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Rule2024-07773

PFAS National Primary Drinking Water Regulation

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Published

April 26, 2024

Effective

June 25, 2024

Issuing agencies

Environmental Protection Agency

Abstract

In March 2023, the U.S. Environmental Protection Agency (EPA) proposed and requested comment on the National Primary Drinking Water Regulation (NPDWR) and health-based Maximum Contaminant Level Goals (MCLGs) for six per- and polyfluoroalkyl substances (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 perfluorobutane sulfonic acid (PFBS). After consideration of public comment and consistent with the provisions set forth under the Safe Drinking Water Act (SDWA), the EPA is finalizing NPDWRs for these six PFAS. Through this action, the EPA is finalizing MCLGs for PFOA and PFOS at zero. Considering feasibility, the EPA is promulgating individual Maximum Contaminant Levels (MCLs) for PFOA and PFOS at 4.0 nanograms per liter (ng/L) or parts per trillion (ppt). The EPA is also finalizing individual MCLGs and is promulgating individual MCLs for PFHxS, PFNA, and HFPO-DA at 10 ng/L. In addition to the individual MCLs for PFHxS, PFNA, and HFPO-DA, in consideration of the known toxic effects, dose additive health concerns and occurrence and likely co-occurrence in drinking water of these three PFAS, as well as PFBS, the EPA is finalizing a Hazard Index (HI) of 1 (unitless) as the MCLG and MCL for any mixture containing two or more of PFHxS, PFNA, HFPO-DA, and PFBS. Once fully implemented, the EPA estimates that the rule will prevent thousands of deaths and reduce tens of thousands of serious PFAS-attributable illnesses.

Full Text

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[Federal Register Volume 89, Number 82 (Friday, April 26, 2024)]
[Rules and Regulations]
[Pages 32532-32757]
From the Federal Register Online via the Government Publishing Office [<a href="http://www.gpo.gov">www.gpo.gov</a>]
[FR Doc No: 2024-07773]

[[Page 32531]]

Vol. 89

Friday,

No. 82

April 26, 2024

Part II

Environmental Protection Agency

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40 CFR Parts 141 and 142

PFAS National Primary Drinking Water Regulation; Final Rule

Federal Register / Vol. 89 , No. 82 / Friday, April 26, 2024 / Rules
and Regulations

[[Page 32532]]

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Parts 141 and 142

[EPA-HQ-OW-2022-0114; FRL 8543-02-OW]
RIN 2040-AG18

PFAS National Primary Drinking Water Regulation

AGENCY: Environmental Protection Agency (EPA).

ACTION: Final rule.

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SUMMARY: In March 2023, the U.S. Environmental Protection Agency (EPA)
proposed and requested comment on the National Primary Drinking Water
Regulation (NPDWR) and health-based Maximum Contaminant Level Goals
(MCLGs) for six per- and polyfluoroalkyl substances (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 perfluorobutane sulfonic acid (PFBS). After
consideration of public comment and consistent with the provisions set
forth under the Safe Drinking Water Act (SDWA), the EPA is finalizing
NPDWRs for these six PFAS. Through this action, the EPA is finalizing
MCLGs for PFOA and PFOS at zero. Considering feasibility, the EPA is
promulgating individual Maximum Contaminant Levels (MCLs) for PFOA and
PFOS at 4.0 nanograms per liter (ng/L) or parts per trillion (ppt). The
EPA is also finalizing individual MCLGs and is promulgating individual
MCLs for PFHxS, PFNA, and HFPO-DA at 10 ng/L. In addition to the
individual MCLs for PFHxS, PFNA, and HFPO-DA, in consideration of the
known toxic effects, dose additive health concerns and occurrence and
likely co-occurrence in drinking water of these three PFAS, as well as
PFBS, the EPA is finalizing a Hazard Index (HI) of 1 (unitless) as the
MCLG and MCL for any mixture containing two or more of PFHxS, PFNA,
HFPO-DA, and PFBS. Once fully implemented, the EPA estimates that the
rule will prevent thousands of deaths and reduce tens of thousands of
serious PFAS-attributable illnesses.

DATES: This final rule is effective on June 25, 2024. The incorporation
by reference of certain publications listed in the rule is approved by
the Director of the Federal Register as of June 25, 2024.

ADDRESSES: The EPA has established a docket for this action under
Docket ID No. EPA-HQ-OW-2022-0114. All documents in the docket are
listed on the <a href="http://www.regulations.gov">http://www.regulations.gov</a> website. Although listed in
the index, some information is not publicly available, e.g.,
Confidential Business Information (CBI) or other information whose
disclosure is restricted by statute. Certain other material, such as
copyrighted material, is not placed on the internet and will be
publicly available only in hard copy form. Publicly available docket
materials are available electronically through <a href="https://www.regulations.gov">https://www.regulations.gov</a>.

FOR FURTHER INFORMATION CONTACT: Alexis Lan, Office of Ground Water and
Drinking Water, Standards and Risk Management Division (Mail Code
4607M), Environmental Protection Agency, 1200 Pennsylvania Avenue NW,
Washington, DC 20460; telephone number 202-564-0841; email address:
<a href="/cdn-cgi/l/email-protection#04544245574a54405356446174652a636b72">[email&#160;protected]</a>.

SUPPLEMENTARY INFORMATION:

Executive Summary

The Environmental Protection Agency (EPA) is issuing an adaptive
and flexible National Primary Drinking Water Regulation (NPDWR) under
the Safe Drinking Water Act (SDWA) to manage risks of per- and
polyfluoroalkyl substances (PFAS) in drinking water. The EPA is
establishing drinking water standards for six PFAS in this NPDWR to
provide health protection against these individual and co-occurring
PFAS in public water systems. The EPA's final rule represents data-
driven drinking water standards that are based on the best available
science and meet the requirements of SDWA. For the six PFAS, the EPA
considered PFAS health effects information, evidence supporting dose-
additive health concerns from co-occurring PFAS, as well as national
and state data for the levels of multiple PFAS in finished drinking
water. SDWA provides a framework for the EPA to regulate emerging
contaminants of concern in drinking water. Under the statute, the EPA
must act based on the ``best available'' science and information. Thus,
the statute recognizes that the EPA may act in the face of imperfect
information. It also provides a mechanism for the EPA to update
standards as more science becomes available. For the PFAS covered by
this rule, the EPA concluded that the state of the science and
information has sufficiently advanced to the point to satisfy the
statutory requirements and fulfill SDWA's purpose to protect public
health by addressing contaminants in the nation's public water systems.
PFAS are a large class of thousands of organic chemicals that have
unique physical and chemical properties. These compounds are designed
to be stable and non-reactive because of the applications in which they
are used: certain industrial and manufacturing processes; stain and
water repellants in clothing, carpets, and other consumer products, as
well as certain types of fire-fighting foams. PFAS tend to break down
slowly and persist in the environment, and consequently, they can
accumulate in the environment and the human body over time. Current
scientific research and available evidence have shown the potential for
harmful human health effects after being exposed to some PFAS. Although
some PFAS have been phased out of use in the United States, they are
still found in the environment and in humans based on biomonitoring
data.
Drinking water is one of several ways people can be exposed to
PFAS. The EPA's examination of drinking water data shows that different
PFAS can often be found together and in varying combinations as
mixtures. Additionally, decades of research demonstrates that exposure
to mixtures of different chemicals can elicit dose-additive health
effects: even if the individual chemicals are each present at levels
considered ``safe,'' the mixture may cause significant adverse health
effects. The high likelihood for different PFAS to co-occur in drinking
water; the additive health concerns when present in mixtures; the
diversity and sheer number of PFAS; and their general presence and
persistence in the environment and the human body are reflective of the
environmental and public health challenges the American public faces
with PFAS, which poses a particular threat for overburdened communities
that experience disproportionate environmental impacts. The final NPDWR
includes:

1. Individual Maximum Contaminant Levels (MCLs)
a. Perfluorooctanoic acid (PFOA) MCL = 4.0 nanograms per liter or
parts per trillion (ng/L or ppt)
b. Perfluorooctane sulfonic acid (PFOS) MCL = 4.0 ng/L
c. Perfluorohexane sulfonic acid (PFHxS) MCL = 10 ng/L
d. Perfluorononanoic acid (PFNA) MCL = 10 ng/L
e. Hexafluoropropylene oxide dimer acid (HFPO-DA) MCL = 10 ng/L
2. A Hazard Index MCL to account for dose-additive health effects
for mixtures that could include two or more of four

[[Page 32533]]

PFAS (PFHxS, PFNA, HFPO-DA, and perfluorobutane sulfonic acid (PFBS)).
The Hazard Index MCL defines when the combined levels of two or more of
these four PFAS requires action. A PFAS mixture Hazard Index less than
or equal to 1 (unitless) indicates a level at which no known or
anticipated adverse effects on the health of persons occur and allows
for an adequate margin of safety with respect to health risk associated
with a mixture of PFAS in finished drinking water. A PFAS mixture
Hazard Index greater than 1 (unitless) indicates an exceedance of the
health protective level. To calculate the Hazard Index, a ratio is
developed for each PFAS by dividing the measured level of the PFAS in
drinking water by the level (in ng/L or ppt) below which adverse health
effects are not likely to occur (i.e., the Health Based Water
Concentration or HBWC). The HBWCs for each PFAS in the Hazard Index
are:

a. PFHxS = 10 ng/L or ppt
b. PFNA = 10 ng/L
c. HFPO-DA = 10 ng/L
d. PFBS = 2,000 ng/L

The individual PFAS ratios are then summed across the mixture to
yield the Hazard Index MCL as follows:
[GRAPHIC] [TIFF OMITTED] TR26AP24.000

Based on the administrative record for the final PFAS NPDWR and as
discussed above, certain PFAS (including PFHxS, PFNA, HFPO-DA, and
PFBS) have been shown to be toxicologically similar; i.e., elicit the
same or similar profile of adverse effects in several biological organs
and systems (see USEPA, 2000a; USEPA, 2007; USEPA, 2024a; USEPA, USEPA,
2024c; and section IV.B of this preamble). Studies with PFAS and other
classes of chemicals support the health-protective conclusion that
chemicals that have similar observed adverse effects following
individual exposure should be assumed to act in a dose-additive manner
when in a mixture unless data demonstrate otherwise (USEPA, 2024a).
Additionally, the record further supports that there is a substantial
likelihood that PFBS, PFHxS, PFNA, and HFPO-DA co-occur as mixtures in
drinking water at levels of public health concern (see USEPA, 2024b and
sections VI.C and D of this preamble). Though the EPA is not
promulgating an individual MCL or Maximum Contaminant Level Goal (MCLG)
for PFBS at this time as it is for PFHxS, PFNA, and HFPO-DA (see
section III.A of this preamble for specific discussion), based on these
evaluations, the agency is establishing a Hazard Index MCL that
addresses PFBS as part of mixtures where its co-occurrence with other
PFAS (PFHxS, HFPO-DA, and/or PFNA) can affect health endpoints when
present in these mixtures.
The individual and Hazard Index MCLs are independently applicable
for compliance purposes.
Additionally, the EPA is finalizing important public ``right to
know'' provisions of the EPA's SDWA regulations, specifically, public
notification (PN) and Consumer Confidence Report (CCR) requirements.
The changes under this rule will strengthen risk communication and
education for the public when elevated levels of these PFAS are found.
Finally, the EPA is finalizing monitoring and reporting requirements
that enable public water systems (PWSs) and primacy agencies to
implement and comply with the NPDWR.
Consistent with the timelines set out under SDWA, PWSs are required
to conduct their initial monitoring by April 26, 2027, and to conduct
PN and include PFAS information in the CCR. After carefully considering
public comment, the EPA is extending the compliance deadline for all
systems nationwide to meet the MCL to allow additional time for capital
improvements. As such, PWSs are required to make any necessary capital
improvements and comply with the PFAS MCLs by April 26, 2029.
As part of its Health Risk Reduction and Cost Analysis (HRRCA), the
EPA evaluated quantifiable and nonquantifiable health risk reduction
benefits and costs associated with the final NPDWR. At a two percent
discount rate, the EPA estimates the quantifiable annual benefits of
the final rule will be $1,549.40 million per year and the quantifiable
costs of the rule will be $1,548.64 million per year. The EPA's
quantified benefits are based on the agency's estimates that that there
will be 29,858 fewer illnesses and 9,614 fewer deaths in the
communities in the decades following actions to reduce PFAS levels in
drinking water. While the modeled quantified net benefits are nearly at
parity, under SDWA, the EPA must consider whether the costs of the rule
are justified by the benefits based on all statutorily prescribed costs
and benefits, not just the quantified costs and benefits (see SDWA
1412(b)(3)(c)(i)).
The EPA expects that the final rule will result in additional
nonquantifiable costs, including costs with generally greater
uncertainty, which the EPA has examined in quantified sensitivity
analyses in the Economic Analysis for the final rule. First, the EPA
had insufficient nationally representative data to precisely
characterize occurrence of HFPO-DA, PFNA, and PFBS. In an effort to
better consider and understand the costs associated with treatment of
these regulated compounds at systems both with and without PFOA, PFOS
and PFHxS occurrence in exceedance of the MCLs, the EPA performed a
quantitative sensitivity analysis of the costs associated with Hazard
Index and/or MCL exceedances resulting from HFPO-DA, PFNA, and PFBS.
The EPA expects that the quantified national costs, which do not
include HFPO-DA, PFNA, and PFBS treatment costs are marginally
underestimated (on the order of 5 percent). Second, stakeholders have
expressed concern to the EPA that a hazardous substance designation for
certain PFAS may limit their disposal options for drinking water
treatment residuals (e.g., spent media, concentrated waste streams)
and/or potentially increase costs. The EPA has conducted a sensitivity
analysis and found that should all water systems use hazardous waste
disposal options national costs would increase by 7 percent.
The EPA anticipates significant additional benefits that cannot be
quantified, will result from avoided negative developmental,
cardiovascular, liver, immune, endocrine, metabolic, reproductive,
musculoskeletal, and carcinogenic effects as a result of reductions in
the levels of the regulated PFAS and other co-removed contaminants. For
example, elevated

[[Page 32534]]

concentrations of both PFOA and PFOS negatively impact the immune and
endocrine systems, impacts which the agency is unable to quantify at
this time. As another example, the EPA assessed the developmental
benefits associated with PFNA exposure reductions semi-quantitively in
sensitivity analysis, and the analysis demonstrates significant
additional benefits associated with reductions in PFNA. There are other
nonquantifiable benefits for other PFNA health endpoints, and numerous
endpoints for PFHxS, HFPO-DA, PFBS, and other PFAS that are anticipated
to be removed as a result of the final NPDWR. Additionally, as a result
of the ability for available treatment technologies to remove co-
occurring contaminants, there are benefits not quantified for removal
of co-occurring contaminants for this regulation (e.g., certain
pesticides, volatile organic compounds). Considering both quantifiable
and nonquantifiable costs and benefits of the rule, the EPA is
reaffirming the Administrator's determination at the time of proposal,
that the quantifiable and nonquantifiable benefits of the final rule
justify the quantifiable and nonquantifiable costs.
To help communities on the frontlines of PFAS contamination, the
passage of the Infrastructure Investment and Jobs Act (IIJA), also
referred to as the Bipartisan Infrastructure Law (BIL), invests
billions of dollars over a 5-year period. BIL appropriates over $11.7
billion in the Drinking Water State Revolving Fund (DWSRF) General
Supplemental; $4 billion to the DWSRF for Emerging Contaminants; and $5
billion in grants to the Emerging Contaminants in Small or
Disadvantaged Communities. These funds will assist many disadvantaged
communities, small systems, and others with the costs of installation
of treatment when it might otherwise be cost-challenging.

Table of Contents

I. General Information
A. What are the EPA's final rule requirements?
B. Does this action apply to me?
II. Background
A. What are PFAS?
B. Human Health Effects
C. Statutory Authority
D. Statutory Framework and PFAS Regulatory History
E. Bipartisan Infrastructure Law
F. EPA PFAS Strategic Roadmap
III. Final Regulatory Determinations for Additional PFAS
A. Agency Findings
B. Statutory Criterion 1--Adverse Health Effects
C. Statutory Criterion 2--Occurrence
D. Statutory Criterion 3--Meaningful Opportunity
E. The EPA's Final Determination Summary
IV. MCLG Derivation
A. MCLG Derivation for PFOA and PFOS
B. MCLG Derivation for Additional PFAS
V. Maximum Contaminant Levels
A. PFOA and PFOS
B. PFAS Hazard Index: PFHxS, PFNA, HFPO-DA, and PFBS
C. Individual MCLs: PFHxS, PFNA and HFPO-DA
VI. Occurrence
A. UCMR 3
B. State Drinking Water Data
C. PFAS Co-Occurrence
D. Occurrence Relative to the Hazard Index
E. Occurrence Model
F. Combining State Data With Model Output To Estimate National
Exceedance of Either MCLs or Hazard Index
G. UCMR 5 Partial Dataset Analysis
VII. Analytical Methods
A. Analytical Methods and Practical Quantitation Levels (PQLs)
for Regulated PFAS
VIII. Monitoring and Compliance Requirements
A. What are the Monitoring Requirements?
B. How are PWS Compliance and Violations Determined?
C. Can Systems Use Previously Collected Data To Satisfy the
Initial Monitoring Requirement?
D. Can systems composite samples?
E. Can primacy agencies grant monitoring waivers?
F. When must systems complete initial monitoring?
G. What are the laboratory certification requirements?
H. Laboratory Quality Assurance/Quality Control
IX. Safe Drinking Water Act (SDWA) Right To Know Requirements
A. What are the Consumer Confidence Report requirements?
B. What are the Public Notification (PN) requirements?
X. Treatment Technologies
A. What are the best available technologies?
B. PFAS Co-Removal
C. Management of Treatment Residuals
D. What are Small System Compliance Technologies (SSCTs)?
XI. Rule Implementation and Enforcement
A. What are the requirements for primacy?
B. What are the record keeping requirements?
C. What are the reporting requirements?
D. Exemptions and Extensions
XII. Health Risk Reduction and Cost Analysis
A. Public Comment on the Economic Analysis for the Proposed Rule
and EPA Response
B. Affected Entities and Major Data Sources Used To Develop the
Baseline Water System Characterization
C. Overview of the Cost-Benefit Model
D. Method for Estimating Costs
E. Nonquantifiable Costs of the Final Rule
F. Method for Estimating Benefits
G. Nonquantifiable Benefits of PFOA and PFOS Exposure Reduction
H. Nonquantifiable Benefits of Removal of PFAS Included in the
Final Regulation and Co-Removed PFAS
I. Benefits Resulting From Disinfection By-Product Co-Removal
J. Comparison of Costs and Benefits
K. Quantified Uncertainties in the Economic Analysis
XIII. Statutory and Executive Order Reviews
A. Executive Order 12866: Regulatory Planning and Review and
Executive Order 14094 Modernizing Regulatory Review
B. Paperwork Reduction Act (PRA)
C. Regulatory Flexibility Act (RFA)
D. Unfunded Mandates Reform Act (UMRA)
E. Executive Order 13132: Federalism
F. Executive Order 13175: Consultation and Coordination With
Indian Tribal Governments
G. Executive Order 13045: Protection of Children From
Environmental Health and Safety Risks
H. Executive Order 13211: Actions That Significantly Affect
Energy Supply, Distribution, or Use
I. National Technology Transfer and Advancement Act of 1995
J. Executive Order 12898: Federal Actions To Address
Environmental Justice in Minority Populations and Low-Income
Populations and Executive Order 14096: Revitalizing Our Nation's
Commitment to Environmental Justice for All
K. Consultations With the Science Advisory Board, National
Drinking Water Advisory Council, and the Secretary of Health and
Human Services
L. Congressional Review Act (CRA)
XIV. Severability
XV. Incorporation by Reference
XVI. References

I. General Information

A. What are the EPA's final rule requirements?

The Safe Drinking Water Act (SDWA) provides a framework for the
Environmental Protection Agency (EPA) to regulate emerging contaminants
of concern in drinking water. Under the statute, the EPA may act based
on the ``best available'' science and information. Thus, the statute
recognizes that the EPA may act in the face of imperfect information
and provides a mechanism for the EPA to update standards as more
science becomes available. For the per- and polyfluoroalkyl substances
(PFAS) covered by this rule, the EPA concluded that the state of the
science and information has sufficiently advanced to the point to
satisfy the statutory requirements and fulfill SDWA's purpose to
protect public health by addressing contaminants in the nation's public
water systems. In this final action, the EPA is finalizing the PFAS
National Primary Drinking Water Regulation (NPDWR) that is based upon
the best available peer-reviewed

[[Page 32535]]

science. The final NPDWR for PFAS establishes Maximum Contaminant Level
Goals (MCLGs) and enforceable Maximum Contaminant Levels (MCLs) for six
PFAS compounds: 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 perfluorobutane sulfonic acid (PFBS). The
final rule requirements and references to where additional discussion
can be found on these topics are summarized here:
The EPA is finalizing MCLGs for PFOA and PFOS at zero (0) and
enforceable MCLs for PFOA and PFOS at 4.0 ng/L (ng/L or ppt). Please
see section IV of this preamble on the MCLG derivation for PFOA and
PFOS. Additionally, please see section V of this preamble for
discussion on the MCL for PFOA and PFOS.
The EPA is finalizing individual regulatory determinations to
regulate PFHxS, PFNA, and HFPO-DA (commonly known as ``GenX
Chemicals''). The EPA is deferring the individual regulatory
determination to regulate PFBS in drinking water. Concurrent with the
final determinations, the EPA is promulgating individual MCLGs and MCLs
for PFHxS, PFNA, and HFPO-DA at 10 ng/L each.
Additionally, the EPA is finalizing a regulatory determination for
mixtures of PFHxS, PFNA, HFPO-DA, and PFBS due to their substantial
likelihood for co-occurrence and dose-additive health concerns when
present as a mixture in drinking water. Concurrent with this final
determination, the EPA is finalizing a Hazard Index (HI) of 1 as the
MCLG and enforceable MCL to address mixtures of PFHxS, PFNA, HFPO-DA,
and PFBS where they co-occur in drinking water. Please see section III
of this preamble for discussion on the EPA's final regulatory
determinations; section IV of this preamble for discussion on the MCLG
derivation for these additional compounds; and section V of this
preamble for a discussion on the final MCLs.
This action also lists feasible technologies for public water
systems (PWSs) that can be used to comply with the MCLs. The EPA notes
that systems are not required to use the listed technologies to meet
the MCL; rather, the MCL is a numeric regulatory limit systems must
meet that is developed while considering treatment feasibility and
cost. Please see section X for additional discussion on feasible
treatment technologies.
The EPA is finalizing SDWA Right-to-Know requirements for the final
rule, including Consumer Confidence Report (CCR) and Public
Notification (PN) requirements. Community water systems (CWSs) must
prepare and deliver to its customers an annual CCR in accordance with
40 CFR part 141, subpart O. Under this rule, CWSs will be required to
report detected PFAS in their CCRs and provide health effects language
in the case of MCL violations. Additionally, under the final rule, MCL
violations require Tier 2 public notification, or notification provided
as soon as practicable but no later than 30 days after a system learns
of the violation, as per 40 CFR 141.203. Additionally, monitoring and
testing procedure violations require Tier 3 notification, or notice no
later than one year after the system learns of the violation. Please
see section IX of this preamble for additional discussion on SDWA
Right-to-Know requirements.
Additionally, the EPA is finalizing monitoring and reporting
requirements for PWSs to comply with the NPDWR. PWSs are required to
sample each EP using a monitoring regime generally based on the EPA's
Standard Monitoring Framework (SMF) for Synthetic Organic Contaminants
(SOCs). As a part of these requirements, to establish baseline levels
of regulated PFAS, water systems must complete initial monitoring
within three years following rule promulgation and/or use results of
recent, previously acquired monitoring to satisfy the initial
monitoring requirements. Following initial monitoring, beginning three
years following rule promulgation, to demonstrate that finished
drinking water does not exceed the MCLs for regulated PFAS, PWSs will
be required to conduct compliance monitoring for all regulated PFAS at
a frequency specifically based on sample results. Compliance with the
NPDWRs will be based on analytical results obtained at each sampling
point. PWSs are required to report to primacy agencies the results of
all initial and compliance monitoring to ensure compliance with the
NPDWRs. Please see section VIII of this preamble for additional
discussion on these requirements.
Finally, the EPA is exercising its authority under SDWA section
1412(b)(10) to implement a nationwide capital improvement extension to
comply with the MCL. All systems must comply with the MCLs by April 26,
2029. All systems must comply with all other requirements of the NPDWR,
including initial monitoring, by April 26, 2027. For additional
discussion on extensions and exemptions, please see section XI.

B. Does this action apply to me?

Entities regulated by this action are CWSs and non-transient non-
community water systems (NTNCWSs). A PWS, as defined in 40 CFR 141.2,
provides water to the public for human consumption through pipes or
``other constructed conveyances, if such system has at least fifteen
service connections or regularly serves an average of at least twenty-
five individuals daily at least 60 days out of the year.'' A PWS is
either a CWS or a non-community water system (NCWS). A CWS, as defined
in Sec. 141.2, is ``a public water system which serves at least
fifteen service connections used by year-round residents or regularly
serves at least twenty-five year-round residents.'' The definition in
Sec. 141.2 for a NTNCWS is ``a public water system that is not a [CWS]
and that regularly serves at least 25 of the same persons over 6 months
per year.'' The following table provides examples of the regulated
entities under this rule:

------------------------------------------------------------------------
Examples of potentially affected
Category entities
------------------------------------------------------------------------
Public water systems.............. CWSs; NTNCWSs.
State and Tribal agencies......... Agencies responsible for drinking
water regulatory development and
enforcement.
------------------------------------------------------------------------

This table is not intended to be exhaustive, but rather provides a
guide for readers regarding entities likely to be regulated by this
action. This table includes the types of entities that the EPA is now
aware could potentially be regulated by this action. To determine
whether your entity is regulated by this action, this final rule should
be carefully examined. If you have questions regarding the
applicability of this action to a particular entity, consult the person
listed in the FOR FURTHER INFORMATION CONTACT section.
All new systems that begin operation after, or systems that use a
new source of water after, April 26, 2024, must demonstrate compliance
with the MCLs

[[Page 32536]]

within a period of time specified by the Primacy Agency. The EPA has
defined in 40 CFR chapter I, subchapter D, part 141, Sec. 141.2, a
wholesale system as a PWS that supplies finished PWSs and a consecutive
system as a PWS that buys or otherwise receives some or all its
finished water from a wholesale system. In this action, the EPA
reiterates that all CWS and NTNCWS must comply with this regulation.
This includes consecutive CWS and NTNCWS systems; however, the
requirements these consecutive systems must implement to comply with
the regulation may be, and often are, much less extensive. For finished
water that is provided through a system interconnection, the wholesale
systems will be responsible for conducting the monitoring requirements
at the entry point (EP) to the distribution system. The final
regulation does not require that any monitoring be conducted at a
system interconnection point. Where a violation does occur, the
wholesale system must notify any consecutive systems of this violation
and it is the responsibility of the consecutive system to provide PN to
their customers pursuant to Sec. 141.201(c)(1). In addition, wholesale
systems must also provide information in Subpart O to consecutive
systems for developing CCRs (Sec. 141.201(c)(1)). Consecutive systems
are responsible for providing their customers with the reports (Sec.
141.153(a)).

II. Background

A. What are PFAS?

Per- and polyfluoroalkyl substances (PFAS) are a large class of
thousands of synthetic chemicals that have been in use in the United
States and around the world since the 1940s (USEPA, 2018a). The ability
for PFAS to withstand heat and repel water and stains makes them useful
in a wide variety of consumer, commercial, and industrial products, and
in the manufacturing of other products and chemicals. This rule applies
directly to six specific PFAS: perfluorooctanoic acid (PFOA),
perfluorooctane sulfonic acid (PFOS), perfluorononanoic acid (PFNA),
hexafluoropropylene oxide dimer acid (HFPO-DA, commonly known as GenX
Chemicals), perfluorohexane sulfonic acid (PFHxS), and perfluorobutane
sulfonic acid (PFBS). Due to their widespread use, physicochemical
properties, and prolonged persistence, many PFAS co-occur in air,
water, ice, and soil, and in organisms, such as humans and wildlife.
Exposure to some PFAS can lead to bioaccumulation in tissues and blood
of aquatic as well as terrestrial organisms, including humans (Domingo
and Nadal, 2019; Fromme et al., 2009). Pregnant and lactating women, as
well as infants and children, may be more sensitive to the harmful
effects of certain PFAS, such as PFOA, PFOS, PFNA, and PFBS. For
example, studies indicate that PFOA and PFOS exposure above certain
levels may result in adverse health effects, including developmental
effects to fetuses during pregnancy or to breast- or formula-fed
infants, increased risk for certain cancers, and negative immunological
effects, among others (USEPA, 2024c; USEPA, 2024d). It has been
documented that exposure to other PFAS are associated with a range of
adverse health effects (USEPA, 2021a; USEPA, 2021b; ATSDR, 2021; NASEM,
2022).
The Environmental Protection Agency (EPA) is aware that PFAS still
enter the environment and there are viable pathways for human exposure.
Most United States production of PFOA, PFOS, and PFNA, along with other
long-chain PFAS, was phased out and then generally replaced by
production of PFHxS, HFPO-DA, PFBS, and other PFAS. The EPA is also
aware of ongoing use of PFOA, PFOS, PFNA, and other long-chain PFAS
(USEPA, 2000b; ATSDR, 2021). Long-chain PFAS are typically defined as
including perfluoroalkyl sulfonic acids containing >= 6 carbons, and
perfluoroalkyl carboxylic acids with >=7 carbons. While domestic
production and import of PFOA has been phased out in the United States
by the companies participating in the 2010/2015 PFOA Stewardship
Program, small quantities of PFOA may be produced, imported, and used
by companies not participating in the PFOA Stewardship Program (USEPA,
2021c). The EPA is also aware of ongoing use of PFAS available from
existing stocks or newly introduced via imports (see USEPA, 2022a).
Additionally, the environmental persistence of these chemicals and
formation as degradation products from other compounds may contribute
to their ongoing release in the environment (ATSDR, 2021).
The six PFAS in this rule and their relevant Chemical Abstract
Service registry numbers (CASRNs) are:

<bullet> PFOA (C<INF>8</INF>F<INF>15</INF>O<INF>2</INF>-;
CASRN: 45285-51-6)
<bullet> PFOS (C<INF>8</INF>F<INF>17</INF>SO<INF>3</INF>-;
CASRN: 45298-90-6)
<bullet> PFHxS (C<INF>6</INF>F<INF>13</INF>SO<INF>3</INF>-;
CASRN: 108427-53-8)
<bullet> PFNA (C<INF>9</INF>F<INF>17</INF>O<INF>2</INF>-;
CASRN: 72007-68-2)
<bullet> HFPO-DA (C<INF>6</INF>F<INF>11</INF>O<INF>3</INF>-;
CASRN: 122499-17-6)
<bullet> PFBS (C<INF>4</INF>F<INF>9</INF>SO<INF>3</INF>-;
CASRN: 45187-15-3)

These PFAS may exist in multiple forms, such as isomers or
associated salts, and each form may have a separate CAS registry number
or no CASRN at all. Additionally, these compounds have various names
under different classification systems. However, at environmentally
relevant pHs, these PFAS are expected to dissociate in water to their
anionic (negatively charged) forms. For instance, International Union
of Pure and Applied Chemistry substance 2,3,3,3-tetrafluoro-2-
(heptafluoropropoxy) propanoate (CASRN: 122499-17-6), also known as
HFPO-DA, is an anionic molecule which has an ammonium salt (CASRN:
62037-80-3), a conjugate acid (CASRN: 13252-13-6), a potassium salt
(CASRN: 67118-55-2), and an acyl fluoride precursor (CASRN: 2062-98-8),
among other variations. At environmentally relevant pHs these all
dissociate into the propanoate/anion form (CASRN: 122499-17-6). Each
PFAS listed has multiple variants with differing chemical connectivity,
but the same molecular composition (known as isomers). Commonly, the
isomeric composition of PFAS is categorized as `linear,' consisting of
an unbranched alkyl chain, or `branched,' encompassing a potentially
diverse group of molecules including at least one, but potentially
more, offshoots from the linear molecule. While broadly similar,
isomeric molecules may have differences in chemical properties. This
rule covers all salts, isomers and derivatives of the chemicals listed,
including derivatives other than the anionic form which might be
created or identified.

B. Human Health Effects

The publicly available landscape of human epidemiological and
experimental animal-based exposure-effect data from repeat-dose studies
across PFAS derive primarily from carboxylic and sulfonic acid species
such as PFOA, PFOS, PFHxS, PFNA, HFPO-DA, and PFBS (ATSDR, 2021; USEPA,
2021a; USEPA, 2021b; USEPA, 2024c; USEPA, 2024d). Many other PFAS have
some human health effects data available (Mahoney et al., 2022) and
some PFAS, such as PFBS, HFPO-DA, PFNA, and PFHxS, have sufficient data
that has allowed Federal agencies to publish toxicity assessments
(USEPA, 2021a; USEPA, 2021b; USEPA, 2024c; USEPA, 2024d; ATSDR, 2021)
and derive toxicity values (e.g., a reference dose), which is an
estimate of daily exposure to the human population

[[Page 32537]]

(including sensitive populations) that is likely to be without an
appreciable risk of deleterious effects during a lifetime). The adverse
health effects associated with exposure to such PFAS include (but are
not limited to): effects on the liver (e.g., liver cell death), growth
and development (e.g., low birth weight), hormone levels, kidney, the
immune system (reduced response to vaccines), lipid levels (e.g., high
cholesterol), the nervous system, and reproduction, as well as
increased risk of certain types of cancer.
Exposure to PFAS may have disproportionate health effects on
children. Adverse health effects relevant to children associated with
exposure to some PFAS include developmental effects to fetuses during
pregnancy or to breast-fed infants, cardiovascular effects, immune
effects, endocrine effects, and reproductive effects. Additionally,
PFAS are known to be transmitted to the fetus via the placenta and to
the newborn, infant, and child via breast milk (USEPA, 2021a; USEPA,
2021b; USEPA, 2024c; USEPA, 2024d; ATSDR, 2021).
Please see sections III.B and IV of this rule for additional
discussion on health considerations for the six PFAS the EPA is
regulating in this document.

C. Statutory Authority

Section 1412(b)(1)(A) of SDWA requires the EPA to establish
National Primary Drinking Water Regulations (NPDWRs) for a contaminant
where the Administrator determines that the contaminant: (1) may have
an adverse effect on the health of persons; (2) is known to occur or
there is a substantial likelihood that the contaminant will occur in
PWSs (public water systems) with a frequency and at levels of public
health concern; and (3) in the sole judgment of the Administrator,
regulation of such contaminant presents a meaningful opportunity for
health risk reduction for persons served by PWSs.

D. Statutory Framework and PFAS Regulatory History

Section 1412(b)(1)(B)(i) of the Safe Drinking Water Act (SDWA)
requires the EPA to publish a Contaminant Candidate List (CCL) every
five years. The CCL is a list of contaminants that are known or
anticipated to occur in PWSs, are not currently subject to any proposed
or promulgated NPDWRs and may require regulation under the drinking
water program. In some cases, developing the CCL may be the first step
in evaluating drinking water contaminants. The EPA uses the CCL to
identify priority contaminants for regulatory decision-making (i.e.,
regulatory determinations), and for data collection. Publishing a CCL
does not impose any requirements on PWSs. The EPA included PFOA and
PFOS on the third and fourth CCLs published in 2009 (USEPA, 2009a) and
2016 (USEPA, 2016a). The EPA then included PFAS as a chemical group in
its most recent list, the fifth CCL (CCL 5) (USEPA, 2022b). This group
is inclusive of the PFAS the EPA is regulating through this action;
however, the fifth CCL did not include PFOA and PFOS as they had
already had final positive regulatory determinations completed for them
in March 2021 (USEPA, 2021d).
The EPA collects data on the CCL contaminants to better understand
their potential health effects and to determine the levels at which
they occur in PWSs. SDWA 1412(b)(1)(B)(ii) requires that, every five
years and after considering public comments on a ``preliminary''
regulatory determination, the EPA issues a determination to regulate or
not regulate at least five contaminants on each CCL. In addition,
section 1412(b)(1)(B)(ii)(III) authorizes the EPA to make a
determination to regulate a contaminant not listed on the CCL at any
time so long as the contaminant meets the three statutory criteria
based on available public health information. SDWA 1412(b)(1)(B)(iii)
requires that ``each document setting forth the determination for a
contaminant under clause (ii) shall be available for public comment at
such time as the determination is published.'' To implement these
requirements, the EPA issues preliminary regulatory determinations
subject to public comment and then issues a final regulatory
determination after consideration of public comment. Section
1412(b)(1)(E) requires that the EPA propose an NPDWR no later than 24
months after a final determination to regulate. The statute also
authorizes the EPA to issue a proposed rule concurrent with a
preliminary determination to regulate. The EPA must then promulgate a
final regulation within 18 months of the proposal (which may be
extended by 9 additional months).
The EPA also implements a monitoring program for unregulated
contaminants under SDWA 1445(a)(2) that requires the EPA to issue a
list once every five years of priority unregulated contaminants to be
monitored by PWSs. This monitoring is implemented through the
Unregulated Contaminant Monitoring Rule (UCMR), which collects data
from community water systems (CWSs) and non-transient community water
systems (NTNCWSs) to better improve the EPA's understanding of the
frequency of unregulated contaminants of concern occurring in the
nation's drinking water systems and at what levels. The first four
UCMRs collected data from a census of large water systems (serving more
than 10,000 people) and from a statistically representative sample of
small water systems (serving 10,000 or fewer people).
Between 2013-2015, water systems collected monitoring data for six
PFAS (PFOA, PFOS, PFHxS, PFNA, PFBS, and perfluoroheptanoic acid
(PFHpA)) as part of the third UCMR (UCMR 3) monitoring program. The
fifth UCMR (UCMR 5), published December 2021, requires sample
collection and analysis for 29 PFAS, including PFOA, PFOS, PFHxS, PFNA,
HFPO-DA, and PFBS, to occur between January 2023 and December 2025
using drinking water analytical methods developed by the EPA. Section
2021 of America's Water Infrastructure Act of 2018 (AWIA) (Pub. L. 115-
270) amended SDWA and specifies that, subject to the availability of
the EPA appropriations for such purpose and sufficient laboratory
capacity, the EPA must require all public water systems (PWSs) serving
between 3,300 and 10,000 people to monitor and ensure that a nationally
representative sample of systems serving fewer than 3,300 people
monitor for the contaminants in UCMR 5 and future UCMR cycles. All
large water systems continue to be required to participate in the UCMR
program. Section VI of this preamble provides additional discussion on
PFAS occurrence. While the complete UCMR 5 dataset was not available to
inform this rule and thus not a basis for informing the agency's
decisions for the final rule, the EPA acknowledges that the small
subset of data released (7 percent of the total results that the EPA
expects to receive) as of July 2023 confirms the EPA's conclusions
supported by the extensive amount of data utilized in its UCMR 3, state
data, and modelling analyses. This final rule allows utilities and
primacy agencies to use the UCMR 5 data to support implementation of
monitoring requirements. Sections VI and VIII of this preamble further
discusses these occurrence analyses as well as monitoring and
compliance requirements, respectively.
After careful consideration of public comments, the EPA issued
final regulatory determinations for contaminants on the fourth CCL (CCL
4) in March of 2021 (USEPA, 2021d) which included determinations to
regulate two contaminants, PFOA and PFOS, in drinking water. The EPA
found that PFOA and PFOS may have

[[Page 32538]]

an adverse effect on the health of persons; that these contaminants are
known to occur, or that there is a substantial likelihood that they
will occur, in PWSs with a frequency and at levels that present a
public health concern; and that regulation of PFOA and PFOS presents a
meaningful opportunity for health risk reduction for persons served by
PWSs. As discussed in the final Regulatory Determinations 4 Notice for
CCL 4 contaminants (USEPA, 2021d) and the EPA's PFAS Strategic Roadmap
(USEPA, 2022c), the agency has also evaluated additional PFAS chemicals
for regulatory consideration as supported by the best available
science. The agency finds that additional PFAS compounds also meet SDWA
criteria for regulation. The EPA's regulatory determination for these
additional PFAS is discussed in section III of this preamble.
Section 1412(b)(1)(E) provides that the Administrator ``may publish
such proposed regulation concurrent with the determination to
regulate.'' The EPA interprets this provision as allowing concurrent
processing of a preliminary determination with a proposed rule, not a
final determination (as urged by some commenters--see responses in
section III of this preamble). Under this interpretation, section
1412(b)(1)(E) authorizes the EPA to issue a preliminary determination
to regulate a contaminant and a proposed NPDWR addressing that
contaminant concurrently and request public comment at the same time.
This represents the only interpretation that accounts for the statutory
language in context and is the only one that fulfills Congress's
purpose of permitting the agency to adjust its stepwise processes where
appropriate to avoid any unnecessary delay in regulating contaminants
that meet the statutory criteria. To the extent the statute is
ambiguous, the EPA's interpretation is the best interpretation of this
provision for these same reasons. As a result, this rule contains both
a final determination to regulate four PFAS contaminants (individually
and/or as part of a PFAS mixture), and regulations for those
contaminants as well as the two PFAS contaminants (PFOA and PFOS) for
which the EPA had already issued a final Regulatory Determination. The
EPA developed an MCLG and an NPDWR for six PFAS compounds pursuant to
the requirements under section 1412(b)(1)(B) of SDWA. The final Maximum
Contaminant Level Goals (MCLGs) and NPDWR are discussed in more detail
in the following section.

E. Bipartisan Infrastructure Law

The passage of the Infrastructure Investment and Jobs Act (IIJA),
often referred to as the Bipartisan Infrastructure Law or BIL, invests
over $50 billion to improve drinking water, wastewater, and stormwater
infrastructure--the single largest investment in water by the Federal
Government. This historic investment specific to safe drinking water
includes $11.7 billion in the Drinking Water State Revolving Fund
(DWSRF) General Supplemental (referred to as BIL DWSRF General
Supplemental); $4 billion to the Drinking Water SRF for Emerging
Contaminants (referred to as BIL DWSRF EC); and $5 billion in grants
for Emerging Contaminants in Small or Disadvantaged Communities
(referred to as EC-SDC) from Federal fiscal years 2022 through 2026
(USEPA, 2023a). For the BIL DWSRF General Supplemental and BIL DWSRF
EC, states must provide 49% and 100%, respectively, as additional
subsidization in the form of principal forgiveness and/or grants. The
EC-SDC grant has no cost-share requirement. Together, these funds will
assist many disadvantaged communities, small systems, and others with
the costs of addressing emerging contaminants, like PFAS, when it might
otherwise be cost-challenging. This financial assistance can be used to
address emerging contaminants in drinking water through actions such as
technical assistance, certain water quality testing, operator and
contractor training and equipment, and treatment upgrades and
expansion. Investments in these areas which will allow communities
additional funding to meet their obligations under this regulation and
help ensure protection from PFAS contamination of drinking water. The
Drinking Water SRF can be used by water systems to reduce the public
health concerns around PFAS in their drinking water and is already
being successfully utilized. Additionally, to support BIL
implementation, the EPA is offering water technical assistance
(WaterTA) to help communities identify water challenges and solutions,
build capacity, and develop application materials to access water
infrastructure funding (USEPA, 2023b). The EPA collaborates with
states, Tribes, territories, community partners, and other stakeholders
with the goal of more communities with applications for Federal
funding, quality water infrastructure, and reliable water services.

F. EPA PFAS Strategic Roadmap

In October 2021, the EPA published the PFAS Strategic Roadmap (or
Roadmap) that outlined the whole-of-agency approach to ``further the
science and research, to restrict these dangerous chemicals from
getting into the environment, and to immediately move to remediate the
problem in communities across the country'' (USEPA, 2022c). The Roadmap
offers timelines by which the EPA acts on key commitments the agency
made toward addressing these contaminants in the environment, while
continuing to safeguard public health. These include the EPA proposing
to designate certain PFAS as Comprehensive Environmental Response,
Compensation, and Liability Act (CERCLA) hazardous substances; issuing
advance notice of proposed rulemakings on various PFAS under CERCLA;
and issuing updated guidance on destroying and disposing of certain
PFAS and PFAS-containing materials. Additionally, the EPA is issued a
memorandum to states in December 2022 that provides direction on how to
use the National Pollutant Discharge Elimination System (NPDES) program
to protect against PFAS (USEPA, 2022d; USEPA, 2022e). The EPA also
announced revisions to several Effluent Limitation Guidelines (ELGs)
including, Organic Chemical, Plastic, Synthetic Fibers manufacturing,
Metal Finishing & Electroplating, and Landfills to address PFAS
discharge from these point source categories. These ELGs collectively
will, if finalized, restrict and reduce PFAS discharges to waterways
used as sources for drinking water. The EPA is taking numerous other
actions to advance our ability to understand and effectively protect
people from PFAS, such as the October 11, 2023, rule finalized under
the Toxic Substances Control Act (TSCA) that will provide the EPA, its
partners, and the public with a dataset of PFAS manufactured and used
in the United States. The rule requires all manufacturers (including
importers) of PFAS and PFAS-containing articles in any year since 2011
to report information to the extent known or reasonably ascertainable:
chemical identity, uses, volumes made and processed, byproducts,
environmental and health effects, worker exposure, and disposal to the
EPA. With this final NPDWR, the EPA is delivering on another key goal
in the Roadmap to ``establish a National Primary Drinking Water
Regulation'' for PFAS. This rule will protect the American people
directly from everyday PFAS exposures that might otherwise occur from
PFAS-contaminated drinking water,

[[Page 32539]]

complementing the many other actions in the Roadmap to protect public
health and the environment from PFAS.

III. Final Regulatory Determinations for Additional PFAS

A. Agency Findings

As noted earlier, in 2021, the EPA made a determination to regulate
two per- and polyfluoroalkyl substances--perfluorooctanoic acid (PFOA)
and perfluorooctane sulfonic acid (PFOS)--in drinking water under the
Safe Drinking Water Act. This section describes the EPA's regulatory
determination findings with respect to three additional PFAS and
mixtures of four PFAS.
Pursuant to sections 1412(b)(1)(A) and 1412(b)(1)(B)(ii)(II) of
SDWA, the EPA is making a final determination to individually regulate
as contaminants PFHxS, PFNA, and HFPO-DA and is publishing Maximum
Contaminant Level Goals (MCLGs) and promulgating National Primary
Drinking Water Regulations (NPDWRs) for these compounds individually.
Under this authority, the EPA is also making a final determination to
regulate as a contaminant a mixture of two or more of the following:
perfluorononanoic acid (PFNA), hexafluoropropylene oxide dimer acid
(HFPO-DA, commonly known as GenX Chemicals), perfluorohexane sulfonic
acid (PFHxS), and perfluorobutane sulfonic acid (PFBS), and is
publishing an MCLG and promulgating an NPDWR for mixtures of these
compounds. The agency has determined that PFHxS, PFNA, and HFPO-DA may
have individual adverse health effects, and any mixture of these three
PFAS and PFBS may also have dose-additive adverse effects on the health
of persons; that there is a substantial likelihood that PFHxS, PFNA,
and HFPO-DA occur individually with a frequency and at levels of public
health concern and that mixtures of these three PFAS and PFBS occur and
co-occur in public water systems (PWSs) with a frequency and at levels
of public health concern; and that, in the sole judgment of the
Administrator, individual regulation of PFHxS, PFNA, and HFPO-DA, and
regulation of mixtures of these three PFAS and PFBS, presents a
meaningful opportunity for health risk reduction for persons served by
PWSs. The EPA refers to ``mixtures'' in its regulatory determinations
to make clear that its determinations cover all the combinations of
PFHxS, PFNA, HFPO-DA, and PFBS that could co-occur in a mixture but
that each regulated mixture is itself a contaminant.
While the final determination includes mixtures of PFBS in
combinations with PFHxS, HFPO-DA, and PFNA, the EPA is deferring the
final individual regulatory determination for PFBS to further evaluate
it individually under the three SDWA regulatory determination criteria;
consequently, the agency is not promulgating an individual MCLG or
NPDWR for PFBS in this action. The EPA is deferring its final
individual regulatory determination because after considering the
public comments, the EPA has decided to further consider whether
occurrence information supports a finding that there is a substantial
likelihood that PFBS will individually occur in public water systems
and at levels of health concern. However, as stated previously, when
evaluating PFBS in mixtures combinations with PFHxS, PFNA, and/or HFPO-
DA, the EPA has determined that based on the best available information
it does meet all three statutory criteria for regulation when a part of
these mixtures, including that it is anticipated to have dose-additive
adverse health effects (see sections III.B and IV.B.1), there is a
substantial likelihood of its co-occurrence in combinations with PFHxS,
PFNA, and/or HFPO-DA with a frequency and at levels of public health
concern (see sections III.C, VI.C, VI.D, and USEPA 2024b), and there is
a meaningful opportunity for health risk reduction by regulating
mixture combinations of these four PFAS (see section III.D of this
preamble). Hence, although the agency is deferring the individual final
regulatory determination for PFBS, it is included in the final
determination to regulate mixture combinations containing two or more
of PFHxS, PFNA, HFPO-DA, and PFBS.
This section describes the best available science and public health
information used by the agency to support the regulatory
determinations. The MCLGs and NPDWR, including the MCLs, are discussed
further in sections IV and V of this preamble.
1. Proposal
The agency proposed preliminary determinations to regulate PFHxS,
PFNA, HFPO-DA, and PFBS individually, and to regulate mixtures of these
four PFAS contaminants, in drinking water. In the proposal, the agency
concluded that PFHxS, PFNA, HFPO-DA, and PFBS, and mixtures of these
PFAS, may cause adverse effects on the health of persons; there is a
substantial likelihood that they will occur and co-occur in PWSs with a
frequency and at levels of public health concern, particularly when
considering them in a mixture; and in the sole judgment of the
Administrator, regulation of PFHxS, PFNA, HFPO-DA, PFBS, and mixtures
of these PFAS, presents a meaningful opportunity for health risk
reductions for people served by PWSs.
Within the proposal, the agency described section 1412(b)(1)(E)
which provides that the Administrator may publish a proposed drinking
water regulation concurrent ``with the determination to regulate.''
This provision authorizes a more expedited process by allowing the EPA
to make concurrent the regulatory determination and rulemaking
processes. As a result, for the proposal, the EPA interpreted the
relevant reference to ``determination to regulate'' in section
1412(b)(1)(E) as referring to the regulatory process in
1412(b)(1)(B)(ii) that begins with a preliminary determination. Under
this interpretation, section 1412(b)(1)(E) authorizes the EPA to issue
a preliminary determination to regulate a contaminant and a proposed
NPDWR addressing that contaminant concurrently and request public
comment at the same time. This allows the EPA to act expeditiously
where appropriate to issue a final determination to regulate
concurrently with a final NPDWR to avoid delays to address contaminants
that meet the statutory criteria.
Additionally, as part of the proposal, the EPA explained why
mixtures of PFAS qualify as a ``contaminant'' for purposes of section
1412. SDWA section 1401(6) defines the term ``contaminant'' to mean
``any physical, chemical or biological or radiological substance or
matter in water.'' A mixture of two or more of the regulated PFAS
qualifies as a ``contaminant'' because the mixture itself is ``any
physical, chemical or biological or radiological substance or matter in
water'' (emphasis added). Therefore, pursuant to the provisions
outlined in section 1412(b)(1)(A) and 1412(b)(1)(B) of SDWA, the agency
made a preliminary determination to regulate PFHxS, PFNA, HFPO-DA,
PFBS, and any mixtures of these PFAS as a contaminant in drinking
water. In the past and in this instance, the EPA's approach to
regulating contaminant groups or mixtures under SDWA considers several
factors, including health effects, similarities in physical and
chemical properties, contaminant co-occurrence, ability for treatment
technology co-removal, or where such a regulatory structure presents a
meaningful opportunity to improve public health protection.

[[Page 32540]]

2. Summary of Major Public Comments and EPA Responses
The EPA requested comments on its preliminary regulatory
determinations for PFHxS, PFNA, HFPO-DA, and PFBS, and mixtures of
these PFAS, including the agency's evaluation of the statutory criteria
and any additional data or studies the EPA should consider that inform
the preliminary regulatory determinations for these contaminants and
their mixtures. The EPA also requested comment on its preliminary
determination that regulation of PFHxS, PFNA, HFPO-DA, PFBS, and their
mixtures, in addition to regulation of PFOA and PFOS, will also provide
protection from PFAS (e.g., PFDA, PFDoA, PfHpA, PFHxA, PFHpS, PFPeS)
that will not be regulated because the treatment technologies that
would be used to ensure compliance for these PFAS are also effective in
reducing concentrations of other unregulated PFAS.
Many commenters expressed support for the EPA's preliminary
regulatory determinations, including that the EPA has appropriately
determined that the three statutory criteria for regulation have been
met for all four contaminants and their mixtures using the best
available information. Many other commenters did not agree that the
agency presented sufficient information to make a preliminary
determination to regulate PFHxS, PFNA, HFPO-DA, PFBS, and their
mixtures, with some commenters recommending that that the agency
withdraw the portion of the proposed rule associated with these four
PFAS because in their view there is insufficient health effects and/or
occurrence data at this time to support the EPA's action. For some of
the four contaminants and their mixtures, a few commenters stated that
the EPA had not met the statutory criteria for regulation or that data
suggests a determination not to regulate is more appropriate. The EPA
disagrees with these commenters because there is information to support
individual regulation of PFHxS, PFNA, and HFPO-DA, as well as mixtures
of these three PFAS and PFBS, based on the three statutory criteria
(these findings are discussed in this section).
As discussed earlier in this section, after consideration of all
the public comments on this issue, the agency is deferring the
determination to individually regulate PFBS for further evaluation
under the statutory criteria. This determination is informed by public
comment suggesting that the three statutory criteria for individual
regulation of PFBS, particularly related to the occurrence criterion
have not been met. The EPA will continue to consider other available
occurrence information, including from UCMR 5, to determine whether the
information supports a finding that there is a substantial likelihood
that PFBS will individually occur in PWSs and at a level of public
health concern. The record demonstrates that exposure to a mixture with
PFBS may cause adverse health effects; that there is a substantial
likelihood that PFBS co-occurs in mixtures with PFHxS, PFNA, and/or
HFPO-DA in PWSs with a frequency and at levels of public health
concern; and that, in the sole judgment of the Administrator,
regulation of PFBS in mixtures with PFHxS, PFNA, and/or HFPO-DA
presents a meaningful opportunity for health risk reduction for persons
served by PWSs.
Furthermore, the EPA is making a final determination to regulate
PFHxS, PFNA, and HFPO-DA individually. While the EPA recognizes there
will be additional health, occurrence, or other relevant information
for these PFAS and others in the future, the EPA has determined that
there is sufficient information to make a positive regulatory
determination and the agency concludes that these three PFAS currently
meet all of the statutory criteria for individual regulatory
determination. Therefore, the agency is proceeding with making final
determinations to regulate these contaminants both individually and as
part of mixtures with PFBS and is concurrently promulgating individual
MCLs for PFHxS, PFNA, and HFPO-DA (see section V of this preamble). For
detailed information on the EPA's evaluation of the three regulatory
determination statutory criteria for PFHxS, PFNA, and HFPO-DA
individually and mixtures of these three PFAS and PFBS, as well as more
specific comments and the EPA responses related to each of the three
statutory criteria, see subsections III.B, C, and D.
Several commenters requested that the EPA evaluate additional
occurrence data to further inform its analysis for the regulatory
determinations. In response to public comments on the proposal, the EPA
evaluated updated and new occurrence data and the updates are presented
within subsection III.C. and section VI of this preamble. These
additional occurrence data further confirm that the SDWA criteria for
regulation have been met for PFHxS, PFNA, and HFPO-DA as individual
contaminants and for mixtures of PFHxS, PFNA, HFPO-DA, and/or PFBS.
A couple of commenters questioned the EPA's rationale for selecting
PFHxS, PFNA, HFPO-DA, and PFBS for regulation. The agency's process is
allowable under SDWA and, as described within this section of the
preamble, there is available health, occurrence, and other meaningful
opportunity information for three PFAS (PFHxS, PFNA, and HFPO-DA) to
meet the SDWA statutory criteria for regulation individually and four
PFAS (PFHxS, PFNA, HFPO-DA, and PFBS) as a mixture. The EPA disagrees
with commenters who suggested that the agency should not develop
national regulations that differ from state-led actions. While states
may establish drinking water standards for systems in their
jurisdiction prior to regulation under SDWA, once an NPDWR is in place,
SDWA 1413(a)(1) requires that states or Tribes adopt standards that are
no less stringent than the NPDWR to maintain primacy. Moreover, the
agency further notes that all four PFAS the EPA is regulating
individually or as a mixture are currently regulated by multiple states
as shown in table 4-17 of USEPA, 2024e.
The EPA received several comments related to the EPA's
interpretation in the proposal that the agency may, as it did here,
issue a preliminary regulatory determination concurrent with a proposed
NPDWR. Many stated that the EPA is authorized under SDWA to process
these actions concurrently and agreed with the EPA's interpretation of
the statute, noting that the EPA has followed all requirements under
SDWA including notice and opportunity for public comment on both the
preliminary regulatory determination and proposed NPDWR, and that
simultaneous public comment periods are not precluded by SDWA. Several
other commenters expressed disagreement with the EPA's interpretation.
These dissenting commenters contend that the statute only allows the
EPA to ``publish such proposed regulation concurrent with the
determination to regulate'' (i.e., in their view, the final
determination), not the ``preliminary determination to regulate.''
Moreover, some of these commenters further indicated that they believe
the EPA's final determination to regulate must precede the EPA's
proposed regulation. The EPA disagrees with commenters who stated that
the EPA cannot issue a preliminary determination concurrent with a
proposed NPDWR. Section 1412(b)(1)(e) states that ``[t]he Administrator
shall propose the maximum contaminant level goals and national primary
drinking water regulation for a contaminant not later than 24 months

[[Page 32541]]

after the determination to regulate under subparagraph (B), and may
publish such proposed regulation concurrent with the determination to
regulate'' (emphasis added). The EPA maintains its interpretation that
``determination to regulate'' in the second phrase of 1412(b)(1)(E)
allows for concurrent processing of a preliminary determination and
proposed rule, not a final determination and proposed rule.
The first clause of the provision provides an enforceable 24-month
deadline for the EPA to issue a proposed rule once it has decided to
regulate. Contrary to the suggestion of some commenters, the statutory
language providing that the EPA ``shall'' propose an NPDWR ``not later
than 24 months after the determination to regulate'' states when the 24
months to issue a proposed rule begins, i.e., the deadline is 24 months
after making a final determination to issue a proposed regulation. The
phrase ``after the determination to regulate'' here simply identifies
when SDWA's deadline begins to run; there is no textual or other
indication in the language that Congress meant it to constitute the
beginning of an exclusive 24-month window in which the EPA is permitted
to propose an NPDWR. Further, though the EPA's reading is clear on the
face of the provision, it is also supported by language elsewhere in
SDWA illustrating that when Congress intends to provide a window for
action (as opposed to a deadline for action) it knows how to do so
clearly. In fact, Congress did so in this very provision when it
required the EPA to ``publish a maximum contaminant level goal and
promulgate a national primary drinking water regulation within 18
months after the proposal thereof.'' See also, 42 U.S.C. 1448
(providing, among other things, that petitions for review of the EPA
regulations under SDWA ``shall be filed within the 45-day period
beginning on the date of the promulgation of the regulation . . .'')
(emphasis added). In addition, the phrase ``not later than,'' expressly
acknowledges that the EPA may issue a proposed rule concurrent with a
final determination. And because this language only provides a deadline
without a beginning trigger, the language in the first clause of this
provision would also not preclude the EPA from issuing a proposed rule
at any time prior to the expiration of the 24 months after a final
regulatory determination, including issuing the proposed rule on the
same day as the preliminary regulatory determination.
The second clause, which states that the Administrator ``may
publish such proposed regulation concurrent with the determination to
regulate'' should not be read to limit when the EPA can issue a
proposed rule prior to a final determination. First, Congress's use of
the phrase ``determination to regulate'' elsewhere in SDWA is not
consistent, requiring the agency to discern its meaning based on
statutory context. Second, reading ``determination to regulate'' to
refer to a final determination would, without good reason, hinder
Congress' goal in enacting this provision, to accelerate the EPA action
under SDWA. Finally, the EPA's interpretation to allow for concurrent
processes is fully consistent with, and indeed enhances, the
deliberative stepwise process provided in the statute for regulating
new contaminants.
Language throughout the statute demonstrates that Congress did not
use the term ``determination to regulate'' consistently. In fact,
``preliminary determination'' only appears once in the entire
provision, ``final determination'' is never used, and the remainder of
the references simply refer to ``determination.'' Specifically, section
1412(b)(1)(B)(ii)(I) expressly requires public comment on a
``preliminary'' regulatory determination made as part of the
contaminant candidate listing process. The rest of section
1412(b)(1)(B)(ii) and (iii) as well as the title of the provision only
refer to a ``determination to regulate'' or ``determination.'' For
example, 1412(b)(1)(B)(iii) states that ``[e]ach document setting forth
the determination for a contaminant under clause (ii) shall be
available for public comment at such time as the determination is
published.'' \1\ Although this provision only refers to a
``determination for a contaminant under clause (ii),'' this language
clearly refers to public comment on a preliminary determination and not
a final determination to regulate. The EPA has interpretated
``determination'' in this paragraph to refer to ``preliminary
determination'' because that is the best interpretation to effectuate
Congressional intent to provide public comment prior to issuing a final
determination. The EPA has done the same with section 1412(b)(1)(E)
here, as only a reading that allows for, in appropriate cases,
concurrent processing of a preliminary determination to regulate and
proposed NPDWR allows for rulemaking acceleration by the EPA as
Congress envisioned. To the extent there is ambiguity, the EPA's
reading of section 1412(b)(1)(E) is the best one to effectuate these
purposes.
---------------------------------------------------------------------------

\1\ Even the first clause of section 1412(b)(1)(E) setting the
24-month deadlines use ``regulatory determination'' without further
clarifying whether it is preliminary or final. Again, it is clear
when viewed in context that the term refers to a final
determination, as triggering a deadline to propose regulations on a
preliminary decision to regulate would not be reasonable, as the
agency may change its mind after reviewing publicv comment,
obviating the need for a proposed NPDWR.
---------------------------------------------------------------------------

The EPA could issue a proposed rule concurrent with a final
determination; there is nothing in the statute or the APA that requires
the EPA to wait. The SDWA gives the EPA 24 months to act after a final
determination but does not require the agency to wait 24 months. The
``no later than'' language in the first clause of section
1412(b)(1)(E), expressly acknowledges that the EPA may issue a proposed
rule concurrent with a final determination. Therefore, construing the
second phrase of section 1412(b)(1)(E) simply to authorize the EPA to
issue a proposed rule concurrent with a final determination renders
that provision of the statute authorizing the EPA to publish such
proposed regulation concurrent with the determination to regulate a
nullity. The well-known tools of statutory construction direct the
agencies and courts not to construe statutes so as to render Congress's
language mere surplusage, yet that it is what commenters'
interpretation would do. The EPA's construction is the one which gives
meaning to that language.
Moreover, the EPA's interpretation of ``determination to regulate''
in the phrase ``may publish such proposed regulation concurrent with
the determination to regulation'' in section 1412(b)(1)(E) to be a
preliminary determination best effectuates Congress' goal in enacting
this provision, to accelerate the EPA action under SDWA when the EPA
determines such a step is necessary and the EPA has, as it does here, a
sufficient record to proceed with both regulatory determination and
regulation actions concurrently. In addition to authorizing concurrent
processes, Congress' intent to expedite regulatory determinations when
necessary is evidenced more generally by the text and structure of
section 1412(b)(1)(B)(ii). The statute contemplates regulatory
determinations could be made as part of the 5-year cycle for the
contaminant candidate list under section 1412(b)(1)(B)(ii)(I) but may
also be made at any time under section 1412(b)(1)(B)(ii)(III). The fact
that Congress provided the EPA with express authority to make a
regulatory determination at any time is a recognition that the EPA may
need to act expeditiously to address public

[[Page 32542]]

health concerns between the statutory periodic 5-year cycle. The EPA's
interpretation of the relevant language in section 1412(b)(1)(E) best
effectuates all provisions of the statute because simultaneous public
processes for off-cycle regulatory determinations and NPDWRs allow for
administrative efficiency that may be needed to address pressing public
health concerns.
Finally, the EPA's interpretation of the statute allowing for
concurrent processes is fully consistent with the stepwise process for
issuing an NPDWR set out by the statute. Here, the EPA provided for
public comment on an extensive record for both the regulatory
determinations and the proposed regulatory levels and it is not clear
what further benefit would be provided by two separate public comment
periods. This is especially true given the D.C. Circuit's ruling in
NRDC v. Regan, 67 F.4th 397 (D.C. Cir 2023), which held that the EPA
cannot withdraw a final determination to regulate a contaminant. Thus,
even if the EPA were to provide two separate comment periods, the
information provided on a proposed rule cannot be used to undo a final
regulatory determination. Indeed, although not required by the statute,
the EPA in proposing actions concurrently provides commenters with much
more information to evaluate the preliminary regulatory determinations.
This is because the EPA has provided not just the information to
support the preliminary determinations to regulate but also the full
rulemaking record and supporting risk, cost, occurrence, and benefit
analysis that supports the proposed Maximum Contaminant Levels (MCLs).
Further, the EPA has a much more comprehensive record for the
regulatory determinations to ensure that the final determination, which
cannot be withdrawn, is based on the comprehensive record provided by
the rulemaking and Health Risk Reduction and Cost Analysis (HRRCA)
development processes.
The EPA received comments on its statutory authority to regulate
mixtures of PFHxS, PFNA, HFPO-DA, and/or PFBS, specifically the
agency's interpretation under section 1401(6) that a mixture of two or
more contaminants also qualifies as the definition of a contaminant
under SDWA since a mixture itself meets the same definition. A few
commenters disagreed and contended that a mixture does not meet the
definition of being a single contaminant under SDWA. The EPA disagrees
with these commenters, as the SDWA definition of a contaminant does not
specify that a contaminant is only a singular chemical. The SDWA
definition is very broad, specifically stating that a contaminant is
``any physical, chemical or biological or radiological substance or
matter'' (emphasis added), with no specific description or requirement
for how it is formed. Matter for example, by definition, is comprised
of either pure substances or mixtures of pure substances. A pure
substance is either an element or compound, which would include any
PFAS chemical. The statute encompasses ``matter'' which is a broad term
that includes mixtures and therefore definitionally includes PFAS
mixtures, comprised of a combination of PFAS (chemical substances), as
itself qualifying as a ``contaminant'' under SDWA. Moreover, other
provisions of the statute, would be restricted in a manner inconsistent
with Congressional intent if the EPA were to adopt the cabined approach
to ``contaminant'' suggested by some commenters. For example, section
1431 of SDWA provides important authority to the EPA to address
imminent and substantial endangerment to drinking water supplies posed
by ``a contaminant'' that is present in or threatened those supplies.
Congress clearly intended this authority to be broad and remedial, but
it would be significantly hampered if the EPA would be restricted to
only addressing individual chemicals and not mixtures threatening a
water supply. For these reasons, the EPA's interpretation of the
definition of contaminant is the only reading that is consistent with
the statutory definition and use of the term in context and at to the
extent the definition of contaminant is ambiguous, the EPA's
interpretation represents the best interpretation of that term.
Finally, even if a mixture is considered a group, as some commenters
suggest, Congress clearly contemplated that the EPA could regulate
contaminants as groups. See H.R. Rep. No 93-1185 (1974), reprinted in
1974 U.S.C.C.A.N. 6454, 6463-64) (noting the tens of thousands of
chemical compounds in use commercially, with many more added each year,
of which many will end up in the nation's drinking water and finding
that ``[i]t is, of course, impossible for EPA to regulate each of these
contaminants which may be harmful to health on a contaminant-by-
contaminant basis. Therefore, the Committee anticipates that the
Administrator will establish primary drinking water regulations for
some groups of contaminants, such as organic and asbestos.'') Thus, the
EPA has the authority to regulate a mixture as a contaminant under
SDWA.
The commenters also suggested that the EPA has not followed its
Supplementary Guidance for Conducting Health Risk Assessment of
Chemical Mixtures (USEPA, 2000a), specifically that the agency did not
use a ``sufficiently similar mixture'' where ``components and
respective portions exist in approximately the same pattern'' and
suggested that there has to be consistent co-occurrence of the mixture
components. The EPA disagrees with these comments. It is not possible
or necessary to use a whole-mixture approach for PFHxS, PFNA, HFPO-DA,
and PFBS or a ``sufficiently similar mixture.'' Instead, the EPA is
using a longstanding component-based mixture approach called the Hazard
Index, which was endorsed in the context of assessing potential risk
associated with PFAS mixtures by the Science Advisory Board (SAB)
during its 2021 review of the EPA's Draft Framework for Estimating
Noncancer Health Risks Associated with Mixtures of Per- and
Polyfluoroalkyl Substances (PFAS) (USEPA, 2021e) (see section IV of
this preamble). The goal of this component-based approach is to
approximate what the whole-mixture toxicity would be if the whole
mixture could be tested and relies on toxicity information for each
individual component in a mixture (USEPA, 2000a). A whole-mixture
approach for regulating these four PFAS in drinking water is not
possible because it would entail developing a single toxicity value
(e.g., a reference dose (RfD)) for one specific mixture of PFHxS, PFNA,
HFPO-DA, and PFBS with defined proportions of each PFAS. Toxicity
studies are typically conducted with only one test substance to isolate
that particular substance's effects on the test organism, and whole-
mixture data are exceedingly rare. There are no known whole-mixture
studies for PFHxS, PFNA, HFPO-DA, and PFBS, and even if they were
available, the corresponding toxicity value (i.e., a single RfD for a
specific mixture of these four PFAS) would only be directly applicable
to that specific mixture. Thus, a more flexible approach that takes
into account the four component PFAS in different combinations and at
different concentrations (i.e., the Hazard Index approach) is
necessary. The Hazard Index indicates risk from exposure to a mixture
and is useful in this situation to ensure a health-protective MCLG in
cases where the mixture is spatially and/or temporally variable. For a
more detailed discussion on whole-mixture and component-based
approaches for PFAS health assessment, please see the EPA's Framework
for

[[Page 32543]]

Estimating Noncancer Health Risks Associated with Mixtures of Per- and
Polyfluoroalkyl Substances (PFAS) (USEPA, 2024a).
Many other commenters supported the EPA's interpretation of
regulating a mixture as a ``contaminant'' that consists of a
combination of certain PFAS, citing the EPA's broad authority under
SDWA to set regulatory standards for groups of related contaminants and
the EPA precedent for doing so under other NPDWRs including
disinfection byproducts (DBPs; for total trihalomethanes [TTHMs] and
the sum of five haloacetic acids [HAA5] (USEPA, 1979; USEPA, 2006a)),
as well as radionuclides (USEPA, 2000c) and polychlorinated biphenyls
(PCBs). The EPA also noted some of these examples within the proposed
rule. One commenter disagreed that these previous EPA grouping
approaches are applicable to the mixture of the four PFAS, noting that
TTHMs and HAA5 are byproducts of the disinfection process and are the
result of naturally occurring compounds reacting with the disinfectants
used in drinking water treatment; thus, their formation cannot be
controlled and is dependent on the presence and amount of disinfectant.
As a result of these factors, measuring them as a class is required;
however, the four PFAS are not byproducts, and the presence of one PFAS
does not change the presence of the other PFAS. Moreover, the commenter
provided that related to radionuclides, alpha particles are identical
regardless of their origination and using this example for PFAS is not
supported since the four PFAS are fundamentally different. The EPA
disagrees with this commenter. As noted above, the SDWA definition of
contaminant is very broad (``any physical, chemical or biological or
radiological substance or matter'' (emphasis added)) with no
limitations, specific description or requirement for how it is formed.
The statute therefore easily encompasses a mixture, comprised of a
combination of PFAS (chemical substances), as itself qualifying as a
``contaminant'' under SDWA. Moreover, as also noted above, to the
extent the mixture is considered a ``group,'' Congress clearly
anticipated that the EPA would regulate contaminants by group. As a
result, even if the PFAS ``group'' is different than other SDWA
regulatory groupings, such a regulation is clearly authorized under the
statute. Furthermore, it makes sense to treat these mixtures as a
``contaminant'' because the four PFAS share similar characteristics: it
is substantially likely that they co-occur; the same treatment
technologies can be used for their removal; they are measured
simultaneously using the same analytical methods; they have shared
adverse health effects; and they have similar physical and chemical
properties resulting in their environmental persistence.
3. The EPA's Final Determination
The EPA is making determinations to regulate PFHxS, PFNA, and HFPO-
DA individually and to regulate mixtures of PFHxS, PFNA, HFPO-DA, and/
or PFBS. A mixture of PFHxS, PFNA, HFPO-DA, and PFBS can contain any
two or more of these PFAS. The EPA refers to ``mixtures'' in its final
regulatory determinations to make clear that its determinations cover
all of the combinations of PFHxS, PFNA, HFPO-DA, and PFBS that could
co-occur in a mixture but that any combination itself qualifies as a
contaminant.
In this preamble, as discussed earlier, the EPA is deferring the
final determination to regulate PFBS individually to further evaluate
the three criteria specified under SDWA 1412(b)(1)(A), particularly
related to its individual known or likely occurrence, but is making a
final determination to regulate PFBS as part of a mixture with PFHxS,
PFNA, and/or HFPO-DA.
To support the agency's regulatory determinations, the EPA
carefully considered the public comments and examined health effects
information from available final peer-reviewed human health assessments
and studies, as well as drinking water monitoring data collected as
part of the UCMR 3 and state-led monitoring efforts. The EPA finds that
oral exposure to PFHxS, PFNA, and HFPO-DA individually, and
combinations of these three PFAS and PFBS in mixtures, may result in a
variety of adverse health effects, including similar or shared adverse
effects on several biological systems including the endocrine,
cardiovascular, developmental, immune, and hepatic systems (USEPA,
2024f). Based on the shared toxicity types, exposure to PFHxS, PFNA, or
HFPO-DA individually, or combinations of these three PFAS and PFBS in a
mixture, is anticipated to affect common target organs, tissues, or
systems to produce dose-additive effects from co-exposures.
Additionally, based on the agency's evaluation of the best available
science, including a review of updated data from state-led drinking
water monitoring efforts discussed in subsection III.C of this
preamble, the EPA finds that PFHxS, PFNA, and HFPO-DA each have a
substantial likelihood to occur in finished drinking water and that
these three PFAS and PFBS are also likely to co-occur in mixtures and
result in increased total PFAS exposure above levels of public health
concern. Therefore, as discussed further in this section, the agency is
determining that:
<bullet> exposure to PFHxS, PFNA, or HFPO-DA individually, and any
mixture of these three PFAS and PFBS, may have adverse effects on the
health of persons;
<bullet> there is a substantial likelihood that PFHxS, PFNA, and
HFPO-DA will occur and there is a substantial likelihood that
combinations of these three PFAS plus PFBS will co-occur in mixtures in
PWSs with a frequency and at levels of public health concern; and
<bullet> in the sole judgment of the Administrator, individual
regulation of PFHxS, PFNA, and HFPO-DA, and mixtures of the three PFAS
plus PFBS, presents a meaningful opportunity for health risk reductions
for persons served by PWSs.
The EPA is making a final individual regulatory determination for
PFHxS, HFPO-DA, and PFNA and promulgating individual MCLGs and NPDWRs
for PFHxS, HFPO-DA, and PFNA. These NPDWRs ensure public health
protection when one of these PFAS occurs in isolation above their MCLs
and also support risk communication efforts for utilities (see section
V of this preamble for more information). The EPA is also making a
final mixture regulatory determination and promulgating a Hazard Index
MCLG and NPDWR for mixtures containing two or more of PFHxS, PFNA,
HFPO-DA, and PFBS. The Hazard Index is a risk indicator and has been
shown to be useful in chemical mixtures decision contexts (USEPA,
2023c).\2\ Individual NPDWRs do not address dose additive risks from
co-occurring PFAS. However, the Hazard Index NPDWR accounts for PFAS
co-occurring in mixtures where the individual concentrations of one or
more PFAS may not exceed their individual levels of public health
concern, but the combined levels of these co-occurring PFAS result in
an overall exceedance of the health-protective level. In this way, the
Hazard Index NPDWR protects against dose-additive effects. This
approach also recognizes that exposure to the PFAS included in the
Hazard Index is associated with adverse health effects at differing
potencies (e.g., the toxicity reference value for PFHxS is lower than

[[Page 32544]]

the one for PFBS) and that, regardless of these potency differences,
all co-occurring PFAS are included in the hazard calculation (i.e., the
health effects and presence of lower toxicity PFAS are neither ignored
nor are they over-represented). Furthermore, the approach accounts for
all the different potential combinations of these PFAS that represent a
potential public health concern that would not be addressed if the EPA
only finalized individual NPDWRs and considered individual PFAS in
isolation.
---------------------------------------------------------------------------

\2\ Some describe the Hazard Index as an indicator of potential
hazard because it does not estimate the probability of an effect;
others characterize the Hazard Index as an indicator of potential
risk because the measure integrates both exposure and toxicity
(USEPA 2000c; USEPA, 2023c).
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B. Statutory Criterion 1--Adverse Health Effects

The agency finds that exposure to PFHxS, PFNA, and HFPO-DA
individually, and any mixture of these three PFAS and PFBS, may have an
adverse effect on the health of persons. Following is a discussion of
health effects information for each of these four individual PFAS and
the levels at which those health effects may be adverse. The agency
developed health reference levels (HRLs) for PFHxS, PFNA, HFPO-DA, and
PFBS as part of its effort to identify the adverse effects each
contaminant may have on the health of persons. In this instance, the
EPA identified the HRL as the level below which adverse health effects
over a lifetime of exposure are not expected to occur, including for
sensitive populations and life stages, and allows for an adequate
margin of safety. The HRLs are also used as health-based water
concentrations (HBWCs) in the calculation of the Hazard Index MCLG (see
section IV).
1. PFHxS
Studies have reported adverse health effects, including on the
liver, thyroid, and development, after oral exposure to PFHxS (ATSDR,
2021). For a detailed discussion on adverse effects associated with
oral exposure to PFHxS, please see ATSDR (2021) and USEPA (2024f).
The EPA derived the individual HRL/HBWC for PFHxS using a chronic
reference value of 0.000002 (2E-06) mg/kg/day based on adverse thyroid
effects (follicular epithelial hypertrophy/hyperplasia), a sensitive
noncancer effect determined to be adverse and relevant to humans,
observed in male rats after oral PFHxS exposure during adulthood
(ATSDR, 2021; USEPA, 2024f). The EPA applied a bodyweight-adjusted
drinking water intake (DWI-BW) exposure factor for adults within the
general population (0.034 L/kg/day; 90th percentile direct and indirect
consumption of community water, consumer-only two-day average, adults
21 years and older) and a relative source contribution (RSC) of 0.20 to
calculate the HRL/HBWC (USEPA, 2024f). The HRL/HBWC for PFHxS is 10 ng/
L which was used to evaluate individual occurrence of PFHxS for the
final regulatory determination as discussed in section III.C of this
preamble.
2. PFNA
Studies have reported adverse health effects, including on
development, reproduction, immune function, and the liver, after oral
exposure to PFNA (ATSDR, 2021). For a detailed discussion of adverse
effects associated with oral exposure to PFNA, please see ATSDR (2021)
and USEPA (2024f).
The EPA derived the HRL/HBWC for PFNA using a chronic reference
value of 0.000003 (3E-06) mg/kg/day based on decreased body weight gain
and impaired development (i.e., delayed eye opening, delayed sexual
maturation) in mice born to mothers that were orally exposed to PFNA
during gestation (with presumed continued indirect exposure of
offspring via lactation) (ATSDR, 2021; USEPA, 2024f). These sensitive
noncancer effects were determined to be adverse and relevant to humans
(ATSDR, 2021; USEPA, 2024f). The EPA applied a DWI-BW exposure factor
for lactating women (0.0469 L/kg/day; 90th percentile direct and
indirect consumption of community water, consumer-only two-day average)
and an RSC of 0.20 to calculate the HRL/HBWC (USEPA, 2024f). The HRL/
HBWC for PFNA is 10 ng/L which was used to evaluate individual
occurrence of PFNA for the final regulatory determination as discussed
in section III.C of this preamble.
3. HFPO-DA
Animal toxicity studies have reported adverse health effects after
oral HFPO-DA exposure, including liver and kidney toxicity and immune,
hematological, reproductive, and developmental effects (USEPA, 2021b).
The EPA determined that there is Suggestive Evidence of Carcinogenic
Potential after oral exposure to HFPO-DA in humans, but the available
data are insufficient to derive a cancer risk concentration for oral
exposure to HFPO-DA. For a detailed discussion of adverse effects of
oral exposure to HFPO-DA, please see USEPA (2021b).
The most sensitive noncancer effects observed among the available
data were the adverse effects on liver (e.g., increased relative liver
weight, hepatocellular hypertrophy, apoptosis, and single-cell/focal
necrosis), which were observed in both male and female mice and rats
across a range of exposure durations and dose levels, including the
lowest tested dose levels and shortest exposure durations. The EPA
derived the HRL/HBWC for HFPO-DA from a chronic oral RfD of 0.000003
(3E-06) mg/kg/day that is based on adverse liver effects, specifically
a constellation of liver lesions including cytoplasmic alteration,
single-cell and focal necrosis, and apoptosis, observed in parental
female mice following oral exposure to HFPO-DA from pre-mating through
day 20 of lactation (USEPA, 2021b). The EPA applied a DWI-BW exposure
factor for lactating women (0.0469 L/kg/day; 90th percentile direct and
indirect consumption of community water, consumer-only two-day average)
and an RSC of 0.20 to calculate the HRL/HBWC (USEPA, 2024f). The HRL/
HBWC for HFPO-DA is 10 ng/L which was used to evaluate individual
occurrence of HFPO-DA for the final regulatory determination as
discussed in section III.C of this preamble.
4. PFBS
Toxicity studies of oral PFBS exposure in animals have reported
adverse health effects on development, as well as on the thyroid and
kidneys (USEPA, 2021a). Human and animal studies evaluated other health
effects following PFBS exposure including effects on the immune,
reproductive, and hepatic systems and lipid and lipoprotein
homeostasis, but the evidence was determined to be equivocal (USEPA,
2021a). No studies evaluating the carcinogenicity of PFBS in humans or
animals were identified. The EPA concluded that there is Inadequate
Information to Assess Carcinogenic Potential for PFBS and its potassium
salt (K + PFBS) by any route of exposure based on the EPA's Guidelines
for Carcinogen Risk Assessment (USEPA, 2005a). For a detailed
discussion on adverse effects after oral exposure to PFBS, please see
USEPA (2021a).
As noted previously, the agency is deferring the final individual
regulatory determination for PFBS. For the purposes of evaluating PFBS
in mixture combinations with PFHxS, PFNA, and HFPO-DA (see section
III.B.5 of this preamble), the EPA derived the HRL/HBWC for PFBS from a
chronic RfD of 0.0003 (3E-04) mg/kg/day that is based on adverse
thyroid effects (decreased serum total thyroxine) observed in newborn
mice following gestational exposure to the potassium salt of PFBS
(USEPA, 2021a). The EPA applied a DWI-BW exposure factor for women of
child-bearing age (0.0354 L/kg/day; 90th percentile direct and indirect
consumption of community water, consumer-only two-day average) and an

[[Page 32545]]

RSC (relative score contribution) of 0.20 to calculate the HRL/HBWC
(USEPA, 2024f). The HRL/HBWC for PFBS is 2000 ng/L.
5. Mixtures of PFHxS, PFNA, HFPO-DA, and PFBS
Exposure to per- and polyfluoroalkyl acids (PFAAs), a subclass of
PFAS that includes PFHxS, PFNA, HFPO-DA, and PFBS, can disrupt
signaling of multiple biological pathways, resulting in a shared set of
adverse effects, including effects on thyroid hormone levels, lipid
synthesis and metabolism, development, and immune and liver function
(ATSDR, 2021; EFSA et al., 2018; EFSA et al., 2020; USEPA, 2021a;
USEPA, 2021b; USEPA, 2024f; see further discussion in section III.B.6.e
of this preamble).
Studies with PFAS and other classes of chemicals support the
health-protective conclusion that chemicals that have similar observed
adverse effects following individual exposure should be assumed to act
in a dose-additive manner when in a mixture unless data demonstrate
otherwise (USEPA, 2024a). Dose additivity means that the combined
effect of the component chemicals in the mixture (in this case, PFHxS,
PFNA, HFPO-DA, and/or PFBS) is equal to the sum of their individual
doses or concentrations scaled for potency (USEPA, 2000a). In other
words, exposure to these PFAS, at doses that individually would not
likely result in adverse health effects, when combined in a mixture may
result in adverse health effects. See additional discussion of PFAS
dose additivity in section IV of this preamble.
The EPA used a Hazard Index (HI) HRL of 1 (unitless) to evaluate
co-occurrence of combinations PFHxS, PFNA, HFPO-DA, and PFBS in
mixtures for the final regulatory determination as discussed in section
III.C of this preamble. For technical details on the Hazard Index
approach, please see section IV of this preamble, USEPA (2024a), and
USEPA (2024f).
6. Summary of Major Public Comments and EPA Responses
Commenters referred to the HRLs and HBWCs interchangeably, so
comments related to those topics are addressed in this section. (Other
comments related to the MCLGs are addressed in section IV of this
preamble.)
Many commenters expressed support for the EPA's derivation of HRLs/
HBWCs and use of best available peer-reviewed science, specifically the
use of the final, most recently published Agency for Toxic Substances
and Disease Registry (ATSDR) minimal risk levels for PFHxS and PFNA as
chronic reference values. Other commenters criticized the EPA for using
ATSDR minimal risk levels and stated that they are inappropriate for
SDWA rulemaking.
The EPA finds that the ATSDR minimal risk levels for PFHxS and PFNA
currently represent the best available, peer-reviewed science for these
chemicals. SDWA specifies that agency actions must rely on ``the best
available, peer-reviewed science and supporting studies conducted in
accordance with sound and objective scientific practices.'' At this
time, the 2021 ATSDR Toxicological Profile for Perfluoroalkyls, which
covers 10 PFAS including PFHxS and PFNA, represents the best available
peer-reviewed scientific information on the human health effects of
PFHxS and PFNA. ATSDR minimal risk levels for PFHxS and PFNA are
appropriate for use under SDWA because ATSDR uses scientifically
credible approaches, its work is internally and externally peer-
reviewed and undergoes public comment, and its work represents the
current best available science for these two chemicals. The 2021 ATSDR
Toxicological Profile for Perfluoroalkyls underwent intra- and
interagency review and subsequent external peer review by seven experts
with knowledge of toxicology, chemistry, and/or health effects.
The agency acknowledges that ATSDR minimal risk levels and EPA RfDs
are not identical. The two agencies sometimes develop toxicity values
for different exposure durations (e.g., intermediate, chronic) and/or
apply different uncertainty/modifying factors to reflect data
limitations. Additionally, ATSDR minimal risk levels and EPA RfDs are
developed for different purposes: ATSDR minimal risk levels are
intended to serve as screening levels and are used to identify
contaminants and potential health effects that may be of concern at
contaminated sites, whereas EPA RfDs are used to support regulatory and
nonregulatory actions, limits, and recommendations in various
environmental media. However, from a practical standpoint, an oral
minimal risk level and an oral RfD both represent the level of daily
oral human exposure to a hazardous substance for a specified duration
of exposure below which adverse health effects are not anticipated to
occur. The EPA has routinely used and continues to use ATSDR minimal
risk levels in human health assessments when they represent the best
available science--for example, in the context of Clean Air Act section
112(f)(2) risk assessments in support of setting national emission
standards for Hazardous Air Pollutants (HAPs), developing Clean Water
Act ambient water quality criteria, evaluating contaminants for the
CCL, and site evaluations under the Resource Conservation and Recovery
Act (RCRA) and the Comprehensive Environmental Response, Compensation,
and Liability Act (CERCLA).
Some commenters questioned the EPA's external peer-review process
for the four underlying final toxicity assessments used to calculate
the HRLs/HBWCs. Some commenters noted that the EPA does not yet have
completed Integrated Risk Information System (IRIS) assessments for
PFHxS and PFNA, questioning the EPA's use of non-EPA assessments (see
above). The EPA notes that all four toxicity assessments containing the
toxicity values (RfD or minimal risk level) used to calculate the HRLs/
HBWCs (i.e., the EPA human health toxicity assessments for HFPO-DA and
PFBS (USEPA, 2021a; USEPA, 2021b) and the ATSDR toxicity assessments of
PFNA and PFHxS (ATSDR, 2021)) underwent rigorous, external peer review
(ATSDR, 2021; USEPA, 2021a; USEPA, 2021b). The EPA is not required
under SDWA to exclusively use EPA assessments to support an NPDWR, and
in fact, SDWA's clear direction in section 1412(b)(3)(A)(i) is to use
the best available, peer-reviewed science when developing NPDWRs
(emphasis added). Final EPA assessments for PFHxS and PFNA are under
development but are not currently available; final, peer reviewed ATSDR
assessments are available.
Other commenters offered critical comments on the HRLs/HBWCs for
PFHxS, PFNA, HFPO-DA, and PFBS and raised technical and process
concerns with the underlying human health assessments. Some commenters
asserted that the human health toxicity values (EPA RfDs, ATSDR minimal
risk levels) upon which the HRLs/HBWCs are based have too much
uncertainty (e.g., inappropriately apply a composite uncertainty factor
(UF) of 3,000) and are therefore inadequate to support a SDWA
regulatory determination. The EPA disagrees with these comments. The
HRLs/HBWCs are data-driven values that incorporate UFs based on the EPA
guidance and guidelines thus, represent the levels below which adverse
health effects are not expected to occur over a lifetime. According to
the EPA guidelines and longstanding practices (USEPA, 2002a; USEPA,
2022f), UFs reflect the limitations of the data across the five areas
used in the current EPA human health risk assessment development: (1)
human interindividual

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variability (UF<INF>H</INF>); (2) extrapolation from animal to human
(UF<INF>A</INF>); (3) subchronic-to-chronic duration extrapolation
(UF<INF>S</INF>); (4) lowest-observed-adverse-effect level-to-no-
observed-adverse-effect level (LOAEL-to-NOAEL) extrapolation
(UF<INF>L</INF>); and (5) database uncertainty (UF<INF>D</INF>). In
minimal risk level development, ATSDR also applies uncertainty factors
as appropriate to address areas of uncertainty, with the exception of
subchronic-to-chronic duration extrapolation (ATSDR, 2021). For the
ATSDR minimal risk levels on which the HRLs/HBWCs for PFNA and PFHxS
are based, ATSDR utilized UF<INF>H</INF>s, UF<INF>A</INF>s, and what
ATSDR calls a modifying factor to address database deficiencies
(equivalent to the EPA's UF<INF>D</INF>) (ATSDR, 2021). The EPA
carefully reviewed ATSDR's application of uncertainty and modifying
factors for PFNA and PFHxS and applied additional uncertainty factors
as warranted. Specifically, the EPA applied an additional UF
(UF<INF>S</INF>) for PFHxS to extrapolate from subchronic to chronic
duration per agency guidelines (USEPA, 2002a) and standard practice
because the critical effect was not observed during a developmental
lifestage (i.e., the effect was in parental male rats). A chronic
toxicity value (i.e., RfD, MRL) represents the daily exposure to the
human population (including sensitive subgroups) that is likely to be
without an appreciable risk of deleterious effects during a lifetime;
the EPA is using a chronic toxicity value to derive the MCLG to ensure
that it is set at a level at or below which no known or anticipated
adverse effects on human health occur and allowing an adequate margin
of safety. The EPA guidelines indicate that the composite (total) UF
may be equal to or below 3,000; composite UFs greater than that
represent ``excessive uncertainty'' (USEPA, 2002a; USEPA, 2022f). In
the case of this final NPDWR, a composite UF of 3,000 was appropriately
applied to derive toxicity values used to develop HRLs/HBWCs for two of
the four PFAS (HFPO-DA and PFHxS) following peer-reviewed agency
guidance and longstanding practice (see USEPA (2024f) for complete
discussion of UF application for all four PFAS). The EPA has previously
developed an MCLG for a chemical that had a composite UF of 3,000
applied to derive a toxicity value (e.g., thallium [USEPA, 1992]).
Further, a composite uncertainty factor of 3,000 has been applied in
the derivation of oral RfDs for several chemicals that have been
evaluated within the EPA's IRIS (Integrated Risk Information System)
program (e.g., fluorene, cis- and trans-1,2-dichloroethylene, 2,4-
dimethylphenol; please see the EPA's IRIS program website [<a href="https://www.epa.gov/iris">https://www.epa.gov/iris</a>] for further information).
Some commenters opposed the EPA's application of a 20 percent RSC
(relative source contribution) in the HRL/HBWC calculations and stated
that it was a ``conservative default'' approach not supported by
available information and that adequate exposure data exist to justify
an RSC other than 20 percent (although commenters did not offer a
suggested alternative RSC). The EPA disagrees with these comments. The
EPA applies an RSC to account for potential aggregate risk from
exposure routes and exposure pathways other than oral ingestion of
drinking water to ensure that an individual's total exposure to a
contaminant does not exceed the daily exposure associated with toxicity
(i.e., threshold level or reference dose). Application of the RSC in
this context is consistent with EPA methods (USEPA, 2000d) and long-
standing EPA practice for establishing drinking water MCLGs and NPDWRs
(e.g., see USEPA, 1989; USEPA, 2004; USEPA, 2010). The RSC represents
the proportion of an individual's total exposure to a contaminant that
is attributed to drinking water ingestion (directly or indirectly in
beverages like coffee, tea, or soup, as well as from dietary items
prepared with drinking water) relative to other exposure pathways. The
remainder of the exposure equal to the RfD (or minimal risk level) is
allocated to other potential exposure sources (USEPA, 2000d). The
purpose of the RSC is to ensure that the level of a contaminant (e.g.,
MCLG) in drinking water, when combined with other identified potential
sources of exposure for the population of concern, will not result in
total exposures that exceed the RfD (or minimal risk level) (USEPA,
2000d). This ensures that the MCLG under SDWA meets the statutory
requirement that it be a level of a contaminant in drinking water at or
below which no known or anticipated adverse effects on human health
occur and allowing an adequate margin of safety.
To determine the RSCs for the four HRLs/HBWCs, the agency assessed
the available scientific literature on potential sources of human
exposure other than drinking water. The EPA conducted literature
searches and reviews for each of the four HRLs/HBWCs to identify
potential sources of exposure and physicochemical properties that may
influence occurrence in environmental media (Deluca et al., 2022;
USEPA, 2024f). Considering this exposure information, the EPA followed
its longstanding, peer-reviewed Exposure Decision Tree Approach in the
EPA's Methodology for Deriving Ambient Water Quality Criteria for the
Protection of Human Health (USEPA, 2000d) to determine the RSC for each
PFAS. As discussed by the EPA in the Hazard Index MCLG document (USEPA,
2024f), the EPA carefully evaluated studies that included information
on potential exposure to these four PFAS (PFHxS, PFNA, HFPO-DA, and
PFBS) via sources other than drinking water, such as food, soil,
sediment, and air. For each of the four PFAS, the findings indicated
that there are significant known or potential uses/sources of exposure
beyond drinking water ingestion (e.g., food, indoor dust) (Box 6 in the
EPA Exposure Tree; USEPA, 2000d), but that data are insufficient to
allow for quantitative characterization of the different exposure
sources (Box 8A in USEPA, 2000d). The EPA's Exposure Decision Tree
approach states that when there are insufficient environmental and/or
exposure data to permit quantitative derivation of the RSC, the
recommended RSC for the general population is 20 percent (Box 8B in
USEPA, 2000d). This means that 20 percent of the exposure equal to the
RfD is allocated to drinking water, and the remaining 80 percent is
attributed to all other potential exposure sources.
Some commenters disagreed with the bodyweight-adjusted drinking
water intake (DWI-BWs) that the EPA used to calculate the HRLs/HBWCs
and thought the selected DWI-BWs were too high (overly health
protective). One commenter stated that the DWI-BW used in the
calculation of the HRL/HBWC for HFPO-DA is inappropriate and that the
EPA should have used a DWI-BW for general population adults instead of
for lactating women. The EPA disagrees with this comment. To select an
appropriate DWI-BW for use in derivation of the HRL/HBWC for HFPO-DA,
the EPA considered the HFPO-DA exposure interval used in the oral
reproductive/developmental toxicity study in mice that served as the
basis for chronic RfD derivation (the critical study). In this study,
parental female mice were dosed from pre-mating through lactation,
corresponding to three potentially sensitive human adult life stages
that may represent critical windows of HFPO-DA exposure: women of
childbearing age, pregnant women, and lactating women (Table 3-63 in
USEPA, 2019a). Of these three, the highest DWI-BW, for lactating women
(0.0469 L/kg/day), is anticipated to be protective of the other two
sensitive life

[[Page 32547]]

stages and was used to calculate the HRL/HBWC for HFPO-DA (USEPA,
2024f).
Other commenters urged the EPA to consider infants as a sensitive
life stage for PFHxS, PFNA, and PFBS and use the DWI-BW for infants to
calculate the HRLs/HBWCs. The EPA disagrees with this comment. The
EPA's approach to DWI-BW selection includes a step to identify the
sensitive population(s) or life stage(s) (i.e., those that may be more
susceptible or sensitive to a chemical exposure) by considering the
available data for the contaminant, including the adverse health
effects observed in the toxicity study on which the RfD/minimal risk
level was based (known as the critical effect within the critical or
principal study). Although data gaps can complicate identification of
the most sensitive population (e.g., not all windows or life stages of
exposure and/or health outcomes may have been assessed in available
studies), the critical effect and point of departure (POD) that form
the basis for the RfD (or minimal risk level) can provide some
information about sensitive populations because the critical effect is
typically observed at the lowest tested dose among the available data.
Evaluation of the critical study, including the exposure window, may
identify a sensitive population or life stage (e.g., pregnant women,
formula-fed infants, lactating women). In such cases, the EPA can
select the corresponding DWI-BW for that sensitive population or life
stage from the Exposure Factors Handbook (USEPA, 2019a). DWI-BWs in the
Exposure Factors Handbook are based on information from publicly
available, peer-reviewed studies, and were updated in 2019. In the
absence of information indicating a sensitive population or life stage,
the DWI-BW corresponding to the general population may be selected.
Following this approach, the EPA selected appropriate DWI-BWs for each
of the four PFAS included in the Hazard Index MCLG (see USEPA, 2024f).
The EPA did consider infants as a sensitive life stage for all four
PFAS; however, the agency did not select the infant DWI-BW because the
exposure intervals of the critical studies supporting the chronic
toxicity values did not correspond to infants. Instead, the exposure
intervals were relevant to other sensitive target populations (i.e.,
lactating women or women of childbearing age) or the general
population. (See also comments related to DWI-BW selection under PFBS
section III.B.6.d. of this preamble).
a. PFHxS
Some commenters noted a typographical error in the HRL/HBWC
calculation for PFHxS which was reported as 9.0 ng/L in the proposal.
The agency has corrected the value in this NPDWR and within the
requirements under 40 CFR part 141, subpart Z. The correct HRL/HBWC for
PFHxS is 10 ng/L.
Two commenters questioned the human relevance of thyroid effects
(i.e., changes in tissue structure (e.g., enlarged cells; increased
numbers of cells) in the thyroids of adult male rats) observed in the
critical study used to derive the ATSDR minimal risk level and the
EPA's PFHxS HRL/HBWC because, as noted in the ATSDR Toxicological
Profile for Perfluoroalkyls, this observed effect may have been
secondary to liver toxicity and, therefore, the commenters state that
its significance is unclear. The EPA disagrees with this comment. SDWA
requires that the EPA use ``the best available, peer reviewed science''
to inform decision making on drinking water regulations. Although there
is some uncertainty regarding the selection of thyroid alterations as
the critical effect (as the ATSDR toxicological profile notes), at this
time, the 2021 ATSDR toxicological profile represents the best
available peer reviewed scientific information regarding the human
health effects of PFHxS. As the most sensitive known effect as
supported by the weight of the evidence, the thyroid effect was
appropriately selected by ATSDR as the critical effect. Additionally,
published studies in rats have shown that PFHxS exposure results in
other thyroid effects, including decreases in thyroid hormone
(primarily T4) levels in serum (NTP, 2018a; Ramh[oslash]j et al.,
2018). Similarly, peer-reviewed final EPA assessments of other PFAS,
including PFBS (USEPA, 2021a) and perfluorobutanoic acid (PFBA) (USEPA,
2022g), have concluded that these changes in rodents are adverse and
human-relevant, and appropriate for RfD derivation. Furthermore, it is
appropriate to use other health protective (toxicity) values developed
by other authoritative governmental agencies, including ATSDR minimal
risk levels, if available, as these agencies use scientifically
credible approaches and their work is peer-reviewed (the ATSDR
toxicological profile underwent intra- and interagency review and
external peer review by seven experts with knowledge of toxicology,
chemistry, and/or health effects). The ATSDR minimal risk levels
reflect the best available, peer-reviewed science.
Furthermore, the EPA's draft IRIS Toxicological Review of
Perfluorohexanesulfonic Acid (PFHxS) and Related Salts (Public Comment
and External Review Draft) (USEPA, 2023d), which is in the public
domain, preliminarily provides confirmatory evidence that PFHxS
significantly affects human development (emphasis added): ``Overall,
the available evidence indicates that PFHxS exposure is likely to cause
thyroid and developmental immune effects in humans, given sufficient
exposure conditions. For thyroid effects, the primary supporting
evidence for this hazard conclusion included evidence of decreased
thyroid hormone levels, abnormal histopathology results, and changes in
organ weight in experimental animals. For immune effects, the primary
supporting evidence included decreased antibody responses to
vaccination against tetanus or diphtheria in children.'' Although the
EPA did not rely on this draft IRIS toxicological review for PFHxS in
this rule, the draft is available to the public and offers confirmation
that PFHxS elicits developmental effects in humans.
b. PFNA
Some commenters questioned the human relevance of developmental
effects observed in PFNA animal studies (i.e., decreased body weight
gain, delayed eye opening, delayed sexual maturation) used to derive
the ATSDR minimal risk level and the EPA's PFNA HRL/HBWC. The EPA
disagrees with this comment. At this time, the 2021 ATSDR Toxicological
Profile for Perfluoroalkyls represents the best available peer-reviewed
scientific information regarding the human health effects of PFNA. In
addition, according to the March 2023 Interagency PFAS Report to
Congress, PFNA is documented to affect the developmental health domain
(United States OSTP, 2023), and a recently published meta-analysis
(Wright et al., 2023) specifically supports decreases in birth weight
as an effect of PFNA exposure in humans. Published studies have shown
that PFNA exposure results in statistically significant, dose-
responsive developmental effects, including reduced fetal/pup
bodyweight, reduced fetal/pup survival, changes in fetal/pup liver gene
expression, increased fetal/pup liver weight, and delayed onset of
puberty. Also, the EPA's 1991 Guidelines for Developmental Toxicity
Risk Assessment (USEPA, 1991a; pp. vii-ix and pp. 1-2) cites evidence
that, in the absence of clear evidence to the

[[Page 32548]]

contrary, developmental effects observed in experimental animals are
interpreted as relevant to humans.
c. HFPO-DA
A few commenters submitted critical comments related to the adverse
health effects associated with exposure to HFPO-DA and how these health
effects are quantified to derive the RfD in the human health toxicity
assessment for HFPO-DA (USEPA, 2021b). Commenters claimed that the RfD
for HFPO-DA is not scientifically sound, and cited one or more of the
following reasons why: (1) the selected critical effect from the study
(constellation of liver lesions) includes different liver effects that
were not consistently observed across male and female mice and were not
necessarily all adverse; (2) the hepatic effects in mice (the selected
critical effect) are mediated by a rodent specific MOA, peroxisome
proliferator-activated receptor alpha (PPAR[alpha]), and therefore not
relevant to humans; (3) the EPA incorporated results of a pathology
working group which misapplied diagnostic criteria classifying
apoptotic and necrotic lesions; and (4) the EPA misapplied uncertainty
factors (UFs) (i.e., the subchronic to chronic UF and database UF)
according to agency guidance resulting in the maximum possible UF of
3,000 (USEPA, 2002a; USEPA, 2022f). Another commenter thought that the
interspecies UF should be further increased. Also, some commenters
stated that the EPA did not properly consider all available
epidemiological data. These comments are addressed in this preamble.
Overall, the EPA disagrees with the commenters and maintains that
the final published peer-reviewed human health toxicity assessment that
derived the RfD for HFPO-DA is appropriate and sound, reflects the best
available peer-reviewed science, and is consistent with agency
guidance, guidelines, and best practices for human health risk
assessment. Notably, the EPA sought external peer review of the
toxicity assessment twice (USEPA, 2018b; USEPA, 2021f), released the
draft toxicity assessment for public comment and provided responses to
public comment (USEPA, 2021g), and engaged a seven-member pathology
working group at the National Institutes of Health--an entirely
separate and independent organization--to re-analyze pathology slides
from two critical studies (USEPA, 2021b, appendix D), all of which
supported the EPA's conclusions in the toxicity assessment, including
the RfD derivation.
Regarding critical effect selection: the EPA's approach to critical
effect selection for the RfD derivation considers a range of factors,
including dose at which effects are observed, biological variability
(which can produce differences in effects observed between sexes), and
relevance of the effect(s) seen in animals to human health. The EPA
maintains that selection of the constellation of liver lesions as the
critical effect for HFPO-DA RfD derivation is appropriate and
scientifically justified, and that the constellation of liver lesions
represents an adverse effect. The EPA engaged a pathology working group
within the National Toxicology Program (NTP) at the National Institutes
of Health to perform an independent analysis of the liver tissue
slides. The pathology working group determined that the tissue slides
demonstrated a range of adverse effects and that the constellation of
liver effects caused by HFPO-DA exposure, which included cytoplasmic
alteration, apoptosis, single cell necrosis, and focal necrosis,
constitutes an adverse liver effect in these studies (USEPA, 2021b,
appendix D). The EPA evaluated the results of the pathology working
group and determined that the effects were relevant to humans according
to the best available science (e.g., Hall et al., 2012). Additionally,
the EPA convened a second independent peer-review panel of human health
risk assessment experts to review the EPA's work on HFPO-DA, including
critical effect selection. The panel unanimously agreed with the
selection of the constellation of liver lesions as the critical effect,
the adversity of this effect and its relevance to humans (USEPA,
2021f).
The commenters' assertion that the hepatic effects observed in mice
are not relevant to humans because they are PPAR[alpha]-mediated is
unsupported. The commenter claims that one specific effect--apoptosis--
can be PPAR[alpha]-mediated in rodents (a pathway that some data
suggest may be of limited or no relevance to humans). However, in
supporting studies cited by commenters, a decrease in apoptosis is
associated with a PPAR[alpha] MOA, with Corton et al. (2018) stating,
``[t]he data indicate that a physiological function of PPAR[alpha]
activation is to increase hepatocyte growth through an increase in
hepatocyte proliferation or a decrease in apoptosis or a combination of
both effects'' while HFPO-DA is associated with increased apoptosis
(USEPA, 2021b). Therefore, the commenter's claim that apoptosis is
associated with the known PPAR[alpha] MOA is unsupported. the critical
study selected by the EPA, and indeed other studies as well, reported
not only apoptosis but also other liver effects such as necrosis that
are not associated with a PPAR[alpha] MOA and therefore are relevant
for human health (Hall et al., 2012). Further, according to the
available criteria, effects such as cytoplasmic alteration in the
presence of liver cell necrosis are considered relevant to humans (Hall
et al., 2012). Additionally, commenters asserted that a 2020 study by
Chappell et al. reported evidence demonstrating that the rodent liver
effects are not relevant to humans, and that the EPA failed to consider
this study. It is important to note that while Chappell et al. (2020)
was published after the assessment's literature search cut-off date
(USEPA, 2021b, appendix A; USEPA, 2022h), the EPA considered this paper
initially through the Request for Correction process (USEPA, 2022h) and
noted that this study specifically assessed evidence for PPAR[alpha]-
driven apoptosis and did not investigate other potential modes of
action or types of cell death, specifically necrosis. The authors state
that they could ``not eliminate the possibility that necrotic cells
were also present.'' The EPA again considered Chappell et al., (2020),
in addition to other studies submitted through public comment (Heintz
et al., 2022; Heintz et al., 2023; Thompson et al., 2023), and
determined that these studies do not fully explore a necrotic/cytotoxic
MOA with Thompson et al., 2023 stating that ``there are no gene sets
for assessing necrosis in transcriptomic databases.'' Critically, the
commenter and these cited studies fail to recognize that increased
apoptosis is a key criterion to establish a cytotoxic MOA. As outlined
in the toxicity assessment (USEPA, 2021b), Felter et al., (2018)
``identified criteria for establishing a cytotoxicity MOA, which
includes: . . . (2) clear evidence of cytotoxicity by histopathology,
such as presence of necrosis and/or increased apoptosis.'' Overall, the
EPA has determined that these studies support the mechanistic
conclusions of the toxicity assessment ``that multiple MOAs could be
involved in the liver effects observed after GenX chemical exposure''
including PPAR[alpha] and cytotoxicity (USEPA, 2021b).
With respect to claims that the EPA misapplied diagnostic criteria
classifying apoptotic and necrotic lesions: as mentioned above, the EPA
engaged a pathology working group within the NTP at the National
Institutes of Health to perform an independent analysis of the liver
tissue slides. Seven pathologists--headed by Dr. Elmore, who was the
lead author of the pathology criteria that the

[[Page 32549]]

commenter cites (Elmore et al., 2016)--concluded that exposure to HFPO-
DA caused a ``constellation of liver effects'' that included
cytoplasmic alteration, apoptosis, single cell necrosis, and focal
necrosis, and that this full ``constellation of lesions'' should be
considered the adverse liver effect within these studies. The EPA then
used the established Hall criteria (Hall et al., 2012) to determine
that since liver cell death was observed, all effects, including
cytoplasmic alteration, were considered adverse and relevant to humans.
The EPA disagrees with the commenters' assertion about UF
application. As noted above, agency guidance (USEPA, 2002a; USEPA,
2022f) have established the appropriateness of the use of UFs to
address uncertainty and account for data limitations. UFs reflect the
limitations of the data across the five areas used in the current EPA
human health risk assessment development (referenced above); all
individual UFs that are applied are multiplied together to yield the
composite or total UF. The EPA guidance dictates that although a
composite UF greater than 3,000 represents ``excessive uncertainty''
(USEPA, 2002a; USEPA, 2022f), a composite UF can be equal to 3,000. For
HFPO-DA, a composite UF of 3,000 was appropriately applied to account
for uncertainties, including variability in the human population,
database uncertainties, and possible differences in the ways in which
humans and rodents respond to HFPO-DA that reaches their tissues.
Furthermore, the composite UF of 3,000 and specifically the database UF
and subchronic-to-chronic UF used for HFPO-DA was peer-reviewed by a
panel of human health risk assessment experts, and the panel supported
the application of the database UF of 10 and the subchronic-to-chronic
UF of 10 (USEPA, 2021f). Additionally, a UF<INF>A</INF> of 3 was
appropriately applied, consistent with peer-reviewed EPA methodology
(USEPA, 2002a), to account for uncertainty in characterizing the
toxicokinetic and toxicodynamic differences between rodents and humans.
As noted in the toxicity assessment for HFPO-DA (USEPA, 2021b), in the
absence of chemical-specific data to quantify residual uncertainty
related to toxicokinetics and toxicodynamic processes, the EPA's
guidelines recommend use of a UF<INF>A</INF> of 3.
Finally, some commenters claimed that the EPA did not consider
available epidemiological evidence showing no increased risk of cancers
or liver disease attributable to exposure to HFPO-DA. The EPA disagrees
with this comment because the agency considered all available
scientific evidence, including epidemiological studies (USEPA, 2021b).
The exhibit submitted by the commenter presents an observational
analysis comparing cancer and liver disease rates in North Carolina to
rates in other states. It does not present the results of a new
epidemiological study that included HFPO-DA exposure measures, health
outcome measures, or an assessment of association between exposure and
health outcome. The exhibit submitted by the commenter consists of a
secondary analysis of disease rate information that was collected from
various sources and does not provide new, high-quality scientific
information that can be used to assess the impact of exposure to
concentrations of HFPO-DA on human health.
d. PFBS
A few commenters suggested that the EPA lower the HRL/HBWC for PFBS
to account for thyroid hormone disruption during early development and
cited the Washington State Action Level for PFBS, which is 345 ng/L.
Washington State used the same RfD (3E-04 mg/kg-d) but a higher DWI-BW
to develop their Action Level as compared to the EPA's HRL/HBWC
(Washington State used the 95th percentile DWI-BW of 0.174 L/kg/day for
infants, whereas the EPA selected the 90th percentile DWI-BW of 0.0354
L/kg/day for women of child-bearing age). The EPA disagrees that the
infant DWI-BW is more appropriate for HRL/HBWC calculation. The EPA
selected the thyroid hormone outcome (decreased serum total thyroxine
in newborn mice seen in a developmental toxicity study) as the critical
effect in its PFBS human health toxicity assessment (USEPA, 2021a).
Notably, the RfD derived from this critical effect included application
of a 10X UF to account for life-stage-specific susceptibility
(UF<INF>H</INF>). To select a DWI-BW for use in deriving the HRL/HBWC
for PFBS, the EPA followed its established approach of considering the
PFBS exposure interval used in the developmental toxicity study in mice
that was the basis for chronic RfD derivation. In this study, pregnant
mice were exposed throughout gestation, which is relevant to two human
adult life stages: women of child-bearing age who may be or become
pregnant, and pregnant women and their developing embryos or fetuses
(Table 3-63 in USEPA, 2019a). To be clear, the critical study exposed
mice to PFBS only during pregnancy and not during postnatal
development; newborn mice in early postnatal development, which would
correspond to the human infancy life stage, were not exposed to PFBS.
Of the two relevant adult stages, the EPA selected the 90th percentile
DWI-BW for women of child-bearing age (0.0354 L/kg/day) to derive the
HRL/HBWC for PFBS because it is the higher of the two, and therefore
more health-protective. Please see additional information related to
DWI-BW selection above.
Other commenters stated that the EPA's human health toxicity
assessment for PFBS is overly conservative, uncertain, and that the
confidence in the chronic RfD is low. The EPA disagrees with these
comments. Confidence in the critical study (Feng et al., 2017) and
corresponding thyroid hormone critical effect in newborn mice was rated
by the EPA as `High;' this rating was a result of systematic study
evaluation and risk of bias analysis by a team of EPA experts. The Feng
et al. (2017) study, the critical effect of thyroid hormone disruption
in offspring, dose-response assessment, and corresponding RfD were
subjected to extensive internal EPA, interagency, and public/external
peer review. While confidence in the critical study was rated `High,'
the `Low' confidence rating for the PFBS chronic RfD was in part a
result of the lack of a chronic exposure duration study in any
mammalian species; this lack of a chronic duration study was one of the
considerations that resulted in the EPA applying a UF of 10 to account
for database limitations (UF<INF>D</INF>). Based on the EPA's human
health assessment practices, the lowest confidence rating across the
areas of consideration (e.g., existent hazard/dose-response database)
is assigned to the corresponding derived reference value (e.g., RfD).
Thus, the EPA has high confidence in the critical study (Feng et al.,
2017) and critical effect/thyroid endpoint, but the database is
relatively limited. Although the PFBS RfD was based on best available
peer-reviewed science, there is uncertainty as to the hazard profile
associated with PFBS after prolonged (e.g., lifetime) oral exposure. In
the toxicity assessment for PFBS (USEPA, 2021a), the EPA noted data
gaps in specific health effects domains, as is standard practice.
Toxicity assessments for most chemicals identify data gaps; the issue
of uncertainty due to toxicological study data gaps is not unique to
PFBS. Data gaps are considered when selecting the UF<INF>D</INF>
because they indicate the potential for exposure to lead to adverse
health effects at doses lower than the POD derived from the
assessment's critical

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study. There is a potential that effects with greater dose-response
sensitivity (i.e., occurring at lower daily oral exposures) might be
discovered from a chronic duration exposure study. Due to this
uncertainty, the EPA applied a UF<INF>D</INF> of 10.
One commenter questioned the EPA's approach to estimating the human
equivalent dose (HED) from the animal data using toxicokinetic (TK)
data rather than using default body-weight scaling and suggested that
the default allometric approach is more appropriate for estimating an
HED. The EPA disagrees with this comment. In human health risk
assessment practice, the EPA considers a hierarchical approach to
cross-species dosimetric scaling consistent with technical guidance to
calculate HEDs (USEPA, 2011; see pp. X-XI of the Executive Summary in
`Recommended Use of Body Weight3/4 as the Default Method in Derivation
of the Oral Reference Dose'). The preferred approach is physiologically
based toxicokinetic (PBTK) modeling; however, there are rarely
sufficient chemical-specific data to properly parameterize such a
model. In the absence of a PBTK model, the EPA considers an
intermediate approach in which chemical-specific data across species,
such as clearance or plasma half-life, are used to calculate a
dosimetric adjustment factor (DAF) (USEPA, 2011). If chemical-specific
TK data are not available, only then is a default approach used wherein
allometric scaling, based on body weight raised to the \3/4\ power, is
used to calculate a DAF. The human health toxicity assessment for PFBS
invoked the intermediate approach, consistent with guidance, as TK data
were available for humans and rodents.
e. Mixtures of PFHxS, PFNA, HFPO-DA, and PFBS
Comments on the EPA's preliminary regulatory determination on the
mixtures of PFHxS, PFNA, HFPO-DA, and/or PFBS were varied. Many
commenters supported the EPA's proposal to regulate a mixture of these
PFAS and agreed with the EPA's scientific conclusions about PFAS dose
additivity. Many commenters urged the EPA to consider making a
determination to regulate for additional PFAS (in a mixture) or all
PFAS as a class. As described throughout section III of this preamble,
the agency is required to demonstrate a contaminant meets the SDWA
statutory criteria to make a regulatory determination. In this
preamble, in addition to PFOA and PFOS which the EPA has already made a
final determination to regulate, the agency is making final
determinations for all PFAS with sufficiently available information to
meet these statutory criteria either individually and/or as part of
mixture combinations. As information becomes available, the agency will
continue to evaluate other PFAS for potential future preliminary
regulatory determinations.
Many commenters opposed the EPA's conclusion about PFAS dose
additivity and use of the Hazard Index approach to regulate co-
occurring PFAS. A few commenters agreed with the EPA's decision to
regulate mixtures of certain PFAS and the EPA's conclusion about dose
additivity but questioned the EPA's use of the general Hazard Index,
and instead, suggested alternative approaches. Please see section IV of
this preamble for a summary of comments and the EPA responses on the
Hazard Index MCLG and related topics.
There is substantial evidence that PFHxS, PFNA, HFPO-DA, and PFBS
act in a dose additive manner, that these four PFAS elicit similar
health effects, and that exposure to mixtures of these PFAS may have
adverse health effects. Following is a discussion of dose additivity
and similarity of adverse effects of PFHxS, PFNA, HFPO-DA, and PFBS.
As noted in this section, the available data indicate that PFHxS,
PFNA, HFPO-DA, and PFBS, while not necessarily toxicologically
identical, elicit many of the same or similar adverse health effects
across different levels of biological organization, tissues/organs,
lifestages, and species (ATSDR, 2021; EFSA et al., 2018; EFSA et al.,
2020; USEPA, 2021d; USEPA, 2021f; USEPA, 2024f). Each of these PFAS
disrupts signaling of multiple biological pathways, resulting in a
shared set of adverse effects including effects on thyroid hormone
levels, lipid synthesis and metabolism, development, and immune and
liver function (ATSDR, 2021; EFSA et al., 2018; EFSA et al., 2020;
USEPA, 2021d; USEPA, 2021f; USEPA, 2024f). Please also see USEPA
(2024a) for an overview of recent studies that provide supportive
evidence of similar effects of PFAS.
Available health effects studies indicate that PFAS mixtures act in
a dose-additive manner when the individual components share some health
endpoints/outcomes. Individual PFAS, each at doses that are not
anticipated to result in adverse health effects, when combined in a
mixture may result in adverse health effects. Dose additivity means
that when two or more of the component chemicals (in this case, PFHxS,
PFNA, HFPO-DA, and/or PFBS) exist in one mixture, the risk of adverse
health effects following exposure to the mixture is equal to the sum of
the individual doses or concentrations scaled for potency (USEPA,
2000a). Thus, exposure to these PFAS, at doses that individually would
not likely result in adverse health effects, when combined in a mixture
may pose health risks.
Many commenters supported the EPA's scientific conclusions about
PFAS dose additivity and agreed that considering dose-additive effects
is a health-protective approach. Many other commenters disagreed with
the EPA's scientific conclusions regarding PFAS dose additivity and a
few commenters questioned the agency's external peer-review process and
whether the agency sufficiently responded to SAB (Science Advisory
Board) comments. For example, these commenters stated that the evidence
base of PFAS mixture studies is too limited to support dose additivity
for these four PFAS and recommended that the EPA re-evaluate its
conclusion about dose additivity as new data become available. A few
commenters stated that the EPA failed to adequately follow the SAB
recommendation that ``discussion of studies of toxicological
interactions in PFAS mixtures in the EPA mixtures document be expanded
to also include studies that do not indicate dose additivity and/or a
common MOA [mode of action] for PFAS.'' The EPA's responses to these
comments are summarized in this section.
The EPA continues to support its conclusion that PFAS that elicit
similar adverse health effects following individual exposure should be
assumed to act in a dose-additive manner when in a mixture unless data
demonstrate otherwise. Numerous published studies across multiple
chemical classes, biological effects, and study designs support a dose-
additive mixture assessment approach for PFAS because they demonstrate
that experimentally observed responses to exposure to PFAS and other
chemical mixtures are consistent with modeled predictions of dose
additivity (see the EPA's Framework for Estimating Noncancer Health
Risks Associated with Mixtures of Per- and Polyfluoroalkyl Substances
(PFAS) (USEPA, 2024a)). Since the EPA's draft PFAS Mixtures Framework
underwent SAB review in 2021, new studies from the EPA and others have
published robust evidence of combined toxicity of PFAS in mixtures,
corroborating and confirming earlier findings (e.g., Conley et al.,
2022a; Conley et al., 2022b; USEPA, 2023c; see USEPA, 2024a for
additional examples). Additionally, the National Academies of

[[Page 32551]]

Sciences, Engineering, and Medicine (NASEM, 2022) recently recommended
that clinicians apply an additive approach for evaluating patient
levels of PFAS currently measured in the National Health and Nutrition
Examination Survey (NHANES) in order to protect human health from
additive effects from PFAS co-exposure.
The EPA directly asked the SAB for feedback on PFAS dose additivity
in the charge for the 2021 review of the EPA's draft PFAS Mixtures
Framework. Specifically, the EPA asked the SAB to, ``[p]lease comment
on the appropriateness of this approach for a component-based mixture
evaluation of PFAS under an assumption of dose additivity'' (USEPA,
2022i). The SAB strongly supported the scientific soundness of this
approach when evaluating PFAS and concurred that it was a health
protective conclusion. For example, the SAB said:

. . . The information included in the draft framework supports the
conclusion that toxicological interactions of chemical mixtures are
frequently additive or close to additive. It also supports the
conclusion that dose additivity is a public health protective
assumption that typically does not underestimate the toxicity of a
mixture . . . (USEPA, 2022i)
The SAB Panel agrees with use of the default assumption of dose
additivity when evaluating PFAS mixtures that have similar effects
and concludes that this assumption is health protective. (USEPA,
2022i)

Regarding the commenters' assertion that the agency did not
adequately follow the SAB recommendation to expand its discussion of
PFAS mixtures study results that did not show evidence of dose
additivity and/or a common MOA, the EPA disagrees. The EPA reviewed all
studies provided by the SAB and in response, included a discussion of
relevant additional studies in its public review draft PFAS Mixtures
Framework (see section 3 in USEPA, 2023w). Since then, the EPA has
included additional published studies and those findings further
confirm dose additive health concerns associated with PFAS mixtures
(see section 3 in USEPA, 2024a). Data from in vivo studies that
rigorously tested accuracy of Dose Additivity (DA), Integrated Addition
(IA), and Response Additivity (RA) model predictions of mixtures with
components that disrupted common pathways demonstrated that DA models
provided predictions that were better than or equal to IA and RA
predictions of the observed mixture effects (section 3.2 in USEPA,
2024a). The National Academy of Sciences (NAS) conclusions on
phthalates (and related chemicals) (NRC, 2008) and systematic reviews
of the published literature (Boobis et al., 2011 and Martin et al.,
2021; see also section 3.2 in USEPA, 2024a) support DA as the default
model for estimating mixture effects in some circumstances, even when
the mixtures included chemicals with diverse MOAs (but common target
organs/effects) (Boobis et al., 2011; Martin et al., 2021; USEPA,
2024a). Recent efforts to investigate in vitro and in vivo PFAS mixture
effects have provided robust evidence that PFAS behave in a dose-
additive manner (see section 3 in USEPA, 2024a).
As supported by the best available science, the SAB, the agency's
chemical mixtures guidance (USEPA, 1991b; USEPA, 2000a), and the EPA
Risk Assessment Forum's Advances in Dose Addition for Chemical
Mixtures: A White Paper (USEPA, 2023c), the EPA proposed a Hazard Index
MCLG for a mixture of up to four PFAS (PFHxS, PFNA, HFPO-DA, and PFBS)
based on dose additivity because published studies show that exposure
to each of these individual four PFAS elicits some of the same or
similar adverse health effects/outcomes. As noted above, many
commenters, as well as the SAB (USEPA, 2022i), supported this
conclusion of dose additivity based on similarity of adverse effects.
While the SAB also noted that there remain some questions about
PFAS interaction in mixtures (USEPA, 2022i), the available data justify
an approach that accounts for PFAS dose additivity. Studies that have
assessed PFAS mixture-based effects do not offer evidence for
synergistic/antagonistic effects (USEPA, 2024a). For example, Martin et
al. (2021), following a review of more than 1,200 mixture studies
(selected from > 10,000 reports), concluded that there was little
evidence for synergy or antagonism among chemicals in mixtures and that
dose additivity should be considered as the default. Experimental data
demonstrate that PFAS disrupt signaling in multiple biological pathways
resulting in common adverse effects on several of the same biological
systems and functions including thyroid hormone signaling, lipid
synthesis and metabolism, developmental toxicity, and immune and liver
function (USEPA 2024a). Additionally, several EPA Office of Research
and Development (ORD) studies provide robust evidence that PFAS behave
in a dose-additive manner (Conley et al., 2022a; Conley et al., 2022b;
Conley et al., 2023; Gray et al., 2023).
Several commenters opposed the conclusion of dose additivity based
on similarity of adverse effects and stated that the EPA failed to
establish that the four PFAS included in the Hazard Index (PFHxS, PFNA,
HFPO-DA, and PFBS) elicit similar adverse health effects. The EPA
disagrees with these comments because the available epidemiology and
animal toxicology studies demonstrate that these four PFAS (PFHxS,
PFNA, HFPO-DA, and PFBS) have multiple health endpoints and outcomes in
common (USEPA, 2024f). Further, these four PFAS are well-studied PFAS
for which the EPA or ATSDR have developed human health assessments and
toxicity values (i.e., RfDs, minimal risk levels). As shown in Table 1,
available animal toxicological data and/or epidemiological studies
demonstrate that PFHxS, PFNA, HFPO-DA, and PFBS are documented to
affect at least five (5) of the same health outcomes for this
evaluation: lipids, developmental, immune, endocrine, and hematologic
(USEPA, 2024g). Similarly, according to the 2023 Interagency PFAS
Report to Congress (United States OSTP, 2023), available animal
toxicological data show that PFHxS, PFNA, HFPO-DA, and PFBS are
documented to significantly affect at least eight (8) of the same major
health effect domains: body weight, respiratory, hepatic, renal,
endocrine, immunological, reproductive, and developmental. In short,
multiple evaluation efforts have clearly demonstrated that each of the
PFAS regulated by this NPDWR impact numerous of the same or similar
health outcomes or domains.

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In summary, there is substantial evidence that mixtures of PFHxS,
PFNA, HFPO-DA, and PFBS act in a dose-additive manner and elicit
multiple similar toxicological effects. Studies by the EPA and others
provide evidence that corroborates the dose-additive toxicity of PFAS
mixtures, and data on different chemical classes and research also
provide support for dose additivity. Additionally, numerous in vivo and
in vitro studies demonstrate that these four PFAS share many common
health effects across diverse health outcome categories (e.g.,
developmental, immunological, and endocrine effects), and that they
induce some of the same effects at the molecular level along biological
pathways (USEPA, 2024f).

C. Statutory Criterion 2--Occurrence

The EPA has determined that there is a substantial likelihood that
PFHxS, PFNA, and HFPO-DA will individually occur and combinations of
these three PFAS and PFBS will co-occur in mixtures in PWSs with a
frequency and at levels of public health concern based on the EPA's
evaluation of the best available occurrence information. In this
preamble, while the EPA is making a final determination to regulate
PFBS in mixtures with PFHxS, PFNA, and/or HFPO-DA, the agency is
deferring the final individual regulatory determination for PFBS so
that the agency can continue to evaluate this contaminant relative to
the SDWA criteria for regulation, particularly related to its
individual known or likely occurrence. For the other three PFAS, the
EPA is making a final determination to regulate them individually in
this preamble (i.e., PFHxS, PFNA, and HFPO-DA). The EPA recognizes
there will be additional occurrence or other relevant information for
these and other PFAS in the future. The EPA has, however, determined
that there is more than sufficient occurrence information to satisfy
the statutory criterion to regulate PFNA, PFHxS, and HFPO-DA.
The EPA's evaluation of the second statutory criterion for
regulation of PFHxS, PFNA, and HFPO-DA individually and regulation of
combinations of these PFAS and PFBS in mixtures follows a similar
process to previous rounds of regulatory determinations including the
written Protocol developed under Regulatory Determination 3 (USEPA,
2014a) and also described in detail in the Preliminary Regulatory
Determination 4 (USEPA, 2020a). Using the Protocol, and as conducted
for the regulatory determinations in this action, the agency compares
available occurrence data relative to the contaminant HRL, a health-
based concentration against which the agency evaluates occurrence data
when making regulatory determinations, as a preliminary factor in
informing the level of public health concern. For both this regulatory
determination and previous regulatory determinations, this is the first
screening factor in informing if there is a substantial likelihood the
contaminant will occur at a frequency and level of public health
concern. Consistent with the Protocol and similar to all past
regulatory determinations, these regulatory determinations are also
based on other factors, not just the direct comparison to the HRL. As
described clearly in the proposal, the EPA has not been able to
determine a simple threshold of public health concern for all
contaminants the agency considers for regulation under SDWA; rather, it
is a contaminant-specific decision which ``involves consideration of a
number of factors, some of which include the level at which the
contaminant is found in drinking water, the frequency at which the
contaminant is found and at which it co-occurs with other contaminants,
whether there is an sustained upward trend that these contaminant will
occur at a frequency and at levels of public health concern, the
geographic distribution (national, regional, or local occurrence), the
impacted population, health effect(s), the potency of the contaminant,
other possible sources of exposure, and potential impacts on sensitive
populations or lifestages.'' (USEPA, 2023f). It also includes
consideration of production and use trends and environmental fate and
transport parameters which may indicate that the contaminant would
persist and/or be mobile in water. Appropriately, the EPA has
considered these relevant factors in its evaluation

[[Page 32553]]

that there is a substantial likelihood that PFHxS, PFNA, and HFPO-DA
will individually occur and combinations of these three PFAS and PFBS
will co-occur in mixtures in PWSs with a frequency and at levels of
public health concern.
The EPA's evaluation of the second statutory criterion is based on
the best available health information, which includes UCMR 3 data and
more recent PFAS drinking water data collected by several states. Based
on suggestions in public comments to update state occurrence data, the
EPA supplemented the data used to inform the rule proposal with new
data from states included in the original proposal and additional
states that have made monitoring data publicly available since the rule
proposal (USEPA, 2024b). Consistent with section 1412(b)(1)(B)(II),
this information combined represents best available occurrence data. It
includes results from tens of thousands of samples and the assembled
data represent one of the most robust occurrence datasets ever used to
inform development of a drinking water regulation of a previously
unregulated contaminant. The state data were primarily gathered after
the UCMR 3 using improved analytical methods that could measure more
PFAS at lower concentrations. These additional data demonstrate greater
occurrence and co-occurrence of the PFAS monitored under UCMR 3 (PFHxS,
PFNA, and PFBS) at significantly greater frequencies than UCMR 3 and
the data initially included in the analysis. Furthermore, the state
data show the co-occurrence of PFAS at levels of public health concern,
as well as the demonstrated occurrence and co-occurrence of HFPO-DA
which was not included within UCMR 3. As discussed subsequently, these
data demonstrate that there is a substantial likelihood PFHxS, PFNA,
and HFPO-DA will occur and combinations of PFHxS, PFNA, HFPO-DA, and
PFBS will co-occur in mixtures with a frequency and at levels of public
health concern. When determining that there is a substantial likelihood
PFHxS, PFNA, and HFPO-DA will occur and PFHxS, PFNA, HFPO-DA, and/or
PFBS will co-occur at levels of public health concern, the EPA
considered both the occurrence concentration levels for PFHxS, PFNA,
and HFPO-DA individually, as well as their collective co-occurrence and
corresponding dose additive health concerns from co-exposures with PFBS
for purposes of considering a regulatory determination for mixtures of
these four PFAS. The EPA also considered other factors in evaluating
the second criterion and informing level of public health concern for
PFHxS, PFNA, and HFPO-DA individually and combinations of these three
PFAS and PFBS in mixtures, including the frequency at which the
contaminant is found, the geographic representation of the
contaminant's occurrence, and the environmental fate and transport
characteristics of the contaminant. As the EPA noted previously, while
the agency is not making an individual regulatory determination for
PFBS at this time, PFBS is an important component in mixtures with
PFHxS, PFNA, and HFPO-DA and the EPA presents occurrence information
for PFBS as part of section III.C.5 and its co-occurrence analyses in
sections VI.C and D of this preamble.
The EPA focused the evaluation of the state data on the non-
targeted or non-site specific (i.e., monitoring not conducted
specifically in areas of known or potential contamination) monitoring
efforts from 19 states. Non-targeted or non-site-specific monitoring is
likely to be more representative of general occurrence because its
framework and monitoring results will be less likely to potentially
over-represent concentrations at locations of known or suspected
contamination. Sixteen (16) of 19 states reported detections of at
least three of PFHxS, PFNA, HFPO-DA, or PFBS.
The EPA considered the targeted state monitoring data separately
since a higher rate of detections may occur as a result of specifically
looking in areas of suspected or known contamination. For the targeted
state data nearly all these states also reported detections at systems
serving millions of additional people, as well as at levels of public
health concern, both individually for PFHxS, PFNA, and HFPO-DA, and as
mixtures of these three PFAS and PFBS. State data detection frequency
and concentration results vary for PFHxS, PFNA, HFPO-DA, and PFBS, both
between these four different PFAS and across different states, with
some states showing much higher reported detections and concentrations
of these PFAS than others. The overall results demonstrate the
substantial likelihood that individually PFHxS, PFNA, and HFPO-DA and
mixtures of these three PFAS with PFBS will occur and co-occur at
frequencies and levels of public health concern. Tables 2 and 3 show
the percent of samples with state reported detections of PFHxS, PFNA,
HFPO-DA, and PFBS, and the percentage of monitored systems with
detections of PFHxS, PFNA, HFPO-DA, and PFBS, respectively, across the
non-targeted state finished water monitoring data. The EPA notes that
Alabama is not included in Tables 2 and 3 as only detections were
reported and there was no information on the total number of samples
collected to determine percent detection.
BILLING CODE 6560-50-P

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BILLING CODE 6560-50-C
As shown in Tables 2 and 3, all states except three report sample
and system detections for at least three of the four PFAS. For those
states that reported detections, the percentage of samples and systems
where these PFAS were found ranged from 1 to 39.8 percent and 0.1 to
38.1 percent, respectively. While these percentages show occurrence
variability across states, several of these states demonstrate that a
significant number of samples (e.g., detections of PFHxS in 26.2
percent of New Jersey samples) and systems (e.g., detections of HFPO-DA
in 12.2 percent of monitored systems in Kentucky) contain some or all
four PFAS. This occurrence information, as well as the specific
discussion related to individual occurrence for PFHxS, PFNA, and HFPO-
DA and co-occurrence of these three PFAS and PFBS, supports the
agency's determination that there is a substantial likelihood that
PFHxS, PFNA, HFPO-DA occur and PFHxS, PFNA, HFPO-DA, and PFBS co-occur
in combinations of mixtures with a frequency of public health concern.
Additionally, the agency emphasizes that occurrence and co-occurrence
of these PFAS is not only at a regional or local level, rather it
covers many states throughout the country; therefore, a national level
regulation is necessary to ensure all Americans served by PWSs are
equally protected.
1. PFHxS
The occurrence data presented above, throughout section VI of this
preamble and discussed in the USEPA (2024b) support the agency's final
determination that there is a substantial likelihood PFHxS occurs with
a frequency and at levels of public health concern in drinking water
systems across the United States. PFHxS was found under UCMR 3 in
approximately 1.1 percent of systems, serving 5.7 million people across
25 states, Tribes, and U.S. territories. However, under UCMR 3, the
minimum reporting level for PFHxS was 30 ng/L. As this reporting level
is three times greater than the health-based HRL for PFHxS (10 ng/L),
it is extremely likely there is significantly greater occurrence and
associated population exposed in the range between the HRL of 10 ng/L
and the UCMR 3 minimum reporting level of 30 ng/L (as demonstrated by
both the more recent state data and the EPA's occurrence model
discussed in this section and in section VI of this preamble showing
many results in this concentration range). Through analysis of
available state data, which consisted of approximately 48,000 samples
within 12,600 systems, 18 out of the 19 states that conducted non-
targeted monitoring had reported detections of PFHxS in 1.3 to 32.9
percent of their systems (Tables 2 and 3). These same systems reported
concentrations ranging from 0.2 to 856

[[Page 32556]]

ng/L with median sample concentrations ranging from 1.17 to 12.1 ng/L,
demonstrating concentrations above the HRL of 10 ng/L.
Targeted state monitoring data of PFHxS show similar results. For
example, in its targeted monitoring efforts, California reported 38.5
percent of monitored systems found PFHxS, where concentrations ranged
from 1.1 to 160 ng/L, also demonstrating concentrations above the HRL.
In total, considering both the non-targeted and targeted state data,
PFHxS was found above the HRL in at least 184 PWSs in 21 states serving
a population of approximately 4.3 million people.
The EPA also evaluated PFHxS in a national occurrence model that
has been developed and utilized to estimate national-scale PFAS
occurrence for four PFAS that were included in UCMR 3 (Cadwallader et
al., 2022). The model has been peer reviewed and is described
extensively in Cadwallader et al. (2022). The model and results are
described in section VI.E of this preamble; briefly, both the UCMR 3
and some state data were incorporated into a Bayesian hierarchical
model which supported exposure estimates for select PFAS at lower
levels than were measured under UCMR 3. Hundreds of systems serving
millions of people were estimated to have mean concentrations exceeding
the PFHxS HRL (10 ng/L). Therefore, the UCMR 3 results, the national
occurrence model results, and the substantial state data demonstrate
the substantial likelihood PFHxS occurs at a frequency and level of
public health concern. Finally, UCMR 5 data are being reported to the
EPA while this final rule is being prepared. See section VI of this
preamble for more information on the preliminary results. While these
UCMR 5 PFHxS data are too preliminary to provide the basis for the
regulatory determination, these preliminary UCMR 5 results appear to
confirm state data and model results.
Further supporting this final determination, PFHxS is very stable
and persistent in the environment. While PFHxS was phased out in the
U.S. in the early 2000's there are still detections as previously
demonstrated. In addition, legacy stocks may also still be used,
production continues in other countries, and products containing PFHxS
may be imported into the U.S. (USEPA, 2000b). Since PFHxS is
environmentally persistent and products containing PFHxS are still in
use and may be imported into the United States, the EPA anticipates
environmental contamination to sources of drinking water will continue.
To illustrate this point further, PFOA and PFOS, two of the most
extensively sampled PFAS, are also very environmentally persistent and
have similarly been phased out in the U.S. for many years, though these
two contaminants continue to often be found at levels of public health
concern as discussed in section VI of this preamble. Currently, this
also appears to be a similar trend for PFHxS occurrence, where the
drinking water sample data demonstrates it continues to occur at levels
of public health concern. Therefore, in consideration of factors
relating to the environmental persistence of PFHxS, its presence in
consumer products and possible continued use, and the observed
occurrence trend of PFOA and PFOS, the EPA finds that there is a
substantial likelihood PFHxS occurs or will occur at a frequency and
level of public health concern.
2. PFNA
The occurrence data presented above, throughout section VI of this
preamble, and discussed in USEPA (2024b) support the agency's final
determination that there is a substantial likelihood PFNA occurs with a
frequency and at levels of public health concern in drinking water
systems across the U.S.
PFNA was found under UCMR 3 in approximately 0.28 percent of
systems, serving 526,000 people in 7 states, Tribes, and U.S.
territories, using a minimum reporting level of 20 ng/L. As this
reporting level is two times greater than the health-based HRL of 10
ng/L, the EPA expects there is even greater occurrence and exposed
population in the range between 10 and 20 ng/L. Additionally, through
analysis of the extensive amount of available state data, which
consisted of approximately 57,000 samples within approximately 12,400
systems, 16 of 19 non-targeted monitoring states reported detections of
PFNA within 0.3 to 16.5 percent of their systems (Tables 2 and 3).
These same states reported sample results ranging from 0.23 to 330 ng/
L, demonstrating levels above the HRL of 10 ng/L, with median sample
results ranging from 0.35 to 7.5 ng/L.
Targeted state monitoring data of PFNA are also consistent with
non-targeted state data; for example, Pennsylvania reported 5.8 percent
of monitored systems found PFNA, where concentrations ranged from 1.8
to 18.1 ng/L, also showing concentrations above the HRL. When
considering all available state data, there are at least 480 systems in
19 states serving more than 8.4 million people that reported any
concentration of PFNA, and at least 52 systems in 12 states within
different geographic regions serving a population of 177,000 people
with reported concentrations above the HRL of 10 ng/L. Furthermore,
when evaluating only a subset of the available state data representing
non-targeted monitoring, PFNA was reported in approximately 3.6 percent
of monitored systems; if these results were extrapolated to the nation
and those system subject to the final rule requirements, the agency
estimates that PFNA would be detectable in over 2,300 PWSs serving 24.9
million people. If those results were further compared to the HRL for
PFNA (10 ng/L), PFNA would be detected above the HRL in 228 systems
with 830,000 people exposed. Thus, in addition to the UCMR 3 results,
these extensive state data also reflect there is a substantial
likelihood PFNA occurs at a frequency and level of public health
concern because it is observed or likely to be observed within numerous
water systems above levels of public health concern across a range of
geographic locations. Finally, UCMR 5 data are being reported to the
EPA while this final rule is being prepared. See section VI of this
preamble for more information on the preliminary results. While these
PFNA UCMR 5 data are too preliminary to provide the basis for the
regulatory determination, these preliminary UCMR 5 results appear to
confirm state data discussed above.
Further supporting this final determination, PFNA is very stable
and persistent in the environment. While it has generally been phased
out in the U.S. there are still detections as demonstrated previously.
Additionally, legacy stocks may still be used and products containing
PFNA may still be produced internationally and imported to the U.S.
(ATSDR, 2021). Since PFNA is environmentally persistent and products
containing PFNA are still in use and may be imported into the U.S.,
there is a substantial likelihood that environmental contamination of
sources of drinking water will continue. To illustrate this point
further, PFOA and PFOS, two of the most extensively sampled PFAS, are
also very environmentally persistent and have similarly been phased out
in the U.S. for many years, though these two contaminants continue to
often be found at levels of public health concern as discussed in
section VI of this preamble. Currently, this also appears to be a
similar trend for PFNA occurrence, where the drinking water sample data
demonstrates it continues to occur at levels of public health concern.
Therefore, in consideration of factors relating to the environmental
persistence of PFNA, its presence in

[[Page 32557]]

consumer products and possible continued use, and the observed
occurrence trend of PFOA and PFOS, the EPA finds that there is a
substantial likelihood PFNA occurs or will co-occur at a frequency and
level of public health concern.
3. HFPO-DA
The occurrence data presented above, throughout section VI of this
preamble, and discussed in the USEPA (2024b) support the agency's final
determination that there is a substantial likelihood HFPO-DA occur with
a frequency and at levels of public health concern in drinking water
systems across the U.S. HFPO-DA was not included as a part of the UCMR
3; however, through analysis of available state data, which consisted
of approximately 36,000 samples within approximately 10,100 systems, 10
of the 16 states that conducted non-targeted monitoring had state
reported detections of HFPO-DA within 0.1 to 12.2 percent of their
systems (Tables 2 and 3). These same states reported sample results
ranging from 0.7 to 100 ng/L and median sample results ranging from 1.7
to 29.6 ng/L, demonstrating concentrations above the HRL of 10 ng/L.
Additionally, targeted state monitoring in North Carolina included
sampling across six finished drinking water sites and 438 samples with
HFPO-DA. Concentrations ranged from 9.52 to 1100 ng/L, a median
concentration of 40 ng/L, and 433 (99 percent) samples exceeding the
HRL (10 ng/L). When considering all available state data, there are at
least 75 systems in 13 states serving more than 2.5 million people that
reported any concentration of HFPO-DA, and at least 13 systems in 5
states within different geographic regions of the country serving a
population of 227,000 people with reported concentrations above the HRL
of 10 ng/L. Additionally, when evaluating only a subset of the
available state data representing non-targeted monitoring to ensure
that the data were not potentially over-represented by sampling
completed in areas of known or suspected contamination, HFPO-DA was
reported in approximately 0.48 percent of monitored systems; if these
results were extrapolated to the nation and those system subject to the
final rule requirements, the agency estimates that HFPO-DA would be
detectable in over 320 PWSs serving 9.9 million people. If those
results were further compared to the HRL for HFPO-DA (10 ng/L), HFPO-DA
would be detected above the HRL in 42 systems with at least 495,000
people exposed. Finally, UCMR 5 data are being reported to the EPA
while this final rule is being prepared. See section VI of this
preamble for more information on the preliminary results. While these
HFPO-DA UCMR 5 data are too preliminary to provide the basis for the
regulatory determination, these preliminary UCMR 5 results appear to
confirm the state data discussed above.
Further supporting this final determination, HFPO-DA is very stable
and persistent in the environment. Additionally, unlike PFOA, PFOS,
PFHxS, and PFNA which have been phased out in the U.S, HFPO-DA
continues to be actively produced and used within the country and is
generally considered to have replaced the production of PFOA. Since
HFPO-DA is environmentally persistent and products containing HFPO-DA
are still being actively produced and used, the EPA anticipates that
contamination will continue, if not increase, due to disposal and
breakdown in the environment. To illustrate this point further, PFOA
and PFOS, two of the most extensively sampled PFAS, are also very
environmentally persistent and have been phased out in the United
States for many years, though these two PFAS continue to often be found
at levels of public health concern as discussed in section VI of this
preamble. Therefore, in consideration of factors relating to the
environmental persistence of HFPO-DA, its continued and possibly
increasing presence in consumer products and use, and the observed
occurrence trend of PFOA and PFOS, the EPA anticipates that occurrence
levels of HFPO-DA will similarly continue to be found at least to the
levels described in this preamble demonstrating that there is a
substantial likelihood HFPO-DA will occur at a frequency and level of
public health concern.
As discussed, HFPO-DA continues to be actively produced and used
throughout the U.S., it currently occurs at levels above its HRL, and
it occurs within geographically diverse areas of the country
demonstrating it is not a local or regional issue only. While the
current individual occurrence profile of HFPO-DA is not as pervasive
and is found at somewhat lower frequency as the currently observed
levels of PFOA, PFOS, or PFHxS, based upon the available substantial
amount of state occurrence data and given factors previously described,
the EPA has determined that there is a substantial likelihood HFPO-DA
occurs or will occur at a frequency and level of public health concern.
4. PFBS
The agency is deferring the final individual regulatory
determination for PFBS to further consider whether occurrence
information supports a finding that there is substantial likelihood
that PFBS will individually occur in PWSs and at a level of public
health concern. While current information demonstrates that PFBS
frequently occurs, it has not been observed to exceed its HRL of 2,000
ng/L in isolation. However, when considered in mixture combinations
with other PFAS, including PFHxS, PFNA, and HFPO-DA, PFBS is
anticipated to have dose-additive adverse health effects (based on
available data on PFAS and dose additivity) and there is a substantial
likelihood of its co-occurrence in combinations of mixtures with PFHxS,
PFNA, and HFPO-DA with a frequency and at levels of public health
concern. This is described further in sections III.C.5 and VI.C. and
VI.D of this preamble.
5. Mixtures of PFHxS, PFNA, HFPO-DA, and PFBS
Through the information presented within this section and in USEPA
(2024b), along with the co-occurrence information presented in sections
VI.C and VI.D of this preamble, the EPA's evaluation of all available
UCMR 3 and state occurrence data demonstrates that there is a
substantial likelihood that combinations of PFHxS, PFNA, HFPO-DA, and
PFBS (collectively referred to as ``Hazard Index PFAS'') co-occur or
will co-occur in mixtures at a frequency and level of public health
concern.
As discussed throughout section III.C of this preamble, the EPA has
determined that PFHxS, PFNA, and HFPO-DA each meet the second statutory
criterion for individual regulation. Additionally, as demonstrated in
sections VI.C. and D. of this preamble, the EPA has determined that
these three PFAS also meet the second statutory criterion when present
in mixture combinations. PFBS has not been observed to exceed its HRL
of 2,000 ng/L in isolation; therefore, the EPA is deferring the
individual regulatory determination for this PFAS to further consider
future occurrence information. However, the agency has determined that
PFBS frequently occurs (as shown in Table 2 and Table 3), and that when
considering dose additivity there is a substantial likelihood of its
co-occurrence in mixtures of PFHxS, PFNA, and/or HFPO-DA with a
frequency and at a level of public health concern. Therefore, the
agency has

[[Page 32558]]

determined that PFBS also meets the criterion when present in mixture
combinations with PFHxS, PFNA, and/or HFPO-DA.
In sections VI.C and D of this preamble, the EPA has presented its
evaluation and findings related to the likelihood and frequency of co-
occurrence of the four Hazard Index PFAS, including both through
groupwise and pairwise analyses for the Hazard Index PFAS, in non-
targeted state monitoring datasets. The groupwise co-occurrence
analysis established the broad occurrence frequency of Hazard Index
PFAS through a linkage to the presence of PFOA and PFOS. Because not as
many states have monitored for the Hazard Index PFAS as compared to
PFOA and PFOS, their occurrence information is less extensive than the
occurrence information for PFOA and PFOS. Therefore, though the agency
has previously made a final regulatory determination for PFOA and PFOS,
establishing co-occurrence of Hazard Index PFAS with PFOA and PFOS is
important to better understand the likelihood of Hazard Index PFAS
occurrence. In this analysis, the six PFAS were separated into two
groups--one consisted of PFOS and PFOA and the other group included the
four Hazard Index PFAS. The analysis broke down the systems and samples
according to whether chemicals from the two respective groups were
detected. Given that the groupwise co-occurrence analysis established
that there is a substantial likelihood that the Hazard Index PFAS
frequently occur, particularly alongside PFOA or PFOS, the pairwise co-
occurrence was relevant for understanding how the Hazard Index PFAS co-
occur with each other instead of occurring independently. Pairwise co-
occurrence analysis explored the odds ratios for each unique pair of
PFAS included in the regulation. For every pair of PFAS chemicals
included in the final regulation, the odds ratio, a statistic that, in
this context, quantifies the strength of association between two PFAS
being present, was found to be statistically significantly greater than
1. This means there was a statistically significant increase in the
odds of reporting a chemical as present after knowing that the other
chemical was detected. In most instances the odds appeared to increase
in excess of a factor of ten. Thus, based on the large amount of
available data, the chemicals are clearly demonstrated to frequently
co-occur rather than occur independently of one another, supporting the
agency's determination for mixtures of the four PFAS.
For the groupwise analysis, results generally indicated that when
PFOA and PFOS were found, Hazard Index PFAS were considerably more
likely to also be present. Additionally, for systems that only measured
PFOA and/or PFOS and did not measure the Hazard Index PFAS, it can be
assumed that the Hazard Index PFAS are more likely to be present in
those systems, and that Hazard Index occurrence may be underestimated.
Moreover, while PFOA and PFOS are not included within the Hazard Index
PFAS or the determination to regulate mixtures of these PFAS, the
pervasive occurrence of PFOA and PFOS shown in section VI of this
preamble is a strong indicator that these other Hazard Index PFAS are
also more likely to be found than what has been reported in state
monitoring data to date. In this analysis, comparisons were also made
between the number of Hazard Index PFAS analyzed and the number of
Hazard Index PFAS reported present. As more Hazard Index PFAS were
analyzed, more Hazard Index PFAS were reported present. Systems and
samples where Hazard Index PFAS were found were more likely to find
multiple Hazard Index PFAS than a single Hazard Index PFAS (when
monitoring for three or four Hazard Index PFAS), demonstrating an
increased likelihood of their co-occurrence. Additionally, for both
system-level and sample-level analyses where PFOA and/or PFOS were
reported present and all four Hazard Index PFAS were monitored, two or
more Hazard Index PFAS were reported present more than half of the
time, exhibiting they are more likely to occur together than in
isolation. Furthermore, the EPA notes that when evaluating only a
subset of the available state data representing non-targeted monitoring
where either three or four Hazard Index PFAS were monitored, regardless
of whether PFOA or PFOS were reported present, two or more of the
Hazard Index PFAS were reported in approximately 12.1 percent of
monitored systems; if these results were extrapolated to the nation,
two or more of these four PFAS would co-occur in about 8,000 PWSs (see
section VI.C.1 of this preamble for additional information).
The EPA uses a Hazard Index of 1 as the HRL to further evaluate the
substantial likelihood of the Hazard Index PFAS co-occurring at a
frequency and level of public health concern. As discussed in greater
detail in section VI.D, of this preamble based on available state data
the EPA finds that across 21 states there are at least 211 PWSs serving
approximately 4.7 million people with results above a Hazard Index of 1
for mixtures including two or more of the Hazard Index PFAS.
Specifically evaluating the presence of PFBS, in these same 211 systems
where the Hazard Index was found to be greater than 1, PFBS was
observed at or above its PQL in mixtures with one or more of the other
three Hazard Index PFAS in at least 72 percent (152) of these systems
serving approximately 4.5 million people. Additionally, as described
previously in sections III.C.1-3, PFHxS, PFNA, HFPO-DA, and PFBS are
all very stable and persistent in the environment. All are either still
being actively used or legacy stocks may be used and imported into the
U.S. Consequently, there is a substantial likelihood that environmental
contamination of sources of drinking water from these PFAS will
continue to co-occur to at least the levels described in this preamble.
Therefore, in consideration of the environmental persistence of
these PFAS, their presence in consumer products and continued use, the
findings of both the pairwise and groupwise co-occurrence analyses, and
demonstration of combinations of Hazard Index PFAS mixtures exceeding
the Hazard Index of 1, the EPA has determined there is sufficient
occurrence information available to support the second criterion that
there is a substantial likelihood that combinations of the four Hazard
Index PFAS in mixtures co-occur at frequencies and levels of public
health concern.
6. Summary of Major Public Comments and EPA Responses
The EPA requested comment on its preliminary regulatory
determination for all four PFAS and their mixtures and its evaluation
of the statutory criteria that supports the finding. The EPA also
requested comment on additional occurrence data the agency should
consider regarding its decision that PFHxS, PFNA, HFPO-DA, and PFBS and
their mixtures occur or are substantially likely to occur in PWSs with
a frequency and at levels of public health concern. The EPA received
many comments on the agency's evaluation of the second statutory
criterion under section 1412(b)(1)(A) of SDWA. Many commenters
supported the EPA's preliminary determination that PFHxS, PFNA, HFPO-
DA, and PFBS and mixtures of these four contaminants meet the second
statutory occurrence criterion under SDWA.

[[Page 32559]]

A couple of commenters claimed that the EPA does not have a robust
understanding of available occurrence data that supports any of the
regulatory determinations for the four PFAS in this rule. Additionally,
some commenters suggested that the preliminary determinations were
``rushed'' and ``non-scientific,'' and that the agency should wait
until some or all of the UCMR 5 data is available and considered. The
EPA disagrees. Sufficient occurrence data are available to establish a
substantial likelihood of occurrence at frequencies and levels of
health concern. Per the intent of the statute, the agency used the best
available data in an expeditious manner, which, as the agency described
earlier, was also a very large dataset consisting of tens of thousands
of samples and representing one of the most robust occurrence datasets
ever used to inform development of a drinking water regulation of a
previously unregulated contaminant. The agency also disagrees that the
occurrence analyses undertaken and available in the preamble as well as
the technical support document for occurrence were non-scientific.
Based on publicly available information within the state data, the EPA
verified that the very large majority of samples (at least 97 percent)
were collected using EPA-approved methods; the slight percentage the
agency was unable to verify would not result in different agency
conclusions. Additionally, the EPA notes that the aggregated data were
assessed using precedented statistical metrics and analyses. In
addition, the Cadwallader et al. (2022) model uses a robust, widely
accepted Bayesian statistical approach for modeling contaminant
occurrence. Based on these analyses, the EPA has a clear understanding
of the occurrence of the modeled contaminants. As discussed in section
III.C of this preamble and USEPA, 2024b, the EPA also has sufficient
state data which consist of a greater number of total systems and
samples than that included within the monitoring under UCMR 3, to
confidently establish that there is a substantial likelihood of
occurrence at frequencies and levels of public health concern.
As discussed above, the agency believes that the best currently
available occurrence data demonstrate substantial likelihood of
occurrence for the chemicals included in the final rule as they are
demonstrated at frequencies and levels of public health concern. UCMR 5
data are being reported to the EPA while this final rule is being
prepared. See section VI of this preamble for more information on the
EPA's evaluation of the preliminary results. While these data are too
preliminary to provide the basis for a regulatory determination, these
preliminary UCMR 5 results appear to support the data discussed
previously.
Several commenters disagreed that the available occurrence
information supports a preliminary determination for HFPO-DA, with a
few citing a lack of nationally representative data and suggesting a
delay until UCMR 5 data is collected. The EPA disagrees with these
comments, as the state monitoring data for the proposed rule
demonstrates HFPO-DA occurrence in 13 geographically diverse states,
including at 75 systems serving at least 2.5 million people. Moreover,
non-national datasets may serve to demonstrate occurrence of a
contaminant to warrant a positive determination and subsequent
development of an NPDWR. For example, the best available HFPO-DA state
data consists of approximately 36,000 samples within 10,000 systems and
is representative of multiple geographic locations.
One commenter stated that a regulatory determination for PFNA was
unnecessary as they do not believe it occurred with frequency under
UCMR 3 monitoring, and a couple of other commenters suggested that a
negative determination was appropriate for PFNA citing occurrence
levels. The EPA disagrees that a negative determination is appropriate
for PFNA as it has been demonstrated to occur at levels of public
health concern in at least 52 water systems across 12 states.
Furthermore, as described previously, when evaluating only a subset of
the available state data representing non-targeted monitoring, PFNA was
reported in approximately 3.6 percent of monitored systems and if those
results were extrapolated across the country, PFNA would be detectable
at any concentration in over 2,300 PWSs serving 21.2 million people and
detectable above 10 ng/L in 227 systems serving 711,000 people.
Additionally, PFNA frequently co-occurs with other PFAS, and as
previously discussed in this section, presents dose additive health
concerns with other PFAS demonstrating it is also an important
component of the determination to regulate it in mixtures with PFHxS,
HFPO-DA, and/or PFBS.
Commenters both agreed and disagreed with the EPA's individual
preliminary determination for PFBS. With respect to commenters who
suggested that the EPA has not met the occurrence criterion, while PFBS
occurs at significant frequency, the agency is deferring the individual
determination to regulate PFBS when it occurs individually until it
conducts further evaluation under the statutory criteria. The EPA
further finds that PFBS exposure may cause dose additive adverse health
effects in mixtures with PFHxS, PFNA, and/or HFPO-DA; that there is a
substantial likelihood that PFBS co-occurs in mixtures with PFHxS,
PFNA, and/or HFPO-DA in PWSs with a frequency and at levels of public
health concern; and that, in the sole judgment of the Administrator,
regulation of PFBS in mixtures with PFHxS, PFNA, and/or HFPO-DA
presents a meaningful opportunity for health risk reduction for persons
served by PWSs. Therefore, PFBS will be regulated as part of a mixture
with PFHxS, PFNA, and HFPO-DA.
A few commenters provided feedback on occurrence thresholds the
agency should consider when evaluating the second statutory criterion
for regulatory determinations. Particularly, these commenters
recommended that the EPA should define a threshold for frequency and
level of public health concern that warrants a specific regulatory
determination. A few commenters cited other previous regulatory
determinations where the agency made a determination not to regulate
contaminants with similar or lower levels of occurrence suggesting that
this should be the same for some or all of these four PFAS.
Furthermore, some of these commenters stated that it would be arbitrary
and capricious and conflict with the SDWA if the EPA did not use the
level of adverse health effect (i.e., the HRL) to represent the level
at which a contaminant is considered a public health concern.
The EPA disagrees with these commenters and as demonstrated in the
proposal and noted earlier in section III of this preamble, for this
regulatory determination, as well as past determinations, the agency
did compare available occurrence data relative to the contaminant HRL
as a factor in informing the occurrence level of public health concern.
However, the level of public health concern for purposes of the second
criterion is a contaminant-specific analysis that include consideration
of the HRL, as well as other factors and not solely based on the direct
comparison to the HRL. There is not just one simple threshold used for
public health concern for all contaminants. In the case of PFAS, this
is particularly relevant given the dose-additivity of mixtures.
The EPA also disagrees with these commenters as SDWA does not
define the occurrence level of public health

[[Page 32560]]

concern for contaminants, nor does it prescribe the level of adverse
health effects that must be used for a regulatory determination.
Ultimately, the overall decision to regulate a contaminant considers
all three statutory criteria, including the comprehensive assessment of
meaningful opportunity which is in the Administrator's sole discretion.
In previous EPA regulatory determinations, the agency has considered
the occurrence criteria unique to the contaminant it is evaluating and
has made decisions not to regulate contaminants both where there was
substantial likelihood of occurrence at frequency and/or at levels of
public health concern and where there was limited or no substantial
likelihood of occurrence at frequency and/or at levels of public health
concern. Consistent with this past regulatory history and the
Administrator's authority under the terms of the statute, the decision
considers all three criteria and cannot be determined in the exact same
manner for different contaminants. While the EPA may have made negative
determinations for other contaminants demonstrating occurrence at
different frequencies and levels of public health concern, the basis
for those decisions was specific to those contaminants and does not
apply to these PFAS or any other future contaminants for which the EPA
would make regulatory determinations. Therefore, the statute does not
require, and the EPA does not use a minimum or one-size-fits-all
occurrence thresholds (for either frequency or precise level) for
regulatory determinations.
As described in section VI of this preamble, many commenters
supported the EPA's proposal to regulate mixtures of PFAS. Specific to
occurrence, some of these commenters particularly expressed support for
the EPA's preliminary determination that mixtures of these four PFAS
meet the second statutory occurrence criterion under SDWA, citing that
the agency has used the best available information to determine that
there is a substantial likelihood that combinations of these PFAS will
co-occur in mixtures at a frequency and level of public health concern.
One commenter stated that the additional occurrence data presented by
the EPA in the proposal for the Hazard Index PFAS supports the EPA's
proposed determination that these PFAS should be regulated under the
SDWA. Conversely, several other commenters stated that there was not
supporting evidence for the co-occurrence of the four Hazard Index
PFAS. The EPA disagrees; the extent to which Hazard Index PFAS
chemicals co-occur in the non-targeted state dataset is discussed
extensively in the record for this rule and made evident through the
system level analysis in section VI.C. of this preamble. As also
discussed elsewhere in the record for this rule, in both system level
and sample level analyses where PFOA and/or PFOS were reported present
and all four Hazard Index PFAS were monitored, two or more Hazard Index
PFAS were reported present more than half of the time. Further, the
odds ratios tables in Exhibit 11 provide a statistical examination of
pairwise co-occurrence. The odds ratio is a statistic that quantifies
the strength of association between two events. In the context
described here, an ``event'' is the reported presence of a specific
PFAS contaminant. The odds ratio between PFOA and PFHxS, for example,
reflects the strength of association between PFHxS being reported
present and PFOA being reported present. If an odds ratio is greater
than 1, the two events are associated. The higher the odds ratio, the
stronger the association. For every pair of PFAS chemicals included in
the proposed regulation, the odds ratio was found to be statistically
significantly greater than 1. This means there was a statistically
significant increase in the odds of a PFAS being present if the other
PFAS compound was detected (e.g., if PFOA is detected, PFHxS is more
likely to also be found). In most instances the odds appeared to
increase in excess of a factor of ten. Thus, based on the large amount
of available data, the chemicals are clearly demonstrated to co-occur
rather than occur independently of one another, further supporting the
agency's determination for combinations of mixtures of the four PFAS.
After considering the public comments and additional occurrence
data evaluated as requested by public commenters, the EPA finds that
PFHxS, PFNA, and HFPO-DA individually and mixtures of these three PFAS
and PFBS, meet the second statutory criterion for regulatory
determinations under section 1412(b)(1)(A) of SDWA that the contaminant
is known to occur or co-occur or there is a substantial likelihood that
the contaminant will occur or co-occur in PWSs with a frequency and at
levels of public health concern (USEPA, 2024b).

D. Statutory Criterion 3--Meaningful Opportunity

The agency has determined that individual regulation of PFHxS,
PFNA, and HFPO-DA and regulation of combinations of PFHxS, PFNA, HFPO-
DA, and PFBS in mixtures presents a meaningful opportunity for health
risk reduction for persons served by PWSs. As discussed in section
III.C. of this preamble, the EPA evaluated this third statutory
criterion similarly to previous regulatory determinations using the
Protocol developed under Regulatory Determination 3 (USEPA, 2014b) and
also used in the Regulatory Determination 4. This evaluation includes a
comprehensive assessment of meaningful opportunity for each unique
contaminant including the nature of the health effects, sensitive
populations affected, including infants, children and pregnant and
nursing women, number of systems potentially affected, and populations
exposed at levels of public health concern, geographic distribution of
occurrence, technologies to treat and measure the contaminant, among
other factors. The agency further reiterates that, per the statute,
this determination of meaningful opportunity is in the Administrator's
sole discretion.
Accordingly, the EPA is making this determination of meaningful
opportunity after evaluating health, occurrence, treatment, and other
related information and factors including consideration of the
following:
<bullet> PFHxS, PFNA, and HFPO-DA and combinations of these three
PFAS and PFBS in mixtures may cause multiple adverse human health
effects, often at very low concentrations, on several biological
systems including the endocrine, cardiovascular, developmental, renal,
hematological, reproductive, immune, and hepatic systems as well as are
likely to produce dose-additive effects from co-exposures.
<bullet> The substantial likelihood that PFHxS, PFNA, and HFPO-DA
individually occur or will occur and that mixtures of PFHxS, PFNA,
HFPO-DA, and/or PFBS co-occur or will co-occur together at frequencies
and levels of public health concern in PWSs as discussed in section III
of this preamble above and in section VI of this preamble, and the
corresponding significant populations served by these water systems
which potentially include sensitive populations and lifestages, such as
pregnant and lactating women, as well as children.
<bullet> PFHxS, PFNA, HFPO-DA and combinations of these three PFAS
and PFBS in mixtures are expected to be persistent in the environment,
with some (e.g., PFHxS, PFNA) also demonstrated to be very persistent
in the human body.
<bullet> Validated EPA-approved measurement methods are available
to measure PFHxS, PFNA, HFPO-DA, and

[[Page 32561]]

PFBS. See section VII of this preamble for further discussion.
<bullet> Treatment technologies are available to remove PFHxS,
PFNA, and HFPO-DA and combinations of these three PFAS and PFBS from
drinking water. See section X of this preamble for further discussion.
<bullet> Even though PFBS is very likely to be below its
corresponding individual HRL when it occurs in a mixture, the record
indicates that there is a substantial likelihood that it co-occurs with
the regulated PFAS throughout public water systems nationwide. See
sections III.C.5 and VI.C. of this preamble for further discussion.
According to the 2023 Interagency PFAS Report to Congress (United
States OSTP, 2023), PFBS has been shown to affect the following health
endpoints: body weight, respiratory, cardiovascular, gastrointestinal,
hematological, musculoskeletal, hepatic, renal, ocular, endocrine,
immunological, neurological, reproductive, and developmental. Thus,
including PFBS as a mixture component represents a meaningful
opportunity to reduce PFBS' contributions to the overall hazard of the
mixture and resulting dose additive health concerns. This is
particularly relevant where the exposures of the other three PFAS in
the mixture are also below their respective HRLs but when the hazard
contributions of each mixture component are summed, the total exceeds
the mixture HRL. In this scenario, the inclusion of PFBS allows for a
more accurate picture o

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