Whenever The United States Environmental Protection Agency (EPA) comes out with a new PFAS regulatory proposal, we inevitably get a lot of questions. The National Primary Drinking Water Regulations (NPDWR) proposed for PFAS on March 14th were no exception. While the EPA held a couple of informational webinars that were highly informative, the Pace® PFAS Team wanted to offer some clarity as well.
In this webinar, our PFAS experts delved into the NPDWR proposal, providing their insights, answering frequent questions from Pace® customers, and clearing up a few points of confusion. We’ll recap some of the highlights here, but we also invite you to watch the on-demand recording.
What are the Proposed MCLs?
To set the stage, the team shared the proposed Maximum Contaminant Levels (MCLs) and discussed the surprise addition of four additional PFAS to the proposal. Lindsay Boone, M.Sc. also answered a question from the audience on whether EPA may expand the proposal to include additional PFAS. She said it was possible as EPA is conducting toxicity studies on additional PFAS. Lindsay also noted the sampling that will be done under UCMR 5 in 2023.
When proposing MCLs, EPA looks at two criteria: the likelihood the compound will be found in the nation’s public drinking water systems and the toxicity of the compound. If UCMR 5 sampling finds high levels of a specific PFAS compound and EPA has sufficient evidence of its toxicity, they are compelled to propose MCLs for that compound under the Safe Drinking Water Act (SDWA).
Proposed PFAS MCLs and MCLGs
| Compound | Proposed MCLG | Proposed MCL (enforceable levels) |
| PFOA | Zero | 4.0 parts per trillion (also expressed as ng/L) |
| PFOS | Zero | 4.0 ppt |
| PFNA | 1.0 (unitless) Hazard Index | 1.0 (unitless) Hazard Index |
| PFHxS | ||
| PFBS | ||
| HFPO-DA (commonly referred to as GenX Chemicals) |
What’s the Difference between MCLs and MCLGs?
In the webinar, Dr. Kevin Custer answered questions about the difference between MCLs and MCLGs (Maximum Contaminant Level Goals) and why the MCLGs are set at zero for PFOA and PFOS. MCLs consider both the adverse health effects of a contaminant and the feasibility of treatment technologies to remove or reduce the contaminant in drinking water to an acceptable level. As such, MCLs are the legally enforceable standard water systems must adhere to.
MCLGs, on the other hand, are non-enforceable public health goals. They are the level of a contaminant in drinking water at which no known or anticipated adverse health effects are expected. MCLGs are based solely on potential health risks without considering the technical feasibility or costs of treatment.
In their informational webinars, EPA has said that 4 ppt was the lowest level that could be reliably detected by drinking water labs across the country. This has led to a lot of questions from our customers concerning the reporting limits for the six PFAS in the proposed rule. Our current reporting limit for all six PFAS with associated MCLs is 2.0 ppt which is below the lowest set MCL of 4.0 ppt. Just like all other laboratories, our reporting level is set based on the low end of a calibration curve.
Why Use the Hazard Index?
Perhaps no element of the NPDWR proposal has spurred more questions than the Hazard Index. This tool has been used under other programs, such as CERCLA, but this will be the first time it’s used for NPDWR.
PFOA and PFOS MCLs do not use the Hazard Index as they are deemed “likely cancerous” with MCLGs of zero and MCLs individually set at 4.0 ppt. Conversely, PFNA, PFHxS, PFBA, and HFPO-DA (GenX) are assessed in combination. According to EPA, that's at least partly because these four compounds tend to co-occur in the environment. If one is detected, there's a high likelihood at least one of the others will be too. EPA’s data also suggests their toxicity is cumulative, so a water system can be below the level considered toxic for all four compounds and still be above the Hazard Index limit of 1.0. By popular request, Lindsay walked through a couple of example calculations using the Hazard Index.
What is a PQL?
If it’s not become clear already, the NPDWR has a lot of levels and limits. During her presentation on how to use the Hazard Index, Lindsay also explained Practical Quantitation Limits, or PQLs. A PQL, as defined by the EPA for this proposed rule, is the lowest concentration that can be consistently determined within plus or minus 20% of the true concentration. That's using 75% of the laboratories tested in a performance evaluation.
The EPA determined PQLs in part per trillion for each of the six compounds in the proposed rule. If your detectable level is below the PQL for any of the four PFAS in the Hazard Index, zero is used to calculate the running annual average for that compound.
But not so fast! As Lindsay explained, the EPA is considering defaulting to the trigger level rather than zero for values below the PQL. That begs the question:
What is a Trigger Level?
Trigger levels may be used to help determine sampling frequency. The EPA has proposed a trigger level at 1/3 of each MCL. So, for example, for PFOA and PFOA, the trigger level is 1.3 parts ppt individually. (1/3 of the 4.0 ppt MCL) For the combination of the four PFAS included in the Hazard Index calculation, the trigger level is 1/3 of 1.0, or 0.33.
If your detectable levels of PFAS are above zero but below the trigger levels, your monitoring requirements may be reduced. However, EPA is requesting comment on establishing alternate trigger levels at one half of the proposed MCL.
Other topics and questions covered in the webinar included:
Remember, you can watch the full webinar on-demand here. And as always, if you have questions, don’t hesitate to reach out to us.