Abstract
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 (Aaw) as it changes with soil moisture content (Θw) 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 Aaw(Θw) 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.
Highlights
This paper presents a robust vadose-zone numerical model that can simulate the transport and reactions of per- and polyfluoroalkyl substances (PFAS) under dynamic vadose zone conditions.Such modeling is useful for the characterization of PFAS fate and for implementing vadose zone and groundwater remedial activities
There is a limited number of published unsaturated transport data for PFAS that could be used to validate the performance of the modified model [23,24,37]
The HYDRUS unsaturated flow and contaminant transport model was modified to include the effects of air-water interfacial (AWI) adsorption of PFAS and its dependence on solution concentration
Summary
This paper presents a robust vadose-zone numerical model that can simulate the transport and reactions of per- and polyfluoroalkyl substances (PFAS) under dynamic vadose zone conditions. We demonstrate the utility of the HYDRUS unsaturated flow and solute transport simulator that was recently modified to include the transport of PFAS in the vadose zone [2]. HYDRUS was modified to include air–water interfacial adsorption as a source of retention during transient variably saturated flow, and retains its original capabilities with regard to simulating the effects of solid-phase sorption (i.e., linear and non-linear sorption models) as a concurrent source of PFAS retention. The modified model retains its original nonequilibrium contaminant flow and transport capabilities (e.g., mobile/immobile water, dual-porosity models, and dual-permeability models) and soil sorption kinetics modeling capabilities (e.g., one- and two-site kinetics and two-site sorption models) Another model was presented in the very recent literature [30]. 2019 [2], has the additional ability to simulate scenarios of PFAS transport that are important for actual site evaluations (e.g., complex spatially variable source term/strength scenarios, structured and texturally heterogenous flow and transport domains, 2D representations)
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