A Modified HYDRUS Model for Simulating PFAS Transport in the Vadose Zone

The HYDRUS unsaturated flow and transport model was modified to simulate the effects of non-linear air-water interfacial (AWI) adsorption, solution surface tension-induced flow, and variable solution viscosity on the unsaturated transport of per- and polyfluoroalkyl substances (PFAS) within the vadose zone. These modifications were made and completed between March 2019 and May 2019, and were implemented into both the one-dimensional (1D) and two-dimensional (2D) versions of HYDRUS. Herein, the model modifications are described and validated against the available literature-derived PFAS transport data (i.e., 1D experimental column transport data). The results of both 1D and 2D example simulations are presented to highlight the function and utility of the model to capture the dynamic and transient nature of the temporally and spatially variable interfacial area of the AWI (<inline-formula><math display="inline"><semantics><mrow><msub><mi>A</mi><mrow><mi>a</mi><mi>w</mi></mrow></msub></mrow></semantics></math></inline-formula>) as it changes with soil moisture content (<inline-formula><math display="inline"><semantics><mrow><msub><mi mathvariant="sans-serif">Θ</mi><mi>w</mi></msub></mrow></semantics></math></inline-formula>) and how it affects PFAS unsaturated transport. Specifically, the simulated examples show that while AWI adsorption of PFAS can be a significant source of retention within the vadose zone, it is not always the dominant source of retention. The contribution of solid-phase sorption can be considerable in many PFAS-contaminated vadose zones. How the selection of an appropriate <inline-formula><math display="inline"><semantics><mrow><msub><mi>A</mi><mrow><mi>a</mi><mi>w</mi></mrow></msub><mrow><mo>(</mo><mrow><msub><mi mathvariant="sans-serif">Θ</mi><mi>w</mi></msub></mrow><mo>)</mo></mrow></mrow></semantics></math></inline-formula> function can impact PFAS transport and how both mechanisms contribute to PFAS mass flux to an underlying groundwater source is also demonstrated. Finally, the effects of soil textural heterogeneities on PFAS unsaturated transport are demonstrated in the results of both 1D and 2D example simulations.