Dynamics of PFAS transport in the unsaturated zone: Insight from long-term monitoring of an AFFF-contaminated site
PFAS in the unsaturated zone can remain a long-term source of groundwater contamination. A two-year Danish field study shows that precipitation and infiltration, soil composition and secondary PFAS reservoirs control mobilisation and vertical transport, with distinct behaviour for short- and long-chain PFAS.
PFAS contamination at sites where aqueous film-forming foams (AFFF) have historically been used can represent a long-term source of groundwater contamination. Kolade et al. investigated PFAS transport through the unsaturated zone at a former fire-training site in Korsør, Denmark, where both legacy PFOS-containing and fluorotelomer-based AFFF had been used. A Vadose Zone Monitoring System (VMS) was applied over two years, complemented by soil and groundwater sampling, to evaluate how hydrological conditions influence PFAS mobilisation and vertical transport.
The results demonstrate a strong relationship between precipitation, infiltration, soil moisture and PFAS mobilisation. During wetter periods, increasing water contents in the unsaturated zone promoted PFAS mobilisation and transport to greater depths. The unsaturated zone therefore behaves as a dynamic reservoir in which historically retained PFAS can be remobilised when hydrological conditions change.
Important differences were observed between individual PFAS. Longer-chain compounds, including PFOS and PFOA, showed stronger retention within the soil, whereas short-chain PFAS were generally more mobile and more readily transported with infiltrating water. PFAS migration therefore depends strongly on compound-specific properties, and total PFAS concentrations alone may not adequately describe contaminant transport behaviour.
Historical PFAS contamination remained present in both shallow soils and deeper organic-rich layers, which can function as long-term secondary PFAS sources. Calculated mass fluxes confirmed that infiltration is a major driver of vertical PFAS transport. Consequently, seasonal hydrological variability can substantially influence the amount of PFAS reaching groundwater and the concentrations observed during individual monitoring campaigns.
The study demonstrates that single soil or groundwater sampling events may provide an incomplete representation of PFAS source strength and mass flux. Investigation and risk assessment of AFFF-contaminated sites should therefore consider soil stratigraphy, organic-rich layers, soil moisture, infiltration dynamics, seasonal variability and compound-specific PFAS behaviour. The unsaturated zone should be regarded as a dynamic source–storage system capable of sustaining groundwater contamination long after the original PFAS release has ceased.
The full article is available here.
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