Salinity-driven partitioning of structurally distinct PFAS and implications for ecological risk in urbanized estuaries

Per- and polyfluoroalkyl substances (PFAS) are ubiquitously detected in water, sediment, and biota, yet comparatively few studies have examined their occurrence, behavior, and risk to estuarine/marine systems. Certain PFAS undergo "salting out" as ionic strength increases, yet effects of structure-dependent solubility on partitioning of PFAS across estuarine salinity gradients remain understudied. Importantly, this behavior could alter their bioavailability to ecological receptors at different salinities, even if total PFAS concentrations remain equal. We used an integrated laboratory and field approach to address these knowledge gaps, which included (i) benchtop experiments to assess changes in PFOS, PFNS, PFHxS, PFNA, PFOA, and GenX solubility across salinities ranging from 0 to 45 parts per thousand (‰), (ii) environmental sampling to evaluate whether the partitioning behavior observed in the laboratory corresponded to PFAS distributions in an urbanized estuary, and (iii) a screening-level ecological risk assessment (SLERA) parameterized using lab and field-collected data, in which we developed hazard quotients and indices for aquatic receptors and aquatic-dependent wildlife across the salinity gradient. Functional group identity was a significant predictor of salinity-driven solubility changes, and chain length modified the magnitude of that response, with the greatest effects observed on long-chain sulfonates (PFOS declined by 47.5% at 45‰). By contrast, the effects of salinity-driven partitioning were modest among all carboxylates (maximum decline of 9.2%), as well as the short-chain sulfonate evaluated (maximum decline of 5.1%). Field-collected samples were consistent with this phase distribution, as sediments contained almost exclusively long-chain PFAS (dominated by sulfonates), while short-chain compounds dominated seawater. The SLERA revealed a compartmental crossover in risk with increasing salinity, in which waterborne exposure declined, and sediment borne exposure increased, culminating in a hazard quotient > 1 for benthic invertebrates across the brackish-to-marine range evaluated (28 to 45‰). Results suggest that salinity-driven changes in partitioning can meaningfully alter ecological risk in a manner that would be overlooked using current exposure assessment approaches (i.e., measuring total concentrations in water and bulk tissue).