The overall objective of this project is to demonstrate the application of a high resolution passive profiler (HRPP) to provide high-resolution delineation of per- and polyfluoroalkyl substances (PFAS) in groundwater aquifers. The project team will demonstrate the ability of the HRPP to produce high (< 20 cm) resolution vertical profiles of PFAS (type and concentration) as well as groundwater velocity, geochemical conditions, and co-occurring chemicals for three types of occurrences: 1) source areas, 2) along plume transects, and 3) groundwaters discharging to surface water. Using these scenarios, the project team will demonstrate the ability of the tool to evaluate the impact of the air/water interface and occurrence of other chemicals on PFAS distribution and flux, identify potential for storage and release from source zones in heterogenous horizontal layered geologic formations, evaluate the differential transport of PFAS down-gradient of source zones based on properties of the geologic formation, and identify discharge points of PFAS surfacing from groundwater to surface water. The project team will also evaluate the ability of the HRPP to delineate the role of precursors as a continuing source of perfluoroalkyl acids. The HRPP tool is expected to provide high-resolution data on the distribution and flux of relevant PFAS, which will have applications for site assessment, effective site-specific transport models, risk-based evaluations and site prioritization, and development and assessment of remedial approaches.

The HRPP is a passive sampler, developed, refined, and field validated during SERDP project ER-2419, which can be direct-driven into the saturated subsurface using a standard Geoprobe. The sampler can evaluate at fine-scale (~ 20 cm intervals) the following:

1. chemical of concern types and concentrations (e.g., chlorinated volatile organic compound);
2. the concentrations of biogeochemical species (e.g. dissolved organic carbon, Cl^- , NO₃^- , NO₂, Fe²⁺ , Fe_T, SO₄^2- , S^-2 , CH₄ etc.);
3. pore water velocity;
4. the composition of relevant microbial communities via quantitative polymerase chain reaction analysis of Bio-Sep beads; and
5. biotic and/or abiotic degradative activity via compound specific isotope analysis (CSIA) of chemicals in solution or adsorbed to Bio-Sep beads.

For this project, the project team will focus on the initial three parameters, since CSIA and microbial community structure are currently not deemed relevant for PFAS-impacted sites. PFAS can be measured in the equilibrated ~ 10 mL volumes produced by each duplicate cell/reservoir (at each depth) without using a sorptive matrix with limit of quantification of < 10 ng/L using large volume, direct injection. The HRPP uses equilibrium diffusional sampling to evaluate dissolved species in the porewater. Porewater velocity is estimated using the loss of a conservative tracer and a developed relationship between the mass transfer coefficient and velocity.

Based on extensive preliminary laboratory work as well as four successful field demonstrations of HRPPs at chlorinated solvent sites, the project team believes that the HRPP will provide unique insights into the fate and transport of PFAS. This project is expected to result in the field validation of the HRPP for fine-scale delineation PFAS species and concentrations, along with relevant co-occurring chemicals, in heterogenous aquifers. The tool can be applied at aqueous film-forming foam source areas to better understand the transport behavior of different PFAS. The HRPP is also designed to provide data concerning surface water discharge locations of chemicals, which is an important consideration at many sites where PFAS impact groundwater. In addition, these results and future results based on use of the HRPP will directly address a number of critical PFAS fate and transport data gaps. The HRPP resolution provides an enormous advantage over existing methods for improving site models, improving remedial design and application, and assessing technology effectiveness at Department of Defense PFAS sites. (Anticipated Project Completion - 2025)

McDermett, K.S., J. Guelfo, T.A. Anderson, D. Reible, and A.W. Jackson. 2022. The Development of Diffusive, Equilibrium, High-Resolution Passive Samplers to Measure Perfluoroalkyl Substances (PFAS) in Groundwater. Chemosphere, 303(1):134686. doi.org/10.1016/j.chemosphere.2022.134686.