Aqueous film-forming foam (AFFF) formulations have been used for decades to suppress hydrocarbon fuel-based fires and during firefighter training exercises. Per- and polyfluoroalkyl substances (PFAS) are key constituents of AFFF and perfluoroalkyl acids (PFAAs) are of particular concern for environmental risk assessment. However, it is now known that some polyfluoroalkyl substances are transformed in the subsurface, resulting in the formation of PFAAs. For this reason, many polyfluoroalkyl substances are considered to be PFAA “precursors” that can sustain source zones at AFFF-impacted sites over time.

The goal of this project is to improve the fundamental understanding of precursor fate and transformation in natural systems. Specifically, this project will collect cross-scale data to generate new knowledge on the fate and transformation of precursors in wetland systems, thereby providing new information and insights that will improve risk assessment and site management decisions at U.S. military facilities. The research objectives of this project are to:

• characterize precursor biotransformation rates and pathways across biogeochemical gradients and adsorption/desorption processes across a set of relevant mineral and organic surfaces;
• characterize the relative importance of emergent rhizosphere fate and transformation processes for precursors in model wetland systems;
• characterize the distribution and seasonal dynamics of PFAAs and their precursors in a natural wetland system; and
• develop tools to predict precursor biotransformation pathways, precursor biotransformation rates, and precursor mobility under a variety of field scenarios.

This project addresses specific knowledge gaps through development of a self-consistent biotransformation test system and evaluation of the biotransformation of a wide range of precursors across a gradient of redox and biogeochemical conditions. The project team will specifically explore precursor fate and transformation in the wetland rhizosphere. Wetlands are particularly interesting because the wetland rhizosphere is comprised of extensive oxic-anoxic interfaces with steep redox gradients that are conducive to complex and often specialized microbial assemblages and associated biotransformation processes. Wetlands are also important because they are key landscape features at several U.S. military facilities and likely play an important role in the fate and transformation of precursors at those sites. This project is structured around five scientific tasks:

1. Batch incubation experiments to measure precursor biotransformation rates and pathways
2. Batch experiments to characterize precursor adsorption and reactivity
3. Mesocosm experiments to assess the roles of emergent rhizosphere processes and plant uptake
4. Field sampling to assess seasonal impacts on precursor distribution, transformation, and mobility in a natural wetland system
5. Model development to predict biotransformation pathways, biotransformation rates, and precursor mobility in the subsurface

This project will significantly improve the understanding of PFAS fate and transformation processes. Further, this project will provide remedial project managers and the scientific community with tools to improve risk assessment at AFFF-impacted sites and inform site management decisions. Anticipated deliverables of this project include: (1) self-consistent datasets that describe precursor biotransformation rates and pathways across biogeochemical gradients and adsorption/desorption processes across a set of mineral and organic surfaces; (2) a tool to predict biotransformation pathways for previously unstudied precursors under a variety of field scenarios; and (3) a mathematical model to qualitatively predict precursor biotransformation rates and precursor mobility in the subsurface under a variety of field scenarios. (Anticipated Project Completion - 2026)