Adsorption of PFAAs in the Vadose Zone and Implications for Long-Term Groundwater Contamination.

Perfluoroalkyl acids (PFAAs) are persistent environmental contaminants that sorb to air-water and solid interfaces throughout the vadose zone. These sorption processes lead to decadal leaching of PFAS from the source zones to groundwater systems. While these processes are increasingly well understood, critical gaps exist in describing the vertically variable adsorption in the presence of vadose zone heterogeneity and methods for efficiently upscaling the laboratory observations to predict field-scale PFAA transport and retardation. In this work, we build upon fundamental theories and scalable relationships to define a semi-analytical framework for synthesizing and upscaling PFAA adsorption in heterogeneous vadose zone systems. Solid-phase and air-water interfacial adsorption are quantified mechanistically for several PFAAs and then applied to a contaminated site in Northern Wisconsin. The results highlight the dominance of air-water and organic carbon solid-phase adsorption processes in the vadose zone. Strong sorption heterogeneity─driven by depth-dependent adsorption mechanisms─produces complex spatially variable retardation profiles. We develop vadose zone retardation potentials to quantify this field-scale heterogeneity and propose vertical integration methods to upscale spatially resolved information for transport modeling. This work highlights the importance of accounting for multiscale and multiprocess heterogeneity for accurately describing and predicting the long-term fate and transport of PFAAs in the subsurface.