Dry-wet and freeze-thaw cycles enhance PFOA leaching from subsurface soils

Subsurface soil naturally experiences dry-wet and freeze-thaw cycles, which could affect the leaching of previously adsorbed pollutants. A slow release of poly- and perfluoroalkyl substances (PFAS) from impacted subsurface soil may serve as a long-term diffuse source of PFAS to groundwater. Yet, the extent to which these weathering conditions may affect the subsurface release of PFAS is unknown. We subjected columns packed with soil pre-adsorbed with perfluorooctanoic acid (PFOA) to dry-wet and freeze-thaw cycles and observed a spike in PFOA concentration in leachate following each weathering treatment compared to no weathering treatment. Weathering conditions released a high concentration of soil colloids, which were confirmed by particle-size distribution analysis, SEM-EDS, and XRD. Fractionation of PFOA in the water sample reveals that up to 36 % of leached PFOA was associated with soil colloids. Thus, previous studies that did not account for colloids might have underestimated the leaching of PFAS from the soil. Overall, the results indicate that natural weathering conditions can enhance subsurface leaching of colloids and colloid-associated PFOA. Therefore, current conceptual site models to quantify the leaching of PFAS from source zones should account for weathering and the contribution of colloids.