Multidimensional simulation of PFAS transport and leaching in the vadose zone: Impact of surfactant-induced flow and subsurface heterogeneities

PFAS are emergent contaminants of which the fate and transport in the environment remain poorly understood. As surfactants, adsorption at air-water and solid-water interfaces in soils complicates the retention and leaching of PFAS in the vadose zone. Recent modeling studies accounting for the PFAS-specific nonlinear adsorption processes predicted that the majority of long-chain PFAS remain in the shallow vadose zone decades after contamination ceases -in agreement with many field measurements. However, some field investigations show that long-chain PFAS have migrated to tens to a hundred meters below ground surface. These discrepancies may be attributed to model simplifications such as a one-dimensional (1D) homogeneous representation of the vadose zone. Another potentially critical process that has not been fully examined by the 1D models is how surfactant-induced flow (SIF) influences PFAS leaching in multidimensions. We develop a new three-dimensional model incorporating the PFAS-specific flow and transport processes to quantify the impact of SIF and subsurface heterogeneities. Our simulations and analyses conclude that 1) SIF has a minimal impact on the long-term leaching of PFAS in the vadose zone, 2) preferential flow pathways generated by subsurface heterogeneities lead to early arrival and accelerated leaching of (especially long-chain) PFAS, 3) the acceleration of PFAS leaching in high water content preferential pathways or perched water above capillary barriers is more prominent than conventional contaminants due to the destruction of air-water interfaces, and 4) subsurface heterogeneities are among the primary sources of uncertainty for predicting PFAS leaching and retention in the vadose zone.

Automated summary

PFAS fate and transport in the environment are not well understood, with recent modeling studies suggesting long-chain PFAS remain in the shallow vadose zone. Heterogeneities in the subsurface can lead to accelerated leaching of PFAS, especially in preferential flow pathways. Surfactant-induced flow has minimal impact on long-term PFAS leaching in the vadose zone.