Coupled PFAS Transport and Seepage Models to Evaluate Retardation due to Air-Water-Interface Adsorption

The term Per- and polyfluoroalkyl substances (PFAS) refer to a class of manmade very stable chemicals popular for their ability to withstand heat and moisture. Hence, they are used in manufacturing a variety of goods and firefighting aqueous film-forming foams (AFFF). The discharge of these chemicals into the soil occurs when they are introduced into the environment through various means, such as being used in firefighting foams and similar products or when they get deposited in landfills. In addition to some levels of adsorption to a solid phase, PFAS have a propensity to adhere to the water-air interface, due to surfactant-like nature. This adsorption to air-water interfaces can complicate PFAS transportation through the vadose zone. The persistent properties and potential health risks linked to PFAS provide formidable complexity. Due to the widespread contamination of the environment caused by PFAS, scientists and environmental agencies are intensively studying the fate and flow of these chemicals within the soil environment. In this paper, two numerical models - groundwater seepage and PFAS transport - are developed to examine the impact of air-water interface adsorption on the transport of PFAS within soil's unsaturated (vadose) and saturated (groundwater) environments. For unsaturated soil and the coupled groundwater seepage and PFAS transport models, the degree of saturation is fed into the PFAS transport model from the seepage model; the results are shown through 2D snapshots at certain times that show the spatial distribution of PFAS concentration over time for the two inlet-outlet-combination scenarios. Highlighting the significant impact of air-water interface adsorption (caused by varying degrees of water saturation) on PFAS transport within the groundwater system, the coupled model emphasizes the essential importance of taking site-specific conditions and hydrogeological parameters into account when assessing the risks of PFAS contamination and devising effective remediation strategies.