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Understanding fate and transport processes for per‐ and poly‐fluoroalkyl substances (PFAS) is critical for managing impacted sites. “PFAS Salting Out” in groundwater, defined herein, is an understudied process where PFAS in fresh groundwater mixes with saline groundwater near marine shorelines, which increases sorption of PFAS to aquifer solids. While sorption reduces PFAS mass discharge to marine surface water, the fraction that sorbs to beach sediments may be mobilized under future salinity changes. The objective of this study was to conceptually explore the potential for PFAS Salting Out in sandy beach environments and to perform a preliminary broad‐scale characterization of sandy shoreline areas in the continental U.S. While no site‐specific PFAS data were collected, our conceptual approach involved developing a multivariate regression model that assessed how tidal amplitude and freshwater submarine groundwater discharge affect the mixing of fresh and saline groundwater in sandy coastal aquifers. We then applied this model to 143 U.S. shoreline areas with sandy beaches (21% of total beaches in the USA), indirectly mapping potential salinity increases in shallow freshwater PFAS plumes as low (<10 ppt), medium (10–20 ppt), or high (>20 ppt) along groundwater flow paths before reaching the ocean. Higher potential salinity increases were observed in West Coast bays and the North Atlantic coastline, due to the combination of moderate to large tides and large fresh groundwater discharge rates, while lower increases occurred along the Gulf of Mexico and the southern Florida Atlantic coast. The salinity increases were used to estimate potential perfluorooctane sulfonic acid (PFOS) sorption in groundwater due to salting out processes. Low‐category shorelines may see a 1‐ to 2.5‐fold increase in sorption of PFOS, medium‐category a 2.0‐ to 6.4‐fold increase, and high‐category a 3.8‐ to 25‐fold increase in PFOS sorption. The analysis presented provides a first critical step in developing a large‐scale approach to classify the PFAS Salting Out potential along shorelines and the limitations of the approach adopted highlights important areas for further research. [ABSTRACT FROM AUTHOR]

Understanding the retention of per‐ and polyfluoroalkyl substances (PFAS) in the vadose zone is vital to the management of impacted sites. This paper examines PFAS retention in the unsaturated zone above the water table using a mathematical model, MODFLOW‐USG‐Transport PFAS or "USGT‐PFAS." The USGT‐PFAS model incorporates adsorption onto air‐water interfaces, providing a more comprehensive understanding of PFAS retention near the water table and release to groundwater. Modeling of a hypothetical perfluorooctane sulfonic acid (PFOS) site under various idealized site conditions illustrated that the impacts on PFOS retention from smallest to largest were water table fluctuations, low episodic recharge, constant recharge, moderate episodic recharge, constant recharge with water table fluctuations, and high episodic recharge. PFOS retention also varied by sand type, with greater retention occurring in simulations incorporating coarse sand with low capillary potential versus fine sand with high capillary potential. PFAS management strategies were also explored, including the adaptation of gas sparging, a method traditionally used for volatile organic compounds. Gas sparging can concentrate PFAS in groundwater and the vadose zone around the water table, facilitating retention or removal. Model simulations for a simplified hypothetical site demonstrated that PFAS can be substantially retained in the unsaturated zone once gas sparging results in an upward concentration of PFAS in groundwater and the unsaturated zone near the water table. Modeling can aid in understanding PFAS behavior but requires simulation of multiple interrelated processes to correctly predict PFAS fate and transport in subsurface conditions. [ABSTRACT FROM AUTHOR]

Managed aquifer recharge has become a standard water resources management practice to promote the development of locally sustainable water supplies and combat water scarcity. However, installation of injection wells for replenishment purposes in urban areas with complex hydrogeology faces many challenges, such as limited land availability, potential impacts on municipal production wells and known subsurface contamination plumes, and complex spatially variable hydraulic connections between aquifer units. To assess the feasibility and cost‐effectiveness of injecting advanced treated water (ATW) into a complex urban aquifer system, a Simulation‐Optimization (SO) model was developed to automate a systematic search for the most cost‐effective locations to install new wells for injecting various quantities of ATW, if feasible. The generalized workflow presented here uses an existing MODFLOW groundwater model—along with advanced optimization routines that are publicly available—to flexibly accommodate a multiobjective function, complex constraints, and specific project requirements. The model successfully placed wells for injection of 1 to 4 MGD of ATW in aquifers underlying the study area. The injection well placement was primarily constrained by avoiding excessive impact on environmental sites with underlying groundwater plumes. The largest costs were for well installation and piping to the wells from the existing ATW pipes. This workflow is readily adaptable to other sites with different complexities, decision variables, or constraints. [ABSTRACT FROM AUTHOR]