a minimum of one year up to a maximum of 40 years (i.e., a minimum 
of one application to a

[[Page 40565]]

maximum of 20 applications based on a fixed application frequency of 
once every two years), and (4) a distribution of sewage sludge 
application rates (i.e., amount of sludge applied to the land) ranging 
from 5-10 metric tons per hectare per application. The LADD also 
includes doses from each exposure route (i.e., inhalation and 
ingestion) and body weight. A distribution of body weights for the 
adult and child were taken from the Exposure Factors Handbook.
---------------------------------------------------------------------------

    \4\ Exposure durations representing the residence time in the 
same house were also determined using the Exposure Factors Handbook. 
The lifetime of the individual was assumed to be a fixed value of 70 
years. A fixed value for exposure frequency was assumed to be 350 
days per year, accounting for two weeks away from the farm for 
vacation (USEPA, 2002b). These single values were selected to be 
protective and yet representative of realistic scenarios.
---------------------------------------------------------------------------

    The purpose of the exposure assessment is to estimate the dose to 
an exposed individual by combining media intake estimates with media 
concentrations. Estimates of exposure are based on the potential dose 
(e.g., the dose ingested or inhaled) rather than the applied dose 
(e.g., the dose delivered to the gastrointestinal tract) or the 
internal dose (e.g., the dose delivered to the target organ). Doses 
from individual pathways (e.g., soil, exposed vegetables) were 
calculated by multiplying the contaminant concentration in the food 
product or other exposure media (e.g., air or soil) by the respective 
intake rate on a per kilogram body weight basis. Doses received from 
the various ingestion pathways (e.g., soil and food) were then summed 
over the period of time in which exposure occurs, resulting in an 
average daily dose received from ingestion exposure.

I. How Was Childhood and Infant Exposure Evaluated in the Exposure 
Analysis?

    Children are an important sub-population to consider in a risk 
assessment because they may be more highly exposed than adults; 
compared to adults, children may eat more food and drink more fluids 
per unit of body weight. This higher intake-rate-to-body-weight ratio 
can result in a higher average daily dose of dioxins than adults 
experience. The risk assessment performed for sewage sludge application 
to agricultural land includes an analysis of exposures to 3,000 
individuals whose exposures begin in childhood. To account for intake 
rates varying over different childhood age groups, parameters 
characterizing exposures beginning in childhood were developed.
    The first step in developing the time-weighted parameters is to 
define the start age for the child and the length of exposure for that 
individual. These two values then determine how long the individual is 
in each age group. Four age groups were defined as follows: age group 1 
(1-5 years of age); age group 2 (6-11 years of age); age group 3 (12-19 
years of age); and age group 4 (over 20 years of age). After the 
individual is defined, age appropriate consumption rates are chosen for 
each age group which are selected from the age specific consumption 
rate distribution for each item considered in the analysis. For example 
if the exposure begins at age 3 and continues for 20 years, a 
consumption rate for each age group was selected and weighted to 
represent the number of years spent in each age group to get an average 
intake rate for the entire exposure duration of 20 years (i.e., age 
group 1= 3 years of exposure; age group 2 = 6 years; age group 3 = 7 
years; and age group 4 = 4 years, for a total of 20 years exposure.) 
This time weighted intake rate is then used with the average 
concentration of dioxins for the food item over the entire exposure 
duration, to yield an average daily dose.
    Infants are also an important sub-population to consider in this 
risk assessment because they may be exposed to dioxin-like compounds 
via the ingestion of breast milk. While risks to children and adults 
were integrated to incorporate individuals for whom exposure first 
occurs during childhood but continues into adulthood, the lifetime 
risks to infants were calculated separately from the risks to older 
children (i.e., ages 1 year or older) and adults. For infants, exposure 
during the first year of life was averaged over an expected lifetime of 
seventy years to derive a LADD that was then used to calculate risk. 
The ``lifetime'' risk to infants thus should be thought of as the 
contribution to lifetime risk that occurs during the first year of life 
through ingestion of breast milk for individuals born into a farm 
family exposed to dioxins from land-applied sewage sludge.

J. How Was the Cancer Risk Estimate Calculated?

    Cancer risk is calculated using lifetime excess cancer risk 
estimates to represent the excess probability of developing cancer over 
a lifetime as a result of exposure to the constituent of interest. 
Lifetime excess cancer risk estimates are the product of the lifetime 
average daily dose for each of the four types of individuals exposed to 
dioxin and for each exposure pathway, and the corresponding cancer 
slope factor.
    The exposure assessment estimates delivered doses for each of the 
29 congeners to a farm family individual. Each of these congener doses 
were then converted to TEQ doses by multiplying each congener dose by 
its TEF. These TEQ doses for each of the 29 congeners were then summed 
to yield an overall TEQ dose to the individual for that exposure 
pathway (e.g., inhalation or ingestion). Finally this TEQ dose was 
multiplied by the cancer slope factor to estimate the excess cancer 
risk to the individual for that pathway of exposure.
    Using all samples from the EPA 2001 dioxin update survey, the 
estimated risks and corresponding daily exposure to dioxins for the 
highly exposed farm adult and child are given below in Table 5 for 
various percentiles of exposure within this population. ``Adult'' means 
individuals whose exposure begins when they are adults, and ``child'' 
means individuals whose exposure begins when they are children. In most 
cases exposure which begins during childhood also ends during 
childhood. However, in some instances, exposures which begin when 
individuals are children continued into their adult years.
    Additional risk calculations were performed to estimate the impact 
on the risk if sewage sludge with 300 ppt TEQ dioxin and 100 ppt TEQ 
dioxin were restricted from being land applied. Eliminating sewage 
sludge samples with higher concentrations of dioxins did not change the 
estimated risk. The distribution of risk estimates for scenarios 
excluding samples with dioxin concentrations greater than 300 ppt TEQ 
and 100 ppt TEQ are the same as the distribution below shown in Table 
5, which includes data from all sewage sludge samples.

[[Page 40566]]



Table 5.--Risks and Daily Exposure for Highly Exposed Farm Adult and Child for All Exposure Pathways--(Q*=1.56 x
                                                10-4/pg TEQ/kg-d)
----------------------------------------------------------------------------------------------------------------
                                                                    Adult *                    Child **
                                                         -------------------------------------------------------
                       Percentile                                           Daily                       Daily
                                                              Risk       Exposure pg      Risk      Exposure, pg
                                                                          TEQ/kg-d                    TEQ/kg-d
----------------------------------------------------------------------------------------------------------------
50th....................................................     1 x 10-6           7.3      1 x 10-6           7.3
75th....................................................     4 x 10-6           7.3      3 x 10-6           7.3
90th....................................................     1 x 10-5           7.3      7 x 10-6           7.3
95th....................................................     2 x 10-5           7.3      1 x 10-5           7.3
99th....................................................     4 x 10-5           7.3      2 x 10-5          7.3
----------------------------------------------------------------------------------------------------------------
 * Initial exposure begins when the individual is an adult.
 ** Initial exposure begins when the individual is a child.

K. How Did EPA Analyze the Relative Importance of Inputs to the Risk 
Model?

    In addition to the revised risk assessment, EPA conducted a 
sensitivity analysis to identify the effects of variability and 
uncertainty in the risk model on the risk estimates. These steps are 
performed on the inputs and outputs of the Monte Carlo analysis. In the 
Monte Carlo analysis, probability distributions were assumed for each 
of the variable input parameters, and a distribution of 3,000 media 
concentrations and risk results were generated as outputs in the 
analysis. In the sensitivity analysis, statistical methods were applied 
to this sample of inputs and outputs to evaluate the influence of the 
individual inputs on the model outputs. Several different indices of 
sensitivity were derived from the simulated sample to quantify the 
influence of the inputs and identify the most influential parameters. 
Finally, a regression analysis was applied to a linear equation to 
estimate the relative change in the output of a Monte Carlo simulation 
relative to the changes in the input parameters.
    Table 6 presents the results of the sensitivity analysis for the 
beef and dairy products exposure pathways. The consumption of beef and 
dairy products by the farm family represent over 90 percent of dioxin 
exposure and subsequent cancer risk associated with land application of 
sewage sludge. For the beef products pathway, exposure duration and 
beef consumption rate combine to account for 86 percent of the 
variation in the estimation of dioxin exposure. The two variables which 
account for the next highest contributions to variation in the 
estimation of exposure (i.e., sewage sludge application rate and 
average year that the farm family moves in) combined for 2 percent of 
the variation. Similarly, for dairy products, exposure duration and 
dairy products consumption rate also represent 86 percent of the 
variation in the estimation of exposure, with the next two highest 
variables again representing a combined 2 percent of the variation. A 
detailed discussion of the entire sensitivity analysis can be found in 
the land application risk assessment Technical Background Document 
(USEPA, 2002b).

                Table 6.--Results of Sensitivity Analysis
------------------------------------------------------------------------
                                              Percent of risk accounted
     Pathway and Sensitivity variables             for by variable
------------------------------------------------------------------------
Beef:
      Exposure Duration...................  60
      Consumption Rate....................  26
      Sewage sludge Application Rate......  1
      Average year that the farm family     1
     moves in.
Dairy products:
      Exposure Duration...................  54
      Consumption Rate....................  32
      Average year that the farm family     1
     moves in.
      Sewage sludge Application rate......  1
------------------------------------------------------------------------

L. How Does EPA Characterize the Risk?

    As previously noted, EPA developed a revised risk assessment using 
a probabilistic approach as a basis for the Agency final action on 
development of a numerical standard for dioxins in sewage sludge 
applied to agricultural land. In order to protect the general public 
from adverse health impacts from dioxins in land-applied sewage sludge 
with an adequate margin of safety, the risk assessment calculates the 
risk to the most highly exposed population (i.e., a farm family 
consuming 50 percent of their diet from products grown on sewage sludge 
amended soil) . The following discussion characterizes the key elements 
of EPA's risk assessment and compares them according to the principles 
in EPA's guidance for exposure assessment and for risk characterization 
(USEPA, 1992 and USEPA, 2000b).
    Approximately 95 percent of the U.S. population's exposure to 
dioxins results from the consumption of animal products in the diet 
where dioxin is concentrated in the fatty portion of the meats and 
dairy products (USEPA, 2000a. See Part I, Vol. 3, Chap. 3.). EPA chose 
the farm family as the highly exposed population to be modeled, using a 
key assumption that their diets have significant percentages of meat 
and dairy products from their own farms where sewage sludge is land 
applied as a fertilizer or soil amendment. Members of such a farm 
family are at greater risk from exposure to dioxins associated with 
land application as compared with the overall U.S. population because 
their diets would be based on products from their farm. As previously 
noted, a decision that is protective of this highly exposed modeled 
population is thus protective of the general population from the same 
pathways of dioxin exposure with a greater margin of safety since the 
diet of the general population contains only a small fraction of meat 
and dairy products grown on farms with land-applied sewage sludge.
    The following discussion characterizes the three principal 
components of the risk assessment: the exposure scenario; key 
assumptions and data used in the exposure assessment modeling; and the 
cancer slope factor (Q1* or potency factor). Each of these components 
is characterized as either ``high end'' or ``central tendency.''
    As previously noted, sewage sludge is assumed to be applied at 
agronomic rates to tilled crop land used for the production of 
vegetables, fruits, and root crops, and to pasture land which is not 
tilled. Fifty percent of the farm family's agricultural land is assumed 
to be tilled crop land and the other fifty percent untilled pasture. An 
important assumption in terms of characterizing the risk is that the 
dioxin in each

[[Page 40567]]

application of sewage sludge to pasture is assumed to permanently 
remain in the top two centimeters of the land surface and is not 
diluted over time. This is a key assumption since volatilization from 
soil to the leaf surfaces of crops consumed by animals and humans is 
the principal mechanism by which dioxins are transported from sewage 
sludge applied to the land. This assumption predicts a maximum amount 
of transport of dioxins for subsequent consumption by pastured animals. 
In addition, this pasturing scenario is not varied; EPA assumes that 
the farmer does not rotate the pasture to grow row crops where tilling 
of sewage sludge in the soil would mitigate dioxin volatilization 
transport. Thus, this assumption is likely to contribute to an 
overestimation of risk.
    Another important assumption contributing to the risk estimate is 
that the family is simultaneously exposed to a combination of 
agricultural products produced on the farm. For the purpose of the 
exposure assessment and risk assessment, all pathways of exposure to 
dioxins are summed.
    As previously noted, the cancer slope factor used in the revised 
risk assessment is 1.56 x 10-4/pg TEQ/kg-d. This value is 
characterized as the upper bound (i.e., at the 95th percentile 
confidence level) on the slope of the dose-response curve in the low-
dose region and is generally assumed to be linear. Use of upper bound 
slope factors also results in calculation of high-end risks of cancer 
for individuals in the target population of highly exposed farm 
families (i.e., 95% likelihood that risk to such highly exposed 
individuals is lower) (USEPA, 2000a. See Part III, Chap. 6).
    As described above in the description of the risk assessment, most 
of the parameters used in the Monte Carlo simulations were 
distributions of a range of observed values for each parameter. Where a 
range of data was not available, ``fixed'' data points or assumptions 
were used. The sources of information for the fixed point inputs 
necessary to conduct the risk assessment include the EPA Exposure 
Factors Handbook (USEPA, 1997), peer reviewed scientific literature, 
and other assumptions specifically related to land application of 
sewage sludge based on actual practice.
    The following is a listing of some of the key fixed parameters used 
in the Monte Carlo simulations and their characterizations. Some of the 
fixed assumptions characterized as ``high end'' have the greatest 
impact on the risk estimate based on the results of the sensitivity 
analysis discussed above (see Section VI.K.). These assumptions include 
the farm family simultaneously exposed to multiple pathways including a 
certain percentage of their own products; dioxin remaining in the top 
two centimeters on pasture lands; and the upper bound Q1*. The 
following ``fixed'' parameters are important to note, but have a lesser 
impact on the risk estimate.
Other ``High End'' Assumptions
     Exposure Frequency--350 days per year.
     Fraction of diet for home-caught fish--100%.
     Fraction of soil ingested that is contaminated--100%.
     Fraction of ingested dioxin absorbed by the mother--100%.
     Use of potential dose rather than applied or internal 
dose.
Mean or Central Tendency Values from EPA Exposure Factors Handbook
     Fraction of food preparation loss for exposed fruit, 
exposed vegetables, and root vegetables.
     Percent cooking and percent post-cooking loss for beef and 
poultry.
     Fraction of home-caught fish that are at trophic levels 3 
and 4 (high dioxin bio-accumulating fish).
     Soil ingestion rates for children and adults.
Values from Scientific Literature \5\
---------------------------------------------------------------------------

    \5\ USEPA, 1998a. Methodology for Assessing Health Risks 
Associated with Multiple Pathways of Exposure To Combustor 
Emissions. These values were gathered from various sources and are 
either mean values or representative ranges (not high end).
---------------------------------------------------------------------------

     Biological half life of dioxin in lactating women.
     Concentration of dioxin in aqueous phase of maternal milk.
     Fraction of fat in maternal breast milk. (mean value)
     Fraction of ingested dioxin absorbed by the infant.
     Fraction of mother's weight that is fat. (mean value)
     Proportion of dioxin stored in maternal fat.
    The probabilistic methodology facilitates risk estimates for 
individuals in any percentile of the assessed population. The revised 
land application risk assessment reports high-end estimates of risks 
for individuals at the 50th, 75th, 90th, 95th and 99th percentiles of 
exposure within the population defined for this analysis as ``highly 
exposed.'' USEPA, 2002b. It may also be acceptable to characterize the 
risk assessment as the ``high end of the high end'' within this modeled 
population of highly exposed farm families.
    The incremental cancer risk for land application of sewage sludge 
was estimated considering all exposure pathways for three scenarios: 
baseline (all samples from the EPA 2001 dioxin update survey); 300 ppt 
TEQ cutoff (samples greater than 300 ppt TEQ excluded); and 100 ppt TEQ 
cutoff (samples greater than 100 ppt TEQ excluded). The estimated 
lifetime risks for adults using this cancer slope factor range from 4 x 
10-5 at the 99th percentile to 1 x 10-6 at the 
50th percentile for multi-pathway exposure to dioxins through land-
applied sewage sludge (see Table 5). (As indicated in Table 5, the 
estimated risks for children are less than or equal to the estimated 
risks for adults.) No quantifiable decrease in risk is calculated if 
sewage sludge with greater than 300 ppt TEQ dioxins or greater than 100 
ppt TEQ dioxins were restricted from being land applied. The reason 
that the estimated risk does not decrease when sewage sludge limits of 
300 ppt TEQ dioxins or 100 ppt TEQ dioxins are assumed is that, based 
on the representative sampling, there is so little sewage sludge that 
contains dioxin at or above these concentrations.
    Continual application of sewage sludge with significantly higher 
concentrations of dioxins than currently measured would be necessary to 
predict quantifiable increases in risk. However, comparison of data 
from the 1988 NSSS (USEPA, 1990) and the EPA 2001 dioxin update survey 
(USEPA, 2002a) indicate that ``spikes'' (i.e., higher concentrations) 
of dioxins in sewage sludge appear to be transient. Specifically, all 
ten sewage sludge samples with the highest concentrations of dioxins 
and furans measured in the 1988 Survey (concentrations ranging from 97 
ppt TEQ to 827 ppt TEQ) had greatly reduced concentrations of dioxins 
and furans in the 2001 dioxin update survey (concentrations ranging 
from 2 ppt TEQ to 53 ppt TEQ) (USEPA, 1990 and USEPA, 2002a). 
Conversely, the four sewage sludge samples with the highest 
concentrations of dioxins and furans measured in the 2001 dioxin update 
survey (concentrations ranging from 93 ppt TEQ to 682 ppt TEQ) had 
markedly lower concentrations of dioxins and furans in the 1988 Survey 
(concentrations ranging from 2 ppt TEQ to 41 ppt TEQ) (USEPA, 2002a and 
USEPA, 1990).

    [Note: These comparisons are based on dioxin and furan 
concentrations since only dioxins and furans were measured in the 
1988 Survey.] Thus, it is highly unlikely that a single family would 
be exposed to one of these sewage sludges with a high

[[Page 40568]]

concentration of dioxin long enough to produce a quantifiable 
increase in risk.

    Finally, the Agency calculated the maximum number of cancer cases 
in the highly exposed population that could be predicted from exposure 
to dioxins in land applied sewage sludge (USEPA, 2002c). To make this 
calculation the Agency used data from the EPA Exposure Factors Handbook 
(USEPA, 1997) that indicates that 2 percent of the United States 
population are in farm families whose diets consist of 50 percent of 
products produced on their own farm (5.6 million people). The Agency 
then estimated the maximum percentage of farmland to which sewage 
sludge could be applied annually is 0.2 percent. This estimate was 
derived by dividing the amount of farmland which could receive sewage 
sludge if all 8 million metric tons of sewage sludge produced annually 
in the United States (USEPA, 1999c) were land-applied at an agronomic 
rate of 10 metric tons/hectare (800,000 hectares) by the total amount 
of farmland in the United States (377 million hectares; USDA, 1997). On 
this basis EPA estimates that the highly exposed farm family population 
is no greater than 11,000 (i.e., 0.2% of the 5.6 million people whose 
diets consist of 50% percent of products produced on their own farm). 
The number of lifetime cancer cases is estimated by multiplying the 
risk by the number of individuals in the modeled population. The 
estimated lifetime cancer cases for the modeled population is 0.224 if 
the 95th percentile adult risk from land application of sewage sludge 
(2 x 10-5, see Table 5) is used for this calculation, and 
0.112 using the 90th percentile adult risk (1 x 10-5, see 
Table 5). The number of annual cases is estimated by dividing the 
lifetime cancer cases by 70 years of exposure. The estimated annual 
cancer cases is 0.006 if the 99th percentile adult risk is assumed, 
0.003 if the 95th percentile adult risk is assumed, and 0.002 if the 
90th percentile adult risk is assumed.
    EPA requests comments on the Agency's characterization of the key 
elements of the revised land application risk assessment. EPA will 
consider these comments to characterize the overall estimate of risk to 
the modeled population.

VII. What Are the Implications of EPA's Dioxin Reassessment Process for 
This Rulemaking?

    Since 1991 EPA has been conducting a scientific reassessment of the 
health risks of exposure to dioxin and dioxin-like compounds. EPA began 
this task in light of significant advances in the Agency's scientific 
understanding of mechanisms of dioxin toxicity, significant new studies 
of dioxin's carcinogenic potential in humans, and increased evidence of 
other adverse health effects. These efforts have included the 
involvement of outside scientists as principal authors of several 
chapters, frequent public meetings to report progress and take public 
comment, and publication of early drafts for public comment and peer 
review. The review process for the Dioxin Reassessment has also 
involved extensive use of outside scientists from other federal 
agencies and the general scientific community.
    As previously stated, aspects of the Agency's Draft Dioxin 
Reassessment that are considered state of the science or the best 
available information about dioxin have been incorporated into the 
revised exposure analysis and risk assessment for dioxins in land-
applied sewage sludge. (See Section VI.D. of this Notice). However, the 
Agency has not finalized its policy and/or factual conclusions with 
respect to other aspects of the Draft Dioxin Reassessment, and any 
decisions on these policy and factual conclusions made in part as a 
result of the Dioxin Reassessment could affect the sewage sludge land 
application exposure analysis and risk assessment, and therefore could 
affect the Agency's decisions with respect to this rulemaking. 
Therefore, EPA is seeking comment on the implications of this 
information in the event that, prior to taking final action on the 
Round Two rule, EPA finalizes a cancer slope factor, an approach to 
assessing risk of non-cancer health effects from dioxins, or other 
aspects of the current Draft Dioxin Reassessment. If EPA issues a final 
Dioxin Reassessment that is substantially similar to the current draft 
as discussed in this Notice, EPA does not expect to provide further 
notice and opportunity for public comment with respect to the effect of 
the Dioxin Reassessment on this rulemaking. The following is a brief 
summary of the EPA Dioxin Reassessment process, and a discussion of how 
the Agency may integrate key decisions on dioxins policy resulting from 
the Dioxin Reassessment into the Round Two rulemaking.
    EPA first released the external review drafts of the Dioxin 
Reassessment health effects and exposure documents in September 1994 
(USEPA 1994a). The Agency took public comment on the drafts, followed 
by the Agency's Science Advisory Board (SAB) review of the Draft Dioxin 
Reassessment in May 1995. The documents were revised based on these 
reviews and were again released for external peer review. EPA made 
additional revisions to the documents based on the external peer review 
and submitted them once again to the SAB. After a public meeting on May 
15, 2001, the SAB's Executive Committee endorsed a review report of the 
Draft Dioxin Reassessment contingent upon changes to address some of 
the differing scientific opinions raised in the review report.
    Based on the overall endorsement of the content of the Draft Dioxin 
Reassessment by the SAB, EPA used many aspects of the Reassessment in 
the revised Part 503 exposure analysis and risk assessment. These 
include the TEQ approach based on the toxicity of 2,3,7,8-TCDD, the use 
of the current WHO98 TEQs, and the numerous physical, 
chemical, occurrence, and exposure factors used in the Dioxin 
Reassessment to evaluate and characterize human health risks from 
dioxins.
    Two of the key areas which the SAB identified as having differing 
scientific opinions are the cancer slope factor for 2,3,7,8-TCDD and 
the use of a margin of exposure (MOE) approach to evaluate the 
likelihood that non-cancer effects may occur in the human population at 
environmental exposure levels. The Draft 2000 Dioxin Reassessment notes 
that, while major uncertainties remain, efforts to bring more data into 
the evaluation of cancer potency have resulted in an estimate of 1 x 
10-3/pg TEQ/kg-d. According to the Draft 2000 Dioxin 
Reassessment, this cancer slope factor represents a plausible upper 
bound on risk based on evaluation of human and animal data. These 
values are approximately six times higher than previous estimates 
(USEPA, 1985 and USEPA, 1994a) which were based on fewer data. However, 
the EPA SAB panel was not able to reach consensus on a single value for 
a dioxin potency factor. The SAB panel cited differences of opinion on 
the adequacy of data and modeling approaches and assumptions as their 
reasons for not reaching consensus on a dioxin cancer slope factor.
    The revised Round Two land application risk assessment uses the 
cancer slope factor currently used by EPA in risk assessments (USEPA, 
1994a). If EPA adopts a different cancer slope factor for assessing the 
risk of cancer from dioxin prior to taking final action on the proposed 
Round Two rule, EPA will evaluate the risk of cancer from land-applied 
sewage sludge using any such revised cancer slope factor. Similarly, to 
the extent EPA adopts a policy regarding risks of non-cancer health 
effects from dioxin prior to the

[[Page 40569]]

final decision on the proposed Round Two rule, the Agency will evaluate 
non-cancer effects associated with dioxins in land-applied sewage 
sludge using any such policy.
    In order to give the public an opportunity to understand and 
comment on how the particular approaches contained in the Draft Dioxin 
Reassessment could potentially affect the proposed Round Two 
rulemaking, EPA is presenting a discussion of the potential impacts of 
the revised cancer slope factor and approaches for estimating non-
cancer effects contained in the Draft Dioxin Reassessment on EPA's 
revised land application risk assessment. This includes a discussion of 
background exposures and risks based on information in the Draft Dioxin 
Reassessment, such as existing body burden, although EPA has not made a 
final decision regarding these findings or adopted any policy with 
respect to regulating dioxins in light of background exposures and 
existing body burden.

A. How Would the Dioxin Cancer Risk from Land Application Compare to 
Background Dioxin Cancer Risk?

    Dioxin and dioxin-like compounds always exist in nature as complex 
mixtures. These compounds can be quantified in environmental media and 
their potential effects assessed as a mixture. As previously noted, the 
contribution of the other ``dioxin-like'' compounds is quantified by 
treating each as having a defined ``toxicity equivalence'' to dioxin 
(toxicity equivalent factor, TEF). The TEQ concentration is calculated 
by multiplying the concentration of each congener in the sewage sludge 
by its corresponding TEF, and then summing the resulting products from 
this calculation for all 29 congeners.
    The significance of the incremental exposure and risk due to a 
specific source such as land application of sewage sludge is best 
understood by discussing it in the context of general population 
background exposure to dioxins. The fact that background exposures and 
body burden of dioxins are currently high for the general population 
means that any incremental exposure from a particular source needs to 
be considered in context of its contribution to overall risk. The 
following is a comparison of the dioxin cancer risk the EPA calculated 
from the Agency's revised risk assessment to the background dioxin 
cancer risk estimated from the Agency's 2000 Draft Dioxin Reassessment. 
This comparison considers both the current cancer slope factor the 
Agency has been using since 1985 and the revised cancer slope factor 
contained in EPA's 2000 Draft Dioxin Reassessment.
    The revised risk assessment for land application of sewage sludge 
uses the current cancer slope factor of 1.56 x 10-4/pg TEQ/
kg-d. The estimated upper bound lifetime risks for highly exposed farm 
family adults using this cancer slope factor range from 4 x 
10-5 at the 99th percentile to 1 x 10-6 at the 
50th percentile for multi-pathway exposure to dioxins through land-
applied sewage sludge (see Table 5). As indicated in Table 5, the 
estimated risks for children are less than or equal to the estimated 
risks for adults. These risks correspond to an estimated daily 
exposures (adult) ranging from 0.3 pg TEQ/kg-d at the 99th percentile 
to 0.006 pg TEQ/kg-d at the 50th percentile. Use of the 1 x 
10-3/pg TEQ/kg-d cancer slope factor being considered in the 
2000 Draft Dioxin Reassessment would result in estimated high-end 
multi-pathway lifetime risks for highly exposed farm family adults 
ranging from 2.4 x 10-4 at the 99th percentile to 6 x 
10-6 at the 50th percentile (see Table 7, below). These 
estimated risks using a 1 x 10-3/pg TEQ/kg-d cancer slope 
factor are based on the same daily exposures indicated in Table 5. 
Again, the estimated risks for children would be less than or equal to 
the estimated risks for adults (see table 7).

Table 7.--Risks for Highly Exposed Farm Adult and Child for All Exposure
                   Pathways--(Q*=1 x 10-3 pg TEQ/kg=d)
------------------------------------------------------------------------
                  Percentile                      Adult *      Child **
------------------------------------------------------------------------
50th..........................................     6 x 10-6     6 x 10-6
75th..........................................     2 x 10-5     2 x 10-5
90th..........................................     6 x 10-5     4 x 10-5
95th..........................................     1 x 10-4     6 x 10-5
99th..........................................     2 x 10-4    1 x 10-4
------------------------------------------------------------------------
* Initial exposure begins when the individual is an adult.
** Initial exposure begins when the individual is a child.

    For this comparison EPA considered ``background risk'' to be the 
upper bound risk for the general population. Using the current cancer 
slope factor of 1.56 x 10-4/pg TEQ/kg-d and current body 
burden and exposure levels, the background risk for the general 
population is estimated to be approximately 1 x 10-4. By 
comparison, EPA's 2000 Draft Dioxin Reassessment estimates that the 
upper bound risk for the general population exceeds 1 x 10-3 
using a revised cancer slope factor of 1 x 10-3/pg TEQ/kg-d. 
Note that actual risks for individuals are a function primarily of 
dietary habits and could be higher or lower. Thus, high-end incremental 
risk estimates for highly exposed farm families from land application 
of sewage sludge are approximately an order of magnitude (i.e., ten 
times) lower than background risks for the general population.
    These risk calculations are a function of dioxin TEQ dietary 
intake. Adult daily intakes of dioxins, furans and coplanar PCBs are 
estimated to average 65 picograms toxic equivalents per day (pg TEQ/
day) from all sources for the general population. By comparison, land 
application of sewage sludge results in an estimated incremental intake 
for a highly exposed adult farmer of 0.45 pg TEQ/day at the 50th 
percentile of exposure; 1.7 pg TEQ/day at the 75th percentile; 4.5 pg 
TEQ/day at the 90th percentile; 9.1 pg TEQ/day at the 95th percentile; 
and 19.6 pg TEQ/day at the 99th percentile. These estimates of total 
intake of dioxin for highly exposed adult farmers are calculated by 
multiplying the estimated daily exposures from land application of 
sewage sludge (in pg TEQ/kg-d; see Table 5) by an assumed adult body 
weight of 70 kg.

B. How Would the Non-Cancer Dioxin Risk from Land Application Compare 
to Background Non-Cancer Dioxin Risk?

    EPA traditionally uses a ``reference dose'' (RfD) for evaluating 
the potential for non-cancer effects for an incremental exposure that 
results from a specific source of contamination. The RfD is an estimate 
of a daily oral exposure to the human population that is likely to be 
without an appreciable risk of deleterious non-cancer effects during a 
lifetime. RfDs for a particular contaminant are a useful health 
benchmark when background exposures are low or nonexistent. Background 
exposures for dioxin-like compounds have been quantified by EPA as 
being in the range of 1 pg TEQ/kg body weight-day for adults. On a body 
burden basis, the background exposure for adults in the United States 
has been quantified at 5 ng TEQ/kg whole weight basis (USEPA, 2000a. 
See Part I, Vol. 3, Chap. 4.). The Draft Dioxin Reassessment concluded 
that traditional approaches for setting an RfD would result in an RfD 
for dioxin TEQs that is likely to be substantially below current 
background intakes. For this reason, EPA believes that establishment of 
an RfD that is below typical background exposures is uninformative in 
judging the significance of incremental exposures. Consequently, EPA 
has not developed an RfD in the Draft Dioxin Reassessment (USEPA, 
2000a. See Part III, Chap. 6.)

[[Page 40570]]

    Instead, the Draft Dioxin Reassessment promotes the concept of 
evaluating an incremental percentage increase over background approach 
for assessing potential non-cancer risk. There are two approaches to 
evaluating the incremental percent increase. One is based on dose or 
intake, and the second is based on body burden. The Draft Dioxin 
Reassessment states that body burden, rather than daily dose, is a more 
appropriate metric for quantifying risks of cancer as well as non-
cancer health effects. For long-term exposures to a steady dose (i.e., 
15-20 years or more), dose and body burden are correlated since the 
body burden will tend to approach a steady state with long term steady 
exposures. However, a short term change in dose will not result in the 
same relative change in body burden. For example, a short term elevated 
exposure to dioxin, say an exposure ten times higher on average for one 
year, will not result in a proportional increase in body burden, a ten-
fold increase in body burden in this example. However, over long 
periods of time, 20 years or more for example, a ten-fold increase in 
an average dose will result in a ten-fold increase in body burden.
    High-end incremental dioxin body burdens to the modeled highly 
exposed farm population associated with land application of sewage 
sludge are estimated to be 0.019 ng TEQ/kg body weight at the 50th 
percentile of exposure, 0.072 ng TEQ/kg body weight at the 75th 
percentile of exposure, 0.19 ng TEQ/kg body weight at the 90th 
percentile of exposure, 0.39 ng TEQ/kg body weight at the 95th 
percentile of exposure, and 0.84 ng TEQ/kg body weight at the 99th 
percentile of exposure (Lorber 2002). These body burden estimates are 
based on the estimated daily exposure from land application of sewage 
sludge for highly exposed adult farmers (see Table 5) and an assumed 
exposure time of at least 20 years. As described in the Draft Dioxin 
Reassessment, the general population body burden spans a range of 
younger to older adults. Evidence clearly indicates that older 
individuals have body burdens that are higher than younger individuals, 
mainly because of much higher exposures in past decades. The average 
body burden of younger adults is more likely to be approximately 3 ng 
TEQ/kg body weight, while the body burden of older adults would be 
higher than the overall population average of 5 ng TEQ/kg body weight. 
Women of childbearing age, a population of concern, would more likely 
have body burdens in the range of 3 ng TEQ/kg body weight. (USEPA, 
2000a. See Part I, Vol. 3, Chap. 6.). Using this background body burden 
and the high-end incremental exposures associated with land application 
of sewage sludge, the percentage increases in body burdens of dioxins 
for highly exposed adult farmers from land application of sewage sludge 
are estimated to be 0.6 percent at the 50th percentile of this modeled 
population, 2 percent at the 75th percentile, 6 percent at the 90th 
percentile,13 percent at the 95th percentile and 28 percent at the 99th 
percentile.

VIII. What Is EPA's Assessment of Effects on Ecological Species?

A. What Approach Did EPA Use for the Screening Ecological Risk Analysis 
of Dioxins in Land-Applied Sewage Sludge?

    In response to public and peer review comments EPA performed a 
screening ecological risk analysis (SERA) since the December 1999 Round 
Two proposal. The SERA uses a two-phased approach that includes (1) an 
initial bounding estimate to assess the upper bound potential for 
ecological effects at a high-end of exposure and (2) a deterministic 
assessment focused on representative ecological receptors.
    The risk measurement chosen for this SERA is the hazard quotient 
(HQ), the ratio of the exposure (in dose or concentration) to an 
ecological benchmark. Media concentrations (e.g., sediment, soil) from 
the human health risk assessment modeling simulations were used to 
predict exposure doses, and HQs were calculated on a dioxin TEQ basis. 
Calculation of HQs has a binary outcome: either the chemical 
concentration (or dose) is below the protective ecological benchmark 
(HQ<1), or it is equal to or greater than the benchmark (HQr1). Given 
the assumptions and data inputs for each stage, the HQ results are 
presumed to progress from highly uncertain and highly conservative in 
the first phase to somewhat less conservative and more certain in the 
second phase.
    Screening-level ecological risk assessments are designed to 
provide, for those chemicals and receptors that pass the screen, a high 
level of confidence that there is a low probability of adverse effects 
to ecological receptors (U.S. EPA, 2001c). The SERA was not designed or 
intended to provide definitive estimates of risk; rather, the SERA 
provides insight into the potential for ecological risk. The SERA was 
designed to be consistent with EPA's Guidelines for Ecological Risk 
Assessment (USEPA, 1998b).

B. How Did EPA Conduct the Screening Ecological Risk Analysis?

    The screening ecological risk analysis addresses the 29 dioxin 
congeners modeled in ``Exposure Analysis for Dioxins, Dibenzofurans, 
and Coplanar Polychlorinated Biphenyls in Sewage Sludge'' (USEPA, 
2002b) and was based on media concentrations generated in that 
assessment.
    The analysis phase of the SERA began with a highly conservative 
approach to determine whether any of the habitats, receptor categories, 
and exposure routes might be of concern. The second phase consisted of 
more refined deterministic analyses based on somewhat more 
representative exposure scenarios. Both phases predicted exposure doses 
and compared those estimates to ecological benchmarks (i.e., the HQ). 
HQs greater than 1 in the first phase analysis indicated that a more 
refined analysis was needed to determine whether ecological effects are 
expected.
    The exposure estimates were derived from modeled media 
concentrations generated in the human health risk assessment (USEPA, 
2002b). For the SERA, annual soil, sediment, and surface water 
concentrations were used as the basis for estimating exposure in all 
phases of the analysis. Thus, the SERA inherently assumes a one-year 
exposure duration for ecological receptors. The model calculates 
average annual exposures. We use these values as high end 
representations of exposures over the lifetimes of the evaluated 
receptors.
    Table 8 compares the values and assumptions used in each phase of 
the analysis.

   Table 8.--Values and Assumptions for the Screening Ecological Risk
                                Analysis
------------------------------------------------------------------------
                                                           Phase 2--
            Parameter              Phase 1--High end     deterministic
                                       exposures           exposures
------------------------------------------------------------------------
Cogeners addressed..............  All...............  All.
Receptors.......................  Four highly         35 representative
                                   exposed mammals     mammals and
                                   and birds.          birds.
Dietary composition.............  Diets reflecting    Representative
                                   maximum exposure.   diets.
Biouptake factors...............  Fixed values......  Fixed values

[[Page 40571]]

 
Percent of diet taken from        100%..............  100%.
 contaminated area.
Ecological benchmarks...........  NOAELs............  Maximum allowable
                                                       toxicant level,
                                                       calculated as the
                                                       geometric means
                                                       of the NOAELs and
                                                       LOAELs.
Media concentrations used to      50th and 90th       90th percentile
 estimate exposure.                percentiles and     for modeled
                                   maximum for         concentrations in
                                   sewage sludge.      environmental
                                                       media.
------------------------------------------------------------------------

    The exposure scenarios considered in the SERA include the 
agricultural application of sewage sludge in crop fields and pastures, 
silvicultural application, and application to reclaimed lands. However, 
only the agricultural application in crop fields and pastures was 
assessed quantitatively; the other scenarios were addressed 
qualitatively through comparison with agricultural application. For 
agricultural application, the SERA addressed two types of habitats. The 
first habitat consisted of receptors feeding and foraging in the 
agricultural fields where sewage sludge is applied (i.e., terrestrial 
habitat). These receptors are terrestrial vertebrates that eat the 
crops and pasture vegetation (e.g., the white-tailed deer), or that eat 
small birds and mammals that live and feed in the fields (e.g., the red 
fox). In addition, the agricultural field includes soil invertebrates 
that are exposed through direct contact with the land-applied sewage 
sludge.
    The second type of habitat consisted of receptors exposed through 
living in or feeding from nearby surface water bodies that receive 
dioxin loads through runoff (i.e., waterbody margin habitat). Aquatic 
species, such as fish and aquatic invertebrates, were assumed to be 
exposed through direct contact with dioxins in water and sediment and 
through ingesting sediment and aquatic prey items. Terrestrial species, 
such as the raccoon or the osprey, were assumed to be exposed when they 
eat aquatic prey, such as fish, mussels, and snails from contaminated 
water bodies.
    Exposure in both of these habitat types was based on the common 
characteristics of terrestrial and waterbody margin habitats, 
respectively. Exposure in waterbody margin habitats is influenced by 
variables such as water body size, position in the landscape, water 
flow rate, bed sediment composition, periodicity of flood events, and 
the presence of aquatic vegetation. Exposure in terrestrial systems is 
dependent upon many important factors such as regional location, 
vegetative cover type, wildlife community structure, and adequacy of 
food sources. While the generalized representative habitats are a 
simplification of exposure scenarios, they capture the basic elements 
characteristic of most terrestrial and waterbody margin habitats. The 
use of generalized terrestrial and waterbody margin habitats provided a 
screening-level context for this analysis.
    Given the generalized habitat types for the SERA, the exposed 
ecological species were selected based on the following criteria: (1) 
Represent all trophic levels and relevant feeding guilds (e.g., 
herbivores, carnivores), (2) represent receptors with the potential to 
be highly exposed to dioxins in land-applied sewage sludge, and (3) 
include receptors with as wide a geographic distribution as possible, 
avoiding local receptors or those with narrow ecological niches because 
sewage sludge is land applied throughout the United States. Since 
adequate data were identified only for mammals and birds, assessment 
endpoints (i.e., HQs) were quantitatively screened only for these 
wildlife species populations.
    The most significant pathway for vertebrate exposures to dioxins 
(e.g., mammals, birds, amphibians) is ingestion, and exposure/risk are 
expressed in terms of ingestion dose. Ingestion risk estimates for 
terrestrial vertebrates reflect risk to an individual in a species 
population, and risk to a population of that species is inferred 
through the selection of endpoints relevant to population viability.

C. What Are the Results of the Screening Ecological Risk Analysis?

    Each phase of the SERA was designed to provide insight into the 
potential for adverse ecological effects. Phase 1 was a high-end 
bounding analysis, and Phase 2 was a deterministic analysis based on 
somewhat more representative exposure parameters and somewhat less 
protective benchmarks. In the Phase 1 analysis, HQ values greater than 
1 were calculated, indicating that a more refined analysis was needed.
    For the Phase 2 analysis, no HQ values exceeded the target HQ of 1; 
values range from a minimum of 0.0035 (Canada goose) to a maximum value 
of 0.36 (short-tailed shrew). The median HQ for the receptors assigned 
to waterbody margin habitats was 0.015, and the median HQ for receptors 
assigned to terrestrial habitats was 0.044, indicating that the 
potential for effects on terrestrial receptors may be somewhat higher 
than for receptors in waterbody margin habitats. The results of the 
Phase 2 analysis are summarized below in Table 9.

    Table 9.--Phase 2 Results for Screening Ecological Risk Analysis
------------------------------------------------------------------------
                                    HQ: Terrestrial      HQ: Waterbody
             Species                   habitats         margin habitats
------------------------------------------------------------------------
American kestrel................  3.5E-02...........  not assigned.
American robin..................  1.2E-02...........  not assigned.
American woodcock...............  1.8E-01...........  not assigned.
Bald eagle......................  not assigned......  2.8E-03.
Beaver..........................  not assigned......  2.5E-02.
Belted kingfisher...............  not assigned......  9.0E-03.
Black bear......................  8.1E-02...........  not assigned.
Canada goose....................  3.5E-03...........  not assigned.
Cooper's hawk...................  2.9E-02...........  not assigned.
Coyote..........................  2.2E-01...........  not assigned.
Deer mouse......................  3.0E-01...........  not assigned.

[[Page 40572]]

 
Eastern cottontail rabbit.......  4.4E-02...........  not assigned.
Great blue heron................  not assigned......  3.5E-03.
Green heron.....................  not assigned......  6.3E-03.
Herring gull....................  not assigned......  8.8E-03.
Least weasel....................  1.6E-01...........  not assigned.
Lesser scaup....................  not assigned......  2.1E-02.
Little brown bat................  6.2E-02...........  not assigned.
Long-tailed weasel..............  2.2E-01...........  not assigned.
Mallard.........................  not assigned......  1.0E-02.
Meadow vole.....................  1.7E-02...........  not assigned.
Mink............................  not assigned......  2.3E-02.
Muskrat.........................  not assigned......  8.1E-02.
Northern bobwhite...............  1.3E-02...........  not assigned.
Osprey..........................  not assigned......  3.6E-03.
Prairie vole....................  2.3E-02...........  not assigned.
Raccoon.........................  4.4E-02...........  1.3E-01.
Red fox.........................  1.7E-01...........  not assigned.
Red-tailed hawk.................  1.9E-02...........  not assigned.
River otter.....................  not assigned......  2.6E-02.
Short-tailed shrew..............  3.6E-01...........  not assigned.
Short-tailed weasel.............  1.8E-01...........  not assigned.
Tree swallow....................  2.8E-02...........  not assigned.
Western meadowlark..............  1.7E-02...........  not assigned.
White-tailed deer...............  6.1E-02...........  not assigned.
------------------------------------------------------------------------

    As noted above sewage sludge application to reclaimed lands and 
silvicultural application of sewage sludge were addressed qualitatively 
through comparison with agricultural application. In general, 
reclamation and silviculture applications of sewage sludge are not well 
characterized. Reclamation applications can consist of spreading sewage 
sludge on reformed land surfaces as an amendment to support re-
vegetation or as fill material deposited in excavations. In the former 
case, some tilling may occur with landscaping operations; for the 
latter case, tilling is unlikely. In either case, the dioxins would be 
expected to bind to soil particles and to display fate and transport 
behavior similar to that in agricultural fields. While the application 
rates and frequency are not necessarily comparable, ecological 
exposures are likely to occur in a manner similar to that for 
agricultural fields. Terrestrial vertebrates feeding at reclaimed sites 
would generally be similar to those in an agricultural setting. 
Receptors and pathways of exposure through aquatic systems would also 
be expected to be similar to those modeled in the SERA.
    For silvicultural application of sewage sludge, EPA assumed that 
sewage sludge is land-applied once per site. Tilling is less likely to 
occur except in reforestation projects where site preparation for new 
plantings could include tilling of sewage sludge into the soil. Many of 
the avian and mammalian species considered in the agricultural analysis 
for the field habitat are also expected to feed and forage in forests 
and, therefore, the screening results for field habitats are considered 
relevant to the forest habitats. Although there are forest species that 
are not represented in the agricultural scenario, the major trophic 
elements are substantially represented. For these reasons, EPA believes 
that the results of the SERA also provide a useful indicator for the 
potential for adverse ecological effects at reclamation and 
silvicultural sites.
    Finally, EPA notes the following considerations that should be 
recognized due to the screening nature of this analysis:
     Because the screening methodology is based on the 
exceedance of a target HQ of 1, the outcome of the screen is binary: HQ 
< 1 or HQ r 1. Although large exceedances suggest a greater potential 
for ecological damage, an HQ of 50 is not necessarily five times worse 
than an HQ of 10.
     The potential for adverse ecological effects (as indicated 
by an HQ exceedance) should not be confused with the ecological 
significance of those effects. Regardless of the magnitude of an HQ 
exceedance, screening results can only suggest ecological damage; they 
do not demonstrate actual ecological effects, nor do they indicate 
whether those effects will have significant implications for ecosystems 
and their components.
     Ecological receptors for the screening methodology were 
chosen to represent relatively common species populations. Threatened 
and endangered species and/or habitats were not included in the 
analysis because a different type of spatial resolution would have been 
required (i.e., co-occurrence of threatened and endangered species/
habitats with sewage sludge application sites). Consequently, the 
screening results do not indicate whether endangered species/habitats 
are at risk.
    EPA requests comments on the methodology and data used for the 
screening ecological risk assessment. The Agency also requests comments 
on the results derived from the screening ecological risk analysis 
summarized above.

IX. How Might the New Data and Revised Risk Assessment Affect EPA's 
Proposed Dioxin Concentration Limit for Land-Applied Sewage Sludge and 
the Proposed Monitoring Requirements?

A. Possible Implications for Proposed Concentration Limit for Land-
Applied Sewage Sludge

    As indicated above, the revised risk assessment (probabilistic) for 
land application of sewage sludge estimates that the high-end 
individual excess lifetime risk to the highly exposed modeled 
population using the current cancer slope factor could range from 2 x 
10-5 to 1 x 10-6 (``two in one-hundred thousand'' 
to ``one in one million'') for exposure by multiple pathways. Use of 
the cancer slope factor being considered in the 2000 Draft Dioxin 
Reassessment would result in

[[Page 40573]]

estimated high-end multi-pathway lifetime cancer risks ranging from 1.2 
x 10-4 to 6 x 10-6 for this same highly exposed 
modeled population. By comparison, the risk assessment for the December 
1999 proposal (which used a deterministic methodology and a number of 
different assumptions; see Section VI.D. of this Notice), estimated a 
high-end cancer risk of 1.7 x 10-5 (USEPA, 1999b). As noted 
in the December 1999 proposal, the Agency considers risks in the range 
of 1 x 10-6 to 1 x 10-4 (``one in one million'' 
to ``one in ten thousand'') to be acceptable levels of risk. The 
revised high-end risk estimates continue to fall within this range of 
acceptable risks. The revised risk assessment also shows no measurable 
change in risk from requiring all sewage sludge to meet a 300 ppt TEQ 
limit.

B. Effect on Proposed Monitoring Requirements

    In the December 1999 proposal, the Agency proposed two alternative 
monitoring schedules based on the level of dioxins in sewage sludge to 
be land applied. Specifically, treatment works and other sewage sludge 
preparers that measure the level of dioxin in their sewage sludge to be 
between 300 ppt TEQ and 30 ppt TEQ would be required to monitor 
annually. Treatment works and sewage sludge preparers that measure 
dioxin levels of 30 ppt TEQ or less for two consecutive years would be 
required to monitor every five years thereafter. The proposed 
monitoring schedule was based on the Agency's assumption that the level 
of dioxins in sewage sludge, both nationally and from specific sources, 
is relatively constant over time and may be decreasing. The Agency 
noted that since the concentration of 30 ppt TEQ which would allow less 
frequent monitoring is a full order of magnitude less than the proposed 
numeric standard of 300 ppt TEQ (i.e., one-tenth), the chances that 
such a sewage sludge would exceed the limit are small. Furthermore, the 
Agency noted that any health risks associated with dioxin exposure from 
land application of sewage sludge at these levels would require long-
term exposure (i.e., significantly greater than five years) to 
potentially present unreasonable health risks.
    As noted in Section V.H. of this Notice, the EPA 2001 dioxin update 
survey indicates that dioxin levels in sewage sludge appear to have 
decreased from 1988 to 2001. The new data also indicate that for most 
POTWs, dioxin levels appear to not fluctuate greatly over time. 
However, the sewage sludge samples which had the highest levels of 
dioxins in either the 1988 NSSS or 2001 EPA update survey appeared to 
evidence greater fluctuations in dioxin concentrations than the other 
sewage sludges. As also previously noted, the data for facilities where 
monthly data were available indicate that dioxin concentrations tend to 
corroborate these observations from the EPA 2001 dioxin update survey. 
The data for the facilities where monthly data were available indicate 
that the dioxin concentrations are relatively consistent over time on a 
month-to-month basis, but the variability appeared the greatest for the 
facility with the highest dioxin concentrations measured in its sewage 
sludge (see Section V.K.).
    The Agency continues to believe that if it sets a dioxin limit of 
300 ppt TEQ, this two-tier monitoring schedule in line with the 
December 1999 proposal may be appropriate. For facilities where longer 
term monitoring data was available, the maximum monthly concentration 
of dioxin was within a factor of two to four times the average 
concentration for that facility. By comparison, the proposed monitoring 
schedule would allow reduced monitoring frequency only when two 
consecutive measurements were a factor of ten less than the specified 
limit. Furthermore, no POTWs in the EPA 2001 dioxin update survey had 
consistently high levels of dioxins in their sewage sludge; and the 
revised risk assessment predicts that even long term exposure to 
dioxins in land-applied sewage sludge would result in negligible 
increases in risk.
    Based on the data from the EPA 2001 dioxin update survey, 
approximately 31 percent of POTWs produce sewage sludge with dioxin 
levels between 30 ppt TEQ and 300 ppt TEQ (USEPA, 2002a). These POTWs 
would be required to monitor annually for dioxin under the proposed 
monitoring schedule if their sewage sludge is land applied. (By 
comparison, approximately 61 percent of POTWs previously were estimated 
to produce sewage sludge with dioxin levels between 30 ppt TEQ and 300 
ppt TEQ based on the data available to EPA at the time of the December 
1999 proposal (USEPA, 1999d).)
    The costs associated with monitoring for dioxin annually at 
facilities with sewage sludge concentrations between 30 ppt TEQ and 300 
ppt TEQ previously was estimated to be $1,224,000 based on the sewage 
sludge dioxin data available to EPA at the time of the December 1999 
proposal (USEPA, 1999d). EPA now estimates the costs associated with 
monitoring for dioxin annually at facilities with sewage sludge dioxin 
concentrations between 30 ppt TEQ and 300 ppt TEQ would be 
approximately $656,000 (USEPA, 2002d).
    Based on the new data, EPA is considering whether alternatives to 
the proposed monitoring scheme would be more appropriate. Because the 
data continue to show periodic ``spikes,'' and the data indicates that 
these higher levels of dioxin may not remain for long periods of time, 
a different monitoring schedule may be indicated. Similarly, the data 
indicates that sewage sludge with lower levels of dioxins may not 
fluctuate as greatly, which may indicate a different threshold or 
monitoring frequency than those proposed. For example, monitoring every 
two years rather than annually; or at some other interval may be more 
appropriate.
    The percentage of land-applied sewage sludge which would have to be 
monitored annually would be reduced if the threshold for annual dioxin 
monitoring was set at a higher concentration than 30 ppt TEQ. Likewise, 
the percentage of land-applied sewage sludge which would have to be 
monitored annually would be greater if the threshold for annual dioxin 
monitoring was set at a lower concentration than 30 ppt TEQ. As an 
example, 13 percent of POTWs produce sewage sludge between 50 ppt TEQ 
and 300 ppt TEQ based on data from the EPA 2001 dioxin update survey 
(USEPA, 2002a). This compares to 31 percent of POTWs with sewage sludge 
dioxin concentrations between 30 ppt TEQ and 300 ppt TEQ, as noted 
above.
    The Agency requests comments on the proposed monitoring schedule 
and the threshold concentration of dioxin that would allow for more or 
less frequent monitoring. Specifically, EPA requests comments on 
whether other schedules which would require more or less frequent 
monitoring would be more appropriate. EPA also requests comment on 
whether a monitoring requirement in lieu of a numeric limit should be 
considered.

X. How Might the New Data and Revised Risk Assessment Affect EPA's 
Proposal for Small Entities?

    EPA proposed to exclude from the proposed land application 
requirements relating to dioxins, sewage treatment works with a 
wastewater flow of one MGD or less and sewage sludge-only entities 
which prepare 290 dry metric tons or less of sewage sludge annually for 
land application. (EPA estimates that a one MGD treatment works 
produces approximately 290 dry metric tons of sewage sludge annually.) 
Sewage sludge from these small preparers would be excluded from the 
limitation on dioxins

[[Page 40574]]

in sewage sludge. Such preparers could continue to land apply their 
sewage sludge with no further restriction due to the sewage sludge's 
dioxin content.
    The December 1999 proposal indicated that EPA believes that this 
exclusion is appropriate for several reasons. First, less than eight 
percent of the total sewage sludge that is land applied is produced by 
sewage treatment works with flow rates of one MGD or less (USEPA, 
1990). Second, the probability that this small amount of sewage sludge 
(i.e., 42 dry metric tons per facility annually, which is the average 
amount of sewage sludge produced by POTWs less than one MGD) could 
unreasonably increase health risks for any individual is extremely 
small. EPA specifically requested comment on the Agency's proposal to 
exclude small preparers from any requirements relating to dioxins in 
sewage sludge to be land applied.
    The new data that EPA collected on the levels of dioxins found in 
sewage sludge (USEPA, 2002a) and the revised land application risk 
assessment (USEPA, 2002b), provide additional information which the 
Agency believes supports the proposal to exclude sewage treatment works 
with a wastewater flow of one MGD or less and sewage sludge-only 
entities which prepare 290 dry metric tons or less of sewage sludge 
annually for land application.
    The levels of dioxins in sewage sludge from treatment works with a 
wastewater flow of one MGD or less was measurably less than the levels 
of dioxins in sewage sludge from facilities with a wastewater flow 
greater than one MGD (USEPA, 2002a). The highest observed level of 
dioxins from treatment works with a wastewater flow of one MGD or less 
was 78.6 ppt TEQ. This compares to the highest observed value of 718 
ppt TEQ for dioxins for facilities with a wastewater flow greater than 
one MGD. The average (mean) and 95th percentile values dioxins for 
treatment works with a wastewater flow of one MGD or less also were 
measurably less compared to treatment works with flows greater than one 
MGD: 26.5 ppt TEQ and 67.1 ppt TEQ, respectively for treatment works 
with a wastewater flow of one MGD or less compared to 44.1 ppt TEQ and 
94.8 ppt TEQ, respectively for treatment works with a wastewater flow 
greater than one MGD.
    The revised risk assessment methodology does not allow EPA to make 
a separate risk estimate for treatment works with wastewater flows of 
one MGD or less because, other than the dioxin levels in sewage sludge 
discussed above, there are no relevant factors considered in the risk 
assessment which vary specifically based on the capacity of the 
treatment works . However, the Agency believes the revised risk 
assessment provides further indication that the minimal amounts of 
sewage sludge from treatment works with wastewater flows of one MGD or 
less would be very unlikely to produce an unreasonable increase in 
health risks for any individual.
    The revised risk assessment estimates that the high-end incremental 
adult lifetime risk for highly exposed farm families associated with 
dioxins in land-applied sewage sludge ranges from 4 x 10-5 
at the 99th percentile to 1 x 10-6 at the 50th percentile, 
which equates to less than 0.006 cancer cases annually. The key 
variable in this risk estimate that can be related to treatment 
facility size is the distribution of farm sizes to which the sewage 
sludge is land-applied. The revised risk assessment used a distribution 
of median farm sizes for 41 meteorologic regions ranging from 24.2 
acres to 1241.7 acres (USDA, 1997). For this distribution, the average 
farm size is 487 acres and the median farm sizes is 120 acres. By 
comparison, the average amount of sewage sludge produced by a treatment 
works with a wastewater flow of one MGD or less (i.e., 42 dry metric 
tons annually) would be applied to approximately 10 acres of farmland 
when applied at agronomic rates (i.e., 4 metric tons per acre 
annually). Thus, the acreage impacted by treatment works with a 
wastewater flows of one MGD is significantly less than that which would 
result in an estimated risk of 1 x 10-6. On this basis, EPA 
believes that the amount of sewage sludge produced by treatment works 
with a wastewater flow of one MGD or less is not sufficient to result 
in an unreasonable risk to potentially exposed populations. Again, EPA 
specifically invites comment on the Agency's proposal to exclude small 
entities from any limit for dioxins in sewage sludge to be land 
applied.

XI. How Does the New Data and Revised Risk Assessment Affect EPA's Cost 
Estimates?

    As noted in the December 1999 proposal, the increased costs which 
would be imposed by the proposed regulation are the costs for initially 
monitoring for dioxins by all land-applying treatment works greater 
than one MGD, annual monitoring at those facilities with dioxin levels 
between 30 ppt TEQ and 300 ppt TEQ, and switching to co-disposal with 
municipal solid waste for current land appliers whose sewage sludge 
contains over 300 ppt TEQ of dioxins. The Agency assumed that the cost 
of measuring dioxins in sewage sludge is $2,000 per sample and the cost 
to switch to co-disposal with municipal solid waste was $189 per dry 
metric ton in 1998 dollars. For the proposal, EPA estimated that the 
annualized cost of this regulation nationwide would be approximately 
$18 million. Of this amount, 13 percent was for monitoring, and the 
balance is for switching use or disposal practices (USEPA, 1999d).
    EPA has updated these cost estimates (USEPA 2002d). The Agency 
assumes that the cost to switch to co-disposal with municipal solid 
waste has risen to $197 per dry metric ton in year 2000 and that the 
cost of measuring dioxins in sewage sludge remains at $2,000 per 
sample. On this basis, EPA now estimates that the annualized cost of 
this regulation Nationwide would be approximately $4.5 million if the 
dioxin limit for land application of sewage sludge is 300 ppt TEQ. The 
decrease in the estimated cost results from the smaller percentage of 
sewage sludge that would exceed a 300 ppt TEQ dioxin limit based on the 
data from the EPA 2001 dioxin update survey (i.e., 1% vs. 5%). The 
estimated benefits of a 300 ppt limit would be very low, since such a 
limit would not likely produce a detectable change in lifetime cancer 
risk, even to highly exposed farm families and using conservative 
assumptions, and no species in the SERA has a HQ above 1, even in the 
baseline with no limits.
    XII. Identification and Control of Dioxin Sources that Contribute 
to Elevated Dioxin Levels in Sewage Sludge.
    Both the EPA 2001 dioxin update survey and the 2001 AMSA Survey 
found a small percentage of sewage sludge samples with dioxin 
concentrations which were significantly higher than most of the other 
the sewage sludge samples in the survey. The EPA 2001 dioxin update 
survey found only 1 percent of the samples with a dioxin concentration 
greater than 100 ppt TEQ (compared to an average (mean) of 31.6 ppt 
TEQ). The AMSA 2001 survey found less than 5 percent of the samples 
analyzed in their survey with a dioxin concentration greater than 100 
ppt TEQ (compared to an average (mean) of 48.6 ppt TEQ.)
    Even though relatively few sewage sludge samples have elevated 
concentrations of dioxins, those that do can have levels which are much 
higher than the values typically observed. The highest dioxin 
concentration measured in the 2001 EPA and AMSA surveys were 718 ppt 
TEQ and 3,590 ppt TEQ,

[[Page 40575]]

respectively. In addition, as discussed previously in this Section of 
today's notice, higher levels of dioxins in sewage sludge appear to be 
transient and may not be consistently identified. While the revised 
risk assessment shows no measurable change in the risk from eliminating 
these spikes to individuals exposed through land application of sewage 
sludge, the Agency believes it may be beneficial to develop a procedure 
to identify the sources contributing to higher levels of dioxins in 
sewage sludges. Relatively high levels of dioxin in sewage sludge may 
be an indication of sources in the treatment works' service area with 
even higher levels of dioxins.
    The Agency is requesting comments on a methodology to assist 
communities in identifying sources of elevated dioxins in their sewage 
sludge. This methodology relies on two complementary elements to 
identify sources of dioxin: (1) Identification of sources known to be 
generators or sinks for dioxin (e.g., specific chemical manufacturing 
operations, combustion sources or contaminated landfills); and (2) 
comparison of the mix of the 29 dioxin congeners measured in a 
particular sewage sludge sample to the ``fingerprint'' of 29 dioxin 
congeners for known sources of dioxins. The methodology would be used 
by communities to reduce levels of dioxins in their sewage sludge by 
eliminating these sources of dioxins from the collection system or 
remediating contaminated sites.
    The first element of this methodology is identification of local 
industrial, commercial and other sources with inputs to municipal 
sanitary sewers which have a potential to contain significant levels of 
dioxins. The primary database used to make these identifications would 
be the Agency's updated 2001-2002 Toxics Release Inventory. The Toxics 
Release Inventory is a valuable source of nationwide information 
regarding toxic chemicals that are being used, manufactured, treated, 
transported or released into the environment. Toxics Release Inventory 
data includes the local discharges of chemicals to sanitary sewers by 
industrial and commercial establishments. Other potential local sources 
of dioxins in sewage sludge include leachate from landfills, 
contaminated manufacturing and disposal sites, and scrubber water from 
combustion operations.
    Identification of possible sources of dioxins in sewage sludge also 
will be aided by reviewing data available from local pretreatment 
programs and the results of detailed studies conducted in any 
communities which have attempted to identify sources of dioxins in 
their sewage sludge. Industry listings for local pretreatment programs 
will be reviewed to determine which are likely sources of elevated 
dioxins in sewage sludge. With respect to community-specific studies, 
EPA has received information which indicates that elevated 
concentrations of dioxins in the sewage sludge may be due to non-point 
source contamination. Non-point source contamination comes from 
erodible soils that contain elevated levels of dioxins and periodically 
enter either sanitary sewers as a result of infiltration during 
precipitation, or combined sewers through normal stormwater flows.
    The second element of a methodology to identify sources which 
contribute to elevated dioxins in sewage sludge is to compare the mix 
of dioxin congeners in a particular sewage sludge to the mix of dioxin 
congeners in known sources of dioxins. Mixtures of the 29 congeners of 
dioxins have distinct patterns (profiles or ``fingerprints'') of 
relative proportions for each of the congener classes (i.e., dioxins, 
dibenzofurans and coplanar PCBs) depending on the source of dioxins. 
For example, dioxins produced by combustion have a different 
``fingerprint'' than dioxins produced by chemical processes such as 
pulp and paper mill bleaching with chlorine or pentachlorophenol 
manufacturing. By examining these congener ``fingerprints'', it is 
possible to identify likely manufacturing, chemical or combustion 
processes that produced that particular profile. Dioxin congener 
profiles from the sewage sludge samples with elevated dioxin 
concentrations from the 2001 EPA and AMSA surveys will be compared 
against known dioxin profiles of samples from various manufacturing, 
chemical and combustion and chemical processes. These comparisons can 
be used in the source identification portion of the methodology 
described above.
    EPA is inviting comments on this overall methodology to identify 
and reduce or eliminate sources of dioxins entering wastewater 
treatment plants that contribute to elevated levels of dioxins in 
sewage sludge. In particular, comments are invited on the two phase 
approach to identify these sources described above. Note that EPA is 
not proposing use of this methodology in a regulatory context, but 
rather developing it as a tool for use by POTWs and/or communities on a 
voluntary basis.

XIII. Request for Public Comments

    While EPA is requesting comments on all of the information 
discussed in this Notice, the Agency hopes that public comments will 
also focus specifically on the following aspects of this Notice:
    (1) The significance of the differences in dioxin concentrations in 
sewage sludge measured at facilities with wastewater flows greater than 
one MGD compared to dioxin concentrations in sewage sludge at 
facilities with wastewater flows less than one MGD (V.G.).
    (2) The significance of the differences in dioxin concentrations in 
sewage sludge measured in the EPA 2001 dioxin update survey compared to 
dioxin concentrations in sewage sludge measured in the 1988 NSSS 
(V.H.).
    (3) Choice of the highly exposed farm family as the modeled 
population for the revised risk assessment and the assumptions related 
to this choice of modeled population. (VI.D.).
    (4) All of the assumptions related to exposure, fate and transport 
used in the revised risk assessment , including the specific 
assumptions related to the farming and grazing practices used by the 
modeled farm family (VI.D.),
    (5) The treatment of non-detects in the revised risk assessment and 
the effect on estimating risk (VI.E.).
    (6) The assumptions and values used to estimate how much dioxins 
are being transported to individuals in the modeled farm family (e.g., 
the sources [store-bought versus farm-produced], types and dioxin 
contamination levels of poultry feeds.) (VI.F.)
    (7) The methodology and data used for the screening ecological risk 
assessment (VIII.A. and VIII.B); and the results derived from the 
screening ecological risk analysis (VIII.C.).
    (8) The significance of the finding that setting a 300 ppt TEQ 
limit would make no detectable difference in the risk of cancer to the 
highly exposed farm family.
    (9) Taking no action with respect to regulating dioxins for land 
application (IX.).
    (10) The proposed monitoring schedule and the threshold 
concentration of dioxin that would allow for less frequent monitoring, 
and specifically, on whether other schedules which would require more 
or less frequent monitoring would be more appropriate (IX.).
    (11) Excluding small entities from the limit for dioxins in sewage 
sludge to be land applied (X.).
    (12) A methodology to assist communities in voluntarily identifying 
and reducing or eliminating sources of dioxins entering wastewater 
treatment plants that contribute to elevated levels of dioxins in 
sewage sludge (XII.).

[[Page 40576]]

XIV. List of References

AMSA 2001. The AMSA 2000/2001 Survey of Dioxin-like Compounds in 
Biosolids: Statistical Analyses
Green, et al. 1995. Comments on Estimating Exposure to Dioxin-Like 
Compounds: Review Draft, Jan. 12, 1995. 204 pp. Addendum. May 11, 1995. 
23 pp.
Lorber, M.N., 2002. Evaluating Non-Cancer Risk from Land Application of 
Sewage Sludge Using an Increment Over Background Approach. Memorandum 
from Matthew Lorber, National Center for Environmental Assessment, 
Office of Research and Development, USEPA, Washington, DC to Alan B. 
Hais, Health and Ecological Criteria Division, Office of Science and 
Technology, Office of Water, USEPA, Washington, DC. April, 2002.
USDA, 1997. Census of Agriculture. Washington, DC.
USEPA, 1985. Health Assessment Document for Polychlorinated Dibenzo-p-
Dioxins. EPA/600/8-84/014F. Final Report. Office of Health and 
Environmental Assessment. Washington, DC September, 1985.
USEPA, 1989. Interim Procedures for Estimating Risks Associated with 
Exposure to Mixtures of Chlorinated Dibenzo-p-dioxins and -
dibenzofurans (CDDs and CDFs) and 1989 Update. EPA/625/3-89/016. Risk 
Assessment Forum. Washington, DC March 1989.
USEPA, 1990. National Sewage Sludge Survey; Availability of Information 
and Data, and Anticipated Impacts on Proposed Regulations; Proposed 
Rule. Federal Register 55 (218): 47210-47283.
USEPA, 1992. Guidelines for Exposure Assessment, EPA/600Z-92/001, 
National Center for Environmental Assessment, Washington, DC.
USEPA, 1994a. Health Assessment for 2,3,7,8-TCDD and Related Compounds. 
External Review Draft. EPA/600/BP-92/001a-c, ( Vol. I: 420 pp., Vol. 
II: 685 pp., Vol. III: 125 pp.) and Estimating Exposure to Dioxin-Like 
Compounds. Volume I. Executive Summary. 128 pp. Volume II. Properties, 
Sources, Occurrence, and Background Exposures 424 pp. + 260 pp. Volume 
III. Site-Specific Assessment Procedures. 452 pp. External Review 
Draft. EPA/600/6-88/005Ca-c. National Center for Environmental 
Assessment. Washington, DC.
USEPA, 1994b. EPA Method 1613: Dioxins and Furans by Isotope Dilution 
High-resolution Gas Chromatography/ Mass Spectrometry, Revision B (EPA 
821-B-94-005, October 1994.
USEPA, 1997. Exposure Factors Handbook. National Center for 
Environmental Assessment. Washington, DC EPA/600/P-95/002F(a-c). Vol. 
I: 208 pp. Vol. II: 336 pp. Vol. III: 340 pp. Also available at NTIS 
(Vol. I PB98-124225, Vol. II PB98-124233, Vol. III PB98-124241, The Set 
PB98-124217). See also http://www.epa.gov/ncea/exposfac.htm
USEPA, 1998a. Methodology for Assessing Health Risks Associated with 
Multiple Pathways of Exposure to Combustion Emissions. EPA/600/P-98/
137. Washington, DC.
USEPA, 1998b. Guidelines for Ecological Risk Assessment (Final). EPA/
630/R-95/002F. Risk Assessment Forum. Washington, DC.
USEPA, 1999a. EPA Method 1668: Polychlorinated Biphenyls by Isotope 
Dilution High-resolution Gas Chromatography/Mass Spectrometry, Revision 
A , EPA-821-R-00-002, December 1999).
USEPA, 1999b. Risk Analysis for the Round Two Biosolids Pollutants. 
Office of Science and Technology. Washington, DC.
USEPA, 1999c. Biosolids Generation, Use, and Disposal in the United 
States. EPA 530-R-99-009. Office of Solid Waste and Emergency Response. 
Washington, DC.
USEPA, 1999d. Costs Associated with Regulating Dioxins, Furans, and 
PCBs in Biosolids. Office of Science and Technology. Washington, DC.
USEPA, 2000a. Exposure and Human Health Reassessment of 2,3,7,8-
Tetrachlorodibenzo-p-Dioxin (TCDD) and Related Compounds. Parts I-III. 
Draft. Prepared by the National Center for Environmental Assessment, 
Office of Research and Development. Washington, DC (EPA/600/P-00/001 
Bb, Bc, Bd, Be, Bg). Available online at http://www.epa.gov/ncea.
USEPA, 2000b. Risk Characterization Handbook, EPA 100-B-00-002, Science 
Policy Council, Washington, DC.
USEPA, 2001a. Sampling Procedures for the 2001 National Sewage Sludge 
Survey, Office of Science and Technology, Washington, DC.
USEPA, 2001b. Analytical Data for Dioxins in Sewage Sludge Submitted by 
Three Wastewater Treatment Plants, Office of Science and Technology, 
Washington, DC.
USEPA, 2001c. The Role of Screening-Level Risk Assessments and Refining 
Contaminants of Concern in Baseline Ecological Assessments. EPA ECO 
Update, Publication 9345.0-14. EPA/540/F-01/014. Office of Solid Waste 
and Emergency Response, U.S. EPA, Washington, DC.
USEPA, 2002a. Statistical Support Document for the Development of Round 
2 Biosolids Use or Disposal Regulations , Office of Science and 
Technology, Washington, DC.
USEPA, 2002b. Exposure Analysis for Dioxins, Dibenzofurans, and 
Coplanar Polychlorinated Biphenyls in Sewage Sludge-Technical 
Background Document, Office of Science and Technology, Washington, DC.
USEPA, 2002c. Estimate of Population Exposed to Dioxins from the Land 
Application of Sewage Sludge and Corresponding Number of Annual Cancer 
Cases from this Exposure, Office of Science and Technology, Washington, 
DC.
USEPA, 2002d. Costs Associated with Regulating Dioxins, Furans, and 
PCBs in Biosolids. Office of Science and Technology. Washington, DC.
Van den Berg M, et al. 1998. Toxic Equivalency Factors (TEFs) for PCBs, 
PCDDs, and PCDFs for Humans and Wildlife. Environ. Health Perspect. 
106(12): 775-792.

    Dated: June 5, 2002.
G. Tracy Mehan III,
Assistant Administrator for Water.
[FR Doc. 02-14761 Filed 6-11-02; 8:45 am]
BILLING CODE 6560-50-P