Modeling PFAS Leaching to Groundwater

(updated June 16, 2021)

As discussed on these webpages, the major current concern being raised related to PFAS and biosolids and residuals management is the potential for leaching of traces of PFAS from soils where these materials are applied. State regulators and biosolids managers are interested in establishing screening values for PFAS in biosolids. However, the science is still limited; for example, sorption values (Koc, Kd) reported in the literature vary widely. PFAS behave differently, and more research is needed.

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NEBRA had been looking at ways to advance understanding of potential PFAS leaching. When states in New England began to take dramatic regulatory steps related to PFAS in the spring of 2019, including Maine’s requirement for testing for PFAS in biosolids and soils using screening values that NEBRA had critiqued and found to be inappropriate for biosolids, NEBRA moved forward in contracting initial formal, robust modeling.

In late July, 2019, Stone Environmental of Montpelier, VT delivered to NEBRA a memorandum of initial results. They used a formal, widely-accepted model (Pesticide Root Zone Model, PRZM) to estimate migration to groundwater of PFOA and PFOS (the two most prominent PFAS chemicals and the ones most regulated), providing conservative (worst-case) inputs to the model that were tailored for Maine. The memo, and a slide presentation and poster presenting the same information, are provided for download below. NEBRA provides this information as a starting point, with the caveat that these are preliminary results and are based on highly conservative assumptions – most significantly: using the lowest lab Kd values from the literature. Presumably, more precise inputs coming from research will provide greater confidence and precision. In the meantime, we are seeing where protective, conservative interim screening values for PFAS might be. Use of such values in the short term, as research continues, may be useful.

“I have been conversing with Paul Sanders of the NJ DEP that is also on expert at Sesoil and AT123D. I asked Paul why NJ does not have soil leaching values?

According to Paul, Sesoil needs a Koc value and the model to derive the Koc is for nonpolar organics. PFAS compounds are like soaps. They have an ionized end and a nonpolar tail, thus the Koc model does not work. There also is a method to derive a Kd value that can be used in Sesoil, but that model also has issues with PFAS compounds. Paul said that he recently saw a DoD request for proposals to develop a method/model to derive a Koc for PFAS. How long that will take….

My understanding is that at this time, we will not be using Sesoil/AT123D to derive soil leaching values. I’m not sure how this will affect your program to deal with PFAS in residuals.”
— David B. Larson, M.P.H, Health Risk Assessor, NH DES

In March 2020, Arcadis, NCASI, NACWA, WEF & AFPA published a review of models for evaluating PFAS fate and transport in land-applied biosolids and residuals systems. In June, an updated version came out (V. 1.1). It notes the value of PRZM modeling and included it in its second tier of acceptable models, noting it is best for screening purposes. The primary recommended models are more complex and perhaps more thorough; they also cost something, while PRZM is available at no cost and is widely used in permitting through U. S. EPA. Download the report.

When the initial version of the report was published in March 2020, Stone Environmental provided NEBRA the following comments on the initial version of the report, which gave minimal consideration to PRZM: “The reasons for rejecting PRZM as a model to study potential PFAS leaching appear incorrectly justified….” See their full comments here. Discussions between the report authors and Stone Environmental led to the updated V.1.1.

In May 2021, Stone Environmental provided NACWA, NCASI, and AF&PA with a guidance document for using the PRZM modeling to estimate leaching of PFAS from biosolids. This builds on the prior work Stone Environmental completed for NEBRA. This guidance is helpful for biosolids managers and regulators. Download this modeling guidance here.

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