The overarching objective of this project is to better understand factors affecting the mobility and accumulation of per- and polyfluoroalkyl substances (PFASs) in groundwater. Specifically, the research team will characterize how perfluoroalkyl acid precursor transport and biotransformation are affected by varying geochemical conditions in groundwater environments. This work will be conducted at the Joint Base Cape Cod (JBCC) site and surrounding areas that historically received PFAS inputs from Aqueous Film Forming Foams (AFFFs) used during fire-training exercises. The researchers will leverage from an ongoing collaboration with the U.S. Geological Survey for this project, using their detailed understanding of subsurface hydrology of this area and geochemical modeling expertise to inform the work. Short term objectives of the research are to: (A) Characterize the mobility of perfluoroalkyl acid (PFAA) precursors in groundwater environments and interactions with geochemical conditions; (B) To assess factors affecting biotransformation of PFAA precursors and quantify the potential rates of these reactions in natural environments; (C) To formalize understanding of PFAA precursor chemistry in groundwater environments within a reactive transport model to quantify the mobility of these compounds; (D) To employ passive samplers that will provide time-weighted average concentrations of PFASs travelling from the source unsaturated zone to the groundwater, and monthly average PFAS concentrations and correlations with seasonal and associated geochemical variations in surface waters.
The technical approach involves the development of column experiments to determine partition coefficients for PFASs, with a focus on PFAA precursors. These experiments will provide a dynamic, non-equilibrium comparison to groundwater transport without interferences from bubble formation. The research team will use these data to test the hypothesis that dissolved organic carbon enhances PFAA mobility, while non PFAS components of AFFF retard transport. A second set of experiments will measure biotransformation rates under simulated groundwater conditions to examine the roles of varying dissolved organic carbon and redox conditions. This work will test the hypothesis that biotransformation rates for some precursors are rapid and measureable experimentally. The third task involves parameterizing an existing reactive transport model for the JBCC site for PFAA precursors to better understand what scenarios act to immobilize the PFAS plume. After evaluation against field data, the model will be used to test the hypothesis that mobile iron and manganese formed under reducing conditions and a decrease in sediment organic carbon content both facilitate precursor transport. The final task will characterize the uptake of PFASs (including precursors) by passive samplers in the lab and field. This work will be used to test the hypothesis that passive sampling tubes provide time-integrated measurements of PFASs in groundwater and surface water.
Results of this project will provide broad insights into how site characteristics such as organic carbon content of sediment and soils, dissolved organic carbon and redox conditions affect PFAA precursor transport and transformation in groundwater environments. The reactive plume transport model developed as part of this work will be readily extendable to other impacted sites and the technology for PFAS precursor measurements using passive samplers will facilitate deployment and detection of these compounds in other areas. (Anticipated Completion - January 2021)