After several months of initial monitoring for PFAS under the National Primary Drinking Water Regulations (NPDWR), water systems across the country continue to ask questions about how to interpret these rules, especially the Hazard Index calculation. Late last month, I did a national webinar in which I drilled down into some of the more confusing aspects of the PFAS MCLs and the initial monitoring period. This webinar is now available on-demand:
Watch: Unlock PFAS Maximum Contaminant Levels (MCLs) – What, When, How
As usual, we got several new questions during the Q&A portion of the webinar, so I wanted to share answers to some of them here. I’ll also add some of the frequently asked questions I covered during the presentation. If you have additional questions, don’t hesitate to reach out to us.
Why did the EPA include PFBS in the Hazard Index but not assign it an individual MCL?
Prior to finalizing the primary drinking water rules for PFAS, the EPA had four compounds in the hazard index that were not assigned individual MCLs. In the final rule, all but PFBS were assigned individual MCLs. While I do not believe they have publicly shared their reasoning, it is probable the agency did not feel it had enough data to assign an individual MCL to PFBS prior to rule finalization.
In the PFAS MCL webinar, you went through several examples of using significant figures in compliance monitoring. Should we also be rounding our quarterly results?
No, your quarterly Hazard Index values should not be rounded. Since it is only the Running Annual Average (RAA) that determines an exceedance, the significant figures requirements only apply to the RAA.
What is a PQL, and how does it impact my results?
The Practical Quantitation Limit (PQL) is the lowest level at which the quantity of the substance can be reliably measured, taking into account routine laboratory precision and recovery. Each compound has its own PQL: PFOA (4 ppt), PFOS (4 ppt), PFNA (4 ppt), PFHxS (3 ppt), HFPO-DA (5 ppt), PFBA (3 ppt). If your results are less than the compound’s PQL, you would use a value of zero in your RAA or Hazard Index calculation.
In our webinar, we used an example where a water system had quarterly results for PFOA of 2.0, 1.5, 5.0, and 1.5 ng/L. Because the PQL for PFOA is 4 ppt, these results are converted to 0.0, 0.0, 5.0, and 0.0 for the purposes of the calculation. Averaging these, we get 1.25, but because the reporting requirement for PFOA is two significant figures, the RAA is reported as 1.3. The PQL is NOT applied to the RAA.
We sampled twice in Q3 and got slightly different results. Which value do we use for our RAA?
You use both sets of results and divide by five to get your RAA.
We conducted four quarters of initial monitoring and none of the compounds were over the MCLs. Are we done monitoring?
No. If the results were between the Trigger Levels (half the MCLs) and the MCLs then annual sampling must be done. If results were below the Trigger Levels, then the system can apply to sample on a triennial basis.
Should private well owners test for PFAS?
EPA mandates for the analysis of PFAS in drinking water do not cover private wells, as it has no purview over private well water. However, private wells are just as susceptible to PFAS contamination as other drinking water systems, so testing private wells in areas found to have high levels of PFAS may be warranted. We recommend using the EPA’s PFAS Analytical Tools to see whether drinking water systems in areas surrounding your property have been found to contain PFAS. If levels exceed the national primary drinking water limits or if the well may be impacted in any way by upstream sources of PFAS contamination such as a fire training facility, private well testing is a good idea.
Learn more about Pace® PFAS testing services for private wells.
Got Questions?
Again, don’t hesitate to contact us if you still have questions. On the surface, the PFAS MCLs can seem straightforward, but as they say, the devil is in the details.
