Baseline Data Acquisition and Numerical Modeling to Evaluate the Fate and Transport of Per- and Polyfluoroalkyl Substances within the Vadose Zone

Jeff Silva | GSI North America Inc.



The per- and polyfluoroalkyl substances (PFASs) possess surface active characteristics that allow them to partition to and accumulate at air-water interfaces (AWIs) within the vadose zone. This accumulation represents a potentially significant source of PFAS retention within vadose zone soils that may be contributing to the observed longevity of PFAS contamination beneath aqueous film-forming foam (AFFF) application sites. Additionally, accumulation of PFASs at AWIs will likely reduce pore-water surface tensions sufficient to promote pore-water drainage and redistribution, which can in turn impact the rate and degree of groundwater contamination. Depending on accumulated concentration, these surface-active chemicals may also spread laterally within the vadose zone and along the capillary fringe, increasing the scale of PFAS contamination and that of co-contaminants. These mechanisms have yet to be rigorously evaluated for PFASs and therefore represents a key data gap in the current understanding of the environmental fate and transport of these chemicals in the near and long term.

The objectives of this proof-of-concept project are to: (1) generate fundamental physico-chemical properties data with which to evaluate the significance of AWI accumulation as a source of retention and as an environmental sink for PFASs; (2) investigate the potential for interfacial tension-induced flow and lateral spreading of PFASs in the vadose zone to determine whether these processes could increase the scale of PFAS contamination; and (3) incorporate these mechanisms into the current version of the commercially-available HYDRUS unsaturated flow and transport model. The modified HYDRUS model will thereafter be used to numerically evaluate the dynamics and significance of these mechanisms within porous media beyond the experimental work scope and at field scale.

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Technical Approach

The technical approach of this project will consist of laboratory measurement of surface tension isotherms for homologous PFASs, precursors, and AFFF co-surfactants that will be used to calculate surface concentration isotherms, AWI partitioning coefficients, and retardation factors for AWI partitioning (Task 1). These results will be used to select PFASs to be used in one-dimensional (1D) moisture redistribution experiments that will physically characterize the significance of surface tension-driven moisture redistribution for PFASs (Task 2). Finally, the results of Task 1 will be used to modify the HYDRUS model to include these mechanisms and the results of Task 2 will be used in validating the modified numerical model (Task 3). 

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The results of this project will benefit the DoD and the larger scientific community by documenting the significance of these mechanisms for PFASs and co-contaminants typical of AFFF source zones and by providing analytical relationships and a numerical model to consider AWI accumulation and tension-driven flows as a part of site characterization and remedial actions. (Anticipated Project Completion - 11/2019)

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Silva, J.A.K., W.A. Martin, J.L. Johnson, and J.E. McCray. 2019. Evaluating Air-Water and NAPL-Water Interfacial Adsorption and Retention of Perfluorocarboxylic Acids with the Vadose Zone. Journal of Contaminant Hydrology, 223:103472.

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Points of Contact

Principal Investigator

Dr. Jeff A.K. Silva

GSI North America Inc.

Phone: 808-354-3051

Program Manager

Environmental Restoration