Thin water films in controlling PFAS fate and transport in soils: Interfacial processes, pore-scale modeling, and upscaling

PFAS are emerging contaminants that have been widespread in the environment. A growing body of site investigations suggests that PFAS have accumulated significantly in soils at contamination sites, threatening to contaminate the groundwater underneath. Quantifying PFAS leaching in soils and mass discharge to groundwater is therefore critical for characterizing, managing, and mitigating long-term contamination risks.

Many PFAS are surfactants that adsorb at air–water and solid–water interfaces, which leads to complex retention of PFAS in soils. Soils have abundant air-water interfaces (AWI), which generally consist of two types: one is associated with the bulk water between soil grains (i.e., bulk AWI) and the other arises from the thin water films covering the soil grains. The latter contributes to over 90% of AWIs in soils under many field-relevant wetting conditions. This talk will discuss two unique complexities introduced by thin water films for PFAS fate and transport in soils: 1) slow mass transfer along the thin water films affects the accessibility of the water films and the film-associated AWI by PFAS; and 2) the interactions between the solid surface and air-water interface change the chemical potential and adsorption capacity of PFAS at the air-water interface. Both phenomena can substantially modify the overall retention and transport behavior of PFAS in soils, which have important implications for quantifying the risks of PFAS contamination to groundwater at the field scale.