Modeling 1-D aqueous film forming foam transport through the vadose zone under realistic site and release conditions

Aqueous film forming foams (AFFFs) have been used to extinguish fires since the 1960s, leading to widespread subsurface contamination by per- and polyfluoroalkyl substances (PFAS), an essential component of AFFF. This study presents 1-D simulations of PFAS migration in the vadose zone resulting from AFFF releases. Simulation scenarios used soil profiles from three US Air Force (USAF) installations, encompassing a range of climatic conditions and hydrogeologic environments. A three-component mixture, representative of major constituents of AFFF, facilitated the exploration of competitive and synergistic effects of co-constituents on PFAS migration. To accurately capture unsaturated transport of PFAS in porous media, the model considers (1) surfactant-induced flow, (2) non-linear sorption to the solid phase, (3) competitive accumulation at the air-water interface, and (4) the moisture-dependence of the air-water interfacial area. Defined PFAS releases were consistent with fire training exercises, emergency responses, and accidental spills of record. Simulation results illustrate the importance of hydrogeologic, climatic, geochemical, and AFFF release conditions on PFAS transport and retention. Comparison of field observations and model simulations for Ellsworth AFB indicate that much of the PFOA and PFOS mass is associated with the air-water interface and the solid phase, which limits their migration potential in the vadose zone. Results also show that rates of migration in the aqueous phase are largely controlled by hydrogeologic properties, including recharge rates and hydraulic conductivity. AFFF spill scenarios varying in volume, concentration, and frequency reveal the importance of release characteristics in determining rates of PFAS migration and concentration peaks. Variability is attributed to non-linear sorption processes, where, contrary to simple linear partitioning formulations, transport is strongly affected by the concentration of PFAS species. Simulations also demonstrate the importance of modeling the AFFF as a mixture since competitive interfacial accumulation effects are shown to enhance the mobility of less surface-active PFAS compounds.