Multiphysics Numerical Modeling for PFAS Transport within Vadose and Saturated Zones

The extent and severity of wildfires have increased around the world, necessitating a greater understanding of the consequences of wildfire and post-fire impacts on soil and groundwater. Wildfire suppression techniques like aqueous film-forming foams (AFFF) can also contaminate the soil with per- and polyfluoroalkyl substances (PFAS), which can contribute to human and environmental health concerns. PFAS are dangerous man-made chemical compounds that are persistent, mobile, poisonous, and a major cause of soil and groundwater contamination. In addition to contamination by aqueous film-forming foams, PFAS has accumulated in the environment as a result of being utilized in numerous other goods over time. PFAS are popular because of a number of physiochemical characteristics that make them beneficial in a range of products and industries, furthering their spread. There are several uncertainties about the fate and transport of PFAS in unsaturated zones, as well as how the subsurface groundwater is impacted. This is because of PFAS’s tendency for biotransformation, bioaccumulation, and partitioning, as well as persistence in the environment due to their robust C-F bond. Therefore, concerns are raised about their fate, transport, and adverse impacts on the ecosystem, people, and other biota. In this research, the fate and transport of PFAS (specifically Perfluorooctane Sulfonic acid, PFOS) in both saturated and unsaturated zones are investigated through numerical modeling using the finite-difference method. This study investigates the effect of various transport processes (advection, diffusion, and adsorption) on the fate of PFAS in soil and groundwater. The numerical model is developed to simulate the transport of PFAS in the vadose and saturated soils. After development, the sensitivity of the model results to the spatial and temporal discretization (i.e., selection of time, dt, and space, dz) resolution was analyzed. The results demonstrate very low (less than 2%) sensitivity to dt in the range of 2 to 20 seconds (actually tested at 2, 5, 10, and 20 seconds) and to dz in the range of 0.001 to 0.02 m. (actually tested at 0.001, 0.0025, 0.005, 0.0075, and 0.01 m), respectively. To qualitatively test and verify the model and comprehend the fate and mobility of PFAS in both vadose and saturated zones, a number of scenarios were then explored using the model.