A field study to assess the role of air-water interfacial sorption on PFAS leaching in an AFFF source area.

Field-deployed lysimeters were used to measure the concentrations of poly- and perfluoroalkyl substances (PFASs) in soil porewater at a site historically impacted with aqueous film forming foam (AFFF). Samples collected over a 49-day period showed that perfluorooctane sulfonate (PFOS) and perfluorohexane sulfonate (PFHxS) were the PFASs with the highest concentrations in porewater, with concentrations of approximately 10,000 and 25,000 ng L, respectively. The corresponding average mass flux to underlying groundwater observed for PFOS and PFHxS was 28,000 ± 11,000 and 92,000 ± 32,000 ng m d, respectively. Employing the use of batch desorption isotherms (soil:water slurries) to determine desorption K values resulted in an overestimation of PFAS porewater concentrations by a factor for 1.4 to 4. However, using the desorption K values from the batch desorption isotherms in combination with a PFAS mass balance that incorporated PFAS sorption at the air-water interface resulted in improved predictions of the PFAS porewater concentrations. This improvement was most notable for PFOS, where inclusion of air-water interfacial sorption resulted in a 58% reduction in the predicted PFOS porewater concentration and predicted PFOS porewater concentrations that were identical (within the 95% confidence interval) to the lysimeter measured PFOS porewater concentration. Overall these results highlight the potentially important role of air-water interfacial sorption on PFAS migration in AFFF-impacted unsaturated soils in an in situ field setting.