Fate and transport of perfluorooctane sulfonic acid (PFOS) within heterogenous riparian floodplains

Per- and polyfluoroalkyl substances (PFAS) have emerged as a significant global concern due to their persistence in the environment and potential health risks. Understanding the fate and transport of PFAS in the environment is quite challenging as characteristics of the sedimentary aquifers, the PFAS itself, and the solution all can influence its behavior. This study examines perfluorooctane sulfonic acid (PFOS) fate and transport in heterogeneous riparian floodplains. We investigate the role of physical and geochemical heterogeneity on PFOS fate and transport during river stage fluctuations. Our simulations combine saturated and unsaturated flow dynamics with various transport processes which influence PFOS fate, including sorption to sediments and air-water interfaces (AWIs). PFOS behavior was dictated by sediment heterogeneity. Aqueous PFOS concentrations and PFOS at AWIs were highest with larger volume proportions of high-permeability, less sorptive sediments due to increased hydraulic connectivity within riparian floodplains. PFOS adsorbed in the solid phase was maximized with a 50:50 mix of high-permeability sediments, which promote river water infiltration, and low-permeability sediments with favorable geochemical properties for PFOS sorption. Our rigorous sensitivity analyses indicate that sediment permeability is the most sensitive parameter as compared to parameters that control adsorption to sediments and AWIs. Our results imply that accurate characterization of aquifer heterogeneity is critical to understanding PFAS fate and transport within dynamic and heterogeneous riparian floodplains.