The use of aqueous film-forming foam (AFFF) to extinguish fuel-based fires at military, industrial, and municipal sites since the 1970s has greatly impacted those locations with per- and polyfluoroalkyl substances (PFAS), including a wide variety of environmentally persistent perfluoroalkyl acids (PFAA). Field observations of prolonged PFAA persistence may result in part from abiotic and biotic transformations of PFAA precursors introduced during AFFF applications. However, the mechanisms of these processes and their potential contribution as long-term sources are largely unknown, especially in surface soils and heterogeneous aquifers subject to multiple environmental factors. Complicating the issue further are the chemical mixtures that could likely alter the transport and reactivity of PFAA precursors. Research efforts to date have primarily investigated the transformation of PFAA precursors in idealized batch reactor systems. Therefore, the overall goal of this project is to advance the fundamental understanding of abiotic and biotic processes that can transform PFAA precursors under dynamic conditions that are representative of AFFF-impacted sites.

To achieve the project objectives, the research is structured around four technical tasks involving laboratory experiments of increasing scale and complexity that are designed to:

1. determine the influence of environmentally relevant conditions on PFAA precursor biological transformations;
2. examine conditions that promote abiotic transformation of PFAA precursors in naturally-occurring radical-forming systems;
3. investigate the influence of variable water saturation on abiotic and biotic transformation processes in unsaturated columns and intact cores collected from an AFFF-impacted site; and
4. evaluate the influence of dynamic flow and oxygen consumption on the abiotic and biotic transformations of PFAA precursors in multi-dimensional systems incorporating unsaturated and saturated zones.

Tasks 1 and 2 will investigate abiotic and biotic transformation processes involving aerobic and anaerobic defluorination of representative PFAA precursors in AFFF formulations using natural soils and aquifer materials, reactive mineral species, dissolved organic matter, and native microbial communities. Task 3 will utilize unsaturated column systems and laboratory-scale lysimeters with intact cores collected from Loring Air Force Base to further evaluate coupled abiotic and biotic transformation of PFAA precursors under variable water-saturated conditions and to provide data for mathematical model development and validation. PFAA precursor transformation data obtained in Tasks 1-3 will be used to design aquifer cell studies in Task 4 that will examine the impacts of changing redox conditions on these processes under dynamic flow conditions and water table fluctuations. Representative chlorinated solvents and fuel hydrocarbons will be incorporated into the experimental matrix to investigate the effects of co-occurring chemicals on precursor transformation processes.

Outcomes of this research will provide site managers, regulators, researchers, and community stakeholders with: (1) a more complete understanding of the environmental conditions that impact the extent and rate of the coupled processes that govern PFAA precursor transformation at AFFF-impacted sites, (2) quantitative data that can be used to validate and improve the accuracy of risk assessments, and conceptual site models of AFFF-impacted sites, and (3) validated mathematical models that can be used to predict PFAA precursor transformation and transport in the vadose zone. The project team anticipates that the outcomes of this project will serve to guide the selection of scientifically-sound strategies for sustainable long-term management and remediation of complex sites impacted by PFAA precursors and byproducts (including when in mixtures with other organic chemicals). (Anticipated Project Completion - 2027)