Modeling per- and polyfluoroalkyl substance precursor transformation in groundwater at a well-characterized site

Perfluoroalkyl acids (PFAAs) are a class of per- and polyfluoroalkyl substances (PFAS) that are mobile in groundwater, are not known to degrade, and are highly regulated. PFAAs can be generated by the transformation of certain PFAS compounds called precursors. A semi-analytical groundwater model that considers matrix diffusion was calibrated to field PFAS data at a site known to contain both PFAAs and their respective precursor compounds. The modeling effort considered four key PFAAs: perfluorooctane sulfonate (PFOS), perfluorooctanoic acid (PFOA), perfluorohexane sulfonate (PFHxS), and perfluorobutane sulfonate (PFBS). Individual precursor compounds were grouped together according to their ability to transform into one of these four key PFAAs. Model calibration that varied source characteristics, fate and transport parameters, and precursor transformation indicated that precursor transformation could be a critical process for three of the four PFAAs: 40 years after the initial source release approximately 59%, 84%, and 87% of the PFOA, PFHxS, and PFBS plume maximum concentrations, respectively, originated from precursor transformation. In addition, precursor transformation led to PFOA, PFHxS, and PFBS plumes that were up to 17% longer than if precursor transformation did not occur. In contrast, PFOS was not substantially affected by precursor transformation due to high PFOS concentrations in the source zone compared to its precursors. Additional modeling of complete PFAS source removal indicated that the effects of matrix diffusion coupled with no natural degradation of certain PFAAs may result in PFAS plumes at this site that persist and are sustained by precursor transformation.