# PFAS National Primary Drinking Water Regulation Rulemaking > Briefs, arguments, decisions, and more. URL: https://www.frixlaw.com/law-library/documents/fr%3A2023-05471 ## Record - **Collection:** Federal Register - **Document type:** Proposed Rule - **Published:** March 29, 2023 - **Citation:** 88 FR 18638 ## Text ENVIRONMENTAL PROTECTION AGENCY 40 CFR Parts 141 and 142 [EPA-HQ-OW-2022-0114; FRL 8543-01-OW] RIN 2040-AG18 PFAS National Primary Drinking Water Regulation Rulemaking AGENCY: Environmental Protection Agency (EPA). ACTION: Preliminary regulatory determination and proposed rule; request for public comment; notice of public hearing. SUMMARY: The Environmental Protection Agency (EPA) is committed to using and advancing the best available science to tackle per- and polyfluoroalkyl substances (PFAS) pollution, protect public health, and harmonize policies that strengthen public health protections with infrastructure funding to help communities, especially disadvantaged communities, deliver safe drinking water. In March 2021, EPA issued a final regulatory determination to regulate perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) as contaminants under Safe Drinking Water Act (SDWA). In this notice, EPA is issuing a preliminary regulatory determination to regulate perfluorohexane sulfonic acid (PFHxS), hexafluoropropylene oxide dimer acid (HFPO-DA) and its ammonium salt (also known as a GenX chemicals), perfluorononanoic acid (PFNA), and perfluorobutane sulfonic acid (PFBS), and mixtures of these PFAS as contaminants under SDWA. Through this action, EPA is also proposing a National Primary Drinking Water Regulation (NPDWR) and health-based Maximum Contaminant Level Goals (MCLG) for these four PFAS and their mixtures as well as for PFOA and PFOS. EPA is proposing to set the health-based value, the MCLG, for PFOA and PFOS at zero. Considering feasibility, including currently available analytical methods to measure and treat these chemicals in drinking water, EPA is proposing individual MCLs of 4.0 nanograms per liter (ng/L) or parts per trillion (ppt) for PFOA and PFOS. EPA is proposing to use a Hazard Index (HI) approach to protecting public health from mixtures of PFHxS, HFPO-DA and its ammonium salt, PFNA, and PFBS because of their known and additive toxic effects and occurrence and likely co-occurrence in drinking water. EPA is proposing an HI of 1.0 as the MCLGs for these four PFAS and any mixture containing one or more of them because it represents a level at which no known or anticipated adverse effects on the health of persons is expected to occur and which allows for an adequate margin of safety. EPA has determined it is also feasible to set the MCLs for these four PFAS and for a mixture containing one or more of PFHxS, HFPO-DA and its ammonium salt, PFNA, PFBS as an HI of unitless 1.0. The Agency is requesting comment on this action, including this proposed NPDWR and MCLGs, and have identified specific areas where public input will be helpful for EPA in developing the final rule. In addition to seeking written input, the EPA will be holding a public hearing on May 4, 2023. DATES: Comments must be received on or before May 30, 2023. Comments on the information collection provisions submitted to the Office of Management and Budget (OMB) under the Paperwork Reduction Act (PRA) are best assured of consideration by OMB if OMB receives a copy of your comments on or before April 28, 2023. Public hearing: EPA will hold a virtual public hearing on May 4, 2023, at https://www.epa.gov/sdwa/and-polyfluoroalkyl-substances-pfas. Please refer to the SUPPLEMENTARY INFORMATION section for additional information on the public hearing. ADDRESSES: You may send comments, identified by Docket ID No. EPA-HQ-OW-2022-0114 by any of the following methods: • Federal eRulemaking Portal: https://www.regulations.gov/ (our preferred method). Follow the online instructions for submitting comments. • Mail: U.S. Environmental Protection Agency, EPA Docket Center, Office of Ground Water and Drinking Water Docket, Mail Code 2822IT, 1200 Pennsylvania Avenue NW, Washington, DC 20460. • Hand Delivery or Courier: EPA Docket Center, WJC West Building, Room 3334, 1301 Constitution Avenue NW, Washington, DC 20004. The Docket Center's hours of operations are 8:30 a.m. to 4:30 p.m., Monday through Friday (except Federal Holidays). Instructions: All submissions received must include the Docket ID No. for this rulemaking. Comments received may be posted without change to https://www.regulations.gov/, including any personal information provided. For detailed instructions on sending comments and additional information on the rulemaking process, see the “Public Participation” heading of the SUPPLEMENTARY INFORMATION section of this document. 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: PFASNPDWR@epa.gov SUPPLEMENTARY INFORMATION: Executive Summary In March 2021, EPA issued a final regulatory determination to regulate perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) as contaminants under Safe Drinking Water Act (SDWA). EPA is issuing a preliminary regulatory determination to regulate perfluorohexane sulfonic acid (PFHxS), hexafluoropropylene oxide dimer acid (HFPO-DA) and its ammonium salt (also known as a GenX chemicals), perfluorononanoic acid (PFNA), and perfluorobutane sulfonic acid (PFBS), and mixtures of these PFAS as contaminants under SDWA (see section III of this preamble for additional discussion on EPA's preliminary regulatory determination). Through this action, EPA is also proposing a National Primary Drinking Water Regulation (NPDWR) and health-based Maximum Contaminant Level Goals (MCLG) for these four PFAS and their mixtures as well as for PFOA and PFOS. Exposure to these PFAS may cause adverse health effects, and all are likely to occur in drinking water. PFAS are a large family of synthetic chemicals that have been in use since the 1940s. Many of these compounds have unique physical and chemical properties that make them highly stable and resistant to degradation in the environment—colloquially termed “forever chemicals.” People can be exposed to PFAS through certain consumer products, occupational contact, and/or by consuming food and drinking water that contain PFAS (see section II.C of this preamble for additional discussion on PFAS chemistry, production, and uses). Current scientific evidence indicates that consuming water containing the PFAS covered in this proposed regulation above certain levels can result in harmful health effects. Depending on the individual PFAS, health effects can include negative impacts on fetal growth after exposure during pregnancy, on other aspects of development, reproduction, liver, thyroid, immune function, and/or the nervous system; and increased risk of cardiovascular and/or certain types of cancers, and other health impacts (see section II.B and III.B of this preamble for additional discussion on health effects). This proposed PFAS drinking water regulation contains several key features. Based on a review of the best available health effects data, EPA is proposing MCLGs that address six PFAS. An MCLG is the maximum level of a contaminant in drinking water at which no known or anticipated adverse effect on the health of persons would occur, allowing an adequate margin of safety. A contaminant means any “physical, chemical or biological or radiological substance or matter in water.” This proposal addresses contaminants and certain mixtures of contaminants. Through this action, EPA is also proposing enforceable standards which takes the form of maximum contaminant levels (MCLs) in this proposed regulation. An MCL is the maximum level allowed of a contaminant or a group of contaminants ( i.e., mixture of contaminants) in water which is delivered to any user of a public water system (PWS). The SDWA generally requires EPA to set an MCL “as close as feasible to” the MCLG. EPA has also included monitoring, reporting, and other requirements to ensure regulated drinking water systems, known as a PWS, meet the PFAS limits in the regulation. Following a systematic review of available human epidemiological and animal toxicity studies, EPA has determined that PFOA and PFOS are likely to cause cancer ( e.g., kidney and liver cancer) and that there is no dose below which either chemical is considered safe (see section IV.A and V.A through B of this preamble for additional discussion). Therefore, EPA is proposing to set the health-based value, the MCLG, for both of these contaminants at zero. Considering feasibility, including currently available analytical methods to measure and treat these chemicals in drinking water, EPA is proposing individual MCLs of 4.0 nanograms per liter (ng/L) or parts per trillion (ppt) for PFOA and PFOS (see sections VI.C and VIII of this preamble for additional discussion on the MCLs and practical quantitation limits [PQLs]). Due to their widespread use and persistence, many PFAS are known to co-occur in drinking water and the environment—meaning that these compounds are often found together and in different combinations as mixtures (see section III.C and VII of this preamble for additional discussion on occurrence). PFAS disrupt signaling of multiple biological pathways resulting in common adverse effects on several biological systems and functions, including thyroid hormone levels, lipid synthesis and metabolism, development, and immune and liver function. Additionally, EPA's examination of health effects information found that exposure through drinking water to a mixture of PFAS can be assumed to act in a dose-additive manner (see sections III.B and IV.B of this preamble for additional discussion on mixture toxicity). This dose additivity means that low levels of multiple PFAS, that individually would not likely result in adverse health effects, when combined in a mixture are expected to result in adverse health effects. As a result, EPA is proposing to use a Hazard Index (HI) approach to protecting public health from mixtures of four PFAS: PFHxS, HFPO-DA and its ammonium salt (also known as GenX chemicals), PFNA, and PFBS because of their known and additive toxic effects and occurrence and likely co-occurrence in drinking water. PFOA and PFOS are being proposed for separate MCLs and not included in the HI because their individual proposed MCLGs are zero, and the level at which no known or anticipated adverse effects on the health of persons is expected to occur is well below current analytical quantitation levels. Based on our current understanding of health effects, this is not the case for the other covered PFAS. Because of the analytical limitations for PFOA and PFOS, the MCL for these two PFAS is set at the lowest feasible quantitation level and any exceedance of this limit requires action to protect public health, regardless of any mixture in which they are found. As a result, EPA is not proposing to include PFOA or PFOS in the HI. The HI is a commonly used risk management approach for mixtures of chemicals (USEPA, 1986a; 2000a). In this approach, a ratio called a hazard quotient (HQ) is calculated for each of the four PFAS (PFHxS, HFPO-DA and its ammonium salt (also known as GenX chemicals), PFNA, and PFBS) by dividing an exposure metric, in this case, the measured level of each of the four PFAS in drinking water, by a health reference value for that particular PFAS. For health reference values, in this proposal, EPA is using Health Based Water Concentration (HBWCs) as follows: 9.0 ppt for PFHxS, 10.0 ppt for HFPO-DA; 10.0 ppt for PFNA; and 2000 ppt for PFBS (USEPA, 2023a). The individual PFAS ratios (HQs) are then summed across the mixture to yield the HI. If the resulting HI is greater than one (1.0), then the exposure metric is greater than the health metric and potential risk is indicated. EPA's Science Advisory Board (SAB) opined that where the health endpoints of the chosen compounds are similar, it is reasonable to use an HI as “a reasonable approach for estimating the potential aggregate health hazards associated with the occurrence of chemical mixtures in environmental media.” (USEPA, 2022a). The HI provides an indication of overall potential risk of a mixture as well as individual PFAS that are potential drivers of risk (those PFAS(s) with high(er) ratios of exposure to health metrics) (USEPA, 2000a; see section IV.B and V.C of this preamble for additional discussion on the HI and its derivation). Therefore, EPA is proposing an HI of 1.0 as the MCLGs for these four PFAS and any mixture containing one or more of them because it represents a level at which no known or anticipated adverse effects on the health of persons is expected to occur and which allows for an adequate margin of safety. EPA has determined it is also feasible to set the MCLs for these four PFAS and for a mixture containing one or more of PFHxS, HFPO-DA and its ammonium salt, PFNA, PFBS as an HI of unitless 1.0 (see sections V.C and VI.B of this preamble for discussion of the HI MCLG and MCL, respectively). Monitoring is a core component of a NPDWR and assures that water systems are providing necessary public health protections (see section IX of this preamble for additional discussion on monitoring and compliance requirements). EPA is therefore proposing requirements for systems to monitor for PFOA, PFOS, PFHxS, HFPO-DA and its ammonium salt, PFNA, and PFBS in drinking water that build upon EPA's Standardized Monitoring Framework (SMF) for Synthetic Organic Compounds (SOCs) where the monitoring frequency for any PWS depends on previous monitoring results. This proposal includes flexibilities related to monitoring, including flexibilities for systems to use certain, previously collected data to satisfy initial monitoring requirements in this proposal as well as reduced monitoring requirements in certain circumstances (see section IX.E of this preamble for additional discussion on monitoring waivers). In summary, the proposed MCLs for PFOA and PFOS are 4 ng/L (individually), and the proposed MCL of an HI of 1.0 for any mixture containing PFHxS, HFPO-DA and its ammonium salt, PFNA, and/or PFBS. Water systems with PFAS levels that exceed the proposed MCLs would need to take action to provide safe and reliable drinking water. These systems may install water treatment or consider other options such as using a new uncontaminated source water or connecting to an uncontaminated water system. Activated carbon, anion exchange (AIX) and high-pressure membrane technologies have all been demonstrated to remove PFAS, including PFOA, PFOS, PFHxS, HFPO-DA and its ammonium salt, PFNA, and PFBS, from drinking water systems. These treatment technologies can be installed at a water system's treatment plant and are also available through in-home filter options (see section XI of this preamble for additional discussion on available treatment technologies). As part of its health risk reduction and cost analysis, SDWA requires an evaluation of quantifiable and nonquantifiable health risk reduction benefits and costs. SDWA also requires that EPA considers quantifiable and nonquantifiable health risk reduction benefits from reductions in co-occurring contaminants. The SDWA also requires that EPA determine if the benefits of the proposed rule justify the costs. In accordance with these requirements, the EPA Administrator has determined that the quantified and nonquantifiable benefits of the proposed PFAS NPDWR justify the costs (see section XIII of this preamble for additional discussion on EPA's Health Risk Reduction and Cost Analysis [HRRCA]). Among other things, EPA evaluated which entities which would be affected by the rule, quantified costs using available data and statical models, and described unquantifiable costs. EPA also quantified benefits by estimating reduced cardiovascular events ( e.g., heart attacks and strokes), developmental impacts to fetuses and infants, and reduced cases of kidney cancer. EPA has also quantified benefits by estimating reduced bladder cancer cases caused by reduced disinfection byproduct (DBP) formation in some systems that install treatment to meet the requirements of this rule. EPA has also developed a qualitative summary of benefits expected to result from the removal of regulated PFAS and additional co-removed PFAS contaminants. To help communities on the frontlines of PFAS contamination, the passage of the Infrastructure Investment and Jobs Act, also referred to as the Bipartisan Infrastructure Law (BIL), invests over $11.7 billion in the Drinking Water State Revolving Fund (SRF); $4 billion to the Drinking Water SRF for Emerging Contaminants; and $5 billion to Small, Underserved, and Disadvantaged Communities Grants. 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. Public participation and consultations with key stakeholders are critical in developing an implementable and public health protective rule. EPA has engaged with many stakeholders and consulted with entities such as the SAB, and the National Drinking Water Advisory Council (NDWAC) in developing this proposed rule (see section XV of this preamble on EPA's Statutory and Executive Order reviews). The Agency is requesting comment on this action, including this proposed NPDWR and MCLGs, and have identified specific areas where public input will be helpful for EPA in developing the final rule (see section XIV of this preamble on specific topics highlighted for public comment). In addition to seeking written input, EPA will be holding a public hearing on May 4th, 2023. I. Public Participation A. Written Comments Submit your comments, identified by Docket ID No. EPA-HQ-OW-2022-0114, at https://www.regulations.gov (our preferred method), or the other methods identified in the ADDRESSES section. Once submitted, comments cannot be edited or removed from the docket. EPA may publish any comment received to its public docket. Do not submit to EPA's docket at https://www.regulations.gov any information you consider to be Confidential Business Information (CBI), Proprietary Business Information (PBI), or other information whose disclosure is restricted by statute. Multimedia submissions (audio, video, etc.) must be accompanied by a written comment. The written comment is considered the official comment and should include discussion of all points you wish to make. EPA will generally not consider comments or comment contents located outside of the primary submission ( i.e., on the web, cloud, or other file sharing system). Please visit https://www.epa.gov/dockets/commenting-epa-dockets for additional submission methods; the full EPA public comment policy; information about CBI, PBI, or multimedia submissions; and general guidance on making effective comments. B. Participation in Virtual Public Hearing EPA will hold a public hearing on May 4th, 2023, to receive public comment and will present the proposed requirements of the draft NPDWR. The hearing will be held virtually from approximately 11 a.m. until 7 p.m. eastern time. EPA will begin registering speakers for the hearing upon publication of this document in the Federal Register (FR). To attend and register to speak at the virtual hearing, please use the online registration form available at https://www.epa.gov/sdwa/and-polyfluoroalkyl-substances-pfas. The last day to pre-register to speak at the hearing will be April 28, 2023. On May 3, 2023, EPA will post a general agenda for the hearing that will list pre-registered speakers in approximate order at: https://www.epa.gov/sdwa/and-polyfluoroalkyl-substances-pfas. The number of online connections available for the hearing is limited and will be offered on a first- come, first-served basis. To submit visual aids to support your oral comment, please contact PFASNPDWR@epa.gov for guidelines and instructions. Registration will remain open for the duration of the hearing itself for those wishing to provide oral comment during unscheduled testimony; however, early registration is strongly encouraged to ensure proper accommodations and adequate timing. EPA will make every effort to follow the schedule as closely as possible on the day of the hearing; however, please plan for the hearings to run either ahead of schedule or behind schedule. Please note that the public hearing may close early if all business is finished. EPA encourages commenters to provide EPA with a written copy of their oral testimony electronically by submitting it to the public docket at www.regulations.gov, Docket ID: EPA-HQ-OW-2022-0114. Oral comments will be time limited to allow for maximum participation, which may result in the full statement not being heard. Therefore, EPA also recommends submitting the text of your oral comments as written comments to the rulemaking docket. Any person not making an oral statement may also submit a written statement. Written statements and supporting information submitted during the comment period will be considered with the same weight as oral comments and supporting information presented at the public hearing. Please note that any updates made to any aspect of the hearing are posted online at https://www.epa.gov/sdwa/and-polyfluoroalkyl-substances-pfas. While EPA expects the hearing to go forward as set forth above, please monitor our website or contact PFASNPDWR@epa.gov to determine if there are any updates. EPA does not intend to publish a document in the Federal Register announcing updates. If you require any accommodations such as language translation, captioning, or other special accommodations for the day of the hearing, please indicate this as a part of your registration and describe your needs by April 28, 2023. EPA may not be able to arrange accommodations without advance notice. Please contact PFASNPDWR@epa.gov with any questions related to the public hearing. This proposed rule is organized as follows: I. General Information A. What is EPA proposing? B. Does this action apply to me? II. Background A. What are PFAS? B. Definitions C. Chemistry, Production and Uses D. Human Health Effects E. Statutory Authority F. Statutory Framework and PFAS Regulatory History G. Bipartisan Infrastructure Law H. EPA PFAS Strategic Roadmap III. Preliminary 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. EPA's Preliminary Regulatory Determination Summary for PFHxS, HFPO-DA, PFNA, and PFBS F. Request for Comment on EPA's Preliminary Regulatory Determination for PFHxS, HFPO-DA, PFNA, and PFBS IV. Approaches to MCLG Derivation A. Approach to MCLG Derivation for Individual PFAS B. Approach to MCLG Derivation for a PFAS Mixture V. Maximum Contaminant Level Goals A. PFOA B. PFOS C. PFAS Hazard Index: PFHxS, HFPO-DA, PFNA, and PFBS VI. Maximum Contaminant Levels A. PFOA and PFOS B. PFAS Hazard Index: PFHxS, HFPO-DA, PFNA, and PFBS C. Reducing Public Health Risk by Protecting Against Dose Additive Noncancer Health Effects From PFAS D. Regulatory Alternatives E. MCL-Specific Requests for Comment VII. Occurrence A. UCMR 3 B. State Drinking Water Data C. 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 VIII. Analytical Methods A. Practical Quantitation Levels (PQLs) for Regulated PFAS IX. 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? X. 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? XI. 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)? XII. Rule Implementation and Enforcement A. What are the requirements for primacy? B. What are the primacy agency record keeping requirements? C. What are the primacy agency reporting requirements? D. Exemptions and Extensions XIII. Health Risk Reduction and Cost Analysis A. Affected Entities and Major Data Sources Used To Develop the Baseline Water System Characterization B. Overview of the Cost-Benefit Model C. Method for Estimating Costs D. Method for Estimating Benefits E. Nonquantifiable Benefits of PFOA and PFOS Exposure Reduction F. Nonquantifiable Benefits of Removal of PFAS Included in the Proposed Regulation and Co-Removed PFAS G. Benefits Resulting From Disinfection By-Product Co-Removal H. Comparison of Costs and Benefits I. Quantified Uncertainties in the Economic Analysis J. Cost-Benefit Determination XIV. Request for Comment on Proposed Rule Section III—Regulatory Determinations for Additional PFAS Section V—Maximum Contaminant Level Goals Section VI—Maximum Contaminant Levels Section VII—Occurrence Section IX—Monitoring and Compliance Requirements Section X—Safe Drinking Water Right to Know Section XI—Treatment Technologies Section XII—Rule Implementation and Enforcement Section XIII—HRRCA Section XV—Statutory and Executive Order Reviews XV. Statutory and Executive Order Reviews A. Executive Order 12866: Regulatory Planning and Review and Executive Order 13563 Improving Regulation and 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 K. Consultations With the Science Advisory Board, National Drinking Water Advisory Council, and the Secretary of Health and Human Services XVI. References I. General Information A. What is EPA proposing? EPA is proposing for public comment a drinking water regulation that includes six PFAS. EPA is proposing to establish MCLGs and an NPDWR for these PFAS in public drinking water supplies. EPA proposes MCLGs for PFOA and PFOS at zero (0) and an enforceable MCL for PFOA and PFOS in drinking water at 4.0 ppt. Additionally, the Agency is requesting comment on a preliminary determination to regulate additional PFAS to include PFHxS, HFPO-DA 1 (also known as and referred to as “GenX Chemicals” in this proposal), PFNA, and PFBS. Concurrent with this preliminary determination, EPA is proposing an HI of 1.0 as the MCLG and enforceable MCL to address individual and mixtures of these four contaminants where they occur in drinking water. EPA is proposing to calculate the HI as the sum total of component PFAS HQs, calculated by dividing the measured component PFAS concentration in water by the relevant HBWC. In this proposal, EPA is using HBWCs of 9.0 ppt for PFHxS, 10.0 ppt for HFPO-DA; 10.0 ppt for PFNA; and 2000 ppt for PFBS. The proposed approach to calculating the HI for this set of four PFAS compounds is designed to be protective against all adverse effects, not a single outcome/effect, and is a health protective decision aid for use in determining the level at which there are no adverse effects on the health of persons with an adequate margin of safety, thus is appropriate for MCLG development. 1 PFAS may exist in multiple forms, such as acids and organic or metal salts. Each of these forms may be listed as a separate entry in certain databases and have separate Chemical Abstract Service (CAS) Registry numbers. However, PFAS are expected to dissociate in water to their anionic form. For example, the term “GenX Chemicals” acknowledges the “acid” and “ammonium salt” forms of HFPO-DA as two different chemicals. In water, though, these chemicals dissociate and therefore the resulting anion appears as a single analyte for the purposes of detection and quantitation. Please see “definitions” for more information. EPA notes that the chemical HFPO-DA is used in a processing aid technology developed by DuPont to make fluoropolymers without using PFOA. The chemicals associated with this process are commonly known as GenX Chemicals and the term is often used interchangeably for HFPO-DA along with its ammonium salt (USEPA, 2021b). The requirements in this proposal that apply to (1) PFOA, (2) PFOS, and (3) PFHxS, HFPO-DA, PFNA, and PFBS and their mixtures are distinct and capable of operating independently. B. Does this action apply to me? The preliminary regulatory determination to establish drinking water regulations for certain PFAS and their mixtures and the proposed regulation are proposals for public comment and are not requirements or regulations. Instead, this action notifies interested parties of the availability of information supporting the preliminary regulatory determinations for four PFAS and their mixtures, the development of the NPDWR for six PFAS, and proposed rule requirements for public comment. If EPA proceeds to a final regulatory determination and final regulation, once promulgated, this action will potentially affect the following: Category Examples of potentially affected entities Public water systems 2 Community water systems (CWSs); Non-transient, non-community water systems (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 that could be affected by this action once promulgated. To determine whether a facility or activities could be affected by this action, this proposed rule should be carefully examined. Questions regarding the applicability of this action to a particular entity may be directed to the person listed in the FOR FURTHER INFORMATION CONTACT section. 2 The term “public water system” means a system for the provision to the public of water for human consumption through pipes or other constructed conveyances, if such system has at least fifteen service connections or regularly serves at least twenty-five individuals. Such term includes (i) any collection, treatment, storage, and distribution facilities under control of the operator of such system and used primarily in connection with such system, and (ii) any collection or pretreatment storage facilities not under such control which are used primarily in connection with such system. II. Background A. What are PFAS? PFAS are a large class of specialized synthetic chemicals that have been in use since the 1940s (USEPA, 2018a). This proposed regulation only applies to certain PFAS: PFOA, PFOS, PFHxS, HFPO-DA, PFNA, and PFBS. People may potentially be exposed to these PFAS through certain consumer products such as textiles ( e.g., seat covers, sail covers, weather protection (Janousek et al., 2019)), leather shoes as well as shoe polish/wax (Norden, 2013; Borg and Ivarsson, 2017), along with cooking/baking wares (Blom and Hanssen 2015; KEMI, 2015; Glüge et al., 2020), occupational contact, and/or by consuming food and drinking water that contain PFAS. Due to their widespread use, physicochemical properties, and prolonged persistence, many PFAS co-occur in exposure media ( e.g., air, water, ice, sediment), and bioaccumulate in tissues and blood of aquatic as well as terrestrial organisms, including humans (Domingo and Nadal, 2019; Fromme et al., 2009). Industrial workers who are involved in manufacturing or processing fluoropolymers, or people who live or recreate near fluoropolymer facilities, may encounter greater exposures; particularly of PFOA, PFNA, as well as HFPO-DA. Firefighters as well as people who live near airfields or military bases may have especially higher exposure to PFHxS and PFBS due to the use of aqueous foam forming film as a fire suppressant. Pregnant and lactating women, as well as children, may be more sensitive to the harmful effects of certain PFAS, for example, 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, cancer, immunological effects, among others (USEPA, 2023b; USEPA, 2023c). Other PFAS are also documented to result in a range of adverse health effects (USEPA, 2021a; USEPA, 2021b; ATSDR, 2021; NASEM 2022). Although most United States production of PFOS, PFOA, and PFNA, along with other long-chain PFAS, was phased out and then generally replaced by production of PFBS, PFHxS, HFPO-DA and other PFAS, EPA is aware of ongoing use of PFOS, PFOA, PFNA, and other long-chain PFAS. 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 and some uses of PFOS are ongoing (see 40 Code of Federal Regulations (CFR) § 721.9582). EPA is also aware of ongoing use of the chemicals available from existing stocks or newly introduced via imports. Additionally, the environmental persistence of these chemicals and formation as degradation products from other compounds may still contribute to their release in the environment. B. Definitions The six PFAS proposed for regulation and their relevant Chemical Abstract Service (CAS) registry numbers are: • PFOA (C8F15CO2-; CAS: 45285-51-6) • PFOS (C8F17SO3-; CAS: 45298-90-6) • PFHxS (C6F13SO3-; CAS: 108427-53-8) • HFPO-DA (C6F11O3-; CAS: 122499-17-6) • PFNA (C9F17CO2-; CAS: 72007-68-2) • PFBS (C4F9SO3-; CAS: 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 CAS 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 (CAS: 122499-17-6), also known as HFPO-DA, is an anionic molecule which has an ammonium salt (CAS: 62037-80-3), a conjugate acid (CAS: 13252-13-6), a potassium salt (CAS: 67118-55-2), and an acyl fluoride precursor (CAS: 2062-98-8), among other variations. At environmentally relevant pHs these all dissociate into the propanoate/anion form (CAS: 122499-17-6). Each PFAS listed has multiple variants with differing chemical connectivity but the same molecular composition; these are 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. The proposed regulation covers all salts, isomers and derivatives of the chemicals listed, including derivatives other than the anionic form which might be created or identified. C. Chemistry, Production and Uses PFAS are most commonly and widely used to make products resistant to water, heat, and stains. As a result, they are found in industrial and consumer products such as clothing, food packaging, cookware, cosmetics, carpeting, and fire-fighting foam (AAAS, 2020). Facilities associated with PFAS releases into the air, soil, and water include those for manufacturing, chemical as well as well as product production and military installations (USEPA, 2016a; USEPA, 2016b). The chemical structures of some PFAS cause them to repel water as well as oil, remain chemically and thermally stable, and exhibit surfactant properties. PFAS have strong, stable carbon-fluorine (C-F) bonds, making them resistant to hydrolysis, photolysis, microbial degradation, and metabolism (Ahrens, 2011; Beach et al., 2006; Buck et al., 2011). These properties are what make PFAS useful for commercial and industrial applications and purposes. However, these are also what make some PFAS extremely persistent in the human body and the environment (Calafat et al., 2007, 2019). PFOA, PFOS, PFHxS, HFPO-DA, PFNA, and PFBS belong to a subset of PFAS known as perfluoroalkyl acids (PFAAs), all of which consist of a perfluorinated alkyl chain connected to an acidic headgroup. Humans are exposed to PFAS due to wide-ranging commercial and industrial applications along with long range migration from sources. The structure of these PFAS contribute to their persistence in the environment as well as their resistance to chemical, biological, and physical degradation processes. PFOA and PFOS are two of the most widely studied and longest used PFAS. These two compounds have been detected in up to 98 percent of human serum samples taken in biomonitoring studies that are representative of the U.S. general population; however, since PFOA and PFOS have been voluntarily phased out in the U.S., serum concentrations have been declining (CDC, 2019). The sole U.S. manufacturer of PFOS agreed to a voluntary phaseout in 2000, and the last reported production was in 2002 (USEPA, 2000b; USEPA, 2018b; USEPA, 2021c). PFOS has been used as a surfactant or emulsifier in firefighting foam, circuit board etching acids, alkaline cleaners, floor polish, and as a pesticide active ingredient for insect bait traps (HSBD, 2016). PFOA has been used as an emulsifier and surfactant in fluoropolymers (such as in the manufacturing of non-stick products like Teflon©), firefighting foams, cosmetics, grease and lubricants, paints, polishes, and adhesives (HSBD, 2016). PFNA was historically the second most used surfactant for emulsion polymerization (after PFOA) which was its main use (Buck et al., 2012). Fluorinated surfactants improve the physical properties of the polymer as well as improving the polymerization rate (Glüge et al., 2020). Fluoropolymers are used in many applications because of their unique physical properties such as resistance to high and low temperatures, resistance to chemical and environmental degradation, and nonstick characteristics. Fluoropolymers also have dielectric and fire-resistant properties that have a wide range of electrical and electronic applications, including architecture, fabrics, automotive uses, cabling materials, electronics, pharmaceutical and biotech manufacturing, and semiconductor manufacturing (Gardiner, 2014). Although drying processes can release the surfactants when manufacturing is complete, surfactant residues remain in the finished products (KEMI, 2015). Legacy stocks may still be used and products containing PFNA may still be produced internationally and imported to the U.S. (ATSDR, 2021). The voluntary phase out caused a shift to alternatives such as per- and polyfluoroalkyl ether carboxylic acids (PFECAs). The chemical HFPO-DA is the most prevalent of these and is used in a processing aid technology developed by DuPont to make fluoropolymers without using PFOA. The chemicals associated with this process are commonly known as GenX Chemicals and the term is often used interchangeably for HFPO-DA along with its ammonium salt (USEPA, 2021b). The most common use for GenX Chemicals is for emulsion polymerization. Another alternative, PFBS, is mainly used as a water and stain repellent protection for leather, textiles, carpets, and porous hard surfaces, representing 25-50 tons/year of PFBS in mixtures (Norwegian Environment Agency, 2017). PFBS and related chemicals are also used in curatives for fluoroelastomers (Glüge et al., 2020). The curatives are used for manufacturing O-rings, seals, linings, protective clothing, cooking wares, and flame retardants (Norwegian Environment Agency, 2017; Blom and Hanssen, 2015). PFHxS is used in stain-resistant fabrics, fire-fighting foams, flame retardants, insecticides, and as a surfactant in industrial processes (Glüge et al., 2020). Additionally, particle accelerators including the Delphi Detector at Stanford University rely on liquid PFHxS (Glüge et al., 2020). PFHxS production, along with PFOS, was phased out in 2002 nationwide however, production continues in other countries and products containing PFHxS may be imported into the U.S. (USEPA, 2000c). Legacy stocks may also still be used. D. 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 linear carboxylic and sulfonic acid species such as PFOA, PFOS, PFHxS, PFNA, and PFBS (ATSDR, 2021). Many other PFAS have preliminary human health effects data (Mahoney et al., 2022) and some PFAS, such as PFBS and HFPO-DA, have sufficient data that has allowed EPA to derive toxicity values and publish toxicity assessments (USEPA, 2021a; USEPA, 2021b). The adverse health effects observed following oral exposure to such PFAS are significant and diverse and include (but are not limited to): cancer and effects on the liver ( e.g., liver cell death), growth and development ( e.g., low birth weight), hormone levels, kidney, immune system, lipid levels ( e.g., high cholesterol), the nervous system, and reproduction. Please see sections III.B, IV, and V of this preamble for additional discussion on health considerations for the six PFAS EPA is proposing to regulate in this document. E. Statutory Authority Section 1412(b)(1)(A) of SDWA requires EPA to establish 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 with a frequency and at levels of public health concern; and (3) where in the sole judgment of the Administrator, regulation of such contaminant presents a meaningful opportunity for health risk reduction for persons served by PWSs. F. Statutory Framework and PFAS Regulatory History Section 1412(b)(1)(B)(i) of SDWA requires 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 and are not currently subject to any proposed or promulgated NPDWRs. EPA uses the CCL to identify priority contaminants for regulatory decision-making ( i.e., regulatory determinations), and information collection. Contaminants listed on the CCL may require future regulation under SDWA. EPA included PFOA and PFOS on the third and fourth CCLs published in 2009 (USEPA, 2009a) and 2016 (USEPA, 2016c). The Agency published the fifth CCL (CCL 5) earlier this year and it includes PFAS as a chemical group (USEPA, 2022b). 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, EPA issue a final regulatory determination to regulate or not regulate at least five contaminants on each CCL. In addition, Section 1412(b)(1)(B)(ii)(III) authorizes EPA to make a determination to regulate a contaminant not listed on the CCL 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, EPA issues preliminary regulatory determinations subject to public comment and then issues a final regulatory determination after consideration of public comment. For any contaminant that EPA determines meets the criteria for regulation under SDWA 1412(b)(1)(A), Section 1412(b)(1)(E) requires that EPA propose a NPDWR within two years and promulgate a final regulation within 18 months of the proposal (which may be extended by 9 additional months). EPA implements a monitoring program for unregulated contaminants under SDWA 1445(a)(2) which requires that once every five years, EPA issue a list of priority unregulated contaminants to be monitored by PWSs. This monitoring is implemented through the Unregulated Contaminant Monitoring Rule (UCMR), which collects data from CWSs and NTNCWSs. 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). Water system monitoring data for six PFAS were collected during the third UCMR (UCMR3) between 2013 to 2015. The fifth UCMR (UCMR5), published December 2021, requires sample collection and analysis for 29 PFAS to occur between 2023 and 2025 using analytical methods developed by EPA and consensus organizations. 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 EPA appropriations for such purpose and sufficient laboratory capacity, EPA must require all 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 VII of this preamble provides additional discussion on PFAS occurrence. Additionally, while the UCMR 5 information will not be available to inform this proposal, EPA is proposing to consider the UCMR 5 data to support implementation of monitoring requirements under the proposed rule. Section IX of this preamble further discusses monitoring and compliance requirements. After careful consideration of public comments, EPA issued final regulatory determinations for contaminants on the fourth CCL in March of 2021 (USEPA, 2021d) which included determinations to regulate two contaminants, PFOA and PFOS, in drinking water. EPA found that PFOA and PFOS may have 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 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 preliminarily finds that additional PFAS compounds also meet SDWA criteria for regulation. EPA's preliminary 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 a proposed drinking water regulation concurrent “with a determination to regulate.” This provision authorizes a more expedited process by allowing EPA to make concurrent the regulatory determination and rulemaking processes. As a result, EPA interprets the 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 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 EPA to act efficiently to issue a final determination to regulate concurrently with a final NPDWR to avoid delays to address contaminants that meet the statutory criteria. As a result, this proposal contains both a preliminary determination to regulate four PFAS contaminants and proposed regulations for those contaminants as well as the two PFAS contaminants (PFOA and PFOS) for which EPA has already issued a final Regulatory Determination. EPA developed a proposed MCLG and a proposed NPDWR for six PFAS compounds pursuant to the requirements under section 1412(b)(1)(B) of SDWA. The proposed MCLGs and proposed NPDWR are discussed in more detail below. G. Bipartisan Infrastructure Law The Agency notes that the passage of the Infrastructure Investment and Jobs Act, also referred to as the BIL, invests over $11.7 billion in the Drinking Water SRF; $4 billion to the Drinking Water SRF for Emerging Contaminants; and $5 billion to Small, Underserved, and Disadvantaged Communities Grants. 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. These funds can also be used to address emerging contaminants like PFAS in drinking water through actions such as technical assistance, water quality testing, and contractor training, which will allow communities supplemental funding to meet their obligations under this proposed regulation and help ensure protection from PFAS contamination of drinking water. H. EPA PFAS Strategic Roadmap In October 2021, EPA published the PFAS Strategic Roadmap that outlined the Agency's plan 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). Described in the Roadmap are key commitments the Agency made toward addressing these contaminants in the environment. With this proposal, EPA is delivering on a key commitment in the Roadmap to “establish a National Primary Drinking Water Regulation” for proposal and is working toward promulgating the final NPDWR in Fall of 2023. III. Preliminary Regulatory Determinations for Additional PFAS Since 2021 when EPA determined to regulate two PFAS contaminants, PFOA and PFOS, EPA has evaluated additional PFAS compounds for regulatory consideration and has preliminarily determined that an additional four individual PFAS and mixtures of these PFAS meet SDWA criteria for regulation. 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 “contaminants” 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 is making a preliminary determination to regulate PFHxS, HFPO-DA, PFNA, and PFBS in drinking water, and mixtures of these PFAS contaminants. PFHxS, HFPO-DA, PFNA, and PFBS, and mixtures of these PFAS, are known to cause adverse human health effects; there is 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, HFPO-DA, PFNA, PFBS and mixtures of these PFAS present a meaningful opportunity for health risk reductions for people served by PWSs. This section describes the best available science and information used by the Agency to support this preliminary Regulatory Determination. The proposed MCLG and enforceable standard for these four PFAS and mixtures of these PFAS are discussed further in sections V to VI of this preamble. A. Agency Findings To support the Agency's preliminary Regulatory Determination, EPA examined health effects information from available peer reviewed human health assessments as well as drinking water monitoring data collected as part of the UCMR 3 and state-led monitoring efforts. EPA finds that oral exposure to PFHxS, HFPO-DA, PFNA, and PFBS may individually and in a mixture each result in adverse health effects, including disrupting multiple biological pathways that result in common adverse effects on several biological systems including the endocrine, cardiovascular, developmental, immune, and hepatic systems (USEPA, 2023a). PFAS, including PFHxS, HFPO-DA, PFNA, and PFBS and their mixtures are 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, EPA finds that PFHxS, HFPO-DA, PFNA, and PFBS each have a substantial likelihood to occur in finished drinking water and that these PFAS are also likely to co-occur as mixtures and result in increased exposure above levels of health concern. Therefore, given this high occurrence and co-occurrence likelihood and that adverse health effects arise as a result of both these PFAS individually and as mixtures, the Agency is preliminarily determining that PFHxS, HFPO-DA, PFNA, and PFBS and their mixtures may have adverse human health effects; there is a substantial likelihood that PFHxS, HFPO-DA, PFNA, PFBS and mixtures of these PFAS, will occur and co-occur in PWSs with a frequency and at levels of public health concern; and in the sole judgment of the Administrator, regulation of PFHxS, HFPO-DA, PFNA, and PFBS, and their mixtures, presents a meaningful opportunity for health risk reductions for persons served by PWSs. B. Statutory Criterion 1—Adverse Health Effects The Agency finds that PFHxS, HFPO-DA, PFNA, PFBS and their mixtures may have an adverse effect on the health of persons. Discussion related to health effects for each of the four PFAS is below. For this proposal, the Agency is developing HBWCs for PFHxS, HFPO-DA, PFNA and PFBS, defined as a level protective of health effects over a lifetime of exposure, including sensitive populations and life stages. Each of the four HBWCs is used in this proposal to evaluate occurrence data and the likelihood of potential risk to human health to justify the agency's preliminary regulatory determinations for PFHxS, HFPO-DA, PFNA and PFBS. The chemical-specific HBWCs are also used to assess the potential human health risk associated with mixtures of the four PFAS in drinking water using the HI approach. Additional details on the HBWC for PFHxS, HFPO-DA, PFNA and PFBS are found in section IV of this preamble. More information supporting EPA's preliminary regulatory determination relating to adverse health effects for these PFAS and the HI approach for mixtures is available in section V of this preamble. 1. PFHxS Toxicity studies of oral PFHxS exposure in animals have reported adverse health effects on the liver, thyroid, and development (ATSDR, 2021). EPA has not yet classified the carcinogenicity of PFHxS. For a detailed discussion on adverse effects of oral exposure to PFHxS, please see ATSDR (2021) and USEPA (2023a). The HBWC for PFHxS is derived using a chronic reference value based on an Agency For Toxic Substances And Disease Registry (ATSDR) intermediate-duration oral Minimal Risk Level, which was based on thyroid effects seen in male rats after oral PFHxS exposure (ATSDR, 2021). The most sensitive non-cancer effect observed was thyroid follicular epithelial hypertrophy/hyperplasia in parental male rats exposed to PFHxS for 42-44 days, identified in the critical developmental toxicity study selected by ATSDR (no observed adverse effect level (NOAEL) of 1 mg/kg/day) (Butenhoff et al., 2009; ATSDR, 2021). To derive the intermediate-duration Minimal Risk Level for PFHxS, ATSDR calculated a human equivalent dose (HED) of 0.0047 mg/kg/day from the NOAEL of 1 mg/kg/day identified in the principal study. Then, ATSDR applied a total uncertainty factor (UF)/modifying factor (MF) of 300X (10X UF for intraspecies variability, 3X UF for interspecies differences, and a 10X MF for database deficiencies) to yield an intermediate-duration oral Minimal Risk Level of 0.00002 mg/kg/day (ATSDR, 2021). Per Agency guidance (USEPA, 2002), to calculate the HBWC, EPA applied an additional UF of 10 to adjust for subchronic-to-chronic duration (UF S ) because the effect was not in a developmental life stage ( i.e., thyroid follicular epithelial hypertrophy/hyperplasia in parental male rats). The resulting chronic reference value was 0.000002 mg/kg/day. No sensitive population or life stage was identified for bodyweight-adjusted drinking water intake (DWI-BW) selection for PFHxS because the critical effect on which the ATSDR Minimal Risk Level was based (thyroid alterations) was observed in adult male rats. Since this exposure life stage does not correspond to a sensitive population or life stage, a DWI-BW 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) was selected for HBWC derivation (USEPA, 2019a). EPA calculated the HBWC for PFHxS using a relative source contribution (RSC) of 0.20. This means that 20% of the exposure—equal to the chronic reference value—is allocated to drinking water, and the remaining 80% is attributed to all other potential exposure sources. This was based on EPA's determination that the available data on PFHxS exposure routes and sources did not permit quantitative characterization of PFHxS exposure. In such cases, an RSC of 0.20 is typically used (USEPA, 2000c). See U.S.EPA (2023a) for complete details on the RSC determination for PFHxS. As further described in USEPA (2023a) and section V of this preamble below, the HBWC for PFHxS is calculated to be 9.0 ppt. This HBWC of 9.0 ppt is also used as the health reference level (HRL) for this preliminary regulatory determination. 2. HFPO-DA EPA's 2021 Human Health Toxicity Assessment for GenX Chemicals describes potential health effects associated with oral exposure to HFPO-DA (USEPA, 2021b). Toxicity studies in animals indicate that exposures to HFPO-DA may result in adverse health effects, including liver and kidney toxicity and immune system, hematological, reproductive, and developmental effects (USEPA, 2021b). There is Suggestive Evidence of Carcinogenic Potential of oral exposure to HFPO-DA in humans, but the available data are insufficient to derive a cancer risk concentration in water for HFPO-DA. For a detailed discussion on adverse effects of oral exposure to HFPO-DA, please see USEPA (2021b). EPA's noncancer HBWC for HFPO-DA is derived from a reference dose (RfD) that is based on liver effects observed following oral exposure of mice to HFPO-DA (USEPA, 2021b). The most sensitive noncancer effect observed was a constellation of liver lesions in parental female mice exposed to HFPO-DA by gavage for 53-64 days, identified in the critical reproductive/developmental toxicity study selected by EPA (NOAEL of 0.1 mg/kg/day) (DuPont, 2010; USEPA, 2021b). To develop the chronic RfD for HFPO-DA, EPA derived an HED of 0.01 mg/kg/day from the NOAEL of 0.1 mg/kg/day identified in the principal study. EPA then applied a composite UF of 3,000 ( i.e., 10X for intraspecies variability, 3X for interspecies differences, 10X for extrapolation from a subchronic to a chronic dosing duration, and 10X for database deficiencies) to yield the chronic RfD (USEPA, 2021b). To select an appropriate DWI-BW for use in derivation of the noncancer HBWC values for HFPO-DA, EPA considered the HFPO-DA exposure interval used in the oral reproductive/developmental toxicity study in mice that was 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 exposure for HFPO-DA: women of childbearing age, pregnant women, and lactating women (Table 3-63 in USEPA, 2019a). Of these three, the DWI-BW for lactating women (0.0469 L/kg/day) is anticipated to be protective of the other two sensitive life stages. Therefore, EPA used the DWI-BW for lactating women to calculate the HBWC for the proposed regulation, which is also used for the HRL for the preliminary regulatory determination. The HBWC value for HFPO-DA was calculated using an RSC of 0.20. This means that 20% of the exposure—equal to the RfD—is allocated to drinking water, and the remaining 80% is attributed to all other potential exposure sources (USEPA, 2022d). Selection of this RSC was based on EPA's determination that the available exposure data for HFPO-DA did not enable a quantitative characterization of relative HFPO-DA exposure sources and routes. In such cases, an RSC of 0.20 is typically used (USEPA, 2000c). As further described in USEPA (2023a) and USEPA (2022d), the HBWC for HFPO-DA is calculated to be 10.0 ppt. This value is consistent with EPA's 2022 drinking water health advisory for HFPO-DA (USEPA, 2022d), but was derived from EPA's 2021 Human Health Toxicity Assessment for HFPO-DA (USEPA, 2021b). This HBWC of 10 ppt is also used as the HRL for this preliminary Regulatory Determination for HFPO-DA. 3. PFNA Animal toxicity studies have reported adverse health effects, specifically on development, reproduction, immune function, and the liver, after oral exposure to PFNA (ATSDR, 2021). EPA has not yet classified the carcinogenicity of PFNA. For a detailed discussion on adverse effects of oral exposure to PFNA, please see ATSDR (2021) and USEPA (2023a). The HBWC for PFNA is derived using a chronic reference value based on an ATSDR intermediate-duration oral Minimal Risk Level, which was based on developmental effects seen in mice after oral PFHxS exposure (ATSDR, 2021). The most sensitive non-cancer effects were decreased body weight (BW) gain and developmental delays ( i.e., delayed eye opening, preputial separation, and vaginal opening) in mice born to mothers that were gavaged with PFNA from gestational days (GD) 1-17, with continued exposure through lactation and monitoring until postnatal day (PND) 287, identified in the critical developmental toxicity study selected by ATSDR (NOAEL of 1 mg/kg/day) (Das et al., 2015; ATSDR, 2021). To derive the intermediate-duration Minimal Risk Level, ATSDR calculated an HED of 0.001 mg/kg/day from the NOAEL of 1 mg/kg/day identified in the principal study. Then, ATSDR applied a total UF/MF of 300X (total UF of 30X and a MF of 10X for database deficiencies) to yield an intermediate-duration Minimal Risk Level of 0.000003 mg/kg/day. EPA did not apply an additional UF to adjust for subchronic-to-chronic duration ( i.e., UF S ) to calculate the chronic reference value because the critical effects were observed during a developmental life stage (USEPA, 2002). The chronic reference value of 0.000003 mg/kg/day was used to derive the HBWC for PFNA. Based on the life stages of exposure in the principal study from which the intermediate-duration Minimal Risk Level was derived ( i.e., during gestation and lactation), EPA identified three potentially sensitive life stages that may represent critical windows of exposure for PFNA: women of childbearing age (13 to < 50 years), pregnant women, and lactating women (Table 3-63 in USEPA, 2019a). The DWI-BW for lactating women (0.0469 L/kg/day; 90th percentile direct and indirect consumption of community water, consumer-only two-day average) was selected to calculate the HBWC for PFNA because it is the highest of the three DWI-BWs and is anticipated to be protective of the other two sensitive life stages. EPA calculated the HBWC for PFNA using an RSC of 0.20. This means that 20% of the exposure—equal to the chronic reference value—is allocated to drinking water, and the remaining 80% is attributed to all other potential exposure sources. This was based on EPA's determination that the available data on PFNA exposure routes and sources did not permit quantitative characterization of PFNA exposure. In such cases, an RSC of 0.20 is typically used (USEPA, 2000c). See USEPA (2023a) for complete details on the RSC determination for PFNA. As further described in USEPA (2023a), the HBWC for PFNA is calculated to be 100 ppt. This HBWC of 10.0 ppt is also used as the HRL for this preliminary Regulatory Determination for PFNA. 4. PFBS EPA's 2021 PFBS Toxicity Assessmen t describe potential health effects associated with oral PFBS exposure (USEPA, 2021a). Toxicity studies of oral PFBS exposures in animals have reported adverse health effects on development, as well as 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. EPA concluded that there is “Inadequate Information to Assess Carcinogenic Potential” for PFBS and K+PFBS by any route of exposure. For a detailed discussion on adverse effects of oral exposure to PFBS, please see USEPA (2021a). EPA's noncancer HBWC for PFBS is derived from a chronic RfD that is based on thyroid effects observed following gestational exposure of mice to K+PFBS (USEPA, 2021a; USEPA, 2022e). The most sensitive non-cancer effect observed was decreased serum total thyroxine (T4) in newborn (PND 1) mice gestationally exposed to K+PFBS from GD 1-20, identified in the critical developmental toxicity study selected by EPA (benchmark dose lower confidence limit HED or BMDLHED) of 0.095 mg/kg/day) (Feng et al., 2017; USEPA, 2021a). To develop the chronic RfD for PFBS, EPA applied a composite UF of 300 ( i.e., 10X for intraspecies uncertainty factor (UF H ), 3X for interspecies uncertainty factor (UF A ), and 10X for database uncertainty factor (UF D )) to yield a value of 0.0003 mg/kg/day (USEPA, 2021a). To select an appropriate DWI-BW for use in deriving the noncancer HBWC value, EPA considered 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 embryo or fetus (Table 3-63 in USEPA, 2019a). Of these two, EPA selected the DWI-BW for women of child-bearing age (0.0354 L/kg/day) to derive the noncancer HBWC for PFBS because it was higher and therefore more health-protective (USEPA, 2022e). The HBWC value for PFBS was calculated using an RSC of 0.20. This means that 20% of the exposure—equal to the RfD—is allocated to drinking water, and the remaining 80% is attributed to all other potential exposure sources (USEPA, 2022e). This was based on EPA's determination that the available data on PFBS exposure routes and sources did not enable a quantitative characterization of PFBS exposure. In such cases, an RSC of 0.20 is typically used (USEPA, 2000c). As further described in USEPA (2022e), the HBWC for PFBS is calculated to be 2000 ppt. This value is consistent with EPA's 2022 drinking water health advisory for PFBS (USEPA, 2022d), but was derived from EPA's 2021 PFBS Toxicity Assessment (USEPA, 2021a). This HBWC of 2000 ppt is also used as the HRL for this preliminary Regulatory Determination for PFBS. 5. Mixtures of PFHxS, HFPO-DA, PFNA, and PFBS PFAAs, including PFHxS, HFPO-DA, PFNA, and PFBS, disrupt signaling of multiple biological pathways resulting in common adverse effects on several biological systems including thyroid hormone levels, lipid synthesis and metabolism, as well as on development, and immune and liver function (ATSDR, 2021; EFSA, 2018, 2020; USEPA, 2023a). Studies with PFAS and other classes of chemicals support the health protective assumption that a mixture of chemicals with similar observed effects should be assumed to also act in a dose additive manner unless data demonstrate otherwise (USEPA, 2023d). Dose additivity means that each of the component chemicals in the mixture (in this case, PFHxS, HFPO-DA, PFNA, and PFBS) behaves as a concentration or dilution of every other chemical in the mixture differing only in relative toxicity (USEPA, 2000a). See additional discussion of PFAS dose additivity in Section V.C of this preamble. C. Statutory Criterion 2—Occurrence With this proposal, EPA is preliminarily determining that PFHxS, HFPO-DA, PFNA, and PFBS, both individually and as mixtures of these PFAS, meet SDWA's second statutory criterion for regulatory determination: there is a substantial likelihood that the contaminants will occur and co-occur with a frequency and at levels of public health concern in PWSs based on EPA's evaluation of the best available occurrence information. EPA is seeking public comment on whether additional data or studies exist which EPA should consider that support or do not support this preliminary determination. EPA has made its preliminary determination based on the most recent, publicly available data, which includes UCMR 3 data and more recent PFAS drinking water data collected by several states. Informed by these data, EPA determined that there is a substantial likelihood PFHxS, HFPO-DA, PFNA, and PFBS will occur and co-occur with a frequency of public health concern. Additionally, when determining that there is a substantial likelihood these PFAS will occur at levels of public health concern, EPA considered both the occurrence concentration levels for each contaminant individually, as well as their collective co-occurrence and corresponding dose additive health effects from co-exposures. Furthermore, the Agency notes that it does not have a bright-line threshold for occurrence in drinking water that triggers whether a contaminant is of public health concern. A determination of public health concern 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. Given the many possible combinations of factors, a simple threshold is not viable and is a highly contaminant-specific decision that takes into consideration multiple factors. UCMR 3 monitoring occurred between 2013 and 2015 for PFHxS, PFNA, and PFBS. HFPO-DA were not monitored for as part of the UCMR 3. Under the UCMR 3, 36,972 samples from 4,920 PWSs were analyzed for PFHxS, PFNA, and PFBS. The minimum reporting levels (MRLs) for PFHxS, PFNA, and PFBS were 30 ppt, 20 ppt, and 90 ppt, respectively. EPA notes that these UCMR 3 MRLs are higher than those utilized within the majority of state monitoring data and for the upcoming UCMR 5. A total of 233 samples and 70 systems serving a total population of approximately 6.7 million people had reported detections (greater than or equal to the MRL) of at least one of the three compounds. Moreover, the large majority of these UCMR 3 reported detections were found at concentrations at or above levels of public health concern as described previously in section III.B of this preamble and below within this section. USEPA (2023e) presents sample and system level summaries of the results for the individual contaminants. More information supporting EPA's regulatory determination relating to the occurrence of these PFAS and their mixtures is included in section VII.A. of this preamble. EPA has also collected more recent finished drinking water data from 23 states who have made their data publicly available as of August 2021 (USEPA, 2023e). EPA used this cutoff date to allow the Agency to conduct thorough analyses of the state information. EPA further refined this dataset based on representativeness and reporting limitations, resulting in detailed technical analyses using a subset of the available state data ( i.e., all 23 states' data were not included within the detailed technical analyses). For example, a few states only reported results as a combination of analytes which was not conducive for analyzing PFAS. In general, the state data which were more recently collected using newer analytical methods that have lower reporting limits than those under UCMR 3 show widespread occurrence of PFOA, PFOS, PFHxS, PFNA, and PFBS in multiple geographic locations. These data also show that there is a substantial likelihood that these PFAS occur at concentrations below UCMR 3 reporting limits. Furthermore, these data include results for more PFAS than were included in the UCMR 3, including HFPO-DA, and show that PFHxS, HFPO-DA, PFNA, and PFBS, and mixtures of these PFAS, occur and co-occur at levels of public health concern as they are measured at concentrations above their respective individual HRLs or, when considering their dose additive impacts, exceed these levels. The Agency notes that the data vary in terms of quantity and coverage, including that some of these available data are from targeted or site-specific sampling efforts ( i.e., monitoring specifically in areas of known or potential contamination) and thus may be expected to have higher detection rates or not be representative of levels found in all PWSs within the state. Tables 1 and 2 below show the percent of samples with state reported detections of PFHxS, HFPO-DA, PFNA, and PFBS, and the percentage of monitored systems with detections of PFHxS, HFPO-DA, PFNA, and PFBS, respectively, across the non-targeted or non-site specific ( i.e., monitoring not conducted specifically in areas of known or potential contamination) state finished water monitoring data. EPA notes that different states utilized various reporting thresholds or limits when presenting their data, and for some states there were no clearly defined limits publicly provided. Further, the limits often varied within the data for each state depending on the specific analyte, as well as the laboratory analyzing the data. When conducting data analyses, EPA incorporated individual state-specific reporting limits where possible. In some cases, states reported data at concentrations below EPA's proposed rule trigger level for reduced compliance monitoring frequency and/or PQLs described in sections VIII.A., IX.A., and IX.B of this preamble. However, to present the best available occurrence data, EPA collected and evaluated the data based on the information as reported directly by the states. EPA also notes, and as described in further detail in section VIII.A. of this preamble, some laboratories are able to detect and measure the PFAS addressed in this document at lower concentrations than EPA's proposed rule trigger level and PQLs which account for differences in the capability of laboratories across the country. As such, EPA believes this data can reasonably support EPA's evaluation of PFOA, PFOS, PFHxS, HFPO-DA, PFNA, and PFBS occurrence and co-occurrence in drinking water. Specific details on state data reporting thresholds are available in Table 1 within USEPA (2023e). Table 1—Non-Targeted State PFAS Finished Water Data—Summary of Samples With State Reported Detections 1 of PFH x S, HFPO-DA, PFNA, and PFBS State PFHxS (%) PFNA (%) PFBS (%) HFPO-DA (%) Colorado 10.8 0.9 11.0 0.2 Illinois 5.1 0.2 7.8 0.0 Kentucky 8.6 2.5 12.3 13.6 Massachusetts 31.9 4.6 35.5 0.0 Michigan 2.9 0.1 5.2 0.04 New Hampshire 16.6 3.3 31.4 3.8 New Jersey 24.7 8.0 24.9 N/A North Dakota 1.6 0.0 0.0 0.0 Ohio 5.8 0.3 4.7 0.1 South Carolina 13.5 2.1 38.3 6.0 Vermont 2.2 1.7 4.8 0.2 Notes: 1 Detections determined by individual state reported limits which are not defined consistently across all states. Table 2—Non-Targeted State PFAS Finished Water Data—Summary of Monitored Systems With State Reported 1 Detections of PFH x S, HFPO-DA, PFNA, and PFBS State PFHxS (%) PFNA (%) PFBS (%) HFPO-DA (%) Colorado 13.4 1.0 13.4 0.3 Illinois 4.3 0.2 6.6 0.0 Kentucky 8.6 2.5 12.3 13.6 Massachusetts 30.2 8.4 39.4 0.0 Michigan 3.0 0.2 5.3 0.1 New Hampshire 22.5 5.5 37.9 5.1 New Jersey 32.6 13.3 34.0 N/A North Dakota 1.6 0.0 0.0 0.0 Ohio 2.2 0.3 2.4 0.1 South Carolina 20.0 6.1 56.0 10.9 Vermont 1.6 1.3 5.2 0.5 Notes: 1 Detections determined by individual state reported limits which are not defined consistently across all states. As shown in Tables 1 and 2, all states except one 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 0.1 to 38.3 percent and 0.1 to 56.0 percent, respectively. While these percentages show occurrence variability across states, several of these states demonstrate a significant number of samples ( e.g., detections of PFHxS in 31.9 percent of Massachusetts samples) and systems ( e.g., detections of HFPO-DA in 13.9 percent of monitored systems in Kentucky) with some or all of the four PFAS, which supports the Agency's preliminary determination that there is a substantial likelihood these PFAS and their mixtures occur and co-occur with a frequency of public health concern. Specific discussion related to occurrence for each of the four PFAS is below. 1. PFHxS The occurrence data presented above, throughout section VII. of this preamble and discussed in the USEPA (2023e) support the Agency's preliminary 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% of systems using an MRL of 30 ppt. All UCMR 3 reported values are greater than the HRL of 9.0 ppt. Additionally, through analysis of available non-targeted state data all states in Tables 1 and 2 had reported detections of PFHxS within 1.6 to 32.6 percent of their systems and reported concentrations ranging from 0.46 to 310 ppt with median sample concentrations ranging from 2.14 to 11.3 ppt. Results from targeted state monitoring data of PFHxS are also consistent with non-targeted state data. For example, California reported 29.2 percent of monitored systems found PFHxS, where concentrations ranged from 1.1 to 140.0 ppt. Therefore, in addition to the UCMR 3 results, these state data reflect PFHxS at frequencies and levels of public health concern. 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 and results are described in section VII.E of this preamble. Hundreds of systems serving millions of people were estimated to have mean concentrations exceeding the PFHxS HRL (9.0 ppt). Further supporting this preliminary determination, PFAS have dose additive impacts and PFHxS co-occurs in mixtures with other PFAS, including PFOA, PFOS, HFPO-DA, PFNA, and PFBS. More information on PFHxS co-occurrence is available in section VII.C. and VII.D. of this preamble. 2. HFPO-DA The occurrence data presented above, throughout section VII of this preamble, and discussed in the USEPA (2023e) support the Agency's preliminary 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 United States. Through analysis of available non-targeted state data over half of the states in Tables 1 and 2 had state reported detections of HFPO-DA within 0.1 to 13.6 percent of their systems. State reported sample results were also reported above the HRL of 10.0 ppt with sample results ranging from 1.7 to 29.7 ppt and median sample results ranging from 1.7 to 9.7 ppt. Additionally, targeted state monitoring in North Carolina which conducted sampling across six finished drinking water sites where 438 samples showed HFPO-DA ranging from 9.2 to 1100 ppt, with a median concentration of 40 ppt. Therefore, these state data demonstrate concentrations of HFPO-DA at levels of public health concern. Further supporting this preliminary determination, PFAS have dose additive impacts and HFPO-DA occur in mixtures with other PFAS, including PFOA, PFOS, PFHxS, PFNA, and PFBS. More information on HFPO-DA co-occurrence is available in section VII.C. and VII.D. of this preamble. 3. PFNA The occurrence data presented above, throughout section VII of this preamble, and discussed in USEPA (2023e) support the Agency's preliminary 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 United States. PFNA was found under UCMR 3 using an MRL of 20 ppt. Thus, all UCMR 3 reported detections are greater than the HRL of 10.0 ppt. Additionally, through analysis of available non-targeted state data all states except one in Tables 1 and 2 had state reported detections of PFNA within 0.2 to 13.3 percent of their systems, and state reported sample results ranging from 0.25 to 94.2 ppt with median sample results range from 2.1 to 7.46 ppt. 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 ppt. Thus, in addition to the UCMR 3 results, these state data also reflect PFNA concentrations at levels of public health concern. Further supporting this preliminary determination, PFAS have dose additive impacts and PFNA co-occurs in mixtures with other PFAS, including PFOA, PFOS, PFHxS, HFPO-DA, and PFBS. More information on PFNA co-occurrence is available in section VII.C. and VII.D. of this preamble. 4. PFBS The occurrence data presented above, throughout section VII of this preamble, and discussed in USEPA (2023e) support the Agency's preliminary determination that there is a substantial likelihood PFBS occurs with a frequency and at levels of public health concern in drinking water systems across the United States. PFBS was found under UCMR 3 using an MRL of 90 ppt. Additionally, through analysis of available non-targeted state data all states except one in Tables 1 and 2 had state reported detections of PFBS within 2.4 to 56 percent of their systems, with four states finding PFBS in over 34 percent of their systems. Furthermore, PFBS occurred at a greater frequency in all but one state than the other three PFAS. State reported sample results ranged from 1 to 310 ppt with median sample results ranging from 1.99 to 7.26 ppt. Targeted state monitoring data of PFBS are consistent with non-targeted state data. Maryland reported 51.5 percent of monitored systems found PFBS, where concentrations ranged from 1.01 to 21.29 ppt. Further supporting this preliminary determination, PFAS have dose additive impacts and PFBS occurs in mixtures with other PFAS, including PFOA, PFOS, PFHxS, HFPO-DA, and PFNA. Moreover, given the considerable prevalence of PFBS in state data reviewed by EPA and frequency in which it has been shown to have other PFAS co-occurring with it, PFBS may serve as an indicator of broad contamination of other PFAS. Those other PFAS are also likely dose additive to PFBS and other PFAS being proposed for regulation. EPA notes that PFBS concentrations do not exceed their HRL of 2000 ppt when considered in isolation; however, when considering dose additivity and the elevated frequency to which PFBS occurrence has been observed over time, EPA has determined that PFBS is an important component of regulated PFAS mixtures and because of their pervasiveness, there is a substantial likelihood of its occurrence with a frequency and at levels of public health concern. More information on PFBS co-occurrence is available in section VII.C. and VII.D. of this preamble. Based on the occurrence and co-occurrence information above and throughout section VII of this preamble, EPA has preliminarily determined that there is substantial likelihood PFBS occurs with a considerable frequency and at levels of public health concern. 5. Preliminary Occurrence Determination for PFHxS, HFPO-DA, PFNA, and PFBS Through the information presented within this section and in USEPA (2023e), along with the co-occurrence information presented in section VII.C. and VII.D. of this preamble, EPA's evaluation of the UCMR 3 data and state data collected more recently demonstrates that as analytical methods improved, monitoring has increased, and minimum reporting thresholds are lowered, there is a sustained upward trend that there is a substantial likelihood that these contaminants will occur and co-occur at a frequency and at levels of public health concern. The UCMR 3 results showed there were over 6.5 million people served by PWSs that had reported detections of PFHxS, PFNA, and PFBS, with many of the detections for PFHxS and PFNA above the HRLs. EPA's evaluation of monitoring data from multiple states that was primarily gathered following the UCMR 3 using improved analytical methods that could measure more PFAS at lower concentrations found that there is even greater demonstrated occurrence and co-occurrence of these PFAS, as well as for HFPO-DA, at significantly greater frequencies and at levels of public health concern. EPA anticipates that national monitoring with newer analytical methods capable of quantifying PFAS occurrence to lower levels, significant occurrence and co-occurrence of these PFAS are likely to be observed. EPA notes that it focused the evaluation of the state data on the non-targeted monitoring efforts from 12 states, given that these types of monitoring efforts are likely to be more representative of PFHxS, HFPO-DA, PFNA, and PFBS occurrence as they are not specifically conducted in areas of known or potential contamination. In these 12 states, there were reported detections of PFHxS, HFPO-DA, PFNA, or PFBS, with nearly all states reporting detections of at least three of these four PFAS. EPA considered the targeted state 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 additional targeted state data that EPA analyzed, EPA also found that these states reported detections at systems serving millions of additional people, as well as at levels of public health concern, particularly when considering PFAS mixtures and dose additive impacts. State data detection frequency and concentration results vary for PFHxS, HFPO-DA, PFNA, 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 when compared to other states. However, given the overall results, this demonstrates the substantial likelihood that these PFAS and their mixtures will occur at frequencies and levels of public health concern, and where these PFAS have been monitored they are very commonly found. Furthermore, EPA notes that as described in section VII.C.1. of this preamble, when evaluating only a subset of the available state data representing non-targeted monitoring, that one or more of PFHxS, HFPO-DA, PFNA, and PFBS were reported in approximately 13.9 percent of monitored systems; if these results were extrapolated to the nation, one or more of these four PFAS would be detectable in over 9,000 PWSs. Moreover, as shown in section VII.C.2. of this preamble, PFHxS, HFPO-DA, PFNA, and PFBS generally co-occur with each other, as well as with PFOA and PFOS, supporting that there is substantial likelihood that these PFAS will co-occur in mixtures with dose additive impacts. For all of these reasons, EPA has determined that there is sufficient occurrence information available to support this preliminary determination that there is a substantial likelihood that PFHxS, HFPO-DA, PFNA, and PFBS will occur at frequencies and levels of public health concern. D. Statutory Criterion 3—Meaningful Opportunity EPA has preliminarily determined that regulation of PFHxS, HFPO-DA, PFNA, and PFBS, both individually and in a mixture, presents a meaningful opportunity for health risk reduction for persons served by PWSs. EPA has made this preliminary determination after evaluating health, occurrence, treatment, and other related information against the three SDWA statutory criteria including consideration of the following for the four PFAS and their mixtures: • PFHxS, HFPO-DA, PFNA, and PFBS, individually and in a mixture, may cause adverse human health effects on several biological systems including the endocrine, cardiovascular, developmental, immune, and hepatic systems. Additionally, these four PFAS, as well as other PFAS, are likely to produce dose-additive effects from co-exposures. • The substantial likelihood that PFHxS, HFPO-DA, PFNA, and PFBS, individually occur and 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 VII of this preamble, and the corresponding significant populations served by these water systems. • PFHxS, HFPO-DA, PFNA, and PFBS, individually and in a mixture, are expected to be environmentally persistent. • Validated EPA-approved measurement methods are available to measure PFHxS, HFPO-DA, PFNA, and PFBS, individually and in mixtures. See section VIII of this preamble for further discussion. • Treatment technologies are available to remove PFHxS, HFPO-DA, PFNA, and PFBS, and mixtures of these contaminants, from drinking water. See section XI of this preamble for further discussion. • Regulating PFHxS, HFPO-DA, PFNA, and PFBS, in addition to PFOA and PFOS, is anticipated to reduce the overall public health risk from all other PFAS that co-occur and are co-removed. Their regulation is anticipated to provide public health protection at the majority of known sites with PFAS-impacted drinking water. • There are achievable steps to manage drinking water that can be taken to reduce risk. Due to the environmental persistence of these chemicals, there is potential for toxicity at environmentally relevant concentrations as studies show it can take years for many PFAS to leave the human body (NIEHS, 2020). See section III of this preamble above and section V of this preamble for discussion about the human health effects of PFHxS, HFPO-DA, PFNA, and PFBS. Data from both the UCMR 3 and state monitoring efforts demonstrates occurrence or likely occurrence and co-occurrence of PFHxS, HFPO-DA, PFNA, and PFBS, and their mixtures, at frequencies and levels of public health concern. Under UCMR 3, 1.4% of systems serving approximately 6.7 million people had reported detections (greater than or equal to their MRLs) of PFHxS, PFNA, and PFBS of at least one of the three compounds. Additionally, based on the available state monitoring data presented earlier in this section, in the 11 states shown in Table 2 that conducted non-targeted sampling of the four PFAS, monitored systems that reported detections of PFHxS, HFPO-DA, PFNA, and PFBS serve approximate populations of 8.3 million, 1.8 million, 2.6 million, and 8.8 million people, respectively. Further, as demonstrated in the UCMR 3 and state data, concentrations of these PFAS, as well as PFOA and PFOS, and their mixtures co-occur at levels of public health concern as described in more detail in section VII.C. and VII.D. of this preamble and USEPA (2023e). Analytical methods are available to measure PFHxS, HFPO-DA, PFNA, and PFBS in drinking water. EPA has published two multi-laboratory validated drinking water methods for individually measuring PFHxS, HFPO-DA, PFNA, and PFBS: EPA Method 537.1 which measures 18 PFAS and EPA Method 533 which measures 25 PFAS. There are 14 PFAS which overlap between methods and both methods measure PFOA and PFOS). Additional discussion on analytical methods can be found in section VIII of this preamble. EPA's analysis, summarized in section XI of this preamble, found there are available technologies capable of reducing PFHxS, HFPO-DA, PFNA, and PFBS. These technologies include granular activated carbon (GAC), AIX resins, reverse osmosis (RO), and nanofiltration (NF). See discussion in section XI of this preamble for information about these treatment technologies. Due to the inherent nature of sorptive and high-pressure membrane technologies such as these, treatment technologies that remove PFHxS, HFPO-DA, PFNA, and PFBS and their mixtures also have been documented to co-remove other PFAS (Sörengård et al., 2020; McCleaf et al., 2017; Mastropietro et al., 2021). Furthermore, as described in section VII of this preamble, PFHxS, HFPO-DA, PFNA, and PFBS also co-occur with PFAS for which the Agency is not currently making a preliminary regulatory determination. Many of these other emergent co-occurring PFAS are likely to also pose hazards to public health and the environment (Mahoney et al., 2022). Therefore, based on EPA's findings that PFHxS, HFPO-DA, PFNA, and PFBS have a substantial likelihood to co-occur in drinking water with other PFAS and treating for PFHxS, HFPO-DA, PFNA, and PFBS is anticipated to result in removing these and other PFAS, regulation of PFHxS, HFPO-DA, PFNA, PFBS (as well as PFOA and PFOS) also presents a meaningful opportunity to reduce the overall public health risk from all other PFAS that co-occur and are co-removed with PFHxS, HFPO-DA, PFNA, and PFBS. With the ability to monitor for PFAS, identify contaminated drinking water sources and contaminated finished drinking water, and reduce PFAS exposure through management of drinking water, EPA has identified meaningful and achievable actions that can be taken to reduce the human health risk of PFAS. EPA is preliminarily determining that regulation of PFHxS, HFPO-DA, PFNA, and PFBS presents a meaningful opportunity for health risk reduction for persons served by PWSs. E. EPA's Preliminary Regulatory Determination Summary for PFHxS, HFPO-DA, PFNA, and PFBS The statute provides EPA significant discretion when making a preliminary determination under Section 1412(b)(1)(A). This decision to make a preliminary regulatory determination for PFHxS, HFPO-DA, PFNA and PFBS and their mixtures is based on consideration of the evidence supporting the factors individually and as a whole. EPA's preliminary determination that PFHxS, HFPO-DA, PFNA, and PFBS “may have an adverse effect on the health of persons” is strongly supported by numerous studies where multiple health effects are demonstrated following exposure. EPA's preliminary determination regarding occurrence is supported by evidence documenting the trend demonstrated first by the UCMR 3 data and then subsequent state occurrence data that measured occurrence of the four PFAS has increased with more widespread monitoring primarily using EPA approved methods that have, lower reporting thresholds. The statute contemplates that there may be instances where exact occurrence may not be “known” and in these instances EPA need only demonstrate that that it has a basis to determine that there is a “substantial likelihood that the contaminant will occur.” Additional nationwide monitoring data will be conducted between 2023-2025 under the UCMR 5. This data will serve to demonstrate whether the four PFAS are known to occur, however, EPA has sufficient evidence now to support a preliminary determination there is a substantial likelihood that these PFAS will occur frequently and at concentrations where they are likely to exceed their respective HRLs based on the increased occurrence trends documented by available information. This finding is further supported by available dose additive impacts and co-occurrence information that demonstrates that there is a substantial likelihood that these PFAS co-occur in PWSs with a frequency and at levels of public health concern at hundreds of systems serving millions of people. Finally, EPA's preliminary determination that regulating these four PFAS presents a meaningful opportunity for health risks reductions is strongly supported by numerous bases, including the potential adverse human health effects and potential for exposure and co-exposure of these PFAS, and the availability of both analytical methods to measure and treatment technologies to remove these contaminants in drinking water. After considering these factors individually and together, EPA has preliminarily determined that now is the appropriate time to exercise its discretion under the statute to regulate the four PFAS and their mixtures as contaminants under SDWA. EPA recognizes the public health burden of PFHxS, HFPO-DA, PFNA, and PFBS, as well as PFOA, PFOS, and other PFAS, a public urgency to reduce PFAS concentrations in drinking water, and that the proposed regulation provides a mechanism to reduce these PFAS expeditiously and efficiently for regulated utilities, States, and Tribes. Furthermore, in addition to making this preliminary regulatory determination, EPA is concurrently proposing an NPDWR to include all four of these PFAS, in part to allow utilities to consider these PFAS specifically as they design systems to remove PFAS and to ensure that they are reducing these PFAS in their drinking water as effectively and quickly as feasible, maximizing the protection of drinking water for the American public. F. Request for Comment on EPA's Preliminary Regulatory Determination for PFHxS, HFPO-DA, PFNA, and PFBS EPA specifically requests comment on its preliminary regulatory determination for PFHxS, HFPO-DA, PFNA, and PFBS and their mixtures. In particular, EPA requests comment on whether there is additional health information the Agency should consider as to whether PFHxS, HFPO-DA, PFNA, and PFBS and their mixtures may have an adverse effect on the health of persons. EPA also requests comment on whether there are other peer-reviewed health or toxicity assessments for other PFAS the Agency should consider as part of this action. Additionally, EPA requests comment on additional occurrence data the Agency should consider regarding its decision that PFHxS, HFPO-DA, PFNA, and PFBS and their mixtures occur or are substantially likely to occur in PWSs with a frequency and at levels of public health concern. EPA also requests public comment on its evaluation that regulation of PFHxS, HFPO-DA, PFNA, and PFBS and their mixtures, in addition to PFOA and PFOS, will provide protection from PFAS that will not be regulated as part of this proposed PFAS NPDWR. IV. Approaches to MCLG Derivation Section 1412(a)(3) of the SDWA requires the Administrator of the EPA to propose a MCLG simultaneously with the NPDWR. The MCLG is set, as defined in Section 1412(b)(4)(A), at “the level at which no known or anticipated adverse effects on the health of persons occur and which allows an adequate margin of safety”. Consistent with SDWA 1412(b)(3)(C)(i)(V), in developing the MCLG, EPA considers “the effects of the contaminant on the general population and on groups within the general population such as infants, children, pregnant women, the elderly, individuals with a history of serious illness, or other subpopulations that are identified as likely to be at greater risk of adverse health effects due to exposure to contaminants in drinking water than the general population.” Other factors considered in determining MCLGs include health effects data on drinking water contaminants and potential sources of exposure other than drinking water. MCLGs are not regulatory levels and are not enforceable. EPA is proposing individual MCLGs for two PFAS (PFOA and PFOS; see USEPA, 2023b; USEPA, 2023c) and a separate MCLG to account for dose additive noncancer effects for a mixture of four PFAS (PFHxS, HFPO-DA, PFNA, and PFBS; see USEPA, 2023d). The derivation of the proposed MCLG for the mixture is based on an HI approach (USEPA, 2023a). The SAB, discussed further in section XV.K.1. of this preamble below, supported many of EPA's conclusions presented in the PFOA and PFOS MCLG approaches, mixtures framework, and economics benefits documents including health effects and economic benefits analyses (USEPA, 2022a). Regarding the Proposed Approaches to the Derivation of Draft MCLGs for PFOA and PFOS (USEPA, 2021e; USEPA, 2021f), SAB agreed with the selection of the UFs used in deriving the noncancer RfDs, supported the selection of an RSC of 20%, and agreed with the “likely” designation for PFOA carcinogenicity. The SAB commented that EPA should “focus on those health outcomes that have been concluded to have the strongest evidence” and “consider multiple human and animal studies for a variety of endpoints in different populations so as to provide convergent evidence that is more reliable than any single study or health endpoint in isolation.” EPA applied these recommendations when deriving points of departure and selecting critical studies used for toxicity value development in the MCLG documents for PFOA and PFOS (USEPA, 2023b; USEPA, 2023c). Specifically, EPA focused on the five health outcomes with the strongest weight of evidence—liver, immune, cardiovascular, developmental, and cancer—during quantitative analyses. However, the SAB had a number of consensus recommendations and identified “methodological concerns in the draft MCLG documents for PFOA and PFOS.” EPA has addressed these concerns by providing additional clarity and transparency on the systematic literature review process and expanding the systematic review steps included in the health effects assessment. The systematic review protocols, which were developed to be consistent with EPA's Office of Research and Development (ORD) Integrated Risk Information System (IRIS) Staff Handbook (USEPA, 2022f), are available in the Appendices of the MCLG documents for PFOA and PFOS (USEPA, 2023b; USEPA, 2023c). In order to base the MCLG derivation on the best available science, EPA has updated the draft MCLG documents to reflect the results of conducting an update to the literature search and performing new evaluations of models, methods, and data. More information is available in section XV.K.1. of this preamble. EPA expects to conduct a final literature search update before the final rule is promulgated. The SAB input has made this product more scientifically sound and ensures that it reflects the best available science. The updated supporting information can be found in the MCLG documents for PFOA and PFOS (USEPA, 2023b; USEPA, 2023c). A. Approach to MCLG Derivation for Individual PFAS To establish the MCLG, EPA assesses the peer reviewed science examining cancer and noncancer health effects associated with oral exposure to the contaminant. For linear carcinogenic contaminants, where there is a proportional relationship between dose and carcinogenicity at low concentrations, EPA has a long-standing practice of establishing the MCLG at zero (see USEPA, 1998a; USEPA, 2000d; USEPA, 2001). For nonlinear carcinogenic contaminants, contaminants that are suggestive carcinogens, and non-carcinogenic contaminants, EPA typically establishes the MCLG based on an RfD. An RfD is an estimate of a daily exposure to the human population (including sensitive populations) that is likely to be without an appreciable risk of deleterious effects during a lifetime. A nonlinear carcinogen is a chemical agent for which the associated cancer response does not increase in direct proportion to the exposure level and for which there is scientific evidence demonstrating a threshold level of exposure below which there is no appreciable cancer risk. The MCLG is derived depending on the noncancer and cancer evidence for a particular contaminant. Establishing the MCLG for a chemical has historically been accomplished in one of three ways depending upon a three-category classification approach (USEPA, 1985; USEPA, 1991a). The categories are based on the available evidence of carcinogenicity after exposure via ingestion. The starting point in categorizing a chemical is through assigning a cancer descriptor using EPA's current Guidelines for Carcinogen Risk Assessment (USEPA, 2005). The 2005 Guidelines replaced the prior alphanumeric groupings although the basis for the classifications is similar. In prior rulemakings, the Agency typically placed Group A, B1, and B2 contaminants into Category I, Group C into Category II, and Group D and E into Category III based on the Agency's previous cancer classification guidelines ( i.e., Guidelines for Carcinogen Risk Assessment, published in 51 FR 33992, September 24, 1986 (USEPA, 1986b) and the 1999 draft revised final guidelines (USEPA, 1999): • Category I chemicals have “strong evidence [of carcinogenicity] considering weight of evidence, pharmacokinetics, and exposure” (USEPA, 1985; USEPA, 1991a). EPA's 2005 Cancer descriptors associated with this category are: “Carcinogenic to Humans” or “Likely to be Carcinogenic to Humans” (USEPA, 2005). EPA's policy under SDWA is to set MCLGs for Category I chemicals at zero, based on the principle that there is no known threshold for carcinogenicity (USEPA, 1985; USEPA, 1991a; USEPA, 2016d). In cases when there is sufficient evidence to determine a nonlinear cancer mode of action (MOA), the MCLG is based on the RfD approach described below. • Category II chemicals have “limited evidence [of carcinogenicity] considering weight of evidence, pharmacokinetics, and exposure” (USEPA, 1985; USEPA, 1991a). EPA's 2005 Cancer descriptor associated with this category is: “Suggestive Evidence of Carcinogenic Potential” (USEPA, 2005). The MCLG for Category II contaminants is based on noncancer effects (USEPA, 1985; USEPA, 1991a) as described below. • Category III chemicals have “inadequate or no animal evidence [of carcinogenicity]” (USEPA, 1985; USEPA, 1991a). EPA's 2005 Cancer descriptors associated with this category are: “Inadequate Information to Assess Carcinogenic Potential” and “Not Likely to Be Carcinogenic to Humans” (USEPA, 2005). The MCLG for Category III contaminants is based on noncancer effects as described below. For chemicals exhibiting a noncancer threshold for toxic effects ( e.g., Category II or III; e.g., see USEPA, 1985 and USEPA, 1991a) and nonlinear carcinogens ( e.g., see USEPA, 2006a), EPA establishes the MCLG based on a toxicity value, typically an RfD, but similar toxicity values may also be used when they represent the best available science ( e.g., ATSDR Minimal Risk Level). A noncancer MCLG is designed to be protective of noncancer effects over a lifetime of exposure with an adequate margin of safety, including for sensitive populations and life stages, consistent with SDWA 1412(b)(3)(C)(i)(V) and 1412(b)(4)(A). The calculation of a noncancer MCLG includes an oral toxicity reference value such as an RfD (or Minimal Risk Level), DWI-BW, and RSC as presented in the equation below: EP29MR23.060 Where: RfD 3 = reference dose—an estimate (with uncertainty spanning perhaps an order of magnitude) of a daily oral exposure of the human population to a substance that is likely to be without an appreciable risk of deleterious effects during a lifetime. The RfD is equal to a point-of-departure (POD) divided by a composite UF. 3 A reference dose (RfD) is an estimate of the amount of a chemical a person can ingest daily over a lifetime (chronic RfD) or less (subchronic RfD) that is unlikely to lead to adverse health effects in humans. DWI-BW = An exposure factor in the form of the 90th percentile DWI-BW for the identified population or life stage, in units of liters of water consumed per kilogram BW per day (L/kg/day). The DWI-BW considers both direct and indirect consumption of drinking water (indirect water consumption encompasses water added in the preparation of foods or beverages, such as tea or coffee). Chapter 3 of EPA's Exposure Factors Handbook (USEPA, 2019a) provides DWI-BWs for various populations or life stages within the general population for which there are publicly available, peer-reviewed data such as National Health and Nutrition Examination Survey (NHANES) data. RSC = relative source contribution—the percentage of the total exposure attributed to drinking water sources (USEPA, 2000c), with the remainder of the exposure allocated to all other routes or sources. EPA established internal protocols for the systematic review steps of literature search, Population, Exposure, Comparator, and Outcomes (PECO) development, literature screening, study quality evaluation, and data extraction prior to conducting the systematic review for PFOA and PFOS. However, EPA recognizes that while components of the protocols were included in the November 2021 draft Proposed Approaches documents (USEPA, 2021e; USEPA, 2021f), the protocols were only partially described in those documents. EPA has incorporated detailed, transparent, and complete protocols for all steps of the systematic review process into the Proposed MCLG documents (USEPA, 2023b; USEPA, 2023c). Additionally, the protocols and methods have been updated and expanded based on SAB recommendations to improve the transparency of the process used to derive the MCLGs for PFOA and PFOS and to be consistent with the ORD Staff Handbook for Developing IRIS Assessments (USEPA, 2022f). For additional details of EPA's systematic review methods, see USEPA (2023b, 2023c; Chapter 2 and Appendix A). EPA evaluated strengths and limitations of each study to determine an overall classification of high, medium, low, or uninformative with respect to confidence in the quality and reliability of the study (this was done for each endpoint evaluated in each study). High, medium, and low confidence studies were prioritized for qualitative assessments, while only high and medium confidence studies were prioritized for quantitative assessments. Within each health outcome, the evidence from epidemiology and animal toxicity studies was synthesized. For noncancer health outcomes, the animal toxicological and epidemiological evidence for each health outcome was classified as either robust, moderate, slight, indeterminate, or compelling evidence of no effect. The weight of evidence for each health outcome across all available evidence ( i.e., epidemiology, animal toxicity, and mechanistic studies) was classified as either evidence demonstrates, evidence indicates (likely), evidence suggests, evidence inadequate, or strong evidence supports no effect. To characterize the weight of evidence for cancer effects, EPA followed recommendations of the Guidelines for Carcinogen Risk Assessment (USEPA, 2005). Further description of the methods used to make these determinations for PFOA and PFOS is provided in USEPA (2023b; 2023c). Consistent with the recommendations of the SAB and to ensure that the rule reflects the best available science, EPA continues to evaluate the literature using systematic review methods. The approach to select the DWI-BW and RSC for MCLG derivation includes a step to identify sensitive population(s) or life stage(s) ( i.e., populations or life stages that may be more susceptible or sensitive to a chemical exposure) by considering the available data for the contaminant, including the adverse health effects reported in the toxicity study on which the RfD 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 or health outcomes may have been assessed in available studies), the critical effect and 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 within the low dose range among the available data. Evaluation of the critical study, including the exposure window or interval, may identify a sensitive population or life stage ( e.g., pregnant women, formula-fed infants, lactating women). In such cases, EPA can select the corresponding DWI-BW for that sensitive population or life stage from the Exposure Factors Handbook (USEPA, 2019a) to derive the MCLG. In the absence of information indicating a sensitive population or life stage, the DWI-BW corresponding to the general population may be selected for use in MCLG derivation. To account for potential aggregate risk from exposures and exposure pathways other than oral ingestion of drinking water, EPA applies an RSC when calculating MCLGs to ensure that total exposure to a contaminant does not exceed the daily exposure associated with the toxicity value, consistent with USEPA (2000c) and long-standing EPA methodology for establishing drinking water MCLGs and NPDWRs. 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 transfer to 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, 2000c). The purpose of the RSC is to ensure that the level of a contaminant ( e.g., MCLG), when combined with other identified potential sources of exposure for the population of concern, will not result in exposures that exceed the RfD (or Minimal Risk Level) (USEPA, 2000c). To determine the RSC, EPA follows the Exposure Decision Tree for Defining Proposed RfD (or POD/UF) Apportionment in EPA's Methodology for Deriving Ambient Water Quality Criteria for the Protection of Human Health (USEPA, 2000c). EPA considers whether there are significant known or potential uses/sources of the contaminant other than drinking water, the adequacy of data and strength of evidence available for each relevant exposure medium and pathway, and whether adequate information on each exposure source is available to quantitatively characterize the exposure profile. The RSC is developed to reflect the exposure to the general population or a sensitive population within the general population. When exposure data are available for multiple sensitive populations or life stages, the most health-protective RSC is selected. In the absence of adequate data to quantitatively characterize exposure to a contaminant, EPA typically selects an RSC of 20 percent (0.2). When scientific data demonstrating that sources and routes of exposure other than drinking water are not anticipated for a specific pollutant, the RSC can be raised as high as 80 percent based on the available data, thereby allocating the remaining 20 percent to other potential exposure sources (USEPA, 2000c). B. Approach to MCLG Derivation for a PFAS Mixture There has been a lot of work evaluating parameters that best inform the combining of PFAS components identified in environmental matrices into mixtures analyses. Indeed, there is currently no consensus on whether or how PFAS should be combined for risk assessment purposes. EPA considered several approaches to account for dose additive noncancer effects associated with PFHxS, HFPO-DA, PFNA, and PFBS in mixtures. PFAS can affect multiple human health endpoints and differ in their impact ( i.e., potency of effect) on target organs/systems. PFAS disrupt signaling of multiple biological pathways resulting in common adverse effects on several biological systems and functions, including thyroid hormone levels, lipid synthesis and metabolism, development, and immune and liver function (ATSDR, 2021; EFSA, 2018, 2020; EPA, 2023d). For example, one PFAS may be most toxic to the liver, and another may be most toxic to the thyroid but both chemicals affect the liver and the thyroid. Other chemicals regulated as groups operate through a common MOA and predominately affect one human health endpoint. This supports a flexible data-driven approach that facilitates the evaluation of multiple health endpoints, such as the HI. EPA is proposing to establish an MCLG for a mixture of chemicals that are expected to impact multiple endpoints. SDWA requires the agency to establish a health-based MCLG set at, “a level at which no known or anticipated adverse effects on the health of persons occur and which allow for an adequate margin of safety. EPA's SAB opined that where the health endpoints of the chosen compounds are similar, “the HI methodology is a reasonable approach for estimating the potential aggregate health hazards associated with the occurrence of chemical mixtures in environmental media. The HI is an approach based on dose additivity (DA) that has been validated and used by EPA” (USEPA, 2022a). This proposal is based on the Agency's finding that the general HI approach is the most efficient and effective approach for establishing an MCLG for PFAS mixtures consistent with the statutory requirement described above. This finding is based on the level of protection afforded by both the HBWCs for the individual PFAS as components of a mixture and the resulting HI itself, which provides an added margin of safety with respect to potential health hazards of mixtures of these PFAS. An HI greater than 1.0 is generally regarded as an indicator of potential adverse health risks associated with exposure to the mixture (USEPA, 1986a; USEPA, 1991b; USEPA, 2000a). A HI less than or equal to 1.0 is generally regarded as having no appreciable risk (USEPA, 1986a; USEPA, 1991b; USEPA, 2000a). The proposed MCLG is based on using this HI of 1.0, and the HBWCs of each mixture component, which in turn is based on its respective health-based reference value (RfV; RfD or MRL). Because the RfV represents an estimate at which no appreciable risk of deleterious effects exists (USEPA, 1986a, 1991a, 2000a), the use of the HBWCs means that the HI of 1.0 will ensure that there are no known or anticipated effects on the health of persons and allow for an adequate margin of safety. In addition, the resulting HI adds an additional margin of safety for mixtures of the four PFAS, to address the potential for additive toxicity where the contaminants co-occur and the HBWCs for the individual components are less than 1.0. The Agency therefore proposes the general HI approach as the basis for the MCLG, and because treatment to this level is also feasible, the MCL for these PFAS, (see additional discussion in section VI of this preamble) and welcomes public comment on its findings. EPA considered the two main types of HI approaches: (1) the general HI which allows [Text truncated at 120,000 characters. The full text is on the page linked above.] --- Source: Frix Law Library, https://www.frixlaw.com/law-library/documents/fr%3A2023-05471. Public record. Not legal advice.