This objective of this project is to determine whether per- and polyfluoroalkyl substance (PFAS)-impacted groundwater plumes that mix with saline groundwater near marine shorelines undergo significantly increased PFAS sorption via “salting out,” a well-known chemical process that acts on most organics but with more effect on PFAS. If this does occur, then the environmental impact of nearshore PFAS groundwater plumes on the marine environment could be much lower than currently expected.

Surface water PFAS salting-out has been observed in multiple studies where dissolved PFAS in freshwater streams and rivers are “scavenged” and sorbed to suspended solids when mixed with saline estuarine waters. This effect can increase sorption of some PFAS by a factor of eight times or more and greatly decrease dissolved phase concentrations. Despite these significant effects, PFAS salting-out in groundwater has not been reported in the scientific literature to date to the best of our knowledge. PFAS salting-out in groundwater has been suspected for some time by Navy Remedial Project Managers (RPMs) who are familiar with the behavior of PFAS plumes as they approach the shoreline; however, additional resources are needed to help understand the science behind salting-out, how to characterize salting-out in the field, and how to incorporate salting-out in conceptual site models.

Conventional groundwater fate and transport models suggest there should be minimal salting-out because the porous media in aquifers act like “billions of tiny baffles” that prevent mixing of fresh and saline groundwater. However, in actuality, when fresh groundwater approaches a marine shoreline, the amount of mixing increases exponentially due to “tidal pumping,” a relatively underappreciated phenomena but one with strong scientific basis. Overall, the project team hypothesize that the mixing caused by tidal pumping at shorelines could greatly increase the PFAS salting-out process by an order of magnitude or more compared to a no-tidal pumping scenario.

To demonstrate PFAS salting-out at shorelines, this project will conduct a high-resolution site characterization program comprised of a vertical monitoring transect extending from an existing inland monitoring well(s) impacted by PFAS to the downgradient shoreline. In this transect, the project team will obtain a number of carefully paired soil/groundwater samples, each with different salinity levels, and measure PFAS sorption. Samples will be analyzed for key individual PFAS, and precursors will be estimated based on calculations using total oxidizable precursor assay data. Data from paired samples will then be used to estimate field-based sorption partition coefficients normalized to the fraction of organic carbon in the aquifer material as a function of salinity. Mass flux calculations will be performed using site groundwater flow data and general tidal pumping modeling to determine the extent to which potential PFAS mass flux to the marine environment can be reduced by the sorption effect.

Salting-out plus tidal pumping could be reducing nearshore PFAS concentrations in groundwater naturally by sequestering PFAS via sorption before the PFAS enter sensitive bays, estuaries, or open oceans. If true, and if sorption and tidal pumping can be verified at nearshore PFAS sites, then it may be possible to rely on naturally occurring processes to manage some fraction of nearshore PFAS plumes. This could preclude the need for expensive groundwater control systems at every shoreline site, save DoD millions of remediation dollars, and more importantly, help focus scarce resources on sites that truly do pose an environmental threat to the marine environment.