The effects of very slow transformation reactions, particularly abiotic transformations, on the long-term release of chemicals (e.g., chlorinated solvents) in low permeability matrices have been well documented. Such reactions can impact chemical plume intensity and longevity. Recent field studies have shown that elevated levels of anionic, zwitterionic, and cationic perfluoroalkyl acid (PFAA) precursors reside in clayey soils within aqueous film-forming foam (AFFF)-impacted source areas; these precursors often represent the majority of overall per-and polyfluoroalkyl substance (PFAS) mass. However, the extent to which slow abiotic transformations occur for PFAA precursors, and the extent to which PFAAs are formed, is largely unknown. Thus, the overall goal of this project is to identify and quantify the nature, extent, and kinetics of abiotic (and coupled abiotic-biotic) precursor transformation reactions that occur within clays or near the clay-sand interface, and subsequently to determine the impacts of these transformations on diffusive flux through the clays. The focus of this project will be on a wide range of environmental reactants and reaction mechanisms that can occur within clays or near the clay-sand interface.
Bench-scale batch experiments will be performed using a wide range of environmental reactants, including ferrous minerals, electron shuttles, and enzymes, for several individual PFAA precursors that have been shown to persist in clays at AFFF-impacted sites. Three natural clayey soils will be tested. These batch experiments will be used to measure the slow abiotic and coupled abiotic-biotic transformation kinetics (half-lives of >5 years) expected for these reactions. Experiments will be performed under both anoxic and oxic conditions to capture the conditions expected within the clay matrix and at the clay-sand interface, respectively. Of particular interest are the oxidative precursor transformation reactions that are expected based on the generation of hydroxyl radicals that occurs when ferrous minerals are exposed to dissolved oxygen. A detailed series of kinetic tests will be used to evaluate reaction order and estimate rate constants.
Following the batch kinetic testing, additional experiments will be performed to assess the coupled diffusion and transformation of precursors in natural clayey soils. These experiments will be performed using packed columns of clay where the diffusive flux and extent of transformation are determined by measuring the concentration profiles in soil after a pre-determined incubation period. Experiments will be performed with the clay surface exposed to anoxic and oxic conditions, the latter to facilitate the generation of hydroxyl radicals. The impacts of these reactions on diffusive flux and PFAA formation will be directly measured.
The experiments and associated kinetic modeling will provide direct confirmation and quantification of the abiotic and coupled abiotic-biotic PFAA precursor transformations that can occur in clay matrices at AFFF-impacted sites. This information is expected to be a critical component in updating conceptual site models, especially since several studies have shown that precursors often account for a substantial fraction of the overall fluorine mass balance. By obtaining an improved understanding of the fate of precursors in clay, the DoD will be able to better assess the extent to which these low permeability sources are sustaining groundwater plumes via back-diffusion. Because precursor and PFAA mass in clay will likely determine the long-term persistence and intensity of PFAS plumes, improved understanding of the coupled diffusion and transformation processes are critically important for site management. (Anticipated Project Completion - 2026)