The oxic–anoxic transition zone (OATZ) is highly active in diverse biogeochemical processes, reactions, and cycles that likely control the retention, transformation, and degradation of per- and polyfluoroalkyl substances (PFAS) in the subsurface. The overall objective of this project is to systematically investigate the synergies between abiotic and biotic transformation of high-priority polyfluorinated precursors at OATZ at Department of Defense (DoD) sites with historic use of aqueous film-forming foam (AFFF). Specific objectives are to characterize the following:

• production of reactive oxygen species (ROS), such as hydroxyl radicals, by reactive mineral intermediates (RMI) that are favored at OATZ;
• abiotic transformation of precursors by ROS;
• biotic transformation of precursors, particularly the synergies between ROS and aerobe/anaerobe-mediated abiotic and biotic processes; and
• field manifestation of these abiotic and biotic effects at a DoD site with frequent water level fluctuations rich in OATZs.

To directly address challenges in managing AFFF-impacted sites, three representative sites with frequent water level fluctuations will be selected for laboratory investigation. Soils impacted by AFFF will be collected and transported to the laboratory for constructing soil microcosm experiments. Under simulated wetting–drying cycles (WDC) that mimic OATZs, the activation of RMI and associated production of ROS in soil microcosms will be quantified. Pathways and mechanisms of abiotic transformation of high-priority precursors by RMI-induced ROS, direct biotransformation of these precursors by aerobes and anaerobes, and the synergies between abiotic and biotic transformations linked by ROS will be disentangled. Field manifestations at one selected site under simulated WDC will be conducted to compare laboratory and field results. Functional gene-encoded biomarkers involved in different biotransformation pathways in both laboratory and field studies will be identified to infer precursor type, transformation rate and extent, and potential discrepancies between laboratory and field findings.

Findings from this project will provide a scientific foundation for predicting release extent and duration of high-priority PFAS due to precursor transformation at AFFF-impacted sites. Insights into the mechanisms of RMI-centered transformation of precursors will advance the knowledge of abiotic natural attenuation of less recalcitrant chemicals, such as chlorinated solvents, at thousands of sites. The identified biomarkers indicative of precursor transformation will enable remedial project managers to assess potential precursor transformation for risk assessment and mitigation. Teaming up with scientists and engineers from the U.S. Navy, Geosyntec Consultants, and the U.S. Environmental Protection Agency will synergistically ensure successful technology transfer of the project results to stakeholders for managing AFFF-impacted sites. (Anticipated Project Completion - 2026)