Assessing the Impacts of Soil Physical and Chemical Characteristics on PFAS Retention in Unsaturated Soils

Abstract

Per and polyfluoroalkyl substances (PFAS) are a group of compounds with widespread historic application that present a potential threat to the safety of water resources throughout the globe. Understanding the fate and transport of these compounds presents a significant challenge due to the diversity of physical and chemical characteristics within the PFAS group, the recalcitrance of the compounds under field conditions, and the unique transport mechanisms that govern the distribution of PFAS in soils and groundwater. PFAS fate and transport is largely characterized by the surface and interfacial activity brought about by the amphiphilic nature of each compound. This surface and interfacial activity results in significant retention of PFAS within the vadose zone. For a large portion of PFAS, transport through the vadose zone is controlled by a mixture of solid phase retention, air-water interfacial retention, and fluid flow dynamics. At the field scale, the impacts of each of these individual components are further complicated by their interdependence and association with other potential largescale processes.The focus of this project was to assess the coupled physical and chemical characteristics that govern PFAS adsorption to the air-water interface during unsaturated flow. Lab studies were performed to characterize the magnitude and accessibility of air-water interfacial area under the various transport conditions including lower water saturations, surfactant concentrations sufficient for surfactant induced flow, PFAS retention under various soil textures, and the impacts of structure and preferential flow. The results of these studies indicate that increased air water interfacial area at saturations relevant for field conditions drives PFAS retention and chromatographic separation of individual PFAS. The lab studies informed an assessment of PFAS distribution and leaching potential of PFAS from an aqueous film-forming foam (AFFF) application site. Results from a telescoping soil analysis ranging from 100 meters to the surface indicated 98% of the PFAS soil mass was located in the upper 1.5 meters of the vadose zone with total PFAS distributions largely confined to the upper 10 meters below ground surface. For each PFAS detected in the soil beneath the AFFF source zone, the concentration distributions, distribution characteristics, and travel depth were assessed using the spatial moment analysis. Significant chromatographic separation was observed in the center of masses for each of the PFAS components. The longest-chain (larger molar volumes) PFAS remained within the uppermost section of the core, exhibiting minimal leaching, whereas the moderate chain-length PFAS exhibited differential magnitudes of leaching. The shortest-chain (lower molar volume) PFAS accumulated at the bottom of the interval, which coincides with the onset of a calcic horizon. The second moments of the spatial distributions were well described by a normal-distribution function, indicating ideal transport behavior. The vertical spatial distribution of the PFAS within the upper 1.5 meters of soil indicated that large scale processes such as infiltration and evapotranspiration play a large role in the vertical flux of PFAS. Evapotranspiration was found to limit infiltration to a depth coincident with the calcic horizon. The vertical distribution of each PFAS was inversely related to its molar volume suggesting that the impacts of non-polar forces drive differential PFAS retention in the vadose zone.