Insights into the Long-Term Mass Discharge & Transformation of AFFF in the Unsaturated Zone
Charles Schaefer | CDM Smith
The overall goals of this project are to 1) attain insight into the transformation and mass discharge of per- and polyfluoroalkyl substances (PFASs) from aqueous film forming foam (AFFF) sources that reside in the vadose zone and capillary fringe, and to 2) understand how these processes change over time and with AFFF composition and mass. The research team will focus on the unique processes that occur in the vadose zone (e.g., varying saturation due to wetting and drying, aerobic/anaerobic cycling, creation of airwater interfaces, and release of natural colloids) and their impacts on PFAS fate and mass discharge to underlying groundwater. A key aspect of this study is to assess, describe, and quantify the relationship between PFAS mass removal and mass discharge; this functionality is known as the source strength function. Determining the processes that control this relationship, and identification of approaches to characterize this functionality for AFFF-impacted sites, is the focus of this research effort.
The research team hypothesizes that the variable saturation conditions in the unsaturated zone, and the subsequent changes in biogeochemical conditions that occur during wetting and drying cycles, have a significant impact on PFAS discharge and composition in AFFF source areas. They further hypothesize that enhanced flushing of the AFFF unsaturated zone source can be used to provide mechanistic insights into the relationship among PFAS mass discharge, fractional PFAS mass removal, and PFAS composition.
The project will examine PFAS mass discharge from an AFFF source in the unsaturated zone using a combination of field and laboratory data. The overall approach will employ the use of a highly characterized and instrumented test cell emplaced within an aged unsaturated zone AFFF source zone. The test cell will be used to facilitate enhanced water flushing of the unsaturated zone source, thereby serving as a means to evaluate PFAS discharge, composition, and transformation as a function of time and fractional PFAS removal. Use of a naturally aged AFFF source is selected for this study, as this is expected to provide the most realistic and relevant conditions for which to assess future PFAS mass discharge. This information will be used to assess the PFAS source strength function. In addition, both the nature of the PFASs present in pore water and biogeochemical conditions will be assessed under varying saturation conditions to attain insight into how vadose zone processes impact source attenuation and transformation. The research team strongly believe that an experimental field test cell is critical at this juncture to validate current concepts and laboratory data concerning AFFF source areas, and to determine long-term source behavior.
In addition, parallel bench-scale testing will be performed to provide additional mechanistic insight needed to interpret the field results. Specifically, bench-scale testing will be used to assess soil and non-aqueous phase liquid desorption kinetics, PFAS partitioning to natural colloids, micellar PFAS mass discharge, and PFAA precursor transformation under cyclic aerobic-anaerobic conditions.
The information attained in this study will facilitate prediction of the intensity and longevity of the PFAS source, thereby serving as a means to improve site management and identify the extent to which source zone mitigation is required for addressing the underlying groundwater plume. Insights into the mechanisms controlling the long-term PFAS mass discharge also will provide the Department of Defense with improved approaches to mitigate, or enhance (when appropriate for remedial purposes), PFAS mass discharge from the vadose zone, thereby allowing for management and treatment strategies based on scientifically-based and field-observed PFAS fate and transport processes. (Anticipated Project Completion - 2021)