What did we learn about PFAS MCLs?

On April 10, 2024, EPA announced that it had finalized the Maximum Contaminant Levels (MCL) for six per- and polyfluoroalkyl substances (PFAS) under the Safe Drinking Water Act. The proposed National Primary Drinking Water Regulations (NPDWR) for these six PFAS were issued last year and required EPA to review roughly 120,000 comments from stakeholders, along with a review of the economic impact prior to rulemaking. The resulting MCLs and MCL goals (MCLG) are summarized in the table below:

Compound Final MCLG Final MCL
PFOA Zero 4.0 ppt
PFOS Zero 4.0 ppt
PFHxS 10 ppt 10 ppt
PFNA 10 ppt 10 ppt
HFPO-DA (commonly known as Gen-X) 10 ppt 10 ppt
Mixtures containing two or more of PFHxS, PFNA, HFPO-DA, and PFBS 1 (unitless)
Hazard Index
1 (unitless)
Hazard Index
Source: US EPA (https://www.epa.gov/sdwa/and-polyfluoroalkyl-substances-pfas)

Are the MCLs what we expected, or did they change?

For the most part, the final rule is what was expected, with one exception. The proposed MCLs for PFOA and PFOS did not change and remain at 4.0 ppt. The MC had also been previously proposed—this does not indicate that the goal is to just remove these PFAS from drinking water to the MCL, but that purveyors should aim to reduce the relevant contaminants until they are no longer detected to the laboratory limit of detection.

What was not expected was the issuance of individual MCLs for three other PFAS:

  • PFHxS
  • PFNA

These three had originally been proposed to be included in the rule as part of a hazard index approach only. Now they are being regulated both individually and as part of a mixture, along with PFBS. The hazard index approach is new for this type of regulation and can lead to non-compliance even if individual PFAS concentrations are found below their MCLs (see how to calculate the hazard index below). Each concentration for these three PFAS, along with PFBS, is compared to an individual health-based value, called a health-based water concentration (HBWC), and then summed. For example, a hazard index of 1.0 would mean that if three of the four compounds were detected at half of their HBWC, then their sum would be 1.5, and therefore would be above the hazard index requirement.

How is hazard index calculated?

  1. Divide the measured concentration of HFPO-DA by its health-based value of 10 ppt.
  2. Divide the measured concentration of PFBS by its health-based value of 2,000 ppt.
  3. Divide the measured concentration of PFNA by its health-based value of 10 ppt.
  4. Divide the measured concentration of PFHxS by its health-based value of 10 ppt.
  5. Sum the ratios of steps 1–4 to determine the hazard index.

What is the impact?

The overall impact to industries and water purveyors is expected to be significant. Within 3 years, approximately 66,000 public water utilities will need to complete an initial PFAS assessment, and they must remediate any exceedances within 5 years of the final MCL rule. While EPA estimates the collective cost to utilities to be $1.5 billion annually, industry estimates are much higher based on costs for compliance of not just water providers but industrial dischargers adjacent to water treatment facilities. Laboratories analyze drinking water using one of two EPA methods, but, due to their contribution to drinking water sources, other methods will need to be employed for surface water, groundwater, and wastewater, as well as for soils and other solid matrices, not to mention air. The effect of air deposition means emissions will need to be considered, which indicates many more PFAS regulations are on the way.

Trusted Partner in Enthalpy

Here at Enthalpy Analytical, we know that testing will be a crucial step for meeting these new regulations, so we have been preparing for the growing demand by investing in state-of-the-art equipment and expert scientific staffing. Staying abreast of the regulations and understanding how they impact our clients and stakeholders means that we can help guide you through the myriad of options and information so that you can maintain compliance.

Jamie Fox

PFAS Technical Program Manager, Enthalpy Analytical
Jamie Fox is a career analytical chemist with over 25 years of experience supporting environmental laboratory services as an analyst, data validator, laboratory auditor, project manager, laboratory director, and business development. His expertise lies in the ultra-trace analysis of persistent organic pollutants using high resolution GC-MS and LC-MS/MS. He has a B.S. in Biochemistry from the University of Missouri-Columbia.