The US Environmental Protection Agency’s proposed rule is the latest development in its expansive whole-of-agency approach to regulating and reducing PFAS in the environment. The proposed rule is not only the first proposed enforceable federal drinking water regulation for PFAS, but, if finalized, it will be the first National Proposed Drinking Water Regulation (NPDWR) under the Safe Drinking Water Act in decades.
The US Environmental Protection Agency (EPA) released its highly anticipated proposed rule on March 14, seeking to set the first enforceable national drinking water standards for perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS). EPA’s proposed rule further seeks to regulate four other per- and polyfluoroalkyl (PFAS)—perfluorononanoic acid (PFNA), perfluorohexanesulfonic acid (PFHxS), perfluorobutane sulfonic acid (PFBS), and hexafluoropropylene oxide dimer acid and its ammonium salt (HFO-DA) (more commonly known as GenX chemicals)—as a mixture through the novel use of a hazard index (HI) calculation instead of fixed individual concentrations. EPA also proposed nonenforceable, health-based Maximum Contaminant Level Goals (MCLGs) for the six PFAS.
While EPA has issued several interim health advisories and health advisory levels over the last few years for PFOA (0.004 parts per trillion (ppt)) and PFOS (0.02 ppt), as well as final health advisory levels for PFBS (2,000 ppt) and GenX chemicals (10 ppt), health advisories are nonbinding and nonenforceable. Following the issuance of an NPDWR, EPA intends to evaluate and update or withdraw the interim and final health advisories for those chemicals.
EPA’s proposed rule identifies a maximum contaminant level (MCL) of 4 ppt or nanograms per liter (ng/L) for PFOA and PFOA, which is the lowest concentration of these chemicals that most laboratories are able to reliably detect. EPA has also described the 4 ppt level as the “lowest feasible level” for which PFOA and PFOS can be removed from drinking water. This level is also below any state-issued MCLs to date.
If finalized, the NPDWR would preempt any state-issued regulations that allow greater MCLs than those provided in the NPDWR. It also would require any state with primary enforcement responsibilities that does not have a drinking water standard to implement regulations that are at least as strict as the federal MCL. In the meantime, duly promulgated state standards remain in effect and are enforceable.
EPA also intends to set an MCL for any mixture containing one or more of PFNA, PFHxS, PFBS, and GenX chemicals by utilizing the HI and setting the MCL at no greater than 1 (unitless). In a first-of-its-kind approach to NPDWRs, water systems will be required to monitor the amount of each PFAS from this group to the associated health-based water concentration (HBWC) of each chemical (as calculated by EPA):
The comparison values (hazard quotients) of each PFAS contained within the mixture are then added, and any value that exceeds 1.0 would be considered to exceed the MCL for these PFAS. EPA states that the HI provides an indication of overall potential risk of a mixture as well as individual PFAS that are potential drivers of risk, and that an HI of 1.0 represents a level at which no known or anticipated adverse health effects could be anticipated.
EPA also published proposed nonenforceable maximum contaminant level goals (MCLGs) of zero for PFOA and PFOS, based on its determination that each PFOA and PFOS is “likely to cause cancer.” While PFOA was previously identified as a likely carcinogen by EPA, the designation of PFOS as such is new.
MCLGs are the levels at which no adverse health effects are anticipated. MCLs are set as close as possible to the MCLGs, while also taking technical feasibility and cost into account.
EPA also proposed MCLGs for PFNA, PFHxS, PFBS, and GenX chemicals using its novel hazard index approach and set those values at 1.0.
While EPA signaled its intent to stringently regulate PFAS when it released its new PFAS Health Advisory Levels in June 2022—reflecting levels for PFOA and PFOS that were orders of magnitude lower than the 2016 levels—EPA’s proposed NPDWR is a significant next step and likely the first in a series of legally enforceable standards in the agency’s continued aggressive approach to regulating PFAS and reducing PFAS in the environment to the lowest possible levels.
If promulgated, the rule will impose monitoring requirements on water systems that account for variables such as the service size of the utilities and prior testing results in determining the frequency of testing. Public water systems will also be required to provide notice to the public if detected levels exceed the relevant MCLs and to treat drinking water to reduce the levels of PFAS below the MCLs. Although by its terms the proposed rule will be limited to public water systems, the impacts of the rule will not be so focused and will be felt both upstream and downstream and across industries and environmental programs.
EPA’s proposed use of the hazard index approach to regulating PFAS also raises a number of questions and may foreshadow future actions by EPA to regulate PFAS as a class (or subclasses) rather than on a chemical-by-chemical basis. And, while a hazard index approach is not a new concept, EPA’s application and use of this methodology to set an enforceable drinking water standard certainly are.
While EPA has anticipated many potential questions by the regulated community (as reflected in the various fact sheets and FAQs posted on its website), additional questions from the regulated community concerning implementation and calculation of the HI values are nonetheless likely. Challenges to the proposed rule, including to the associated costs of implementation, the procedural mechanisms and associated justification employed by EPA in advancing its preliminary regulatory determination for PFHxS, PFNA, PFBS, and GenX in the proposed rule, and the sufficiency of EPA’s supporting science are also anticipated.
Finally, as we have seen with EPA’s various PFAS health advisories, EPA’s announced MCLGs cannot be overlooked. These goals will likely impact litigation and factor into future regulation and guidance decisions at the federal and state levels, especially as detection and treatment methods continue to advance.
EPA has 18 months from the issuance of the proposed rule to finalize a proposed NPDWR, i.e., until September 15, 2024, although the agency has indicated that it plans to finalize the rule by the end of this year. If the rule is promulgated, water systems will have three years from the date of the final rule to come into compliance with any MCLs issued.
Comments on the proposed rule will be due 60 days after publication in the Federal Register.
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 These numbers are based on the practical quantitation level (PQL), defined as “the lowest concentration of a contaminant that can be reliably achieved within specified limits of prevision and accuracy during routine laboratory operating conditions.” See EPA, Proposed PFAS National Primary Drinking Water Regulation FAQs for Drinking Water Primary Agencies, Question 6.
 The hazard quotient is calculated by dividing the measured level of each of the four PFAS in drinking water by the HBWC for each of the four PFAS. For more detail concerning the calculation of the PFAS mixture HI MCLG and EPA’s associated requests for comment, see EPA, Pre-Publication Federal Register Notice: PFAS National Primary Drinking Water Regulation, at pp. 7-8, 101-06, 113-17.
 EPA has promised to provide water systems with a web-based form that will “automatically calculate the Hazard Index.”
 Of note, only four of the PFAS identified in EPA’s proposed rule—PFOA, PFOS, PFBS and GenX chemicals—have final toxicity values. In EPA’s draft mixtures document, however, EPA suggests that the HI approach “could consider other PFAS toxicity values,” including Agency for Toxic Substances and Disease Registry (ATSDR) minimum risk levels (MRLs).” ATSDR has set MRLs for PFHxS and PFNA.