[Federal Register Volume 89, Number 141 (Tuesday, July 23, 2024)]
[Rules and Regulations]
[Pages 59623-59645]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2024-15807]


-----------------------------------------------------------------------

ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 141

[EPA-HQ-OW-2023-0572; FRL 7946-01-OW]


National Primary Drinking Water Regulations; Announcement of the 
Results of EPA's Fourth Review of Existing Drinking Water Standards

AGENCY: Environmental Protection Agency (EPA).

ACTION: Results of regulatory review.

-----------------------------------------------------------------------

SUMMARY: The Safe Drinking Water Act (SDWA) requires the U.S. 
Environmental Protection Agency (EPA or the agency) to conduct a review 
every six years of existing national primary drinking water regulations 
(NPDWRs) and determine which, if any, are appropriate for revision. The 
purpose of the review, called the Six-Year Review, is to evaluate 
available information for regulated contaminants to determine if any 
new information on health effects, treatment technologies, analytical 
methods, occurrence, exposure, implementation, and/or other factors 
provides a basis to support a regulatory revision that would improve or 
strengthen public health protection. While EPA has recently completed 
several significant revisions to existing regulations and other 
regulatory revisions are currently underway, based on this periodic 
review of all NPDWRs, there are no additional candidates for regulatory 
revision at this time.

DATES: July 23, 2024.

ADDRESSES: EPA is not accepting public comment on the review results.

FOR FURTHER INFORMATION CONTACT: Samuel Hernandez, Environmental 
Protection Agency, Office of Ground Water and Drinking Water, Standards 
and Risk Management Division, (Mail Code 4607M), 1200 Pennsylvania 
Avenue NW, Washington, DC 20460; telephone number: (202) 564-1735; 
email address: [email protected].

SUPPLEMENTARY INFORMATION: 
    Abbreviations and acronyms: The following acronyms and 
abbreviations are used throughout this document.

2,4-D--2,4-Dichlorophenoxyacetic acid
ADWR--Aircraft Drinking Water Rule
BAT--Best Available Technology
CFR--Code of Federal Regulations
CVOC--Carcinogenic Volatile Organic Contaminant
CWS--Community Water System
DBCP--1,2-Dibromo-3-Chloropropane
DBP--Disinfection Byproduct
DEHA--Di(2-ethylhexyl)adipate
DEHP--Di(2-ethylhexyl)phthalate
EPA--U.S. Environmental Protection Agency
EQL--Estimated Quantitation Level
FBRR--Filter Backwash Recycling Rule
GWR--Ground Water Rule
HAA5--Haloacetic Acids (five) (sum of monochloroacetic acid, 
dichloroacetic acid, trichloroacetic acid, monobromoacetic acid, and 
dibromoacetic acid)
ICR--Information Collection Request
IRIS--Integrated Risk Information System
LT2--Long-Term 2 Enhanced Surface Water Treatment Rule
MCLG--Maximum Contaminant Level Goal
MCL--Maximum Contaminant Level
MDBP--Microbial and Disinfection Byproduct
MDL--Method Detection Limit
MRDLG--Maximum Residual Disinfectant Level Goal
MRDL--Maximum Residual Disinfectant Level
MRL--Minimum Reporting Level
NAS--National Academy of Sciences
NCWS--Non-Community Water System
NDWAC--National Drinking Water Advisory Council
NPDWR--National Primary Drinking Water Regulations
NRC--National Research Council
NTP--National Toxicology Program
PCBs--Polychlorinated biphenyls
PCE--Tetrachloroethylene
PQL--Practical Quantitation Limit
PT--Proficiency Testing
PWS--Public Water System
RfD--Reference Dose
RSC--Relative Source Contribution
RTCR--Revised Total Coliform Rule
SDWA--Safe Drinking Water Act
SDWIS--Safe Drinking Water Information System
SWTR--Surface Water Treatment Rule
TCDD--Tetrachlorodibenzo-p-dioxin
TCE--Trichloroethylene
TCR--Total Coliform Rule
TNCWS--Transient Non-Community Water System
TTHM--Total Trihalomethanes (sum of four THMs: chloroform, 
bromodichloromethane, dibromochloromethane, and bromoform)
TT--Treatment Technique
USGS--U.S. Geological Survey

Table of Contents

I. General Information
    A. Does this action apply to me?
    B. How can I get copies of this document and other related 
information?
II. Statutory Requirements for the Six-Year Review
III. Regulations Included in the Six-Year Review 4
IV. EPA's Protocol for Reviewing the NPDWRs Included in This Action
    A. What was EPA's review process?
    B. How did EPA conduct the review of the NPDWRs?
    1. Initial Review
    2. Health Effects

[[Page 59624]]

    3. Analytical Feasibility
    4. Occurrence and Exposure Analysis
    5. Treatment Feasibility
    6. Risk-Balancing
    7. Other NPDWR Revisions
V. Results of EPA's Review of NPDWRs
    A. Overview of Six-Year Review 4 Results
    B. Chemical Phase Rules/Radionuclides Rules
    1. Key Review Outcomes
    2. Summary of Review Results
    3. Select NPDWRs with New Information Not Appropriate for 
Revision
    C. Microbial Contaminants Regulations
VI. References

I. General Information

A. Does this action apply to me?

    This action itself does not impose any requirements on individual 
people or entities. Instead, it notifies interested parties of EPA's 
review of existing national primary drinking water regulations (NPDWRs) 
and its conclusions about which of these NPDWRs may warrant regulatory 
revisions at this time. The Six-Year Review is not a final regulatory 
decision to revise or not revise an NPDWR, but rather a planning 
process that involves more detailed analyses of factors relevant to 
deciding whether a rulemaking to revise an NPDWR should be initiated.

B. How can I get copies of this document and other related information?

    1. Docket. EPA has established a docket for this action under 
Docket ID No. EPA-HQ-OW-2023-0572. Publicly available docket materials 
are available electronically on www.regulations.gov or in hard copy at 
the EPA Docket Center, WJC West Building, Room 3334, 1301 Constitution 
Ave. NW, Washington, DC. The Docket Center's hours of operations are 
8:30 a.m. to 4:30 p.m., Monday through Friday (except Federal 
Holidays). For further information on the EPA Docket Center services 
and the current status see: https://www.epa.gov/dockets.
    2. Electronic Access. You may access this Federal Register document 
electronically from https://www.federalregister.gov.

II. Statutory Requirements for the Six-Year Review

    Under the Safe Drinking Water Act (SDWA), as amended in 1996, EPA 
must periodically review existing NPDWRs and, if appropriate, revise 
them. Section 1412(b)(9) of the SDWA states: ``The Administrator shall, 
not less often than every six years, review and revise, as appropriate, 
each national primary drinking water regulation promulgated under this 
title. Any revision of a national primary drinking water regulation 
shall be promulgated in accordance with this section, except that each 
revision shall maintain, or provide for greater, protection of the 
health of persons.''
    Pursuant to the 1996 SDWA Amendments, EPA completed and published 
the results of its first Six-Year Review (Six-Year Review 1) on July 
18, 2003 (68 FR 42908, USEPA, 2003), the second Six-Year Review (Six-
Year Review 2) on March 29, 2010 (75 FR 15500, USEPA, 2010a) and the 
third Six-Year Review (Six-Year Review 3) on January 11, 2017 (82 FR 
3518, USEPA, 2017a).
    During the Six-Year Review 1, EPA identified the Total Coliform 
Rule (TCR) as a candidate for revision.\1\ In Six-Year Review 2, EPA 
identified four NPDWRs corresponding to acrylamide, epichlorohydrin, 
tetrachloroethylene (PCE), and trichloroethylene (TCE) as candidates 
for revision. In Six-Year Review 3, eight NPDWRs were listed as 
candidates for revision, including: chlorite, Cryptosporidium (under 
SWTRs), Giardia lamblia, haloacetic acids (HAA5), heterotrophic 
bacteria, Legionella, total trihalomethanes (TTHM), and viruses (under 
SWTRs). EPA also announced that the NPDWRs for acrylamide and 
epichlorohydrin were no longer candidates for revision due to low 
opportunity for further reduction of public health risk through 
regulatory revision (82 FR 3525, USEPA, 2017a).
---------------------------------------------------------------------------

    \1\ The NPDWRs apply to specific contaminants/parameters or 
groups of contaminants. Historically, when issuing new or revised 
standards for these contaminants/parameters, EPA has often grouped 
the standards together in more general regulations, such as the 
Total Coliform Rule, the Surface Water Treatment Rule or the Phase V 
rules. In this action, however, for clarity, EPA discusses the 
drinking water standards as they apply to each specific regulated 
contaminant/parameter (or group of contaminants), not the more 
general regulation in which the contaminant/parameter was regulated.
---------------------------------------------------------------------------

    In this document, EPA is announcing the results of the fourth Six-
Year Review (Six-Year Review 4). EPA's announcement of whether to 
identify an NPDWR as a candidate for revision (pursuant to SDWA section 
1412(b)(9)) is not a regulatory decision. Instead, announcing that an 
NPDWR is a candidate for revision formally initiates a regulatory 
process that involves more detailed analyses of health effects, 
analytical constraints, treatment feasibility, occurrence, benefits, 
costs, and other policy considerations relevant to informing an NPDWR 
revision effort. The Six-Year Review results do not obligate the agency 
to revise an NPDWR if EPA determines during the regulatory process that 
revisions are no longer appropriate and discontinues further efforts to 
revise the NPDWR. Similarly, when EPA announces that a particular NPDWR 
has not been identified as a candidate for revision it means that the 
agency has concluded that it is not appropriate for revision at this 
time based on available information.
    The criteria that EPA has applied to help identify when an NPDWR 
might be considered as a ``candidate for revision'' are, at a minimum, 
that the regulatory revision presents a meaningful opportunity to 
improve the level of public health protection, and/or achieve cost 
savings while maintaining or improving the level of public health 
protection.

III. Regulations Included in the Six-Year Review 4

    Table 1 of this document lists all 94 NPDWRs established to date. 
The table also reports the maximum contaminant level goal (MCLG) and, 
where applicable, the maximum contaminant level (MCL). The MCLG is 
``set at the level at which no known or anticipated adverse effects on 
the health of persons occur and which allows an adequate margin of 
safety'' (SDWA section 1412(b)(4)). The MCL for each applicable NPDWR, 
is the maximum permissible level of a contaminant in water delivered to 
any user of a public water system (PWS) and generally ``is as close to 
the maximum contaminant level goal as is feasible'' (SDWA section 
1412(b)(4)(B)). If it is not ``economically or technically feasible to 
ascertain the level of the contaminant,'' EPA can require the use of a 
treatment technique (TT) in lieu of establishing an MCL. The treatment 
technique(s) must prevent known or anticipated adverse health effects 
``to the extent feasible'' (SDWA section 1412(b)(7)(A)).\2\ In the case 
of disinfectants (e.g., chlorine, chloramines, chlorine dioxide), the 
values reported in the table are not MCLGs and MCLs, but maximum 
residual disinfectant level goals (MRDLGs) and maximum residual 
disinfectant levels (MRDLs).
---------------------------------------------------------------------------

    \2\ Under limited circumstances, SDWA section 1412(b)(6)(A) 
gives the Administrator the discretion to promulgate an MCL or TT 
that is less stringent than the most protective feasible standard 
that ``maximizes health risk reduction benefits at a cost that is 
justified by the benefits.'' Similarly, SDWA section 1412(b)(5) 
authorizes the Administrator to promulgate an MCL or TT that is less 
stringent than the most protective feasible standard if the more 
protective standard would increase the level of other contaminants 
in drinking water or interfere with the efficacy of treatment 
techniques or process used for compliance with other NPDWRs. Under 
those circumstances, EPA is to promulgate feasible a MCL or TT rule 
to ``minimize the oversall risk of adverse health effects'' while 
avoiding an increase in health risks from other contaminants.

---------------------------------------------------------------------------

[[Page 59625]]

    As part of the fourth Six-Year Review, EPA did not consider 
information after December 2021, unless otherwise noted. EPA identified 
15 NPDWRs for which there has either been a recently completed, an 
ongoing, or a pending regulatory action. EPA did not conduct a detailed 
review of these 15 NPDWRs for the Six-Year Review 4. These include the 
ongoing Lead & Copper rulemaking activities and the potential revisions 
\3\ of the Microbial and Disinfection Byproduct Rules (MDBP). The MDBP 
effort contemplates potential regulatory revisions for the NPDWRs 
covering the following contaminants: (Bromate, Chloramines, Chlorine 
Dioxide, Chlorine, Chlorite, Cryptosporidium, Giardia lamblia, 
Haloacetic acids, Heterotrophic bacteria, Legionella, Total 
Trihalomethanes, Turbidity, & Viruses).
---------------------------------------------------------------------------

    \3\ Additional information can be found at https://www.epa.gov/system/files/documents/2022-04/mdbp-rule-revisions-charge-to-the-ndwac.pdf.
---------------------------------------------------------------------------

    The EPA did not include in this Six-Year Review cycle the recently 
promulgated per-and polyfluoroalkyl substances (PFAS) regulations.\4\ 
The PFAS regulations, promulgated in April 2024, established 6 new 
NPDWRs. The EPA anticipates that once the PFAS regulations go into 
effect and sufficient information regarding compliance monitoring 
becomes available, those NPDWRs will be subject to a more detailed 
regulatory review under a future Six-Year Review cycle. This document 
describes the detailed review of the remaining 73 NPDWRs. section IV of 
this document describes the Six-Year Review 4 protocol, and section V 
of this document describes the review results. Please see USEPA (2024a) 
for more details.
---------------------------------------------------------------------------

    \4\ On April 26, 2024, the EPA promulgated legally enforceable 
drinking water standards to address PFAS known to occur individually 
and as mixtures in drinking water (89 FR 32532). The NPDWRs sets 
limits for five individual PFAS: (perfluorooctanoic acid (PFOA), 
perfluorooctane sulfonic acid (PFOS), perfluorohexane sulfonic acid 
(PFHxS), perfluorononanoic acid (PFNA), hexafluoropropylene oxide 
dimer acid (HFPO-DA, commonly known as GenX Chemicals)); and also 
established a limit for mixtures of any two or more of the following 
four PFAS: (PFNA, PFHxS, perfluorobutane sulfonic acid (PFBS), and 
HFPO-DA).

                                                                 Table 1--List of NPDWRs
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                             MCL or TT (mg/L) 2 3      Contaminants/                               MCL or TT (mg/L) 2 3
     Contaminants/parameters          MCLG (mg/L) 1 3                                   parameters          MCLG (mg/L) 1 3
--------------------------------------------------------------------------------------------------------------------------------------------------------
Acrylamide......................  0......................  TT.....................  Giardia lamblia     0......................  TT.
                                                                                     \4\.
Alachlor........................  0......................  0.002..................  Glyphosate........  0.7....................  0.7.
Alpha/photon emitters...........  0 (pCi/L)..............  15 (pCi/L).............  Haloacetic acids    n/a \5\................  0.060.
                                                                                     (HAA5).
Antimony........................  0.006..................  0.006..................  Heptachlor........  0......................  0.0004.
Arsenic.........................  0......................  0.010..................  Heptachlor epoxide  0......................  0.0002.
Asbestos........................  7 (million fibers/L)...  7 (million fibers/L)...  Heterotrophic       n/a....................  TT.
                                                                                     bacteria \6\.
Atrazine........................  0.003..................  0.003..................  Hexachlorobenzene.  0......................  0.001.
Barium..........................  2......................  2......................  Hexachlorocyclopen  0.05...................  0.05.
                                                                                     tadiene.
Benzene.........................  0......................  0.005..................  Hexafluoropropylen  10 (ppt)...............  10 (ppt).
                                                                                     e oxide dimer
                                                                                     acid (HFPO-DA).
Benzo[a]pyrene..................  0......................  0.0002.................  Lead..............  0......................  TT.
Beryllium.......................  0.004..................  0.004..................  Legionella........  0......................  TT.
Beta/photon emitters............  0 (millirems/yr).......  4 (millirems/yr).......  Lindane...........  0.0002.................  0.0002.
Bromate.........................  0......................  0.010..................  Mercury             0.002..................  0.002.
                                                                                     (inorganic).
Cadmium.........................  0.005..................  0.005..................  Methoxychlor......  0.04...................  0.04.
Carbofuran......................  0.04...................  0.04...................  Monochlorobenzene   0.1....................  0.1.
                                                                                     (Chlorobenzene).
Carbon tetrachloride............  0......................  0.005..................  Nitrate (as N)....  10.....................  10.
Chloramines (as Cl2)............  4......................  4.0....................  Nitrite (as N)....  1......................  1.
Chlordane.......................  0......................  0.002..................  Oxamyl (Vydate)...  0.2....................  0.2.
Chlorine (as Cl2)...............  4......................  4.0....................  Pentachlorophenol.  0......................  0.001.
Chlorine dioxide (as ClO2)......  0.8....................  0.8....................  Perfluorohexane     10 (ppt)...............  10 (ppt).
                                                                                     sulfonic acid
                                                                                     (PFHxS).
Chlorite........................  0.8....................  1.0....................  Perfluorononanoic   10 (ppt)...............  10 (ppt).
                                                                                     acid (PFNA).
Chromium (total)................  0.1....................  0.1....................  Perfluorooctane     0 (ppt)................  4.0 (ppt).
                                                                                     sulfonic acid
                                                                                     (PFOS).
Copper..........................  1.3....................  TT.....................  Perfluorooctanoic   0 (ppt)................  4.0 (ppt).
                                                                                     acid (PFOA).
Cryptosporidium.................  0......................  TT.....................  PFAS Mixture (HFPO- Hazard Index \12\ of 1.  Hazard Index of 1.
                                                                                     DA, PFBS, PFHxS,
                                                                                     & PFNA).
Cyanide (as free cyanide).......  0.2....................  0.2....................  Picloram..........  0.5....................  0.5.
2,4-Dichlorophenoxyacetic acid    0.07...................  0.07...................  Polychlorinated     0......................  0.0005.
 (2,4-D).                                                                            biphenyls (PCBs).
Dalapon.........................  0.2....................  0.2....................  Radium 226/228      0 (pCi/L)..............  5 (pCi/L).
                                                                                     (combined).
Di(2-ethylhexyl)adipate (DEHA)..  0.4....................  0.4....................  Selenium..........  0.05...................  0.05.
Di(2-ethylhexyl)phthalate (DEHP)  0......................  0.006..................  Simazine..........  0.004..................  0.004.
1,2-Dibromo-3- chloropropane      0......................  0.0002.................  Styrene...........  0.1....................  0.1.
 (DBCP).
1,2-Dichlorobenzene (o-           0.6....................  0.6....................  2,3,7,8-TCDD        0......................  3 x10 -8.
 Dichlorobenzene).                                                                   (Dioxin).
1,4-Dichlorobenzene (p-           0.075..................  0.075..................  Tetrachloroethylen  0......................  0.005.
 Dichlorobenzene).                                                                   e.
1,2-Dichloroethane (ethylene      0......................  0.005..................  Thallium..........  0.0005.................  0.002.
 dichloride).
1,1-Dichloroethylene............  0.007..................  0.007..................  Toluene...........  1......................  1.
cis-1,2-Dichloroethylene........  0.07...................  0.07...................  Total coliforms 7   n/a....................  TT.
                                                                                     8.
trans-1,2-Dichloroethylene......  0.1....................  0.1....................  Total               n/a \9\................  0.080.
                                                                                     Trihalomethanes
                                                                                     (TTHM).
Dichloromethane (methylene        0......................  0.005..................  Toxaphene.........  0......................  0.003.
 chloride).
1,2-Dichloropropane.............  0......................  0.005..................  2,4,5-TP (Silvex).  0.05...................  0.05.
Dinoseb.........................  0.007..................  0.007..................  1,2,4-              0.07...................  0.07.
                                                                                     Trichlorobenzene.
Diquat..........................  0.02...................  0.02...................  1,1,1-              0.2....................  0.2.
                                                                                     Trichloroethane.
E. coli.........................  0......................  MCL,\10\ TT 8 11.......  1,1,2-              0.003..................  0.005.
                                                                                     Trichloroethane.
Endothall.......................  0.1....................  0.1....................  Trichloroethylene.  0......................  0.005.
Endrin..........................  0.002..................  0.002..................  Turbidity \6\.....  n/a....................  TT.
Epichlorohydrin.................  0......................  TT.....................  Uranium...........  0......................  0.030.
Ethylbenzene....................  0.7....................  0.7....................  Vinyl Chloride....  0......................  0.002.
Ethylene dibromide (EDB)........  0......................  0.00005................  Viruses...........  0......................  TT.
Fluoride........................  4.0....................  4.0....................  Xylenes (total)...  10.....................  10.
--------------------------------------------------------------------------------------------------------------------------------------------------------
\1\ MCLG: the maximum level of a contaminant in drinking water at which no known or anticipated adverse effect on the health of persons would occur,
  allowing an adequate margin of safety. Maximum contaminant level goals are nonenforceable health goals.

[[Page 59626]]

 
\2\ MCL: the maximum level allowed of a contaminant in water which is delivered to any user of a public water system. TT: any action, process, or
  procedure required of the water system that leads to the reduction of the level of a contaminant in tap water that reaches the consumer.
\3\ Units are in milligrams per liter (mg/L) unless otherwise noted. Milligrams per liter are equivalent to parts per million. For chlorine,
  chloramines, and chlorine dioxide, values presented are MRDLG and MRDL.
\4\ The current preferred taxonomic name is Giardia duodenalis, with Giardia lamblia and Giardia intestinalis as synonymous names. However, Giardia
  lamblia was the name used to establish the MCLG in 1989. Elsewhere in this document, this pathogen will be referred to as Giardia spp. or simply
  Giardia unless discussing information on an individual species.
\5\ There is no MCLG for all five haloacetic acids. MCLGs for some of the individual contaminants are: dichloroacetic acid (zero), trichloroacetic acid
  (0.02 mg/L), and monochloroacetic acid (0.07 mg/L). Bromoacetic acid and dibromoacetic acid are regulated with this group but have no MCLGs.
\6\ Includes indicators that are used in lieu of direct measurements (e.g., of heterotrophic bacteria, turbidity).
\7\ The Aircraft Drinking Water Rule (ADWR) 40 CFR part 141 subpart X, promulgated October 19, 2009, covers total coliforms and E. coli.
\8\ Under the RTCR, a PWS is required to conduct an assessment if it exceeded any of the TT triggers identified in 40 CFR 141.859(a). It is also
  required to correct any sanitary defects found through the assessment. 40 CFR 141.859(c).
\9\ There is no MCLG for total trihalomethanes (TTHM). MCLGs for some of the individual contaminants are: bromodichloromethane (zero), bromoform (zero),
  dibromochloromethane (0.06 mg/L), and chloroform (0.07 mg/L).
\10\ A PWS is in compliance with the E. coli MCL unless any of the conditions identified under 40 CFR 141.63(c) occur.
\11\ Under the GWR in 40 CFR 141.402, a ground water system that does not provide at least 4-log treatment of viruses and has a distribution system RTCR
  sample that tests positive for total coliform is required to conduct triggered source water monitoring to evaluate whether the total coliform presence
  in the distribution system is due to fecal contamination in the ground water source. The system must monitor for one of three State-specified fecal
  indicators (i.e., E. coli, coliphage, or enterococci).
\12\ The Hazard Index is an approach that EPA uses to determine the health concerns associated with mixtures of certain PFAS in finished drinking water.
  The Hazard Index is made up of a sum of fractions. Each fraction compares the level of each PFAS measured in the water to the associated health-based
  water concentration.

IV. EPA's Protocol for Reviewing the NPDWRs Included in This Action

A. What was EPA's review process?

    This section provides an overview of the process EPA used to review 
the NPDWRs discussed in this document. The protocol document, ``EPA 
Protocol for the Fourth Review of Existing National Primary Drinking 
Water Regulations,'' contains a detailed description of the process the 
agency used to review the NPDWRs (USEPA, 2024c). The foundation of this 
protocol was developed for the Six-Year Review 1 based on the 
recommendations of the National Drinking Water Advisory Council (NDWAC, 
2000) and has undergone minor clarifications during each Six-Year 
Review cycle (USEPA, 2024c). Figure 1 presents an overview of the Six-
Year Review protocol and the possible review outcomes.
    The objective of the Six-Year Review process is to identify and 
prioritize NPDWRs for possible regulatory revision. The two major 
outcomes of the detailed review are either (1) the NPDWR is not 
appropriate for revision and no action is necessary at this time or (2) 
the NPDWR is a candidate for revision.
    The reasons why EPA might list an NPDWR as ``not appropriate for 
revision at this time'' could include:
     Recently completed, ongoing, or pending regulatory action: 
The NPDWR was recently completed, is being reviewed under an ongoing 
action, or is subject to a pending action.
     Ongoing or planned health effects assessment: The 
contaminant or contaminants regulated by the NPDWR has an ongoing or 
planned health effects assessment.
     No new information: EPA did not identify any new relevant 
information for the contaminant since the last Six-Year Review that 
indicates changes to the NPDWR may be appropriate.
     Data gaps/emerging information: New information indicates 
a possible change to the MCLG and/or MCL but changes to the NPDWR are 
not appropriate due to data gaps and emerging information that needs to 
be evaluated.
     Low priority and/or no meaningful opportunity: New 
information indicates a possible change to the MCLG and/or MCL but 
changes to the NPDWR are not appropriate at this time due to one or 
more of the following reasons: (1) possible changes present negligible 
gains in public health protection; (2) possible changes present limited 
opportunity for cost savings while maintaining the same or greater 
level of health protection; and/or (3) possible changes are a low 
priority because of competing workload priorities, limited return on 
the administrative costs associated with rulemaking, and the burden on 
states and the regulated community associated with implementing any 
regulatory change that would result.
    Alternatively, the reasons why an NPDWR could be listed as a 
candidate for revision are that the regulatory revision presents a 
meaningful opportunity to improve the level of public health 
protection, and/or achieve cost savings while maintaining or improving 
the level of public health protection.
    Individual regulatory provisions that are evaluated as part of the 
Six-Year Review process include: MCLG, MCL, MRDLG, MRDL, TT, best 
available technology (BAT), and other requirements, such as monitoring 
requirements.
    For example, the microbial regulations include TT requirements 
because no reliable, affordable, and technically feasible method is 
available to measure the microbial contaminants covered by those 
regulations. These TT requirements rely on the use of indicators that 
can be measured in drinking water, such as detection of total coliforms 
as an indicator of a potential pathway for pathogenic contamination in 
the distribution system. As part of the Six-Year Review 4, EPA 
evaluated new information related to the use of those indicators to 
determine if a meaningful opportunity to improve the level of public 
health protection exists. Results of EPA's review of the microbial 
regulations are presented in section V of this document.
Basic Principles
    EPA applied several basic principles to the Six-Year Review 
process:
     The agency sought to avoid redundant review efforts. 
Because EPA has reviewed information for certain NPDWRs as part of 
recently completed, ongoing, or pending regulatory actions, these 
NPDWRs were not subject to detailed review under the Six-Year Review 
process.
     The agency does not believe it is appropriate to consider 
revisions to NPDWRs for contaminants with an ongoing or planned health 
effect assessment where the MCL is set equal to the MCLG or that were 
set at the level at which health risk reduction benefits were maximized 
at a cost justified by the benefits in accordance with SDWA section 
1412(b)(6)(A)). This principle stems from the fact that any new health 
effects assessment may affect the MCL via a change in the MCLG or the 
assessment of the benefits associated with the MCL. EPA notes that 
these NPDWRs are not appropriate for revision and no action is 
necessary if the health effects assessment would not be completed 
during the review cycle.
     In evaluating the potential for new information to affect 
NPDWRs, EPA assumed no change to existing policies and procedures for 
developing NPDWRs. For example, in determining whether new information 
affected the feasibility of analytical methods for a contaminant, the 
agency assumed no

[[Page 59627]]

change to current policies and procedures for calculating practical 
quantitation limits.
     EPA may consider whether there is new public health risk 
information to justify accelerating review and potential revision of a 
particular NPDWR before the next review cycle.
Procedures
    EPA also applied the following procedures in the review process:
     EPA considered new information from health effects 
assessments that were completed by the information cutoff date. 
Assessments completed after this cutoff date will be reviewed by EPA 
during the next review cycle.
     During the review, EPA identified areas where relevant 
information, which is needed to determine whether a revision to an 
NPDWR may be appropriate, was either: inadequate, unavailable (i.e., 
data gaps), or emerging. To the extent EPA is able to fill data gaps or 
fully evaluate the emerging information, the agency will consider the 
information as part of the next review cycle.
     Finally, EPA assured that the scientific analyses 
supporting the review were consistent with the agency's peer review 
policy (USEPA, 2015a).
[GRAPHIC] [TIFF OMITTED] TR23JY24.000

B. How did EPA conduct the review of the NPDWRs?

    The protocol for the Six-Year Review 4 is organized as a series of 
questions to inform an assessment as to the appropriateness of revising 
an NPDWR. These questions are logically ordered into a decision tree. 
This section provides an overview of each of the review elements that 
EPA considered for each NPDWR during the Six-Year Review 4, including 
the following: initial review, health effects, analytical feasibility, 
occurrence and exposure, treatment feasibility, risk balancing, and 
other NPDWR revisions. The final review combines the findings from all 
these review elements to recommend whether an NPDWR is a candidate for 
revision. Further information about the review elements is described in 
the protocol document (USEPA, 2024c). The results of the Six-Year 
Review are presented in section V of this document.
1. Initial Review
    EPA's initial review of all the contaminants included in the Six-
Year Review 4 involved a simple identification of the NPDWRs that have 
either been recently completed or are being reviewed in an ongoing or 
pending action since the publication of Six-Year Review 3. In addition, 
the initial review also identified contaminants with ongoing health 
effects assessments that have an MCL equal to the MCLG. Excluding such 
contaminants from a more detailed review in the Six-Year Review 4 
prevents duplicative agency efforts.
2. Health Effects
    The principal objectives of the health effects review are to 
identify: (1) contaminants for which a new health effects assessment 
indicates that a change in the MCLG might be appropriate (e.g., because 
of a change in cancer classification or a change in reference dose 
(RfD)), and (2) contaminants for which new health effects information 
indicates a need to initiate a new health effects assessment.
    To meet the first objective, EPA reviewed the results of health 
effects assessments completed since promulgation of each NPDWR. To meet 
the second objective, the agency conducted a systematic literature 
search, to capture more recently published peer-reviewed studies on 
relevant health effects via the oral route

[[Page 59628]]

of exposure for the general population as well as sensitive 
subpopulations including children. The results of the literature search 
were used to survey the health effects literature that has become 
available since the previous review cycle, identify any emerging issues 
for a contaminant, and identify data gaps to inform future health 
assessment nominations.
3. Analytical Feasibility
    When establishing an NPDWR, EPA identifies a practical quantitation 
limit (PQL), which is the lowest achievable level of analytical 
quantitation during routine laboratory operating conditions within 
specified limits of precision and accuracy (50 FR 46880, USEPA, 1985). 
EPA has a separate process in place to approve new analytical methods 
for drinking water contaminants; therefore, review and approval of 
potential new methods is outside the scope of the Six-Year Review 
protocol. EPA recognizes, however, that the approval and adoption in 
recent years of new and/or improved analytical methods may enable 
laboratories to quantify contaminants at lower levels than was possible 
when NPDWRs were originally promulgated. This ability of laboratories 
to measure a contaminant at lower levels could affect its PQL, the 
value at which an MCL is set when it is limited by analytical 
feasibility. Therefore, the Six-Year Review process includes an 
examination of whether there have been changes in analytical 
feasibility that could possibly change the PQL for the subset of the 
NPDWRs that reach this stage of the review.
    To determine if changes in analytical feasibility could possibly 
support changes to PQLs, EPA relied primarily on two approaches to 
develop estimated quantitation levels (EQLs), which are based on either 
(1) minimum reporting levels (MRLs) obtained as part of the Six-Year 
Review 4 Information Collection Request (ICR), or (2) method detection 
limits (MDLs) from EPA-approved laboratory protocols.
    An MRL is the lowest level or contaminant concentration that a 
laboratory can reliably achieve within specified limits of precision 
and accuracy under routine laboratory operating conditions using a 
given method. The MRL values provide direct evidence from actual 
monitoring results about whether quantitation below the PQL using 
current analytical methods is feasible. An MDL is a measure of 
analytical sensitivity, representing the minimum reported concentration 
that can be distinguished from blank results with 99 percent 
confidence. MDLs have been used in the past to derive PQLs for 
regulated contaminants.
    EPA used the EQL as a threshold for occurrence analysis to help the 
agency assess for a meaningful opportunity to improve public health 
protection. It should be noted, however, that the use of an EQL does 
not necessarily indicate the agency's intention to promulgate a revised 
MCL based on the new PQL. Any change in the PQL for a contaminant could 
be part of future rulemaking efforts if EPA decides to initiate a 
regulatory revision for the contaminant.
4. Occurrence and Exposure Analysis
    EPA conducted the occurrence and exposure analysis in conjunction 
with other review elements to determine if an NPDWR revision would 
provide a meaningful opportunity to improve public health by:
     estimating the extent of contaminant occurrence, i.e., the 
number of PWSs in which contaminants occur at levels of interest 
(health-effects-based thresholds or analytical method limits), and;
     evaluating the number of people potentially exposed to 
contaminants at these levels.
    To evaluate national contaminant occurrence under the Six-Year 
Review 4, EPA reviewed data from the Six-Year Review 4 ICR database 
(SYR 4 ICR database) and other relevant sources. EPA collected SDWA 
compliance monitoring data and treatment technique information through 
use of an ICR (84 FR 58381, USEPA, 2019). EPA requested that states, as 
well as Tribes and territories with primacy voluntarily submit their 
compliance monitoring data and treatment technique information for 
regulated contaminants in PWSs. Specifically, EPA requested the 
submission of compliance monitoring data, treatment technique 
information, and related details collected between January 2012 and 
December 2019 for regulated contaminants and related parameters (e.g., 
water quality indicators). Forty-six states plus 13 other jurisdictions 
(Washington, DC, territories, and Tribes) provided data. The assembled 
data constitute the largest, most comprehensive set of drinking water 
compliance monitoring data and treatment technique information ever 
compiled and analyzed by EPA to inform decision making, containing 
almost 71 million analytical records from approximately 140,000 PWSs, 
serving approximately 301 million people nationally. Through extensive 
data management efforts, quality assurance evaluations, and 
communications with state data management staff, EPA established the 
SYR 4 ICR dataset (USEPA, 2019). The number of states and PWSs 
represented in the dataset varies across contaminants because of 
variability in state data submissions and contaminant monitoring 
schedules. EPA considers that these data are of sufficient quality to 
inform an understanding of the national occurrence of regulated 
contaminants and related parameters. Details of the data management and 
data quality assurance evaluations are available in the supporting 
document (USEPA, 2024d). The resulting database is available online on 
the Six-Year Review website at https://www.epa.gov/dwsixyearreview.
5. Treatment Feasibility
    An NPDWR either identifies an MCL or establishes enforceable TT 
requirements. When promulgating an MCL or enforceable treatment 
technique requirements, to determine feasibility, EPA identifies the 
best technology, treatment techniques, and other means which EPA finds, 
after examination for efficacy under field conditions and not solely 
under laboratory conditions, are available (taking cost into 
consideration). When promulgating an MCL, EPA also lists the 
technology, treatment techniques, or other means which are feasible for 
purposes of meeting the MCL. EPA reviews treatment feasibility to 
ascertain if available technologies meet BAT criteria for a 
hypothetical more stringent MCL, or if new information demonstrates an 
opportunity to improve public health protection through revision of an 
NPDWR TT requirement.
    To be a BAT, the treatment technology must meet several criteria 
such as having demonstrated consistent removal of the target 
contaminant under field conditions. Although treatment feasibility and 
analytical feasibility are considered together in evaluating the 
technical feasibility requirement for an MCL, historically, treatment 
feasibility has not been a limiting factor for MCLs. The result of this 
review element is a determination of whether treatment feasibility 
would pose a limitation to revising an MCL or provide an opportunity to 
revise the NPDWR TT requirement.
6. Risk-Balancing
    EPA reviews the risk-balancing analysis underlying some NPDWRs to 
examine how a potential regulatory revision would address tradeoffs in 
risks associated with different contaminants. Under this review, EPA 
considers whether a change to an MCL and/or TT

[[Page 59629]]

will increase the public health risk posed by one or more contaminants, 
and, if so, the agency considers revisions that will balance overall 
risks. This review element is relevant only to the NPDWRs included in 
the microbial and disinfection byproduct (MDBP) rules, which were 
promulgated to address the need for risk-balancing between microbial 
and disinfection byproduct (DBP) requirements, and among differing 
types of DBPs. NPDWRs for microbials and disinfectants and DBPs were 
not reviewed during Six-Year Review 4 due to ongoing regulatory action 
initiated by Six-Year Review 3.
7. Other NPDWR Revisions
    In addition to possible revisions to MCLGs, MCLs, and TTs, EPA 
evaluated whether other revisions are needed to other regulatory 
provisions in NPDWRs, such as monitoring and system reporting 
requirements. EPA focused this review element on issues that were not 
already being addressed through alternative mechanisms, such as a 
recently completed, ongoing, or pending regulatory action. EPA also 
reviewed implementation-related NPDWR concerns that were ``ready'' for 
rulemaking--that is, the problem to be resolved had been clearly 
identified, along with specific options to address the problem that 
could be shown to either clearly improve the level of public health 
protection or represent a meaningful opportunity for achieving cost 
savings while maintaining the same level of public health protection. 
The result of this review element is a determination regarding whether 
EPA should consider revisions to the monitoring and/or reporting 
requirements of an NPDWR.

V. Results of EPA's Review of NPDWRs

A. Overview of Six-Year Review 4 Results

    Table 2 of this document, lists the results of EPA's review of the 
88 NPDWRs assessed during Six-Year Review 4, along with the principal 
rationale for the review outcomes. Table 2 includes the 15 NPDWRs that 
have ongoing or pending regulatory actions.

                                  Table 2--Summary of Six-Year Review 4 Results
----------------------------------------------------------------------------------------------------------------
 
----------------------------------------------------------------------------------------------------------------
Outcome                                   Regulated contaminants
----------------------------------------------------------------------------------------------------------------
Not Appropriate for Revision at   Recently completed, ongoing or pending  Bromate...........  Haloacetic acids
 this Time.                        regulatory action.                     Chloramines (as      (HAA5).
                                                                           Cl2).              Heterotrophic
                                                                          Chlorine Dioxide     bacteria.
                                                                           (as ClO2).         Lead.
                                                                          Chlorine (as Cl2).  Legionella.
                                                                          Chlorite..........  Total
                                                                          Copper............   Trihalomethanes
                                                                          Cryptosporidium      (TTHM).
                                                                           (IE, LT1) \1\.     Turbidity.
                                                                          Giardia lamblia...  Viruses (SWTR, IE,
                                                                                               LT1).\1\
----------------------------------------------------------------------------------------------------------------
Not Appropriate for Revision at   Health effects assessment in process    Alpha/photon        Mercury
 this Time.                        or contaminant nominated for health     emitters.           (inorganic).
                                   assessment.                            Arsenic...........  Polychlorinated
                                                                          Beta/photon          biphenyls (PCBs).
                                                                           emitters.          Radium 226/228
                                                                          Chromium (total)..   (combined).
                                                                          Ethylbenzene......  Uranium.
                                 -------------------------------------------------------------------------------
                                  No new information, NPDWR remains       Asbestos..........  trans-1,2-
                                   appropriate after review.              Benzo(a)pyrene....   Dichloroethylene.
                                                                          Chlorobenzene.....  Dinoseb.
                                                                          Dalapon...........  E. coli.
                                                                          Di(2-               Endrin.
                                                                           ethylhexyl)adipat  Ethylene
                                                                           e (DEHA).           dibromide.
                                                                          Di(2-               2,4,5-TP (Silvex).
                                                                           ethylhexyl)phthal
                                                                           ate (DEHP).
                                                                          1,2-Dibromo-3-
                                                                           chloropropane
                                                                           (DBCP).
                                 -------------------------------------------------------------------------------
                                  New information,    Low priority and/   Acrylamide........  Heptachlor.
                                   but no revision     or no meaningful   Alachlor..........  Heptachlor
                                   recommended         opportunity.       Antimony..........   Epoxide.
                                   because . . .                          Atrazine..........  Hexachlorobenzene.
                                                                          Barium............  Hexachlorocyclopen
                                                                          Benzene...........   tadiene.
                                                                                              Lindane.
                                                                                              Methoxychlor.
                                                                          Beryllium.........  Oxamyl (Vydate).
                                                                          Cadmium...........  Pentachlorophenol.
                                                                          Carbofuran........  Picloram.
                                                                          Carbon              Selenium.
                                                                           Tetrachloride.     Simazine.
                                                                          Chlordane.........  Styrene.
                                                                          Cryptosporidium     Tetrachloroethylen
                                                                           (LT2) \1\.          e (PCE).
                                                                          1,2-                Thallium.
                                                                           Dichlorobenzene.   1,2,4-
                                                                          1,4-                 Trichlorobenzene.
                                                                           Dichlorobenzene.   1,1,1-
                                                                          1,2-Dichloroethane   Trichloroethane.
                                                                          1,1-                1,1,2-
                                                                           Dichloroethylene.   Trichloroethane.
                                                                          cis-1,2-            Toluene.
                                                                           Dichloroethylene.
                                                                          Dichloromethane...
                                                                          2,4-                Total Coliform.
                                                                           Dichlorophenoxyac  Toxaphene.
                                                                           etic acid (2,4-D). Trichloroethylene
                                                                          1,2-Dichoropropane   (TCE).
                                                                          Dioxin (2,3,7,8-    Vinyl Chloride.
                                                                           TCDD).             Xylenes.
                                                                          Diquat............
                                                                          Endothall.........
                                                                          Epichlorohydrin...
                                                                          Glyphosate........
                                                     -----------------------------------------------------------
                                                      Emerging            Cyanide (as free    Nitrate.
                                                       information and/    cyanide).          Nitrite.
                                                       or data gaps.      Fluoride..........
----------------------------------------------------------------------------------------------------------------
Candidate for Revision..........  New information.
                                  None.
----------------------------------------------------------------------------------------------------------------
\1\ Regulation abbreviations: Aircraft Drinking Water Rule (ADWR), Ground Water Rule (GWR), Revised Total
  Coliform Rule (RTCR), Surface Water Treatment Rule (SWTR), Interim Enhanced Surface Water Treatment Rule (IE),
  Long Term 1 Enhanced Surface Water Treatment Rule (LT1), and Long Term 2 Enhanced Surface Water Treatment Rule
  (LT2).


[[Page 59630]]

    EPA has identified no appropriate candidates for revision at this 
time.
    EPA's Office of Ground Water and Drinking Water is currently 
engaged in several ongoing and potential regulatory actions, in 
addition to being involved in the efforts to successfully implement 
recently promulgated rules including:
     Developing a proposal to revise the Microbial and 
Disinfection By-Product Rules, including eight NPDWRs listed as 
candidates for revision in Six-Year Review 3 (85 FR 61680, USEPA, 
2020a).
     On December 6, 2023, EPA published the proposed rule 
``National Primary Drinking Water for Lead and Copper: Improvements'' 
(88 FR 84878, USEPA, 2023a).
     In January 2024, EPA announced its commitment to 
promulgate a National Primary Drinking Water Regulation for Perchlorate 
by May 2027.\5\
---------------------------------------------------------------------------

    \5\ Additional information can be found at https://www.epa.gov/sdwa/perchlorate-drinking-water.
---------------------------------------------------------------------------

     On April 26, 2024, EPA published the PFAS final rule 
``PFAS National Primary Drinking Water Regulation'' (89 FR 32532, 
USEPA, 2024a).
     On May 24, 2024, EPA published the final rule ``National 
Primary Drinking Water Regulations: Consumer Confidence Reports'' (89 
FR 45980, USEPA, 2024b).
    Therefore, when evaluating the review results described in sections 
V.B and V.C of this document, EPA also considered competing workloads 
and potential diversion of resources from these other planned, ongoing, 
and pending higher priority efforts within the drinking water office.

B. Chemical Phase Rules/Radionuclides Rules

    The NPDWRs for chemical contaminants, collectively called the Phase 
Rules, were promulgated between 1987 and 1992, following the 1986 SDWA 
amendments. In December 2000, EPA promulgated final radionuclide 
regulations, which had been issued as interim rules in July 1976.
1. Key Review Outcomes
    EPA has decided that it is not appropriate at this time to revise 
any of the NPDWRs covered under the Phase or Radionuclides Rules (Table 
2 of this document). These NPDWRs were determined not to be candidates 
for revision for one or more of the following reasons:
     ongoing/pending regulatory action warrants waiting for 
further review;
     no new information was identified to suggest possible 
changes in MCLG/MCL;
     new information did not present a meaningful opportunity 
for health risk reduction or cost savings while maintaining/improving 
public health protection;
     emerging information and/or data gaps create substantial 
uncertainty.
    In addition, EPA is announcing that the NPDWRs for 
trichloroethylene (TCE) and tetrachloroethylene (PCE) are no longer 
candidates for revision at this time. In March 2010, as an outcome of 
the second cycle of Six-Year Review, EPA listed the TCE and PCE NPDWRs 
as candidates for revision (75 FR 15500, USEPA, 2010a). TCE and PCE 
were not reviewed under Six-Year Review 3 because regulatory revisions 
were being considered as part of plans to address regulated and 
unregulated Carcinogenic Volatile Organic Contaminants (cVOCs) in a 
group rule (75 FR 3525, January 21, 2010; 82 FR 3531, USEPA, 2017a). 
However, after evaluating currently available information for both of 
these chemicals, the EPA concludes that these NPDWRS are not 
appropriate for revision at this time because minimal reductions in 
health risks would be associated with any revisions to these 
regulations. Given resource limitations, competing workload priorities, 
and administrative costs and burden to states to adopt any regulatory 
changes associated with rulemakings, as well as limited potential 
health benefits, these NPDWRs are considered a low priority and are no 
longer candidates for revision at this time.
    Section V.B.2 of this document describes the results of the review 
organized by each review element. Section V.B.3 of this document 
includes a description of the new information gathered by EPA for 
select contaminants that EPA determined are not candidates for revision 
at this time due to emerging information or data gaps or no meaningful 
opportunity for health risk reduction. The contaminants discussed in 
detail in section V.B.3 of this document are cyanide, fluoride, 
nitrate, nitrite, TCE, and PCE.
    Review results organized by contaminant for the Chemical Phase and 
Radionuclides Rules can be found in the ``Chemical Contaminant 
Summaries for the Fourth Six-Year Review of National Primary Drinking 
Water Regulations'' (USEPA, 2024e).
2. Summary of Review Results
Initial Review
    After conducting the initial review, as described in section IV.B.1 
of this document, EPA identified two chemical contaminants (lead and 
copper) with NPDWRs that were considered as part of a recently 
completed action, and which are also currently part of an ongoing or 
pending regulatory action. EPA published the Lead and Copper Rule 
Revisions in January 2021 and published the proposed Lead and Copper 
Rule Improvements on December 6, 2023. EPA did not evaluate lead and 
copper in Six-Year Review 4 because such effort would be redundant with 
these recent and ongoing rulemakings. EPA also identified contaminants 
with ongoing or planned EPA health effects assessments. As of December 
31, 2021, nine chemical or radiological contaminants reviewed had 
ongoing or planned formal EPA health effects assessments. Table 3 of 
this document below lists the contaminants with ongoing or planned EPA 
assessments at the time of the Six-Year Review 4 cutoff date and the 
current status of those reviews. EPA did not conduct a detailed review 
of these nine chemical and radiological contaminants under Six-Year 
Review 4.

Table 3--Six-Year Review Chemical/Radiological Contaminants With Ongoing
                    or Planned EPA Health Assessments
------------------------------------------------------------------------
     Chemical/radionuclide                     Status \1\
------------------------------------------------------------------------
Alpha/photon emitters.........  EPA Office of Air and Radiation (OAR) is
                                 conducting a review of alpha and beta
                                 photon emitters. Additional information
                                 about this effort can be found at in
                                 the Federal Register (87 FR 15988,
                                 USEPA, 2022a) or at: https://sab.epa.gov/ords/sab/r/sab_apex/sab_bkup/advisoryactivitydetail?p18_id=2616&clear=18&session=8694491614209.
Arsenic.......................  Inorganic arsenic is being assessed by
                                 the EPA IRIS Program. The assessment
                                 status can be found at: https://iris.epa.gov/ChemicalLanding/&substance_nmbr=278.
Beta/photon emitters..........  EPA/OAR is conducting a review of alpha
                                 and beta photon emitters. Additional
                                 information about this effort can be
                                 found at in the Federal Register (87 FR
                                 15988, USEPA, 2022a) or at: https://sab.epa.gov/ords/sab/r/sab_apex/sab_bkup/advisoryactivitydetail?p18_id=2616&clear=18&session=8694491614209.

[[Page 59631]]

 
Chromium VI (as part of total   Chromium VI is being assessed by the EPA
 Cr).                            IRIS Program. The assessment status can
                                 be found at: https://iris.epa.gov/ChemicalLanding/&substance_nmbr=144.
Ethylbenzene..................  Ethylbenzene is being assessed by the
                                 EPA IRIS Program. The assessment status
                                 can be found at: https://iris.epa.gov/ChemicalLanding/&substance_nmbr=51.
Mercury.......................  Inorganic Mercury Salts is being
                                 assessed by the EPA IRIS Program. The
                                 Assessment status can be found at:
                                 https://iris.epa.gov/ChemicalLanding/&substance_nmbr=1522.
PCBs..........................  PCBs are being assessed by the EPA IRIS
                                 Program. The assessment status can be
                                 found at: https://iris.epa.gov/ChemicalLanding/&substance_nmbr=294.
Radium 226/228................  EPA/OAR is conducting a review of
                                 radium. Additional information about
                                 this effort can be found at in the
                                 Federal Register (87 FR 15988, USEPA,
                                 2022a) or at: https://sab.epa.gov/ords/sab/r/sab_apex/sab_bkup/advisoryactivitydetail?p18_id=2616&clear=18&session=8694491614209.
Uranium.......................  Uranium is being assessed by the EPA
                                 IRIS Program. The assessment status can
                                 be found at: https://iris.epa.gov/ChemicalLanding/&substance_nmbr=259.
------------------------------------------------------------------------
\1\ Additional information on the status of EPA IRIS Program assessments
  can be found in the EPA IRIS Program Outlooks at https://www.epa.gov/iris/iris-program-outlook.

    Regarding the ongoing health assessment for Chromium VI (hexavalent 
chromium), on October 20, 2022 the EPA published its draft ``IRIS 
Toxicological Review of Hexavalent Chromium [Cr(IV)]'' (87 FR 63774, 
USEPA, 2022b). This draft health effects assessment, which includes a 
comprehensive evaluation of potential health effects, preliminarily 
categorizes hexavalent chromium as likely carcinogenic to humans via 
the oral exposure pathway. The final IRIS assessment was not available 
as of the publication of this document and for consideration as part of 
Six-Year Review 4. When this human health assessment is final, EPA will 
carefully review the conclusions and consider all relevant information 
to determine whether the NPDWR for chromium is a candidate for 
revision.
    After the initial review was completed, EPA identified 71 chemical 
and radiological NPDWRs that were appropriate for detailed review.
Health Effects
    The principal objectives of the health effects assessment review 
were to identify: (1) contaminants for which a new health effects 
assessment indicates that a change in MCLG might be appropriate (e.g., 
because of a change in cancer classification or an RfD), and (2) 
contaminants for which the agency has identified new health effects 
information suggesting a need to initiate a new health effects 
assessment. For chemicals that were not excluded due to an ongoing or 
planned health effects assessment by EPA, a more detailed review was 
undertaken. Of the chemicals that underwent a more detailed review, EPA 
identified 29 contaminants for which an updated RfD and/or the cancer 
risk assessment (from oral exposure) or new relevant non-EPA 
assessments might support a change to the MCLG. These 29 chemicals were 
further evaluated as part of the Six-Year Review 4 to determine whether 
they were candidates for regulatory revision. Table 4 of this document 
lists the chemicals with available new health effects information and 
the sources of the relevant new information. As shown in this table, 15 
chemical contaminants have information that could support a lower MCLG, 
and 14 contaminants have new information that could support a higher 
MCLG.

   Table 4--Chemicals With New Health Assessments That Could Support a
                             Change in MCLG
------------------------------------------------------------------------
                Chemical                     Relevant new assessment
------------------------------------------------------------------------
           15 Contaminants with Potential to Decrease the MCLG
------------------------------------------------------------------------
Antimony...............................  CalEPA, 2016.
Cadmium................................  ATSDR, 2012.
Carbofuran.............................  USEPA OPP, 2008.
Cyanide................................  USEPA IRIS, 2010b.
cis-1,2-Dichloroethylene...............  USEPA IRIS, 2010c.
Endothall..............................  USEPA OPP, 2015b.
Fluoride...............................  USEPA OW, 2010d.
Hexachlorocyclopentadiene..............  USEPA IRIS, 2001.
Methoxychlor...........................  CalEPA, 2010a.
Oxamyl.................................  USEPA OPP, 2017b.
Selenium...............................  ATSDR, 2003.
Styrene................................  CalEPA, 2010b.
Toluene................................  Health Canada, 2014.
1,2,4-Trichlorobenzene.................  USEPA PPRTV, 2009a.
Xylenes................................  Health Canada, 2014.
------------------------------------------------------------------------
           14 Contaminants with Potential to Increase the MCLG
------------------------------------------------------------------------
Alachlor...............................  USEPA OPP, 2007a.
Atrazine...............................  USEPA OPP, 2018a.
Barium.................................  USEPA IRIS, 2005.
Beryllium..............................  USEPA IRIS, 1998.
2,4-Dichlorophenoxy-acetic acid (2,4-D)  USEPA OPP, 2017c.
1,2-Dichlorobenzene....................  ATSDR, 2006.
1,4-Dichlorobenzene....................  ATSDR, 2006.

[[Page 59632]]

 
1,1-Dichloroethylene...................  USEPA IRIS, 2002.
Diquat.................................  USEPA OPP, 2020b.
Glyphosate.............................  USEPA OPP, 2017d.
Lindane................................  USEPA OPP, 2004.
Picloram...............................  USEPA OPP, 2020c.
Simazine...............................  USEPA OPP, 2018b.
1,1,1-Trichloroethane..................  USEPA IRIS, 2007b.
------------------------------------------------------------------------

    Details of the health effects assessment review of the chemical and 
radiological contaminants are documented in the ``Results of the Health 
Effects Assessment for the Fourth Six-Year Review of Existing Chemical 
and Radionuclide National Primary Drinking Water Standards'' (USEPA, 
2024f).
Analytical Feasibility
    EPA performed analytical feasibility analyses for the contaminants 
that reached this portion of the review. These contaminants included 
the 15 chemical contaminants identified under the health effects 
assessment review as having potential for a lower MCLG. EPA evaluated 
whether there were any analytical limitations to lowering the MCL to 
the potential MCLG. EPA also evaluated an additional 22 contaminants 
with MCLs higher than the current MCLGs due to analytical limitations 
at the time of rule promulgation. The document ``Analytical Feasibility 
Support Document for the Fourth Six-Year Review of National Primary 
Drinking Water Regulations: Chemical Phase and Radionuclides Rules'' 
(USEPA, 2024g) describes the process EPA used to evaluate whether 
changes in PQL are possible in those instances where the MCL may be 
limited by analytical feasibility.
    Table 5 of this document shows the outcomes of EPA's analytical 
feasibility review for two general categories of drinking water 
contaminants: (1) contaminants where health effects assessments 
indicate potential for lower MCLGs, and (2) contaminants where existing 
MCLs were limited by analytical feasibility at the time of promulgation 
and new information indicates a potential to reduce the PQL.
     A health effects assessment indicates potential for lower 
MCLG. This category includes the 15 contaminants identified in the 
health effects review as having potential for a lower MCLG. EPA 
reviewed the available information to determine if analytical 
feasibility could limit the potential for MCL revisions. The current 
PQL is not a limiting factor for seven of the 15 contaminants 
identified by the health effects review as potential candidates for 
lower MCLGs (cis-1,2-dichloroethylene, fluoride, 
hexachlorocyclopentadiene, oxamyl, selenium, toluene, and xylenes). For 
the remaining eight contaminants, the current PQL is higher than the 
potential new MCLG, so EPA evaluated whether there is an opportunity to 
lower the PQL. The evaluations indicated that all but one contaminant 
(antimony) have potential for a lower PQL, although not to the 
potential MCLG. Consequently, analytical feasibility may limit 
potential MCL revisions for the remaining seven contaminants (Table 5 
of this document).
     Existing MCLs are based on analytical feasibility. This 
category includes 22 contaminants with existing MCLs that are greater 
than the associated MCLGs due to analytical constraints at the time of 
rule promulgation. Two of the contaminants (thallium and 1,1,2-
trichloroethane) are non-carcinogenic and have a non-zero MCLG, and the 
remaining 20 contaminants are carcinogens with MCLGs equal to zero. EPA 
evaluated whether the PQL could be lowered for each of these 
contaminants. The evaluations indicated that all but five 
(benzo[a]pyrene, DBCP, DEHP, ethylene dibromide, PCBs) of the 22 
contaminants evaluated have potential for a lower PQL (Table 5 of this 
document).
    Where analytical feasibility evaluations indicated the potential 
for a PQL reduction, Table 5 of this document lists the type of data 
that support this conclusion. The types of data considered include 
laboratory proficiency tests (PT), method detection limits (MDL) from 
EPA-approved methods, and minimum reporting level (MRL) from the SYR 4 
ICR dataset. The methods to evaluate each of these data types to 
identify potential to reduce PQLs are described in the analytical 
feasibility support document (USEPA, 2024g). Where the evaluations 
indicated that the current PQL remained appropriate, Table 5 shows of 
this document ``Data do not support PQL reduction.'' EPA found 
information supporting potentially lower MCLs for 31 out of 37 
contaminants evaluated.

[[Page 59633]]



          Table 5--Analytical Feasibility Reassessment Results
------------------------------------------------------------------------
                                                  Analytical feasibility
                                   Current PQL     reassessment result
          Contaminant             ([micro]g/L)      (and source of new
                                                     information) \1\
------------------------------------------------------------------------
  15 Contaminants Identified Under the Health Effects Review as Having
                        Potential for Lower MCLG
------------------------------------------------------------------------
Antimony.......................               6  Data do not support PQL
                                                  reduction.
Cadmium........................               2  PQL reduction supported
                                                  (MDL, MRL).
Carbofuran.....................               7  PQL reduction supported
                                                  (MDL).
Cis-1,2-dichloroethylene.......               5  PQL not limiting.
Cyanide........................             100  PQL reduction supported
                                                  (MDL).
Endothall......................              90  PQL reduction supported
                                                  (MDL, MRL).
Fluoride.......................             500  PQL not limiting.
Hexachlorocyclopentadiene......               1  PQL not limiting.
Methoxychlor...................              10  PQL reduction supported
                                                  (MDL, MRL, PT).
Oxamyl.........................              20  PQL not limiting.
Selenium.......................              10  PQL not limiting.
Styrene........................               5  PQL reduction supported
                                                  (MDL, MRL, PT).
Toluene........................               5  PQL not limiting.
Xylenes........................               5  PQL not limiting.
1,2,4-Trichlorobenzene.........               5  PQL reduction supported
                                                  (MDL, MRL, PT).
------------------------------------------------------------------------
 22 Contaminants with MCLs Limited by Analytical Feasibility and Higher
                               than MCLGs
------------------------------------------------------------------------
Benzene........................               5  PQL reduction supported
                                                  (MDL, MRL, PT).
Benzo[a]pyrene.................             0.2  Data do not support PQL
                                                  reduction.
Carbon tetrachloride...........               5  PQL reduction supported
                                                  (MDL, MRL, PT).
Chlordane......................               2  PQL reduction supported
                                                  (MDL).
1,2-Dibromo-3-chloropropane                 0.2  Data do not support PQL
 (DBCP).                                          reduction.
1,2-Dichloroethane.............               5  PQL reduction supported
                                                  (MDL, MRL, PT).
Dichloromethane................               5  PQL reduction supported
                                                  (MDL, MRL, PT).
1,2-Dichloropropane............               5  PQL reduction supported
                                                  (MDL, MRL, PT).
Di(2-ethylhexyl)phthalate                     5  Data do not support PQL
 (DEHP).                                          reduction.
Ethylene dibromide.............            0.05  Data do not support PQL
                                                  reduction.
Heptachlor.....................             0.4  PQL reduction supported
                                                  (MDL).
Heptachlor epoxide.............             0.2  PQL reduction supported
                                                  (MDL).
Hexachlorobenzene..............               1  PQL reduction supported
                                                  (MDL, MRL).
Pentachlorophenol..............               1  PQL reduction supported
                                                  (MDL).
PCBs...........................             0.5  Data do not support PQL
                                                  reduction.
2,3,7,8-TCDD (dioxin)..........         0.00003  PQL reduction supported
                                                  (MRL).
Tetrachloroethylene............               5  PQL reduction supported
                                                  (MDL, MRL).
Thallium.......................               2  PQL reduction supported
                                                  (MRL).
Toxaphene......................               3  PQL reduction supported
                                                  (MRL, PT).
1,1,2-Trichloroethane..........               5  PQL reduction supported
                                                  (MDL, MRL, PT).
Trichloroethylene..............               5  PQL reduction supported
                                                  (MDL, MRL, PT).
Vinyl chloride.................               2  PQL reduction supported
                                                  (MDL, MRL, PT).
------------------------------------------------------------------------
\1\ The information source codes refer to the method detection limit
  (MDL), minimum reporting level (MRL), and proficiency testing (PT)
  data analyses. See USEPA (2024g) for further information.

Occurrence and Exposure
    Using the SYR 4 ICR database, EPA conducted an assessment to 
evaluate national occurrence of regulated contaminants and estimate the 
potential population exposed to these contaminants. The details of the 
current chemical occurrence analysis are documented in the ``Analysis 
of Regulated Contaminant Occurrence Data from Public Water Systems in 
Support of the Fourth Six-Year Review of National Primary Drinking 
Water Regulations: Chemical Phase Rules and Radionuclides Rules'' 
(USEPA, 2024h). Based on quantitative benchmarks which were identified 
in the health effects and analytical feasibility analyses, EPA 
conducted the occurrence and exposure analysis for 31 contaminants.
    This analysis shows that 27 of the 31 contaminants assessed rarely 
occur at levels above the identified benchmark (e.g., potential MCLG or 
PQL). For these 27 contaminants, monitoring results only exceeded 
benchmarks in a very small percentage (i.e., less than 0.5 percent) of 
systems, which serve a very small percentage of the population, 
indicating that revisions to NPDWRs are unlikely to provide a 
meaningful opportunity to improve public health protection at the 
national level. Therefore, these 27 contaminants were not further 
considered as candidates for regulatory revision. The other four 
contaminants (cyanide, fluoride, TCE, and PCE) occurred at rates 
ranging from 0.57 to 9.1 percent of systems within the SYR 4 ICR 
dataset and 3.4 to 6.3 percent of the population served by those 
systems. Additional considerations for cyanide, fluoride, TCE, and PCE 
are discussed in section V.B.3 of this document. Table 6 of this 
document lists the numerical benchmarks used to conduct the occurrence 
analysis, the total number of systems with mean concentrations 
exceeding a benchmark, and the estimated population served by those 
systems. These average concentration-based evaluations are intended to 
inform the Six-Year Review, not to assess compliance with regulatory 
standards.

[[Page 59634]]



                     Table 6--Occurrence and Potential Exposure Analysis for Chemical NPDWRs
----------------------------------------------------------------------------------------------------------------
                                                                            Number (and     Population served by
                                                                          percentage) of     systems with a mean
                                          Current MCL    Benchmark \1\    systems with a    concentration higher
              Contaminant                   (ug/L)          (ug/L)      mean concentration   than benchmark (and
                                                                          \2\ higher than    percentage of total
                                                                             benchmark           population)
----------------------------------------------------------------------------------------------------------------
           Contaminants Identified Under the Health Effects Review as Having Potential for Lower MCLG
----------------------------------------------------------------------------------------------------------------
Cadmium...............................               5               1         182 (0.36%)       430,823 (0.16%)
Carbofuran............................              40               5       \3\ 7 (0.02%)    \3\ 49,409 (0.02%)
Cyanide...............................             200              50         328 (0.85%)     8,134,220 (3.43%)
cis-1,2-Dichloroethylene..............              70              10           7 (0.01%)        42,215 (0.02%)
Endothall.............................             100              50                   0                     0
Fluoride \4\..........................           4,000             900       4,479 (9.05%)    17,058,830 (6.30%)
Hexachlorocyclopentadiene.............              50              40                   0                     0
Methoxychlor..........................              40               1          1 (<0.01%)        22,536 (0.01%)
Oxamyl................................             200               9       \3\ 7 (0.02%)    \3\ 52,677 (0.02%)
Selenium..............................              50              30          91 (0.18%)        84,988 (0.03%)
Styrene...............................             100             0.5          89 (0.17%)        27,473 (0.01%)
Toluene...............................           1,000              60          14 (0.03%)        5,256 (<0.01%)
1,2,4-Trichlorobenzene................              70             0.5          15 (0.03%)       126,201 (0.05%)
Xylenes (total).......................          10,000              80          23 (0.05%)        34,728 (0.01%)
----------------------------------------------------------------------------------------------------------------
                  Contaminants with MCLs Higher than MCLGs (Limited by Analytical Feasibility)
----------------------------------------------------------------------------------------------------------------
Benzene...............................               5             0.5          83 (0.16%)       319,633 (0.12%)
Carbon tetrachloride..................               5             0.5          90 (0.17%)       766,891 (0.28%)
Chlordane.............................               2               1          1 (<0.01%)          240 (<0.01%)
1,2-Dichloroethane....................               5             0.5          60 (0.11%)       181,041 (0.07%)
Dichloromethane.......................               5             0.5         215 (0.41%)       360,289 (0.13%)
1,2-Dichloropropane...................               5             0.5          41 (0.08%)        34,800 (0.01%)
Heptachlor............................             0.4             0.1          1 (<0.01%)          900 (<0.01%)
Heptachlor epoxide....................             0.2             0.1           3 (0.01%)        32,710 (0.01%)
Hexachlorobenzene.....................               1             0.1           6 (0.02%)        17,278 (0.01%)
Pentachlorophenol.....................               1             0.9                   0                     0
2,3,7,8-TCDD (dioxin).................         0.00003        0.000005           7 (0.11%)        2,311 (<0.01%)
Tetrachloroethylene (PCE).............               5             0.5         432 (0.83%)    15,811,810 (5.76%)
Thallium..............................               2               1          71 (0.14%)        57,541 (0.02%)
Toxaphene.............................               3               1           2 (0.01%)          335 (<0.01%)
1,1,2-Trichloroethane.................               5               3          2 (<0.01%)           50 (<0.01%)
Trichloroethylene (TCE)...............               5             0.5         297 (0.57%)    12,755,926 (4.65%)
Vinyl chloride........................               2             0.5          24 (0.05%)       307,275 (0.11%)
----------------------------------------------------------------------------------------------------------------
\1\ Benchmark screening levels were set to either potential maximum contaminant level goals (MCLGs) or estimated
  quantitation levels (EQLs), depending on the contaminant. For more information see USEPA (2024g).
\2\ Results are based on long-term means generated by substituting one-half the MRL for each non-detection
  record. For results based on substituting the value of the full MRL or zero see USEPA (2024h).
\3\ Oxamyl and carbofuran have health endpoints associated with acute exposure and are not appropriate for long-
  term mean estimates. Results show the number of systems with at least one detection exceeding the benchmark.
\4\ Estimates represent naturally occurring fluoride concentrations. Quality assurance steps were taken to
  exclude samples from fluoridated water systems. See USEPA (2024i) for details.

    In addition, EPA performed a source water occurrence analysis for 
the 15 chemical contaminants in which updated health effects 
assessments indicated the possibility to increase (i.e., render less 
stringent) the MCLG values. EPA conducted this analysis to assess for 
meaningful opportunity to achieve cost savings while maintaining or 
improving the level of public health protection. The data available to 
characterize contaminant occurrence was limited because a comprehensive 
dataset to characterize drinking water source quality is not available. 
Data from the U.S. Geological Survey (USGS) National Water Quality 
Assessment program and the U.S. Department of Agriculture Pesticide 
Data Program water monitoring survey provide useful insights into 
potential contaminant occurrence in source water. The analysis of the 
available contaminant occurrence data for potential drinking water 
sources indicated relatively low contaminant occurrence in the 
concentration ranges of interest, and consequently, no meaningful 
opportunity for system cost savings by increasing the MCLG and MCL for 
these 15 contaminants. The results of this analysis were documented in 
``Occurrence Analysis for Potential Source Waters for the Fourth Six-
Year Review of National Primary Drinking Water Regulations'' (USEPA, 
2024j).
Treatment Feasibility
    Currently, all of the MCLs for chemical and radiological 
contaminants are either (1) set equal to the MCLGs, (2) limited by 
analytical feasibility, or (3) set at the level at which health risk 
reduction benefits were maximized at a cost justified by the benefits; 
none are currently limited by treatment feasibility. EPA considers 
treatment feasibility after identifying contaminants with the potential 
to lower the MCLG/MCL that constitute a meaningful opportunity to 
improve public health. No such contaminants were identified in the 
occurrence and exposure analysis described above.
    Treatment techniques were promulgated for two of the chemical and 
radiological contaminants that were subject to a detailed review in 
Six-Year Review 4. Acrylamide and epichlorohydrin occur in drinking 
water as treatment impurities and are primarily introduced as residuals 
in polymers and copolymers used for water treatment. There are no 
standardized analytical methods for their measurement in water; instead 
of sampling, water systems must certify to the State in writing that 
they use products meeting the specifications in the NPDWR. To evaluate 
the potential to revise the NPDWRs for these contaminants, EPA obtained 
data from

[[Page 59635]]

NSF on analyses for approval of products against NSF/ANSI Standard 60, 
which are based on EPA's regulation. NSF certification data shows that 
manufactured products contain acrylamide and epichlorohydrin impurity 
levels far below the current regulatory standard. Specifically, the 
mean residual acrylamide concentration of certified products is one-
fifth of the current regulatory level and the 90th percentile is one-
half. There were no samples with detections of residual 
epichlorohydrin. The available data indicates that the majority of 
tested products already pose lower health risks than required under the 
current TT, and therefore, revisions are a low priority. EPA is not 
listing acrylamide and epichlorohydrin as candidates for revision at 
this time. See USEPA (2024k) for details.
Other Regulatory Revisions
    In addition to possible revisions to MCLGs, MCLs, and TTs, as a 
part of the Six-Year Review 4, EPA considered whether other regulatory 
revisions to NPDWRs are needed to address implementation issues, such 
as revisions to monitoring and system reporting requirements. EPA used 
the protocol to evaluate which implementation issues to consider 
(USEPA, 2024c). EPA's protocol focused on items that were not already 
being addressed, or had not yet been addressed, through alternative 
mechanisms (e.g., as a part of a recent or ongoing rulemaking).
    EPA compiled information on implementation-related issues 
associated with the Chemical Phase Rules. EPA also identified 
unresolved implementation issues and concerns from previous Six-Year 
Reviews. The complete list of implementation issues related to the 
Phase and Radionuclides Rules is presented in ``Consideration of Other 
Regulatory Revisions in Support of the Fourth Six-Year Review of the 
National Primary Drinking Water Regulations: Chemical Phase Rules and 
Radionuclides Rules'' (USEPA, 2024l).
    The agency focused on the following five implementation issues in 
the Six-Year Review 4:

 Use of an alternative MCL for nitrate in Noncommunity Water 
Systems (NCWSs)
 Frequency of nitrate monitoring in Transient Noncommunity 
Water Systems (TNCWS)
 Frequency of nitrite monitoring
 Total nitrate-nitrogen plus nitrite-nitrogen MCL
 Total cyanide screening for free cyanide
    Table 7 of this document provides a brief description of the five 
issues and identified potential ways of addressing them. Please see 
section V.B.3. of this document for a discussion of these contaminants 
and their review outcomes. Please see USEPA (2024l) for a more detailed 
description and estimated scope of these issues.

Table 7--Chemical Rule Implementation Issues Identified That Fall Within
                      the Scope of an NPDWR Review
------------------------------------------------------------------------
               Implementation issue                 Description of issue
------------------------------------------------------------------------
Nitrate Alternative MCL in Non-community Water      EPA evaluated the
 Systems.                                            possibility of
                                                     removing or further
                                                     restricting the
                                                     options for some
                                                     NCWSs to use an
                                                     alternative nitrate-
                                                     nitrogen MCL of up
                                                     to 20 mg/L. The
                                                     nitrate-nitrogen
                                                     MCL specified for
                                                     PWSs in 40 CFR
                                                     141.62 is 10 mg/L
                                                     and is based on the
                                                     critical health
                                                     endpoint of
                                                     methemoglobinemia
                                                     in children under
                                                     six months of age.
                                                     40 CFR 141.11
                                                     provides States the
                                                     discretion to use
                                                     an alternative MCL
                                                     of 20 mg/L for non-
                                                     community water
                                                     systems (NCWS).
                                                     This alternative
                                                     MCL is allowed
                                                     under certain
                                                     conditions--includi
                                                     ng that water would
                                                     be unavailable to
                                                     children under six
                                                     months of age.
                                                    Monitoring
                                                     requirements for
                                                     nitrate-nitrogen
                                                     are specified in
                                                     the introductory
                                                     text to 40 CFR
                                                     141.23, which
                                                     states that ``Non-
                                                     transient, non-
                                                     community water
                                                     systems shall
                                                     conduct monitoring
                                                     to determine
                                                     compliance with the
                                                     maximum contaminant
                                                     levels specified in
                                                     Sec.   141.62 in
                                                     accordance with
                                                     this section.
                                                     Transient, non-
                                                     community water
                                                     systems (TNCWS)
                                                     shall conduct
                                                     monitoring to
                                                     determine
                                                     compliance with the
                                                     nitrate and nitrite
                                                     MCL in Sec.  Sec.
                                                     141.11 and 141.62
                                                     (as appropriate) in
                                                     accordance with
                                                     this section.''
                                                    Potential concerns
                                                     with the current
                                                     rule provisions
                                                     were identified as:
                                                        The
                                                        alternative MCL
                                                        does not address
                                                        any nitrate-
                                                        induced health
                                                        concerns beyond
                                                        methemoglobinemi
                                                        a and
                                                        While
                                                        Sec.   141.11
                                                        allows the use
                                                        of the
                                                        alternative MCL
                                                        by all eligible
                                                        NCWS, Sec.
                                                        141.23 implies
                                                        that only TNCWS,
                                                        a subcategory of
                                                        NCWS, are
                                                        eligible to use
                                                        the alternative
                                                        MCL.
                                                    To determine the
                                                     scope of this
                                                     issue, the agency
                                                     reviewed state
                                                     drinking water
                                                     regulations and
                                                     analyzed SYR 4 ICR
                                                     nitrate compliance
                                                     data and identified
                                                     nominal application
                                                     of the alternative
                                                     nitrate MCL by
                                                     NCWSs. In addition,
                                                     the nitrate and
                                                     nitrite human
                                                     health assessments
                                                     are currently being
                                                     evaluated by the
                                                     EPA IRIS program.
                                                     An updated
                                                     assessment could
                                                     inform the
                                                     potential health
                                                     effects of nitrate
                                                     exposure to levels
                                                     between 10 and 20
                                                     mg/L on adult
                                                     populations. EPA
                                                     will consider all
                                                     available and
                                                     updated human
                                                     health assessments
                                                     as it conducts
                                                     future cycles of
                                                     the six-year
                                                     review.
Nitrate Monitoring Frequency in Transient           Currently, community
 Noncommunity Water Systems.                         water systems
                                                     (CWSs) and NTNCWSs
                                                     are required to
                                                     monitor for nitrate
                                                     quarterly if a
                                                     sample is greater
                                                     than or equal to 50
                                                     percent of the
                                                     nitrate MCL (Sec.
                                                     141.23). TNCWSs are
                                                     required to monitor
                                                     for nitrate
                                                     annually (Sec.
                                                     141.23(d)(4)). In
                                                     the preamble to the
                                                     1991 final Phase II
                                                     rule, the agency
                                                     describes TNCWSs as
                                                     being subject to
                                                     the quarterly
                                                     monitoring
                                                     requirement stating
                                                     that ``EPA has
                                                     decided to retain
                                                     the 50 percent
                                                     trigger for
                                                     increased nitrate
                                                     monitoring in the
                                                     case of nitrate and
                                                     also to extend this
                                                     requirement to
                                                     TWSs'' (56 FR 3566,
                                                     USEPA, 1991).
                                                    EPA notes the
                                                     conflict between
                                                     the regulatory text
                                                     and the preamble.
                                                     To evaluate whether
                                                     it may be
                                                     appropriate to
                                                     revise the nitrate
                                                     NPDWR, the agency
                                                     analyzed compliance
                                                     monitoring data
                                                     collected under the
                                                     SYR 4 ICR. EPA
                                                     found that while
                                                     the majority of
                                                     TNCWSs that
                                                     reported detections
                                                     equal or greater
                                                     than 50 percent of
                                                     the nitrate MCL did
                                                     not conduct
                                                     quarterly
                                                     monitoring
                                                     afterward, the
                                                     number of these
                                                     systems appears
                                                     relatively small.
                                                     Due to the limited
                                                     scope of this
                                                     issue, EPA is not
                                                     revising the
                                                     monitoring
                                                     requirements at
                                                     this time but will
                                                     consider monitoring
                                                     requirements if
                                                     NPDWRs are revised
                                                     in the future.

[[Page 59636]]

 
Nitrite Monitoring Frequency......................  According to 40 CFR
                                                     141.23(e)(1), all
                                                     PWSs were required
                                                     to monitor for
                                                     nitrite once
                                                     between January 1,
                                                     1993, and December
                                                     31, 1995. If this
                                                     initial sample was
                                                     less than 50
                                                     percent of the MCL
                                                     (10 mg/L), systems
                                                     ``shall monitor at
                                                     the frequency
                                                     specified by the
                                                     State``. Though the
                                                     nitrite monitoring
                                                     frequency is not
                                                     explicitly stated
                                                     in the CFR, EPA's
                                                     guidance provides
                                                     that this frequency
                                                     should be at least
                                                     once every 9-year
                                                     compliance cycle
                                                     (USEPA, 2020d). EPA
                                                     is aware that some
                                                     States may not
                                                     require systems to
                                                     conduct routine
                                                     nitrite monitoring
                                                     when sample results
                                                     are less than 50
                                                     percent of the MCL.
                                                     Because sample
                                                     results below the
                                                     MCL are not
                                                     reported to EPA,
                                                     the scope of this
                                                     issue is uncertain.
                                                    To address this
                                                     uncertainty, EPA
                                                     analyzed State
                                                     regulations and
                                                     nitrite compliance
                                                     monitoring data to
                                                     characterize the
                                                     frequency of
                                                     nitrite monitoring.
                                                     Results indicated
                                                     that a majority of
                                                     systems monitored
                                                     for nitrite at
                                                     least once during
                                                     the last 9-year
                                                     compliance cycle
                                                     (2011-2019). EPA
                                                     intends to work
                                                     with States to
                                                     encourage more
                                                     systems to sample
                                                     for nitrite at
                                                     least once during
                                                     each 9-year
                                                     compliance cycle.
Total Nitrate and Nitrite Analysis for Nitrate MCL  In 40 CFR 141.62,
 Monitoring.                                         the MCL for nitrate
                                                     is specified as 10
                                                     mg/L and the MCL
                                                     for total nitrate
                                                     and nitrite is also
                                                     specified as 10 mg/
                                                     L. Sampling and
                                                     analytical
                                                     requirements as
                                                     specified in 40 CFR
                                                     141.23, however,
                                                     only included
                                                     nitrate and left
                                                     total nitrate and
                                                     nitrite monitoring
                                                     up to the
                                                     discretion of
                                                     States. Using Safe
                                                     Drinking Water
                                                     Information System
                                                     (SDWIS) compliance
                                                     data, EPA is aware
                                                     that at least half
                                                     of the States allow
                                                     total nitrate/
                                                     nitrite analysis to
                                                     determine
                                                     compliance with the
                                                     nitrate MCL.
                                                    To characterize
                                                     monitoring
                                                     practices for the
                                                     nitrate MCL, the
                                                     Agency analyzed Six-
                                                     Year Review 4
                                                     compliance
                                                     monitoring data for
                                                     both nitrate and
                                                     total nitrate/
                                                     nitrite. This
                                                     evaluation aims to
                                                     serve as a baseline
                                                     to assess nitrate
                                                     monitoring
                                                     practices in the
                                                     future, in response
                                                     to the 2020 EPA
                                                     guidance outlining
                                                     best practices when
                                                     using total nitrate/
                                                     nitrite analysis
                                                     for monitoring
                                                     compliance with the
                                                     nitrate MCL. EPA is
                                                     not revising the
                                                     monitoring
                                                     requirements at
                                                     this time but will
                                                     consider monitoring
                                                     requirements in
                                                     Sec.   141.23 if
                                                     NPDWRs are revised
                                                     in the future, to
                                                     incorporate best
                                                     practices similar
                                                     to those described
                                                     in recent guidance
                                                     (USEPA, 2020e).
------------------------------------------------------------------------

3. Select NPDWRs With New Information Not Appropriate for Revision
    The NPDWRs discussed in this section had new information 
identified, but EPA has determined they are not appropriate for 
revision at this time due to: (1) data gaps or emerging information 
that are necessary for EPA to evaluate as part of a review or; (2) new 
information that suggests low or no meaningful opportunity to provide 
greater public health protection. Examples of data gaps and emerging 
information identified during the review include an analytical 
monitoring challenge, a compliance reporting limitation, and an 
anticipated health effects assessment being developed by another U.S. 
Federal Agency. Specific details about the data gaps and emerging 
information identified during the review for on cyanide, fluoride, 
nitrate, nitrite, TCE, and PCE are provided below.
Cyanide
    EPA published the current MCL and MCLG of 0.2 mg/L (200 [micro]g/L) 
for free cyanide on July 17, 1992 (57 FR 31776, USEPA, 1992). In 2010, 
EPA published an IRIS assessment (USEPA, 2010b), which identified a new 
reproductive health effect endpoint that supports decreasing the MCLG 
from 200 [micro]g/L to 4 [micro]g/L. Analytical feasibility information 
identified in Six-Year Review 3 and Six-Year Review 4 supports a PQL 
reduction to as low as 50 [micro]g/L. In Six-Year Review 3, cyanide was 
listed as ``low priority'' due to low occurrence at levels below the 
current MCL. Analysis of Six-Year Review 4 occurrence data identified 
greater occurrence with 328 systems serving 8.1 million people with 
mean concentrations above 50 [micro]g/L (see Table 6 of this document). 
However, occurrence was limited to few states (USEPA, 2024h). EPA 
considered these occurrence results and the potential for a meaningful 
opportunity to improve the level of public health protection.
    Two analytical monitoring challenges complicate interpretation of 
the occurrence data. As described in section V.B.2 of this document, an 
analytical artifact created by ascorbic acid pretreatment of drinking 
water samples, which had been disinfected with chloramines, can result 
in false positives for free cyanide (USEPA, 2020f). An EPA guidance 
document (USEPA, 2020f) identified solutions to address this analytical 
challenge, but the general awareness of the availability of this 
guidance is uncertain. Second, EPA is aware that some systems analyze 
samples for total cyanide, and if the results are lower than the MCL, 
these systems report the total cyanide results as free cyanide. Systems 
may achieve cost savings by analyzing samples for total cyanide; 
however, using results for total cyanide instead of free cyanide could 
potentially overestimate the actual occurrence of free cyanide. Free 
and total cyanide results cannot be distinguished in the Six-Year 
Review 4 ICR dataset because the Safe Drinking Water Information System 
(SDWIS) State-version that many primacy agencies use to manage SDWA 
compliance monitoring data does not have an analyte code for total 
cyanide. Because the numerical benchmark used for occurrence is 
significantly lower than the current cyanide MCL, some of the reported 
concentrations may be for total cyanide. Therefore, the Six-Year Review 
4 occurrence analysis likely overestimates free cyanide occurrence. For 
these reasons, EPA does not believe it is appropriate to list the 
cyanide NPDWR as a candidate for revision at this time. EPA intends to 
help address these data gaps by continuing to disseminate the 2020 
guidance on analytical methods for cyanide and may consider an 
additional analyte code for total cyanide in the SDWIS reporting 
system. Further discussion of the cyanide monitoring issues can be 
found in USEPA (2024h).
Fluoride
    EPA published the MCL and MCLG of 4.0 mg/L for fluoride on April 2, 
1986 (51 FR 11396, USEPA, 1986) based on the critical health endpoint 
of crippling skeletal fluorosis. EPA also established a secondary MCL/
MCLG at 2.0 mg/L to protect against cosmetically objectionable dental 
fluorosis

[[Page 59637]]

(discoloration and/or pitting of teeth). Certain drinking water systems 
may choose to fluoridate finished water as a public health protection 
measure for reducing the incidence of cavities. The U.S. Public Health 
Service (PHS) recommendation for the optimal community water 
fluoridation level is 0.7 mg/L (U.S. Department of Health and Human 
Services, 2015). The decision to fluoridate a community water supply is 
made by the state or local municipalities and is not required by EPA or 
any other federal entity. Fluoride is also added to various consumer 
products, such as toothpaste and mouthwash.
    EPA has reviewed the NPDWR for fluoride in prior Six-Year Reviews. 
As a result of Six-Year Review 1, EPA requested that the National 
Research Council (NRC) of the National Academies of Sciences (NAS) 
conduct a review of the health and exposure data on orally ingested 
fluoride. In 2006, the NRC published the results of its review and 
concluded that severe dental fluorosis can be an adverse health effect 
(NRC, 2006). The NRC report recommended that EPA develop a dose-
response assessment for severe dental fluorosis as the critical health 
endpoint and update an assessment of fluoride exposure from all 
sources.
    In 2010, EPA published Dose Response Analysis for Noncancer Effects 
(USEPA, 2010d), which was considered under Six-Year Review 3. For more 
information, please see Appendix C of the Six-Year Review 3 Health 
Effects Assessment for Existing Chemical and Radionuclide National 
Primary Drinking Water Regulations--Summary Report (USEPA, 2016). In 
Six-Year Review 3, EPA did not recommend the fluoride NPDWR for 
revisions citing limited agency resources, prioritization of other 
contaminants, ongoing health effects research, and other factors that 
were anticipated to reduce the U.S. population's exposure to fluoride 
via drinking water (82 FR 3531, USEPA, 2017a). In Six-Year Review 4, 
EPA again considered the 2010 EPA assessment to derive a lower 
potential MCLG of 0.9 mg/L. Review results are provided in section 
V.B.2. of this document.
    Available published literature on other health effect categories 
including neurotoxicity and behavior, reproduction and development, 
endocrine effects, and cancer were reviewed in the EPA assessment 
(USEPA, 2010d). However, based on the review of the available 
literature at the time, EPA determined that the data for these other 
health effects associated with fluoride exposure were insufficient to 
support their selection as critical effects for potential MCLG 
derivation (USEPA, 2010d). EPA is aware of ongoing efforts by the 
National Toxicology Program (NTP) to conduct a systematic review and 
meta-analysis of the published literature on developmental 
neurotoxicity for fluoride. In May 2023, NTP released the Draft ``NTP 
Monograph on the State of the Science Concerning Fluoride Exposure and 
Neurodevelopmental and Cognitive Health Effects: A Systematic Review'' 
(NTP, 2023); however, the NTP systematic review and meta-analysis are 
not health assessments that could be used to directly inform the 
derivation of a potential MCLG. Due to emerging research published on 
developmental neurotoxicity after fluoride exposure coupled with 
competing workloads and other ongoing high priority actions (see 
section V.A of this document.), EPA has decided that the fluoride NPDWR 
is not a candidate for revision at this time. In addition, the NTP has 
not made a final decision about the report's developmental 
neurotoxicity systematic review conclusions and has not formally 
released a final report. Following publication of the final NTP report, 
EPA will consider the systematic review and meta-analysis conclusions 
regarding developmental neurotoxicity to inform the agency's future 
development of a health effects assessment for fluoride. See USEPA 
(2024f) Appendix B for more information.
Nitrate and Nitrite
    EPA published the MCLs and MCLGs for nitrate (10 mg/L) and nitrite 
(1 mg/L) based on the critical endpoint of methemoglobinemia (blue baby 
syndrome) on January 30, 1991 (56 FR 3526, USEPA, 1991). Nitrate and 
nitrite were not reviewed in detail under Six-Year Review 3 due to 
ongoing IRIS assessments at that time. Although the development of the 
IRIS assessment for nitrate and nitrite was suspended in December 2018, 
EPA has restarted development of their health assessment for nitrate 
and nitrite as indicated in the October 2023 IRIS Program Outlook. The 
agency recently released the ``Protocol for the Nitrate and Nitrite 
IRIS Assessment (Oral)'' for public comment on November 9, 2023 (88 FR 
77310, USEPA, 2023b). EPA plans to evaluate whether a revision of the 
nitrate and nitrite NPDWRs is appropriate, once the final IRIS 
assessment is available.
Trichloroethylene (TCE) and Tetrachloroethylene (PCE)
    The NPDWR for TCE was published on July 8, 1987 (52 FR 25690, 
USEPA, 1987) and the NPDWR for PCE was published on January 30, 1991 
(56 FR 3526, USEPA, 1991). Both TCE and PCE are classified as 
carcinogens and have MCLGs and MCLs of zero and 5 [micro]g/L, 
respectively. The MCLs were based on analytical feasibility at the time 
of rule promulgation. TCE and PCE were both listed as candidates for 
revision in Six-Year Review 2, based on updated analytical feasibility, 
treatment, and occurrence information.
    In 2011, EPA announced plans to address a group of regulated and 
unregulated carcinogenic volatile organic contaminants (cVOCs) in a 
single regulatory effort. The eight regulated contaminants that were 
evaluated for the cVOCs group regulation included benzene, carbon 
tetrachloride, 1,2-dichloroethane, 1,2-dichloropropane, 
dichloromethane, PCE, TCE, and vinyl chloride. In Six-Year Review 3, 
these contaminants were categorized under recent, ongoing, or planned 
regulatory action and were not reviewed. The cVOC group regulation was 
not promulgated, as a result these eight contaminants were reviewed 
again during Six-Year Review 4. EPA has determined that TCE and PCE are 
no longer candidates for revision at this time based on updated 
information.
    In Six-Year Review 2, EPA assessed analytical information that 
supported reducing the PQL and evaluated occurrence for TCE and PCE at 
0.5 [micro]g/L. As shown in Tables 5 and 6 of this document, EPA 
identified information in Six-Year Review 4 that again supported 
assessing occurrence at that level. The average TCE concentration 
exceeded 0.5 [micro]g/L in 297 systems, representing 0.57 percent of 
the systems assessed nationwide and serving approximately 13 million 
people. Similarly, the average PCE concentration exceeded 0.5 [micro]g/
L in 432 systems, which represent 0.83 percent of the approximately 
50,000 PWSs assessed nationwide and serve approximately 16 million 
people. These occurrence results are consistent with the Six-Year 
Review 2 estimates (75 FR 15500, March 29, 2010, USEPA, 2010a).
    The most recent final IRIS assessments for TCE (USEPA, 2011) and 
PCE (USEPA, 2012) were completed after the Six-Year Review 2 results 
were published and have been selected as the health assessments 
relevant to chronic toxicity for TCE and PCE in Six-Year Review 4 
(USEPA, 2024f). The updated IRIS assessments maintained the 
classification of ``carcinogenic to humans,'' and therefore do not 
support a change to the MCLGs of zero for either TCE or PCE. Based on 
the Six-Year Review 4 occurrence estimates described above, EPA 
considered if there was a potential for an increase in

[[Page 59638]]

human health protection at the lower identified level. To evaluate this 
potential, EPA examined the cancer risk level associated with the 
current MCLs (5 [micro]g/L) and the screening level (0.5 [micro]g/L) 
using updated occurrence and health effects information from Six-Year 
Review 4. The cancer risk levels at the current MCLs for TCE and PCE 
are 1 x 10-5 (USEPA, 2011) and 3.0 x 10-7 (USEPA, 
2012), respectively. These cancer risk levels correspond to excess 
lifetime cancer cases of 10 and 0.3 cases per million people, 
respectively. At the screening level of 0.5 [micro]g/L, the risk per 
million people would be 1 case for TCE and 0.03 cases for PCE. The 
implied number of baseline cancer cases over a 70-year exposure period 
is unlikely to exceed 120 total cases for TCE and 5 total cases for 
PCE. This corresponds to annual averages of 1.7 and 0.07 cases for TCE 
and PCE, respectively. This new information identified since Six-Year 
Review 2 indicates that revising the MCLs for either TCE or PCE would 
result in relatively small health risk reductions among the exposed 
population and would divert significant resources from other planned 
and ongoing work. Therefore, EPA has determined that TCE and PCE are 
considered ``low priority'' and are no longer candidates for revision.

C. Microbial Contaminants Regulations

    As discussed in section III of this document, the initial review 
branch of the review protocol identifies NPDWRs that have recently been 
recently competed or are being reviewed in ongoing or pending 
regulatory actions. Excluding such contaminants from a more detailed 
review in the Six-Year Review 4 prevents duplicative Agency efforts. 
Based on the initial review and considering the ongoing rulemaking 
activities for the Microbial and Disinfection Byproduct Rules, EPA did 
not perform a more detailed review for the Surface Water Treatment Rule 
(SWTR), the Interim Enhanced Surface Water Treatment Rule (IESWTR), the 
Long-Term 1 Enhanced Surface Water Treatment Rule (LT1ESWTR), and the 
Stage 1 and Stage 2 Disinfectants and Disinfection Byproducts Rules. 
The following microbial contaminant regulations were subject to a more 
detailed review for the Six-Year Review 4:

 Revised Total Coliform Rule (RTCR)
 Long Term 2 Enhanced Surface Water Treatment Rule (LT2)
 Ground Water Rule (GWR)
 Aircraft Drinking Water Rule (ADWR)
 Filter Backwash Recycling Rule (FBRR)

    Background information on each of the microbial contaminant 
regulations is presented in the subsequent sections. EPA is conducting 
its first detailed review of the RTCR and the ADWR as part of the Six-
Year Review. The RTCR and the ADWR were excluded from a detailed review 
in Six Year Review 3 because they were promulgated in 2013 and 2009, 
respectively.
    These microbial contaminants regulations establish treatment 
technique (TT) requirements in lieu of MCLs, except in the RTCR, EPA 
also established an MCL for Escherichia coli (E. coli) and TT 
requirements for total coliform. In accordance with the Six-Year Review 
Protocol, during the six-year review process, EPA assesses whether new 
health risk, analytical methods, or treatment information indicate 
possible TT revision. For the RTCR, the regulatory review determines 
whether new information indicates potential revision to the MCL for E. 
coli.
    The elements of the RTCR, LT2, GWR, and ADWR regulations that were 
reviewed for Six-Year Review 4 were: health effects, analytical 
feasibility, occurrence and exposure, and treatment feasibility. For 
the RTCR, LT2, GWR, and ADWR regulations, the EPA did not find any new 
relevant information as it relates to analytical feasibility. For all 
the other elements reviewed a summary of the findings is included in 
the subsequent sections. In addition, detailed information about the 
review is provided in the ``Six-Year Review 4 Technical Support 
Document for Microbial Contaminant Regulations'' (USEPA, 2024m).
    At this time, none of the reviewed microbial contaminant rules are 
being identified as a candidate for regulatory revision.
1. Revised Total Coliform Rule
Background
    EPA promulgated the Revised Total Coliform Rule (RTCR), a revision 
to the Total Coliform Rule, on February 13, 2013 (78 FR 10269, USEPA, 
2013). The Total Coliform Rule (TCR) was promulgated on June 29, 1989 
(54 FR 27544, USEPA, 1989). The purpose of the revision was to increase 
public health protection through the reduction of potential entry 
pathways for fecal contamination into distribution systems. The TCR 
required all public water systems (PWSs) to monitor for the presence of 
total coliforms and Escherichia coli (E. coli)) in the distribution 
system at a frequency dependent on the size (population served by) of 
the system. Under the TCR, a maximum contaminant level (MCL) was 
established based on the presence or absence of total coliforms with 
the intent to address contamination that could enter into distribution 
systems. The RTCR revised the TCR to eliminate the MCL for total 
coliforms and established an MCLG and MCL for E. coli of zero. The RTCR 
also requires PWSs that have an indication of coliform contamination 
(e.g., as a result of total coliform positive samples, E. coli MCL 
violations or performance failure) to find and assess the problem, 
identify sanitary defects and take corrective action. There are two 
levels of assessments (i.e., Level 1 and Level 2) based on the severity 
or frequency of the problem.
Summary of Review Results
    Information available for national occurrence and exposure 
indicates that both routine total coliform and E. coli positive rates 
have decreased after the implementation of RTCR. EPA concludes that no 
regulatory revisions to the RTCR are appropriate at this time based on 
the review of available information.
Health Effects
    Collier et al. (2021) estimated the collective U.S. disease burden 
attributable to over a dozen waterborne illnesses from infectious 
pathogens found in the distribution system (vibriosis, 
campylobacteriosis, cryptosporidiosis, giardiasis, Legionnaire's 
disease, salmonellosis, shigellosis, infections by non-tuberculous 
mycobacteria (NTM), norovirus, Shiga-toxin-producing E. coli, otitis 
externa, pneumonia, and septicemia). These researchers estimated the 
total disease burden at approximately 7.15 million cases annually, with 
an estimated 118,000 hospitalizations and 6,630 deaths. In this 
analysis, waterborne disease is understood to include gastrointestinal, 
respiratory, and systemic disease attributable to both drinking-water 
and non-drinking-water exposure. From further evaluation of this 
study's cases, Gerdes et al. (2023) determined 1.13 million of these 
illnesses were attributable to drinking water. According to the 
estimates presented in these studies, the opportunistic pathogens 
(Legionella, Nontuberculous Mycobacteria (NTM), and Pseudomonas) impose 
a greater public health burden than the fecal pathogens. Of the 
estimated 7.15 million infectious waterborne illnesses in 2014 in the 
United States, drinking water exposure caused 40 percent of 
hospitalizations and 50 percent of deaths.

[[Page 59639]]

Occurrence and Exposure
    To evaluate potential pathogenic contamination in distribution 
systems EPA analyzed national compliance monitoring data from the SYR 4 
ICR dataset (USEPA, 2019. EPA assessed the trends that may be 
associated with the implementation of the RTCR and found a 
statistically significant decline for total coliform positive results 
from years of 2014-2015 to 2018-2019 (i.e., before and after the 
implementation of RTCR respectively). The result suggests that the 
presence of these indicator organisms in the distribution system was 
declining. The trend of declining positive total coliform results was 
observed across different types of public water systems, water sources 
(ground water versus surface water), and system sizes (small versus 
large). With respect to the fecal contamination indicator E. coli, the 
observed decreasing trend was not supported by a statistical test of 
significance. EPA also found that the absolute number of E. coli 
positives were low, suggesting that the treatment techniques are 
effective (USEPA, 2024m).
Treatment Feasibility
    In this section as part of Six-Year Review process, EPA evaluated 
new information about tools and treatment techniques. Since the major 
treatment technique requirements under the RTCR are assessments 
followed by corrective actions (if total coliform and/or E. coli are 
detected), EPA evaluated the effectiveness of such requirements by 
comparing total coliform and E. coli positive rates after completion of 
either Level 1 or Level 2 assessments (USEPA, 2024m).
    EPA found about an 80 percent decrease in both routine total 
coliform and E. coli positive rates, two months after completion of 
RTCR assessments for systems having a monthly monitoring schedule.
    These analytical results and newly compiled information suggest 
that the ``find and fix'' approach prescribed under the provisions of 
assessments and corrective action within RTCR appears to work as 
intended for reducing the microbial occurrence in distribution systems 
and may be improving public health protection from microbial risks (as 
indicated by a substantial drop of the total coliform and E. coli 
positive rates following completion of corrective actions to respond to 
assessments).
2. Long Term 2 Enhanced Surface Water Treatment Rule
Background
    EPA promulgated the Long Term 2 Enhanced Surface Water Treatment 
Rule, hereafter referred to as ``LT2'', on January 5, 2006 (71 FR 654, 
USEPA, 2006a). The LT2 applies to all PWSs that use surface water or 
ground water under the direct influence of surface water. The LT2 
builds upon the IESWTR and the LT1 by improving control of microbial 
pathogens and by focusing on systems with elevated Cryptosporidium 
contamination risk. The purposes of the LT2 are to protect public 
health from illness arising from exposure to Cryptosporidium and other 
microbial pathogens in drinking water and to prevent significant 
increases in risks that might occur when systems implement drinking 
water disinfection byproduct rules.
    Key provisions in the LT2 include: source water monitoring for 
Cryptosporidium (with a screening procedure to reduce monitoring costs 
for small systems); risk-targeted Cryptosporidium treatment by filtered 
systems with the highest source water Cryptosporidium levels; 
inactivation of Cryptosporidium by all unfiltered systems; criteria for 
the use of Cryptosporidium treatment and control processes; and 
covering or treating uncovered finished water storage facilities.
    The LT2 requires PWSs using surface water or ground water under the 
direct influence of surface water to monitor their source waters for 
Cryptosporidium and/or E. coli to identify additional treatment 
requirements. PWSs must monitor their source water (i.e., the influent 
water entering the treatment plant) over two different timeframes 
(defined as Round 1 and Round 2) to determine the occurrence of 
Cryptosporidium. Monitoring results determine the extent of 
Cryptosporidium treatment requirements under the LT2. According to the 
LT2 rule requirements, all PWSs were to complete Round 2 by 2021. To 
reduce monitoring costs, small filtered PWSs (serving fewer than 10,000 
people) which initially monitor for E. coli for one year as a screening 
analysis, are required to monitor for Cryptosporidium only if their E. 
coli levels exceed specified trigger values. Small filtered PWSs that 
exceed the E. coli trigger, as well as small unfiltered PWSs, must 
monitor for Cryptosporidium for one or two years, depending on the 
sampling frequency. The LT2 also requires all unfiltered PWSs to 
provide at least 2 to 3-log (i.e., 99 to 99.9 percent) inactivation of 
Cryptosporidium. Further, under the LT2, unfiltered PWSs must achieve 
their overall inactivation requirements (including Giardia lamblia and 
virus inactivation as established by earlier regulations) using a 
minimum of two disinfectants.
    Under the LT2, PWSs with uncovered finished water reservoirs 
(UCFWR) must either cover the storage facility or treat the water 
leaving the storage facility to achieve inactivation and/or removal of 
4-log virus, 3-log Giardia lamblia and 2-log Cryptosporidium using a 
protocol approved by the state (USEPA, 2006a). Most finished water 
reservoirs for surface water systems are covered. All PWSs with UCFWRs 
are under administrative orders or compliance agreements to cover or 
treat their UCFWR.
Summary of Review Results
    From a review of the literature on Cryptosporidium health effects, 
EPA concludes that there is no new health information to suggest a need 
to modify the LT2. In addition, EPA determined that no regulatory 
revisions to the microbial toolbox options are appropriate at this 
time. During Six-Year Review 4, EPA did not consider disinfection 
profiling information since EPA is evaluating overall filtration and 
disinfection requirements in the SWTRs as part of the on-going 
consideration of potential revisions to the MDBP rules. For more 
information regarding EPA's review of treatment feasibility see the 
``Six-Year Review 4 Technical Support Document for Microbial 
Contaminant Regulations'' (USEPA, 2024m).
Health Effects
    Since 1995, cryptosporidiosis has been a nationally notifiable 
disease, meaning healthcare providers and laboratories that diagnose 
cases of laboratory-confirmed cryptosporidiosis are required to report 
cases to their local or state health departments, which in turn report 
the cases to CDC. Since 2012, there have been four reported outbreaks 
of cryptosporidiosis from public water systems to CDC. The four 
outbreaks together resulted in a total of 201 recorded illnesses, 2 
hospitalizations, and no deaths (CDC, 2022). Although cryptosporidiosis 
is a nationally notifiable disease, additional outbreaks may go 
unreported to CDC or may have been recorded as of uncertain causes. In 
addition, since CDC's National Outbreak Reporting System is 
specifically focused on outbreaks, it does not capture rates of endemic 
disease of cryptosporidiosis from drinking water.

[[Page 59640]]

Occurrence and Exposure
    Based on the LT2 source water monitoring results, filtered systems 
were classified in one of four risk categories (Bins 1-4) to determine 
additional treatment needed. Systems in Bin 1 are not required to 
provide additional Cryptosporidium treatment. Systems in Bins 2-4 must 
achieve 1.0-2.5 log of treatment (i.e., 90 to 99.7 percent reduction 
for Cryptosporidium) over and above that provided by conventional 
treatment, depending on the Cryptosporidium concentrations. Filtered 
PWSs must meet the additional Cryptosporidium treatment requirements in 
Bins 2, 3, or 4 by selecting one or more technologies from the 
microbial toolbox to ensure source water protection and management, 
and/or Cryptosporidium removal or inactivation. All unfiltered water 
systems must provide at least 99 or 99.9 percent (2 or 3-log) 
inactivation of Cryptosporidium, depending on their monitoring results. 
All filtered systems that provide 5.5 log treatment for Cryptosporidium 
are exempt from monitoring and subsequent bin classification.
    Six years after the initial bin classification following a first 
round of monitoring, filtered systems were required to conduct a second 
round of monitoring. Round 2 monitoring began in 2015. Round 2 
monitoring was implemented to understand year-to-year variability for 
occurrence of Cryptosporidium. The difference observed between 
occurrence at the time of the ICR Supplemental Surveys and the LT2 
Round 1 monitoring indicates year-to-year variability (USEPA, 2017a).
    Limited occurrence data for Cryptosporidium was available to EPA in 
response to the SYR 4 ICR since fewer than 1 percent of the 
Cryptosporidium monitoring records provided actual concentration levels 
with units of oocysts/L; however, the data about system binning for 
about 300 PWSs serving populations larger than 10,000 was provided. 
Those data indicate that the percentage of PWSs potentially moving to 
an ``action bin'' based on Round 2 monitoring would not be 
substantially higher than the percentage estimated based on modeling 
conducted during the LT2 review included as part of the Six-Year Review 
3, thus suggesting no change to the review decision made under Six-Year 
Review 3.
Treatment Feasibility
    The LT2 includes a variety of treatment and control options, 
collectively termed the ``microbial toolbox,'' that PWSs can implement 
to comply with the LT2's additional Cryptosporidium treatment 
requirements. Most options in the microbial toolbox carry prescribed 
credits toward Cryptosporidium treatment and control requirements. The 
LT2 Toolbox Guidance Manual (USEPA, 2010e) provides guidance on how to 
apply the toolbox options.
    For the Six-Year Review 4, EPA reviewed additional research into 
the relationship between ultraviolet light (UV) dose and log 
inactivation. Some studies showed the same log inactivation at UV doses 
lower than those reported in previous EPA guidance, and other studies 
showed log inactivation at UV doses higher than those contained in the 
guidance. Since there is not a consensus of log inactivation at levels 
significantly lower than EPA prior published guidance, EPA concludes 
that the new information does not support changes to the UV dose table.
    EPA also reviewed new information pertaining to technologies, which 
have not been included in the existing LT2 toolbox guidance manual, and 
which may be effective for the removal or inactivation of protozoa 
including Cryptosporidium. In addition, EPA also reviewed new 
technologies that water systems may be employing to improve treatment 
performance for complying with the MDBP rules, e.g., turbo coagulation 
and powdered activated carbon. Initiatives by states and EPA's Area 
Wide Optimization Program were evaluated as well. EPA found that this 
new information appears insufficient to develop quantification criteria 
for inactivation and removal credit for Cryptosporidium.
3. Ground Water Rule
Background
    EPA promulgated the Ground Water Rule (GWR) in 2006 (71 FR 65574, 
USEPA, 2006b) to provide protection against microbial pathogens in PWSs 
using ground water sources. The rule establishes a risk-based approach 
to target undisinfected ground water systems that are vulnerable to 
fecal contamination. In addition to the protection provided by the RTCR 
and GWR monitoring requirements, systems that do not disinfect are also 
protected by the sanitary survey provisions of the GWR and the 
treatment technique provisions of the RTCR.
    The GWR required compliance beginning December 1, 2009. Since the 
triggered source water monitoring provision was built upon the 
compliance monitoring results of total coliform and E. coli under the 
TCR and later RTCR, implementation of the GWR was not yet completed for 
the period of time covered by the Six-Year Review 3 ICR (2006-2011). 
The RTCR was promulgated in 2013 and became effective on April 1, 2016. 
EPA expected that implementation of the RTCR might impact the percent 
of ground water systems that would be triggered into source water 
monitoring and taking any corrective actions under the GWR. Therefore, 
the effects of the GWR and the RTCR implementation in addressing 
vulnerable ground water systems were not reviewed during the Six-Year 
Review 3 process.
Summary of Review Results
    The information considered during this review suggest that 
microbial pathogens have been detected in untreated ground water 
samples which show no presence of fecal indicators, however these 
studies are limited in quantity and the prevalence of endemic disease 
from microbial contamination of untreated ground water cannot be well 
characterized with the available information (USEPA, 2024m). Additional 
and more robust studies are needed to further understand the magnitude 
of the issue. EPA concludes that no regulatory revisions to the GWR are 
appropriate at this time.
Health Effects
    Waterborne pathogens can cause mild to severe illnesses (Wallender 
et al., 2014). These illnesses may include; acute gastrointestinal 
illness (AGI) with diarrhea, abdominal pain/discomfort, nausea, 
vomiting, conjunctivitis, aseptic meningitis, and hand-foot-and-mouth 
disease. Infections from some waterborne pathogens (e.g., 
Campylobacter) may cause long-term implications, such as reactive 
arthritis, Guillain-Barr[eacute] syndrome, and irritable bowel syndrome 
(Keithlin et al., 2014). Other more severe illnesses include hemolytic 
uremic syndrome (HUS) (kidney failure), hepatitis, and bloody diarrhea 
(WHO, 2004).
    Some studies have indicated that waterborne pathogens such as 
adenovirus, enteroviruses, hepatitis A, norovirus, rotavirus, 
Salmonella, Giardia, Cryptosporidium, and Shigella have been found in 
untreated ground water samples (Borchardt et al., 2012; Wallender et 
al., 2014; Stokdyk et al., 2020).
    Human enteric viruses have been detected in drinking water free of 
bacterial indicators, such as total coliform. With total coliform 
detections rates similar to the average rate for

[[Page 59641]]

undisinfected community PWSs in the U.S, Borchardt et al. (2012) 
estimated a six to 22 percent attributable risk for enteric illness 
from viruses present in the communities' drinking water. In another 
study, Burch et al. (2022) found that noncommunity wells had higher 
infection risk than community wells. Burch et al. (2022) found the 
annual risk was relatively high for all pathogens combined in the 
study, while the average daily doses for individual pathogens were low, 
indicating that significant risk results from sporadic pathogen 
exposure. Studies by Fout et al. (2017) and Stokdyk et al. (2020) found 
that total coliform (and other indicators like E. coli, somatic phage, 
HF183, and Bacteroidales-like HumM2) tend to have high specificity, 
meaning that absence of the indicator provides relatively strong 
assurance that water is free of viral and other pathogens, but also 
have low sensitivity, meaning that presence of the indicator does not 
necessarily predict presence of pathogens.
Occurrence and Exposure
    Similar to the RTCR, EPA examined the national compliance 
monitoring data collected for the Six-Year Review 4 to understand how 
total coliform and E. coli, indicators of contamination behaved before 
and after implementation of the GWR, as well as understanding how level 
of contamination for high risk undisinfected ground water systems have 
changed.
    As noted, GWR monitoring is based on initial monitoring under the 
RTCR. If a system has a positive total coliform sample (based on 
routine coliform monitoring under the RTCR), the system must test that 
sample for the presence of E. coli. Under the GWR, ground water systems 
that do not provide at least 4-log treatment of viruses and are 
notified of a routine positive total coliform sample collected under 
RTCR must collect and analyze at least one source water sample for E. 
coli or other fecal indicators from each ground water source (well) 
within 24 hours. If the triggered source water sample has a positive 
for E. coli the ground water systems must take corrective action. EPA 
conducted a distribution system total coliform/E. coli data exploration 
and analysis effort to identify findings that could inform the risk 
reduction of the fully implemented GWR, as well as characterize high 
risk systems.
    The national average total coliform and E. coli rates (i.e., total 
number of positives divided by total number of samples) before and 
after implementation of the GWR were calculated using Six-Year Review 3 
and Six-Year Review 4 datasets. The analytical results were grouped by 
system sizes and disinfection status (i.e., disinfecting versus and 
undisinfected). The period of analysis was from 2007-2008 (before the 
GWR was implemented) to 2014-2015 (after the completed implementation 
of the first round of sanitary surveys under the GWR). The total 
coliform rates across different system categories decreased, suggesting 
that there may be less pathogenic contamination pathways and so 
potentially less microbial exposure, corresponding to the period when 
the GWR was being implemented. This downward change is supported by a 
statistical significance test. The declining count of the fecal 
contamination indicator, E. coli was not supported by a test of 
statistical significance. Yet numbers of E. coli positives were 
consistently low, which may indicate low exposure to fecal 
contamination.
    EPA performed a more specific analysis using a statistical model 
focused on the most vulnerable water systems, the undisinfected ground 
water systems. EPA conducted statistical modeling focused on 
examination of total coliform levels in small ground water systems to 
account for their infrequent sampling and relatively low level of 
monitoring observations compared to larger systems that monitor more 
frequently.
    There are approximately 45,000 undisinfected ground water systems 
associated with total coliform records collected and less than 1 
percent population among the population served by the public community 
water systems in the U.S. (based on SYR 4 ICR data). Most undisinfected 
ground water systems serve small permanent populations or transient 
populations.
    EPA found that the smallest systems (serving a population fewer 
than 1,001) have higher median total coliform rates than undisinfected 
larger systems. In addition, the analysis indicates that median 
occurrence rates for many undisinfected transient systems may have 
fallen, from four to three percent total coliform detection rate from 
2011 to 2019. Another finding from the statistical modeling is that the 
number of non-community systems that have high total coliform 
detections in the systems serving fewer than 1,001 people has remained 
roughly the same, about 7,000 undisinfected ground water systems, when 
running a comparison using Six-Year Review 3 and Six-Year Review 4 ICR 
data with a threshold of five percent rate of total coliform positive 
detections, which is the threshold that triggers a Level 1 Assessment 
in the RTCR. For statistical analysis of E. coli detection rates, there 
was not sufficient data to make estimates of averages and numbers of 
systems exceeding high levels.
    Two implications of these modelling results should be noted as it 
relates to estimating potential exposure and occurrence. One is that 
the non-community systems serving fewer than 1,001 have total coliform 
positive rates around two to four percent, while a study of 14 
community systems served by untreated ground water in Wisconsin found 
that a total coliform positive rate of 2.3 percent was associated AGI 
burden (Borchardt et al, 2012). EPA concludes, however, that studies 
indicating microbial disease burden at total coliform positive levels 
found in high-risk systems are limited in number as mentioned in the 
Health Effects section, as well as in geographic scope. Another 
implication from the results of this statistical analysis is that the 
remaining systems with very high total coliform rates could suggest 
compliance challenges among small ground water systems.
    In addition to evaluating trends with indicators under RTCR to 
evaluate protection for vulnerable ground water systems, EPA also 
considered the results from the GWR requirement for triggered source 
water sampling. The sample results indicate that there is a small 
percent of positive source water E. coli detections ranging from 0.76 
percent to 1.99 percent of E. coli samples for non-community systems 
which are primarily undisinfected systems, and 250 out of 270 of source 
water E. coli detections were associated with undisinfected systems 
serving fewer than 500 people. The other fecal indicators, coliphage 
and enterococci were used very infrequently, and data was insufficient 
to evaluate. Low incidence of fecal indicators may indicate low 
exposure to fecal contamination among undisinfected ground water 
systems.
Treatment Feasibility
    Per treatment technique requirements under the GWR, there are two 
scenarios that trigger ground water systems to take corrective actions: 
(1) positive results of the triggered source water monitoring, and (2) 
significant deficiencies found during Sanitary Survey (EPA was not able 
to assess sanitary surveys directly given data limitations). EPA 
evaluated whether treatment was improving under the GWR by using the 
RTCR occurrence analysis data to consider total coliform rates before 
and after the GWR was implemented.

[[Page 59642]]

    EPA developed a systematic approach to identify disinfection status 
of ground water systems for each of the years included in the Six-Year 
Review ICR datasets and found that the percentage of ground water 
systems that were disinfecting had increased consistently from 2007-
2008 (before the GWR was implemented) to 2014-2015. This finding of an 
increasing number of systems disinfecting could be attributable to 
systems taking corrective actions to address positive results after 
triggered source water monitoring. The analytical results presented in 
the ``Six-Year Review 4 Technical Support Document for Microbial 
Contaminant Regulations'' (USEPA, 2024m) also indicate that 
disinfecting ground water systems had substantially lower total 
coliform positive rates than undisinfected ground water systems. In 
addition, EPA also observed that the total coliform positive rates 
decreased after completion of the first round of sanitary surveys under 
the GWR among ground water systems.
4. Aircraft Drinking Water Rule
Background
    EPA promulgated the Aircraft Drinking Water Rule (ADWR) on October 
19, 2009 (74 FR 53590, USEPA, 2009b). The primary purpose of the ADWR 
is to ensure that safe and reliable drinking water is provided to 
aircraft passengers and crew. This entails providing air carriers with 
a feasible way to comply with SDWA and NPDWRs. The existing NPDWRs were 
designed for traditional, stationary public water systems not mobile 
aircraft water systems that are operationally different. For example, 
aircraft fly to multiple destinations throughout the course of any 
given day and may board drinking water from sources at any of these 
destinations. Aircraft board water from airport watering points via 
temporary connections. Aircraft drinking water safety depends on a 
number of factors including the quality of the water that is boarded 
from these multiple sources, the care used to board the water, and the 
operation and maintenance of the onboard water system and the water 
transfer equipment.
    The ADWR's provisions protect against disease-causing 
microbiological contaminants through the required development and 
implementation of aircraft water system operations and maintenance 
plans. The ADWR's provisions include: routine disinfection and flushing 
of the water system, air carrier training requirements for key 
personnel, and periodic sampling of the onboard drinking water, as well 
as self-inspections of each aircraft water system and immediate 
notification of passengers and crew when violations or specific 
situations occur.
Summary of Review Results
    The ADWR is a unique rule within the context of the SDWA. This rule 
applies only to aircraft engaged in interstate commerce with onboard 
systems that provide water for human consumption through pipes. These 
aircraft water systems board finished water for human consumption and 
regularly serve an average of at least twenty-five individuals daily, 
at least 60 days out of the year. Human consumption includes water for 
drinking, hand washing, food preparation, and oral hygiene. From a 
review of available technical information within the scope of the 
review, EPA concludes that there is no new information to suggest that 
regulatory revisions to the ADWR are appropriate at this time.
Health Effects
    Limited new literature is available on the presence of microbial 
pathogens in aircraft drinking water. Handschuh et al. (2015) found 
that long-haul flights were significantly poorer in terms of microbial 
water quality than short haul flights. A follow-up study by Handschuh 
et al. (2017) demonstrated that there is a diversity of microorganisms 
within the aircraft drinking water supply chain.
    Other studies have also found microbial contaminants present in 
aircraft drinking water, including Pseudomonas aeruginosa, enterococci, 
clostridia, and Salmonella (WHO, 2009; Schaeffer et al., 2012). 
Tracking an illness back to contaminated water served on an aircraft 
presents a technical challenge. Most disease incubation periods are 
longer than the duration of a flight, and even if it is possible to 
determine that a disease was incurred in air travel, it may be 
difficult to determine if the route of transmission was from beverages, 
food, or close proximity of people, and to determine whether 
transmission happened on board the aircraft or at an air terminal.
Occurrence and Exposure
    The Aircraft Reporting and Compliance System (ARCS) is used to 
facilitate the reporting of aircraft water system data under the ADWR. 
Air carriers subject to the ADWR must report to EPA about their 
inventory of aircraft water system fleet; the date the operations and 
maintenance plan was developed; the date the coliform sampling plan was 
developed; the date the aircraft water system sampling plan(s) was 
incorporated into the aircraft water system Operations and Maintenance 
plan; the date the Operations and Maintenance plan(s) was incorporated 
into the U.S. Federal Aviation Administration (FAA) accepted air 
carrier Operation and Maintenance program; the frequency for routine 
disinfection and flushing, and the corresponding routine total coliform 
sampling frequency; and the date for routine disinfection and flushing, 
routine coliform sampling dates and results, and corrective actions 
(when applicable).
    For Six-Year Review 4, EPA downloaded and reviewed compliance 
monitoring data available in ARCS as of May 2021. Approximately 140,000 
records of aircraft water systems compliance monitoring data for total 
coliform and E. coli samples were available in ARCS from February 2011 
through May 2021, including results reported for more than 70 different 
makes/models of aircraft. These results were used to characterize the 
positivity rates of total coliform and E. coli in aircraft water 
systems on an annual basis for the years that data were available 
(2011-2021) and for the subset of years 2012 through 2019. This 
approach removes potentially confounding considerations associated with 
evaluating data for calendar year 2020 when a large number of aircraft 
PWS were inactive due to COVID-19, as well as years 2011 and 2021 for 
which the ARCS data evaluated represents partial years.
    Monitoring data broken down by year for the years 2012-2019 shows 
an average annual total coliform positivity rate of 5.46 percent, with 
a median of 5.63 percent, a minimum of 3.76 percent and a maximum of 
7.03 percent. The total coliform positivity rate decreased on an annual 
basis from 2012-2019. The average E. coli positivity rate was 0.26 
percent, and the median rate was also 0.26 percent, with a minimum of 
0.17 percent and a maximum of 0.33 percent. The E. coli positivity rate 
also decreased on an annual basis.
Treatment Feasibility
    Under the ADWR, air carriers routinely disinfect and flush aircraft 
water systems at the frequency recommended by the water system 
manufacturer or, if not specified by the manufacturer, they may choose 
from one of four options. If corrective disinfection and flushing is 
chosen or required, air carriers follow the procedures in their O&M 
plans. Unscheduled flight disruptions to

[[Page 59643]]

perform corrective disinfection and flushing can be minimized by 
shutting off the water or preventing the flow of water to the taps. 
Before allowing unrestricted access to the aircraft water system, a 
complete set of follow-up samples must be collected and submitted for 
analysis after the disinfection and flushing event if triggered by a 
total coliform-positive sample and must be reported as total coliform-
negative if triggered by an E. coli-positive sample. One study was 
identified that examined the effectiveness of disinfection and flushing 
procedures to prevent coliform persistence in aircraft water systems 
(Szabo et al., 2019). That study showed that coliforms were not 
persistent on the aircraft plumbing surfaces, and coliforms were not 
detected after disinfection and flushing. However, it noted an 
exception for the aerator installed in the lavatory faucet which was 
coliform positive after disinfection with ozone and mixed oxidants; 
disinfection with glycolic acid and quaternary ammonia showed no 
detectable coliforms on aerators after 30 minutes of soaking in the 
disinfectants.
    Each aircraft water system must be inspected by the air carrier at 
least every 5 years according to the procedures in their O&M plans. At 
a minimum, the self-inspection procedures for an aircraft water system 
must include inspection of the storage tank, distribution system, 
supplemental treatment, fixtures, valves, and backflow prevention 
devices. Any deficiencies detected must be addressed, and any 
deficiency that is unresolved within 90 days of identification of the 
deficiency must be reported to EPA.
5. Filter Backwash Recycling Rule
    EPA promulgated the Filter Backwash Recycling Rule (FBRR) on June 
8, 2001 (66 FR 31086, USEPA, 2001a). The rule aimed to increase public 
health protection by addressing microbial contaminant risks associated 
with filter backwash recycling practices. The rule required certain 
systems to return recycled filter backwash water, sludge thickener 
supernatant, and liquids from dewatering processes to a location in the 
system such that all filtration processes of a system are employed, or 
at an alternate location if approved by the State. In addition, the 
rule required systems that employ conventional filtration or direct 
filtration to notify States of their recycling practices by June 8, 
2004, and after then to keep and retain records on file about their 
recycle flows for subsequent review and evaluation by the State. There 
are no ongoing monitoring requirements associated with the FBBR.
    EPA reviewed available State data collected under the ICR; however, 
the EPA did not identify any new and relevant information that would 
indicate that revisions to the NPDWR at this time are appropriate.

VI. References

ATSDR. 2003. Toxicological Profile for Selenium. Atlanta, GA: U.S. 
Department of Health and Human Services, Public Health Service, 
Agency for Toxic Substances and Disease Registry (ATSDR). https://hero.epa.gov/hero/index.cfm/reference/details/reference_id/2990677
ATSDR. 2006. Toxicological Profile for Dichlorobenzenes. Atlanta, 
GA: U.S. Department of Health and Human Services, Public Health 
Service, Agency for Toxic Substances and Disease Registry (ATSDR). 
https://hero.epa.gov/hero/index.cfm/reference/details/reference_id/5160103
ATSDR. 2012. Toxicological Profile for Cadmium. Atlanta (GA): U.S. 
Department of Health and Human Services, Public Health Service, 
Agency for Toxic Substances and Disease Registry (ATSDR). https://hero.epa.gov/hero/index.cfm/reference/details/reference_id/2509015
Borchardt, M.A., S.K. Spencer, B.A. Kieke, Jr., E. Lambertini, and 
F.J. Loge. 2012. Viruses in Nondisinfected Drinking Water from 
Municipal Wells and Community Incidence of Acute Gastrointestinal 
Illness. Environmental Health Perspectives, 120(9): 1272-1279.
Burch T.R., J.P. Stokdyk, N. Rice, A.C. Anderson, J.F. Walsh, S.K. 
Spencer, A.D. Firnstahl and M.A. Borchardt. 2022. Statewide 
Quantitative Microbial Risk Assessment for Waterborne Viruses, 
Bacteria, and Protozoa in Public Water Supply Wells in Minnesota. 
Environmental Science & Technology. 56(10): 6315-6324.
CalEPA. 2010a. Public Health Goal for Methoxychlor in Drinking 
Water. EPA-HQ-OW-2016-0627-0033. Sacramento, CA: California 
Environmental Protection Agency (CalEPA), Office of Environmental 
Health Hazard Assessment. https://hero.epa.gov/hero/index.cfm/reference/details/reference_id/10489852
CalEPA. 2010b. Public Health Goal for Styrene in Drinking Water. 
Sacramento, CA: California Environmental Protection Agency (CalEPA), 
Office of Environmental Health Hazard Assessment. https://hero.epa.gov/hero/index.cfm/reference/details/reference_id/10489854
CalEPA. 2016. Public Health Goal for Antimony in Drinking Water: 
2016 Update. Sacramento, CA: California Environmental Protection 
Agency (CalEPA), Office of Environmental Health Hazard Assessment. 
https://hero.epa.gov/hero/index.cfm/reference/details/reference_id/10489864
Centers for Disease Control and Prevention (CDC). 2022. National 
Outbreak Reporting System Dashboard. Atlanta, Georgia: U.S. 
Department of Health and Human Services, CDC. Accessed November 
2022. Available from URL: wwwn.cdc.gov/norsdashboard.
Collier, S.A., L. Deng, E.A. Adam, K.M. Benedict, E.M. Beshearse, 
A.J. Blackstock, B.B Bruce, G. Derado, C. Edens, K.E. Fullerton and 
J.W. Gargano. 2021. Estimate of burden and direct healthcare cost of 
infectious waterborne disease in the United States. Emerging 
Infectious Diseases. 27(1): 140.
Fout, G.S., M.A. Borchardt, B.A. Kieke Jr., and M.R. Karim. 2017. 
Human virus and microbial indicator occurrence in public-supply 
groundwater systems: meta-analysis of 12 international studies. 
Hydrogeology Journal. 25(4): 903.
Gerdes, M.E., S. Miko, J.M. Kunz, E.J. Hannapel, M.C. Hlavsa, M.J. 
Hughes, M.J. Stuckey, L.K.F. Watkins, J.R. Cope, J.S. Yoder, V.R. 
Hill, and S.A. Collier. 2023. Estimating Waterborne Infectious 
Disease Burden by Exposure Route, United States, 2014. Emerging 
Infectious Diseases. 29(7): 1357.
Health Canada. 2014. Guidelines for Canadian Drinking Water Quality. 
Guideline Technical Document. Toluene, Ethylbenzene and Xylenes. 
Ottawa, Ontario: Health Canada. https://hero.epa.gov/hero/index.cfm/reference/details/reference_id/3049488
Handschuh, H., J. O'Dwyer, and C.C. Adley. 2015. Bacteria that 
travel: the quality of aircraft water. International Journal of 
Environmental Research and Public Health, 12(11): 13938-13955.
Handschuh, H., M.P. Ryan, J. O'Dwyer and C. C. Adley. 2017. 
Assessment of the bacterial diversity of aircraft water: 
identification of the frequent fliers. PLoS One, 12(1): e0170567.
Keithlin, J., J. Sargeant, M.K. Thomas, and A. Fazil. 2014. 
Systematic review and meta-analysis of the proportion of 
Campylobacter cases that develop chronic sequelae. BMC Public Health 
14: 1-19.
National Drinking Water Advisory Committee (NDWAC). 2000. 
Recommended Guidance for Review of Existing National Primary 
Drinking Water Regulations. November 2000.
National Research Council (NRC). 2006. Fluoride in drinking-water: A 
Scientific Review of EPA's Standards. The National Academies Press, 
Washington, DC.
National Toxicology Program (NTP). 2023. NTP Board of Scientific 
Counselors Working Group Report on the Draft State of the Science 
Monograph and the Draft Meta-Analysis Manuscript on Fluoride. 
Research Triangle Park, NC: U.S. Department of Health and Human 
Services, National Institutes of Health, National Institute of 
Environmental Health Sciences, NTP.
Schaeffer, F., K. Tower, and A.S. Weissfeld. 2012. What's Up with 
Aircraft Drinking Water? Clinical Microbiology Newsletter, 34(2): 9-
13.
Stokdyk J.P., A.D. Firnstahl, J.F. Walsh, S.K. Spencer, J.R. de 
Lambert, A.C. Anderson,

[[Page 59644]]

L.W. Rezania, B.A. Kieke Jr. and M.A. Borchardt. 2020. Viral, 
bacterial, and protozoan pathogens and fecal markers in wells 
supplying groundwater to public water systems in Minnesota, USA. 
Water Research. 178: 115814.
Szabo, J.; M. Rodgers; J. Mistry; J. Steenbock; and J. Hall. 2019. 
The effectiveness of disinfection and flushing procedures to prevent 
coliform persistence in aircraft water systems. Water Supply. 19 
(5): 1339-1346. https://doi.org/10.2166/ws.2018.195.
U.S. Department of Health and Human Services Federal Panel on 
Community Water Fluoridation. 2015. U.S. Public Health Service 
Recommendation for Fluoride Concentration in Drinking Water for the 
Prevention of Dental Caries. Public Health Reports. 2015 Jul-Aug; 
130(4):318-31. doi: 10.1177/003335491513000408.
USEPA. 1985. National Primary Drinking Water Regulations; Volatile 
Synthetic Organic Chemicals; Final Rule and Proposed Rule. 50 FR 
46880. November 13, 1985.
USEPA. 1986. National Primary and Secondary Drinking Water 
Regulations; Fluoride; Final Rule. 51 FR 11396. April 2, 1986.
USEPA. 1987. National Primary Drinking Water Regulations; Synthetic 
Organic Chemicals; Monitoring for Unregulated Contaminants; Final 
Rule. 52 FR 25690. July 8, 1987.
USEPA 1989. Drinking Water; National Primary Drinking Water 
Regulations; Total Coliforms (including Fecal Coliforms and E. 
coli); Final Rule. 54 FR 27544. June 29, 1989.
USEPA. 1991. National Primary Drinking Water Regulations-Synthetic 
Organic Chemicals and Inorganic Chemicals; Monitoring for 
Unregulated Contaminants; National Primary Drinking Water 
Regulations Implementation; National Secondary Drinking Water 
Regulations; Final Rule. 56 FR 3526. January 30, 1991.
USEPA. 1992. Drinking Water; National Primary Drinking Water 
Regulations-Synthetic Organic Chemicals and Inorganic Chemicals; 
National Primary Drinking Water Regulations Implementation. 57 FR 
31776. July 17, 1992.
USEPA. 1998. Toxicological Review of Beryllium and Compounds. EPA/
635/R-98/008. Washington, DC: U.S. Environmental Protection Agency 
(USEPA). https://hero.epa.gov/hero/index.cfm/reference/details/reference_id/999207.
USEPA. 2001. Integrated Risk Information System (IRIS) Chemical 
Assessment Summary: Hexachlorocyclopentadiene. Washington, DC: U.S. 
Environmental Protection Agency (USEPA), Office of Research and 
Development, National Center for Environmental Assessment. https://hero.epa.gov/hero/index.cfm/reference/details/reference_id/10509468.
USEPA. 2001a. National Primary Drinking Water Regulations: Filter 
Backwash Recycling Rule. 66 FR 31086. June 8, 2001.
USEPA. 2002. IRIS Toxicological Review of 1,1-Dichloroethylene in 
Support of Summary Information. EPA/635/R02/002. Washington, DC: 
U.S. Environmental Protection Agency (USEPA), National Center for 
Environmental Assessment, Office of Research and Development. 
https://hero.epa.gov/hero/index.cfm/reference/details/reference_id/10721895.
USEPA. 2003. National Primary Drinking Water Regulations; 
Announcement of Completion of EPA's Review of Existing Drinking 
Water Standards. Notice. 68 FR 42908. July 18, 2003.
USEPA. 2004. Reregistration Eligibility Decision (RED) for Lindane. 
Washington, DC: U.S. Environmental Protection Agency (USEPA), Office 
of Prevention, Pesticides and Toxic Substances. https://hero.epa.gov/hero/index.cfm/reference/details/reference_id/10492448.
USEPA. 2005. Toxicological Review of Barium and Compounds (CAS No. 
7440-39-3) in Support of Summary Information on the Integrated Risk 
Information System (IRIS) (Revised). EPA/635/R-05/001. Washington, 
DC: U.S. Environmental Protection Agency (USEPA). https://hero.epa.gov/hero/index.cfm/reference/details/reference_id/11311280.
USEPA. 2006a. National Primary Drinking Water Regulations: Long-Term 
2 Enhanced Surface Water Treatment Rule; Final Rule. 71 FR 654. 
January 5, 2006.
USEPA. 2006b. National Primary Drinking Water Regulations: Ground 
Water Rule; Final Rule. 71 FR 65574. November 8, 2006.
USEPA. 2007a. Acetochlor/Alachlor: Revised Cumulative Risk 
Assessment for the Chloroacetanilides to Support the Proposed New 
Uses on Alachlor and Acetochlor. Washington, DC: U.S. Environmental 
Protection Agency (USEPA), Office of Prevention, Pesticides, and 
Toxic Substances. https://hero.epa.gov/hero/index.cfm/reference/details/reference_id/10492629.
USEPA. 2007b. Toxicological Review of 1,1,1-Trichloroethane. EPA/
635/R-03/013. Washington, DC: U.S. Environmental Protection Agency 
(USEPA). https://hero.epa.gov/hero/index.cfm/reference/details/reference_id/3004991.
USEPA. 2008. Carbofuran. HED Revised Risk Assessment for the Notice 
of Intent to Cancel (NOIC). EPA-HQ-OPP-2007-1088-0034. Washington, 
DC: U.S. Environmental Protection Agency (USEPA), Office of 
Prevention, Pesticides, and Toxic Substances. https://hero.epa.gov/hero/index.cfm/reference/details/reference_id/10494332.
USEPA. 2009a. Provisional Peer-Reviewed Toxicity Values for 1,2,4-
Trichlorobenzene, CASRN 120-82-1. EPA/690/R-09/065F. Cincinnati, OH: 
U.S. Environmental Protection Agency (USEPA), Office of Research and 
Development, National Center for Environmental Assessment, Superfund 
Health Risk Technical Support Center. https://hero.epa.gov/hero/index.cfm/reference/details/reference_id/10255709.
USEPA. 2009b. National Primary Drinking Water Regulations: Drinking 
Water Regulations for Aircraft Public Water Systems. 74 FR 53590. 
October 19, 2009.
USEPA. 2010a. National Primary Drinking Water Regulations; 
Announcement of the Results of EPA's Review of Existing Drinking 
Water Standards and Request for Public Comment and/or Information on 
Related Issues. 75 FR 15500. March 29, 2010.
USEPA. 2010b. Toxicological Review of Hydrogen Cyanide and Cyanide 
Salts. EPA/635/R-08/016F. Washington, DC: U.S. Environmental 
Protection Agency (USEPA). https://hero.epa.gov/hero/index.cfm/reference/details/reference_id/723657.
U.S. EPA. 2010c. Integrated Risk Information System (IRIS) Chemical 
Assessment Summary: cis-1,2-Dichloroethylene, CASRN 156-59-2. 
Washington, DC: U.S. Environmental Protection Agency (USEPA), Office 
of Research and Development. https://hero.epa.gov/hero/index.cfm/reference/details/reference_id/10493648.
USEPA. 2010d. Fluoride: Dose-Response Analysis for Non-Cancer 
Effects. EPA/820/R-10/019. Washington, DC: U.S. Environmental 
Protection Agency (USEPA), Office of Water. https://hero.epa.gov/hero/index.cfm/reference/details/reference_id/10493692.
USEPA. 2010e. Long Term 2 Enhanced Surface Water Treatment Rule: 
Toolbox Guidance Manual. EPA 815-R-09-016. April 2010. https://www.epa.gov/dwreginfo/long-term-2-enhanced-surface-water-treatment-rule-documents.
USEPA. 2011. Trichloroethylene; CASRN 79-01-6. Integrated Risk 
Information System (IRIS) Chemical Assessment Summary. Last Revised 
September 28, 2011. Retrieved from https://iris.epa.gov/ChemicalLanding/&substance_nmbr=199.
USEPA. 2012. Tetrachloroethylene (Perchloroethylene); CASRN 127-18-
4. Integrated Risk Information System (IRIS) Chemical Assessment 
Summary. Last Revised February 10, 2012. Retrieved from https://iris.epa.gov/ChemicalLanding/&substance_nmbr=106.
USEPA. 2013. National Primary Drinking Water Regulations: Revisions 
to the Total Coliform Rule; Final Rule. 78 FR 10269. February 13, 
2013.
USEPA. 2015a. Peer Review Handbook 4th Edition. October 2015. 
Available online at: https://www.epa.gov/sites/default/files/2015-10/documents/epa_peer_review_handbook_4th_edition_october_2015.pdf.
USEPA. 2015b. Endothall: Human Health Risk Assessment in Support of 
Registration Review, and the Petition to Re-Evaluate Tolerances for 
Livestock, and Remove the Restriction that Prohibits Livestock from 
Drinking Treated Water. EPA-HQ-OPP-2015-0591-0012. Washington, DC: 
U.S. Environmental Protection Agency (USEPA), Office of Chemical 
Safety and Pollution Prevention, Health Effects

[[Page 59645]]

Division. https://hero.epa.gov/hero/index.cfm/reference/details/reference_id/10494329.
USEPA. 2016. Six-Year Review 3--Health Effects Assessment for 
Existing Chemical and Radionuclide National Primary Drinking Water 
Regulations--Summary Report. EPA 822-R-16-008.
USEPA. 2017a. National Primary Drinking Water Regulations; 
Announcement of the Results of EPA's Review of Existing Drinking 
Water Standards and Request for Public Comment and/or Information on 
Related Issues. 82 FR 3518. January 11, 2017.
USEPA. 2017b. Oxamyl Draft Human Health Risk Assessment in Support 
of Registration Review. Washington, DC: U.S. Environmental 
Protection Agency (USEPA), Office of Chemical Safety and Pollution 
Prevention, Health Effects Division. https://hero.epa.gov/hero/index.cfm/reference/details/reference_id/10532947.
USEPA. 2017c. 2,4-D Revised Human Health Risk Assessment for 
Registration Review. Washington, DC: U.S. Environmental Protection 
Agency (USEPA), Office of Chemical Safety and Pollution Prevention, 
Health Effects Division. https://hero.epa.gov/hero/index.cfm/reference/details/reference_id/10532862.
USEPA. 2017d. Glyphosate: Draft Human Health Risk Assessment in 
Support of Registration Review. EPA-HQ-OPP-2009-0361-0068. 
Washington, DC: U.S. Environmental Protection Agency (USEPA), Office 
of Chemical Safety and Pollution Prevention. https://hero.epa.gov/hero/index.cfm/reference/details/reference_id/10532909.
USEPA. 2018a. Draft Atrazine Human Health Risk Assessment for 
Registration Review. EPA-HQ-OPP-2013-0266-1256. Washington, DC: U.S. 
Environmental Protection Agency (USEPA). https://hero.epa.gov/hero/index.cfm/reference/details/reference_id/10533087.
USEPA. 2018b. Simazine: Human Health Risk Assessment for 
Registration Review and to Support the Registration of Proposed Uses 
on Citrus Fruit (Crop Group 10-10), Pome Fruit (Crop Group 11-10), 
Stone Fruit (Crop Group 12-12), Tree Nuts (Crop Group 14-12), and 
Tolerance Amendment for Almond Hulls. Washington, DC: U.S. 
Environmental Protection Agency (USEPA), Office of Chemical Safety 
and Pollution Prevention. https://hero.epa.gov/hero/index.cfm/reference/details/reference_id/10533123.
USEPA. 2019. Information Collection Request Submitted to OMB for 
Review and Approval; Comment Request; Contaminant Occurrence Data in 
Support of the EPA's Fourth Six-Year Review of National Primary 
Drinking Water Regulations. 84 FR 58381. October 31, 2019.
USEPA. 2020a. Microbial Disinfection Byproducts Rules: Public 
Meeting to Inform Potential Rule Revisions. Notice. 85 FR 61680. 
September 30, 2020.
USEPA. 2020b. Diquat. Human Health Risk Assessment for the 
Establishment Of A Tolerance Without U.S. Registration For Residues 
in/on Crop Subgroup 6C Dried Shelled Pea and Bean (Except Soybean). 
EPA-HQ-OPP-2017-0291-0009. Washington, DC: U.S. Environmental 
Protection Agency (USEPA), Office of Chemical Safety and Pollution 
Prevention. https://hero.epa.gov/hero/index.cfm/reference/details/reference_id/10533339.
USEPA. 2020c. Picloram Draft Human Health Risk Assessment in Support 
of Registration Review. Washington, DC: U.S. Environmental 
Protection Agency (USEPA), Office of Chemical Safety and Pollution 
Prevention, Health Effects Division. https://hero.epa.gov/hero/index.cfm/reference/details/reference_id/10533340.
USEPA. 2020d. ``The Standardized Monitoring Framework: A Quick 
Reference Guide.'' EPA 816-F-20-002. May 2020. https://www.epa.gov/dwreginfo/standardized-monitoring-framework-quick-reference-guide.
USEPA. 2020e. Use of Total Nitrate and Nitrite Analysis for 
Compliance Determinations with the Nitrate Maximum Contaminant Level 
(WSG 213). November 30, 2020. https://www.epa.gov/sites/default/files/2021-01/documents/wsg_213_nitrate_wsg_11-30-2020_signed_508-compliantfinal.pdf.
USEPA. 2020f. Clarification of Free and Total Cyanide Analysis for 
Safe Drinking Water Act (SDWA) Compliance Revision 1.0. EPA 815-B-
20-004. June 2020.
USEPA. 2022a. Request for Nominations for the Science Advisory Board 
Radionuclide Cancer Risk Coefficients Review Panel. 87 FR 15988. 
March 21, 2022.
USEPA. 2022b. Availability of the Draft IRIS Toxicological Review of 
Hexavalent Chromium. 87 FR 63774. October 10, 2022.
USEPA. 2023a. National Primary Drinking Water Regulations for Lead 
and Copper: Improvements (LCRI). 88 FR 84878. December 6, 2023.
USEPA. 2023b. Availability of the Protocol for the Nitrate and 
Nitrite IRIS Assessment (Oral). 88 FR 77310. November 9, 2023.
USEPA. 2024a. PFAS National Primary Drinking Water Regulation. 89 FR 
32532. April 26, 2024.
USEPA. 2024b. National Primary Drinking Water Regulations: Consumer 
Confidence Report Rule Revisions. 89 FR 45980. May 24, 2024.
USEPA. 2024c. EPA Protocol for the Fourth Review of Existing 
National Primary Drinking Water Regulations. EPA 815-R-24-018.
USEPA. 2024d. Data Management and Quality Assurance/Quality Control 
Process for the Fourth Six-Year Review Information Collection 
Request Dataset. EPA 815-R-24-017.
USEPA. 2024e. Chemical Contaminant Summaries for the Fourth Six-Year 
Review of Existing National Primary Drinking Water Regulations. EPA 
815-S-24-002.
USEPA. 2024f. Results of the Health Effects Assessment for the 
Fourth Six-Year Review of Existing Chemical and Radionuclide 
National Primary Drinking Water Standards. EPA 815-R-24-020.
USEPA. 2024g. Analytical Feasibility Support Document for the Fourth 
Six-Year Review of National Primary Drinking Water Regulations. EPA 
815-R-24-015.
USEPA. 2024h. Analysis of Regulated Contaminant Occurrence Data from 
Public Water Systems in Support of the Fourth Six-Year Review of 
National Primary Drinking Water Regulations: Chemical Phase and 
Radionuclides Rules. EPA 815-R-24-014.
USEPA. 2024i. Review of Fluoride Occurrence for the Fourth Six-Year 
Review. EPA 815-R-24-021.
USEPA. 2024j. Occurrence Analysis for Potential Source Waters for 
the Fourth Six-Year Review of National Primary Drinking Water 
Regulations. EPA 815-R-24-019.
USEPA. 2024k. Support Document for the Fourth Six-Year Review of 
Drinking Water Regulations for Acrylamide and Epichlorohydrin. EPA 
815-R-24-023.
USEPA. 2024l. Consideration of Other Regulatory Revisions in Support 
of the Fourth Six-Year Review of the National Primary Drinking Water 
Regulations: Chemical Phase Rules and Radionuclides Rule. EPA 815-R-
24-016.
USEPA. 2024m. Six-Year Review 4 Technical Support Document for 
Microbial Contaminant Regulations. EPA 815-R-24-022.
Wallender, E.K., E.C. Ailes, J.S. Yoder, V.A. Roberts, and J.M. 
Brunkard. 2014. Contributing factors to disease outbreaks associated 
with untreated groundwater. Ground Water. 52(6): 886-97.
World Health Organization (WHO). 2004. Guidelines for Drinking-Water 
Quality, Third Edition. Volume 1: Recommendations. https://www.who.int/publications/i/item/9789241547611.
WHO. 2009. Guide to hygiene and sanitation in aviation, 3rd edition. 
https://www.who.int/publications/i/item/9789241547772.

Michael S. Regan,
Administrator.
[FR Doc. 2024-15807 Filed 7-22-24; 8:45 am]
BILLING CODE 6560-50-P