The Unseen Journey: How PFAS Travel Through Soil to Groundwater
When PFAS are released to the land surface—from AFFF spills, biosolids applications, industrial discharges, or contaminated stormwater—they begin a complex migration through the vadose zone (unsaturated soil above the water table) before ultimately reaching the saturated zone of the aquifer below. Understanding vadose zone PFAS fate and transport—the processes governing how quickly PFAS travel through soil, how much is retained by soil particles, and what chemical transformations occur en route—is fundamental to predicting groundwater contamination extent, assessing human and ecological exposure risks, and designing remediation strategies that address contamination at its source.
Sorption and Retardation in the Vadose Zone
PFAS transport through soil is governed by the interplay between aqueous-phase migration downward with percolating water and reversible sorption to soil mineral and organic matter surfaces that retards transport relative to water flow velocity. PFAS sorption to soil is controlled by several mechanisms: hydrophobic interactions between PFAS fluorinated chains and soil organic matter, electrostatic interactions between PFAS charged functional groups and charged mineral surfaces, and air-water interface partitioning—a PFAS-specific phenomenon where PFAS preferentially concentrate at the thin water films coating unsaturated soil particles due to their surfactant properties.
The air-water interface partitioning mechanism is particularly important in the vadose zone, where the extent of air-water interface surface area is much larger than in saturated soils. Studies have demonstrated that PFAS can be significantly retarded by air-water interface sorption in unsaturated soils—a phenomenon that slows initial transport but also creates a large reservoir of PFAS held in the unsaturated zone that can continue leaching to groundwater for extended periods as conditions change. This retardation mechanism helps explain why PFAS groundwater contamination sometimes appears years to decades after surface PFAS releases—the vadose zone acts as a slow-release reservoir.
PFAS Precursor Transformation in Soil
PFAS precursor compounds—fluorotelomer-based substances and polyfluoroalkyl substances that contain non-fully-fluorinated portions susceptible to oxidation or biodegradation—can transform to terminal PFAS products during vadose zone transport. Microbial and abiotic transformation of fluorotelomer alcohols (FTOHs), fluorotelomer sulfonates (FTSs), and other precursor compounds produces perfluorocarboxylic acids (PFCAs) including PFOA and shorter-chain analogues. This precursor transformation means that groundwater PFAS contamination can develop from surface releases of precursor compounds even in the absence of direct PFOA or PFOS releases—complicating source attribution and extending the effective contamination footprint of PFAS-containing products like AFFF.
Total oxidizable precursor (TOP) assay methods—which chemically oxidize all PFAS precursors to terminal perfluorocarboxylic acids and measure the resulting PFCAs—provide a more comprehensive picture of total PFAS burden in soil and water than target compound analytical approaches that miss precursors. Regulatory guidance is increasingly incorporating TOP assay results into site characterization frameworks, recognizing that target-compound-only analysis understates total PFAS contamination at sites with significant precursor contributions.
Implications for Remediation Design
Vadose zone PFAS dynamics have direct implications for remediation strategy selection and performance prediction. Pump-and-treat groundwater remediation systems must account for the continuing vadose zone PFAS source that will recharge groundwater even as dissolved plume concentrations are reduced by extraction. In-situ treatment approaches targeting the vadose zone—soil washing with surfactant solutions to mobilize sorbed PFAS for extraction, thermal treatment to volatilize PFAS for capture, or in-situ chemical reduction to destroy PFAS in the unsaturated zone—are receiving increasing attention as complements to groundwater-focused remediation that addresses the groundwater plume without eliminating the source. Vadose zone remediation feasibility and cost analyses are now standard components of PFAS site investigations at facilities with documented surface source zones.