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

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 AawΘw 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.

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

“…Numerical simulations of PFOS (Perfluorooctanesulfonic acid, C8HF17O3S) released at a model aqueous film-forming foam (AFFF)-impacted fire training area (FTA) site and the subsequent transport and retention in the vadose zone demonstrated that PFOS can be strongly retained in the vadose zone even under highly transient variably saturated flow driven by timedependent rainfall infiltration. Shortly after, a model similar to that of Guo et al (2020) was also independently reported by Silva et al (2020aSilva et al ( & 2020b, where example simulations were presented in both 1D and 2D domains.…”

PFAS Experts Symposium 2: Key advances in poly‐ and perfluoroalkyl characterization, fate, and transport

“…The impact of air-water interfacial adsorption on retention and transport of PFOS, PFOA, and GenX in unsaturated porous media has been examined in miscible-displacement laboratory studies (Brusseau et al, 2021). Mathematical-model simulations of representative PFAS application scenarios have also illustrated the influence of air-water interfacial adsorption on PFAS retention and migration in the vadose zone (Guo et al, 2020;Silva et al, 2020). In aggregate, these studies demonstrate that air-water interfacial adsorption can be a significant source of retention.…”

Impact of Coal Mining on the Moisture Movement in a Vadose Zone in Open-Pit Mine Areas

“…[55][56][57][58][59]. Among them, HYDRAUS is a widely used software to calculate the vadose zone based on Richard's flow equation [60][61][62]. This paper applied HYDRUS software to compute the generalized model.…”

“…The current computational methods on these aspects are various, such as MODFLOW, STOMP, OpenGeoSys, TOUGH, and HYDRAUS [55][56][57][58][59]. Among them, HYDRAUS is a widely used software to calculate the vadose zone based on Richard's flow equation [60][61][62]. This paper applied HYDRUS software to compute the generalized model.…”