Dynamics of PFAS transport in the unsaturated zone: Insight from long-term monitoring of an AFFF-contaminated site

Per- and polyfluoroalkyl substances (PFAS) are persistent contaminants that threaten groundwater resources and human health, particularly at firefighting training sites where aqueous film-forming foams (AFFFs) have been extensively used. This study investigates PFAS fate and transport in the unsaturated zone (UZ) at a firefighting training site in Korsør, Denmark, where legacy PFOS-based and subsequent fluorotelomer-based foams were historically applied. PFAS transport dynamics were evaluated using a vadose zone monitoring system (VMS), complemented by periodic soil and groundwater sampling. Two years of continuous monitoring of sediment water content and PFAS concentrations revealed a strong relationship between seasonal infiltration and PFAS mobilization. Elevated concentrations of both short- and long-chain PFAS were observed during wet periods with increased infiltration and UZ water content. During drier periods, long-chain PFAS, particularly PFOA and PFOS, decreased markedly in UZ porewater, indicating stronger retention. Soil sampling at the end of the monitoring period showed that legacy PFAS persisted in both shallow and deeper organic-rich layers, representing long-term sources of groundwater contamination. PFAS mass-flux estimates further demonstrated that infiltration-driven hydrological forcing regulates contaminant transfer through the vadose zone and contributes to sustained PFAS loading to underlying groundwater over time.