Multi-compound PFAS transport in the unsaturated zone during infiltration cycles

Per- and Polyfluoroalkyl substances (PFAS) are persistent environmental contaminants known for their long-term retention in subsurface environments. Simultaneous transport of long- and short-chain PFAS (PFOS, PFOA, PFHxA, and PFPeA) was examined in a large (3 m) column designed to simulate transient flow conditions through episodic infiltration cycles in the unsaturated zone. Results indicate that short-chained PFAS (PFHxA and PFPeA) exhibited high mobility and minimal retardation, closely matching the transport of a conservative tracer (Bromide). In contrast, long-chained PFAS (PFOS & PFOA) displayed strong retention, with breakthrough curves showing significant delays and concentration fluctuations corresponding to wetting and drainage cycles. Fluctuations in sediment water content, which are attributed to erratic infiltration events, influence PFAS adsorption-desorption dynamics, particularly through solid-phase and air-water interfacial sorption. Flow and transport modeling, which involved equilibrium-based solid phase and air-water interfacial adsorption parameters (K_AWI and K_d), underestimates the complex transport of long-chained compounds. The simulations suggest that traditional equilibrium-based models do not fully capture the observed transport under non-steady flow conditions. Accordingly, a conceptual model is proposed to explain the interaction between PFAS partitioning and dynamic water content variations, showing how wetting and drainage cycles affect PFAS transition from the solid to the mobile phases. These findings highlight the crucial role of infiltration-driven processes on PFAS mobility and the need to incorporate non-equilibrium sorption kinetics under transient flow conditions in unsaturated zone modeling. This has important implications for understanding the fate and migration properties of PFAS in the subsurface, providing insights into remediation strategies in contaminated sites.