Laboratory validation of a simplified model for estimating equilibrium PFAS mass leaching from unsaturated soils

Modelling per- and polyfluoroalkyl substance (PFAS) fate and transport in the vadose zone is inherently more complex than in the saturated zone due to the highly transient nature and the wetting phase saturation dependent hydraulic flux associated with the vadose zone. The chemical complexity of PFAS impart multiple partitioning processes which complicate the evaluation of PFAS transport in the vadose zone. To date, simplified screening models describing PFAS leaching have been developed to determine PFAS soil cleanup criteria in the vadose zone. Recent work has presented evidence that while PFAS transport in the vadose zone is governed by several non-equilibrium mechanisms, it is possible to predict PFAS mass flux using equilibrium modelling over month to year timescales. We hypothesized that by quantifying important equilibrium partitioning and hydraulic processes, we could simplify vadose zone leaching models for assessing mass flux from the vadose zone to the underlying groundwater. A mass flux, cell-based model which accounts for important partitioning processes (solid and air-water interfacial partitioning) and transience in hydraulic processes (water flux and water content) was developed and validated herein. Column studies were conducted under simulated rainfall conditions to provide transient hydraulic and PFAS leaching data. A HYDRUS 1-D with PFAS module model was calibrated to the hydraulic conditions of the simulated rainfall columns. Forward simulations were carried out using HYDRUS and the mass balance approximation models. The HYDRUS and mass balance approximations performed nearly identically for all PFAS, and both models predicted PFAS mass leaching within a half order of magnitude of most measured data. These results suggest that readily applicable empirical models and simplified numerical models can reasonably estimate month to year scale mass flux from the vadose zone for sites without major heterogeneity or transport non-ideality considerations.