Effects of transient air-water interfacial area on PFOA transport in unsaturated soil

Per- and polyfluoroalkyl substances (PFAS) may be retained in the vadose zone for extended periods due to adsorption on soil and at the air-water interface (AWI). While adsorption to the AWI has been shown to slow PFAS leaching, the magnitude of interfacial area can fluctuate with precipitation events, leading to its reduction or collapse. The objective of this study was to investigate PFAS mass transfer from the AWI into mobile pore water in response to changes in water saturation, and to determine the corresponding effects on PFAS transport through the soil profile. To quantify these processes, a combination of batch experiments and water-saturated and unsaturated column studies were performed with perfluorooctanoic acid (PFOA) and Appling soil. At 56% water saturation, the effluent PFOA breakthrough curve exhibited greater spreading than expected under equilibrium conditions, indicating rate-limited adsorption-desorption at the AWI. Under transient conditions, where the average water saturation increased from approximately 60% to 82%, the corresponding decrease in the AWI area led to a spike in effluent PFOA concentrations. These observations were accurately reproduced by a mathematical model that incorporated (1) symmetric rate-limited adsorption/desorption to/from the AWI, (2) regions of immobile water, as determined from non-reactive tracer data, and (3) a dynamic AWI area, as estimated using the Leverett model with a scaling factor. This work demonstrates the potential for precipitation events to temporarily increase PFAS leaching due to a reduction or collapse of the AWI and shows that these fluctuations can be predicted using an existing multiphase reactive transport simulator.