Simulating PFAS transport in effluent-irrigated farmland using PRZM5, LEACHM, and HYDRUS-1D models

Application of wastewater effluent to agricultural lands can serve as a sustainable approach to meet irrigation and nutrient needs for crop production. While nitrogen and phosphorous loadings can be effectively managed, concerns have been raised regarding the fate of emerging contaminants, including per- and polyfluoroalkyl substances (PFAS), which are widely detected in wastewater effluent. The objective of this paper was to evaluate the ability of three unsaturated flow and transport models, Pesticide Root Zone Model 5 (PRZM5), LEACHM, and HYDRUS-1D, to predict the distribution of PFAS in the soil profile at the Pennsylvania State University Living Filter site, which has received daily wastewater effluent applications for several decades. The models were modified to include adsorption at the air-water interface (AWI), which has been shown to be an important factor governing PFAS transport and phase distribution in the vadose zone. Simulations showed that PRZM5 did not accurately reproduce the observed perfluorooctanesulfonic acid (PFOS) behavior, which was attributed to the "tipping bucket" approach used for water flow that results in the disappearance of AWI during water flow. In contrast, both LEACHM and HYDRUS-1D captured the observed retention of PFOS and perfluorooctanoic acid (PFOA) over a 50-year simulation period. Due to differences in the approach used to calculate the AWI area, LEACHM predicted greater accumulation of PFOS and PFOA at the AWI compared to HYDRUS-1D. These findings indicate that mathematical models that directly account for unsaturated water flow and adsorption at the AWI are able to provide reasonable predictions of long-term PFAS leaching resulting from land application of wastewater effluent.