Influence of chain length on field-measured distributions of PFAS in soil and soil porewater

Soil and porewater concentrations measured for multiple PFAS were compiled from three field studies. The soil:porewater concentration ratios were shown to be functions of molar volume for all three data sets. Remarkable consistency was observed between the three sets of field-based measurements, indicating that PFAS distributions in the three soil systems exhibited similar magnitudes of overall retention. The relative contributions of solid-phase sorption and air-water interfacial adsorption to total retention were examined. The contribution of air-water interfacial adsorption was greater than that of solid-phase sorption for the longer-chain PFAS, whereas it was less than that of solid-phase sorption for the shorter-chain PFAS. These results show that the relative contributions of the two processes can vary as a function of the particular PFAS when the solid-phase sorption functionality deviates from that of air-water interfacial adsorption. This might occur for example when sorption is influenced by addition mechanisms beyond hydrophobic interaction, or when sorption and/or adsorption are nonlinear. Based on the results from all three data sets, soil concentrations are likely to be smaller than porewater concentrations for the shortest-chain PFAS. Conversely, soil concentrations will generally be significantly greater than porewater concentrations for longer-chain PFAS. The results from this study have implications for characterizing and evaluating PFAS distributions in vadose-zone soils.