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Regulatory Update

Proposed MCLGs and MCLs for PFAS

Cat Smith | March 15, 2023


On March 3, 2021, the Environmental Protection Agency (EPA) published a Final Regulatory Determination1 for Contaminants on the Fourth Drinking Water Contaminant Candidate List (86 FR 12272). There were 109 contaminants on the Fourth Contaminant Candidate List, and of those, EPA made final regulatory determinations for eight of those contaminants. Of those eight, only two, perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA), were determined to warrant further regulation following the process in the Safe Drinking Water Act (SDWA). Statutory criteria for pursuing a National Primary Drinking Water Regulation (NPDWR) are as follows:

(a) The contaminant may have an adverse effect on the health of persons;

(b) The contaminant is known to occur or there is a substantial likelihood that the contaminant will occur in public water systems with a frequency and at levels of public health concern; and

(c) In the sole judgment of the Administrator, regulation of such contaminant presents a meaningful opportunity for health risk reduction for persons served by public water systems.

After making the determination that the three criteria apply, as the EPA did for PFOS and PFOA2, the EPA was required to propose a regulation within 24 months, by March 3, 2023, and take action on a final regulation within 18 months of the proposal.

Not sure if you facility is compliant?

TRC can assist with water quality and regulatory compliance.

On March 14, 2023, EPA released a prepublication version of the proposed rule for setting a maximum contaminant level goal (MCLG) and maximum contaminant level (MCL) for PFOS and PFOA as follows: 

Non-enforceable Lifetime Drinking Water Health Advisory (ng/L) Non-enforceable MCLG (ng/L) Enforceable MCL  (ng/L)
PFOS 0.02 0 4.0
PFOA 0.004 0 4.0
ng/L – nanograms per liter (or parts per trillion)

As part of this proposed rule, EPA is intending to use a Hazard Index (HI) approach 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), to account for the additive effect since PFAS compounds tend to occur together. EPA has proposed an HI of 1 as the MCLG for each of these four PFAS compounds and mixtures containing one or more of them.  (PFAS HI MCLG Public Review Draft 09 March 2023.pdf (

NOTE: EPA suggested that setting an MCL equal to 1 for PFHxS, HFPO-DA / GenX and its ammonia salts, PFNA and PFBS was “feasible”, and is specifically soliciting comments on this suggestion.

The HI is calculated using the health-based water concentrations3 (HBWCs) established by EPA as follows:

HFPO-DA (GenX) – 10 ng/L

PFBS – 2000 ng/L

PFNA – 10 ng/L

PFHxS – 9 ng/L

HIs >1 represent a potential concern.

Implications: The PFAS (PFOA/PFOS) MCL rule cleared the Office of Management and Budget (OMB) on March 3, 2023. The official comment period will not start until the document is published in the Federal Register.

  • Once an MCL is established, public water systems generally must comply with the MCLs three years after they are finalized. EPA may allow up to two additional years for compliance if capital improvements are needed. States can also add two additional years on a system-by-system basis.
  • State regulations cannot be less stringent than Federal regulations. If a State has a lower MCL, they can enforce that within the State.
  • Many times, states also adopt MCLs as the default cleanup level for groundwater.
  • If a facility/plant is considered a public water supply by virtue of having onsite wells that supply potable water to the facility/plant, the MCLs will apply to that source also when finalized.
  • Industries should consider whether their effluent discharges into potentially potable water sources (which may not necessarily be drinking water sources but may feed into drinking water sources).

What Can You Do to Prepare?

  • Use this website to locate your facility and review available PFAS data.
  • Comment on the rule. Section XIV of the proposed rule specifies which items EPA is soliciting comment on.
  • Interact with and participate in industry-specific advocacy groups.

TRC can help you:

  • Review facility potable water sources, whether onsite or supplied from nearby, to determine if your facility has the potential to be impacted by PFAS MCLs.
  • Review your potential for inputs of PFAS, historical or current, to drinking water systems and develop a plan to address these.

For more information, please contact Cat Smith, REM;

1 A regulatory determination is a decision about whether or not to begin the process to propose and promulgate a national primary drinking water regulation for an unregulated contaminant.

2 While the Agency is not making regulatory determinations for additional PFAS at this time, the Agency remains committed to filling information gaps, including those identified in the PFAS Action Plan, by completing peer reviewed toxicity assessments and collecting nationally representative occurrence data for additional PFAS to support future regulatory determinations as part of the Unregulated Contaminant Monitoring Rule (UCMR) monitoring program (86 FR 12272).

3The HWBCs for Gen-X and PFBS are equivalent to the current EPA drinking water Health Advisories for these compounds.  The HWBCs for PFNA and PFHxS were calculated based upon the ATSDR’s minimal risk levels (MRLs) for these compounds.

Catriona Smith

Catriona (Cat) Smith has over 30 years of professional experience in the interpretation and application of regulations for industrial facilities relating to compliance, remediation and negotiations with regulatory agencies. Her clients include upstream and downstream oil and gas, chem-pharma, petrochem, manufacturing and federal, state, and local governments. Ms. Smith is a Vice President and Environmental Sector Market Director for TRC, with nationwide responsibilities to manage our clients’ PFAS challenges, support and grow our Oil and Gas clients and bring multi-disciplinary solutions to their complex projects. Contact Cat at

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