Per- and polyfluoroalkyl substance (PFAS) source areas resulting from the use of aqueous film-forming foam (AFFF) products exist fully or partially within the vadose zone; however, transport processes for AFFF solutions and PFAS within the vadose zone are not currently well understood. This project is being conducted in two phases to address this knowledge gap.
During the Phase I effort (one-year project), evaluating the significance of PFAS air-water interfacial (AWI) adsorption as a source of retention within vadose zone source zones and characterizing AWI adsorption for the purposes of modeling PFAS transport in the vadose zone were the primary goals. Specific factors evaluated included: PFAS solution concentration, PFAS surface activity (i.e., PFAS type), and solution ionic strength.
The overriding goal of Phase II is to expand upon the preliminary work completed and evaluate additional potentially significant PFAS vadose zone transport processes observed during Phase I (e.g., adsorption nonidealities and kinetic effects for PFAS as a complex mixture). This evaluation will include additional PFAS types (e.g., environmentally relevant branched forms, polyfluoroalkyls, nonionic forms) and bench-scale transport experiments that were not included in Phase I. Where these processes are significant with respect to transport, mathematical formulations to simulate the processes will be developed, implemented in HYDRUS, and evaluated to improve the completeness, robustness, and accuracy of model predictions and to improve the overall utility of the HYDRUS simulator as a numerical tool. An additional objective of the proposed work is to properly parameterize HYDRUS to simulate the fate and transport of PFAS components from AFFF sources within the vadose zone environment and the infiltration of AFFF solutions.
During Phase I, the technical approach was divided into three tasks aimed at achieving the stated objectives. In Task 1, aqueous surface tension measurements were made to prepare surface tension isotherms (i.e. concentration-dependence on surface tension) for individual PFAS of environmental significance and PFAS mixtures. These isotherms were used to calculate AWI adsorption constants that were used to assess the significance of AWI adsorption in the vadose zone environment. Linear perfluorocarboxylic acids (PFCA) and perfluorosulfonic acids (PFSA) were employed in this work. In Task 2, bench-scale column experiments were performed to assess the potential for AFFF fluids to spread under concentration-dependent capillary pressure gradients imposed by the surface tension of the AFFF solution. This experimental data was used to assist numerical model validation after model modifications were made to include mechanisms specific to AFFF solution transport. In Task 3, the focus was incorporating AWI adsorption as an additional source of retention for PFAS within the HYDRUS unsaturated flow and transport model.
During Phase II, the technical approach includes the following:
- Performing additional surface tension/adsorption isotherm measurement for single- and multi-component PFAS mixtures measurement.
- Measurements to characterize the surface-adsorbed composition for multi-component mixtures at the AWI for comparison with surface tension-derived measurements.
- Measurements to evaluate potential kinetic processes related to AWI adsorption.
- Developing and evaluating mathematical predictive relationships for parameter estimation and inclusion of significant processes into the HYDRUS simulator.
Unsaturated flow and transport experiments will also be performed at environmentally relevant PFAS concentrations to validate the various process modifications made to the HYDRUS simulator throughout the execution of this project. Additional physicochemical properties characterization, mathematical model evaluation, and HYDRUS model modifications will be completed to improve simulations of the initial release of AFFF fluids. Finally, the project team will collaborate with other researchers to evaluate the performance of the modified HYDRUS model predictions against field-scale transport data.
Over 900 individual surface tension measurements were made to complete this work. From these, 68 aqueous surface tension isotherms for PFCAs and PFSAs as single components were prepared for deionized water and four simulated groundwaters of varied ionic strength. An additional 23 surface tension isotherms were developed for PFCA binary mixtures and one ternary mixture. Adsorption constants derived from these isotherms were used to characterize the dependence of AWI adsorption on PFAS concentration, the length of the PFCA/PFSA hydrophobic chain length, and on groundwater ionic strength. The HYDRUS unsaturated flow and transport model was modified to include the dependence of AWI adsorption on bulk pore-water concentration, the area of the AWI (as it changes in response to changes in soil moisture content during drainage and imbibition cycles, and on changes in pore-water ionic strength). A framework analytical approach was presented to address AWI adsorption of PFAS mixtures.
Bench-scale column experiments were completed to evaluate AFFF solution transport in the vadose zone and provide data with which to validate modifications made to the numerical model to simulate AFFF transport. Strong solid-phase sorption of one or more components of the AFFF solution dominated other mechanisms of transport. However, facilitated transport related to a combination of concentration-dependent capillary pressure gradients (i.e., tension-driven flow) and changes in the wettability of porous media surfaces was observed. The HYDRUS model was modified to include the effects of tension-driven flow. A good match of simulated and experimental results was achieved when both Langmuir adsorption and tension-driven flow were included; however, additional research is needed to include the effects of solid-phase wettability changes and multi-component nature of these AFFF solutions.
The results of this work will improve the collective understanding of the fate and transport of PFAS within vadose zone source areas and will provide an improved and performance-evaluated numerical simulator that can be used to assist PFAS site characterization, risk calculation, and remediation decision-making efforts. Further, the experimental work is designed to test the significance of identified AWI retention processes such that processes ultimately included in the model are those most appropriate and essential for simulating PFAS transport from vadose zone source zones over the long-term.
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.