PFAS conceptual site model of an AFFF-impacted firefighting training area informed by high resolution soil, porewater, and groundwater sampling

High spatial resolution sampling of vadose zone soil and porewater as well as near downgradient groundwater at an aqueous film forming foam (AFFF)-impacted former firefighting training area (FTA) was conducted to enable more accurate conceptual site models and mass distribution assessments of poly- and perfluoroalkyl substances (PFASs). The total mass and distribution of all individual detectable PFASs in the source zone was determined from the compiled data set, which included 28 soil borings with multi-depth PFAS samples, 17 porous cup suction lysimeters sampled 3 times over 8 months, and 8 shallow groundwater wells sampled 3 times over 8 months. PFAS analyses consisted of both target (quantified) and suspect (semi-quantified) analytes. Results showed that PFASs varied in their vertical and horizontal distribution within the FTA, and that a majority of the zwitterionic and cationic precursors were retained in shallow soils. While perfluoroalkyl acid (PFAA) precursors comprised 61 % the PFASs in collected soil sample, PFAAs were by far the dominant PFAS class within both porewater and near downgradient groundwater (78 % and 87 %, respectively). PFAS porewater concentrations generally increased with increasing soil moisture content, and these increases were reasonably described for perfluorooctane sulfonate (PFOS) by an equilibrium-based mass balance model that accounted for PFAS accumulation at the air-water interface and soil-water partitioning. PFAS porewater concentrations located within or adjacent to low permeability soils showed greater increases with moisture content than in porewater associated with sandy materials, likely due to the larger role of air-water interfacial area in PFAS phase distribution in these fine-grained soils. Bench-scale batch desorption tests conducted with site material reveal a significant PFAS mass that is not readily desorbed from the soil. Observed K_d values measured in the field are comparable to the bench-scale data if this desorption-resistant PFAS mass is considered. These novel findings at the scale of an FTA highlight potential challenges with evaluating PFAS porewater concentrations and leaching in heterogeneous unsaturated soils.