Modeling the long-term leaching of PFAS in heterogeneous vadose zones

PFAS are emergent contaminants of which the fate and transport in the environment remain poorly understood. A growing body of site investigations have demonstrated that vadose zones serve as significant long-term sources of PFAS to contaminate groundwater. As surfactants, adsorption at air–water and solid–water interfaces in soils complicates the retention and leaching of PFAS. Recent modeling studies accounting for the PFAS-specific nonlinear and rate-limited 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 observations. However, some field investigations show that long-chain PFAS have migrated to tens to a hundred meters below ground surface in the vadose zone. These discrepancies may be attributed to model simplifications such as a homogeneous representation of the vadose zone. The other potentially critical process that has not been fully examined 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) acceleration of PFAS leaching in high water content preferential pathways or perched water above capillary barriers is amplified compared to 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. In addition to the specific findings, this talk will also discuss more generally the challenges and opportunities that arise from understanding and quantifying PFAS leaching in heterogeneous vadose zones.