Characterization of relevant site-specific PFAS fate and transport processes at multiple AFFF sites

The relevance of multiple poly- and perfluoroalkyl substance (PFAS) fate and transport process across multiple sites was established through high-resolution characterization of the spatial distribution of PFAS within aqueous film forming foam (AFFF)-related source areas and the downgradient plumes. The maximum total PFAS concentrations in source area groundwater at the three study sites ranged from 6 to 51 mg/L but consistently decreased by several orders of magnitude with distance from the source area at all sites, indicating that non-destructive attenuation of PFAS occurred along each flow path. The relative distribution of different PFAS classes, including zwitterionic/cationic species, provided site-specific lines of evidence for retardation due to hydrophobic, air-water interfacial, and electrostatic partitioning processes, as well as impacts from biotransformation and matrix diffusion at multiple sites. The only site where one of these processes (air-water interfacial partitioning) was not supported by the data (Site 1) was attributable to disturbance of vadose zone soils as part of historic remedial efforts. In other cases, the magnitude that these processes influenced PFAS transport reflected site-specific conditions. This included apparent salting out of PFAS at Site 2 due to its elevated groundwater salinity, which has implications for plume migration in coastal areas. In addition, PFAS was present in lower-permeability soils at each site, suggesting that longer-term retention of PFAS is occurring in these zones. The finding that multiple processes were active at site-wide scales is consistent with expectations that these are naturally occurring reactions that should be relevant at most AFFF-impacted source zones.