Formatted Title
PFAS Source Term Assessment for Groundwater Modeling
Background/Objectives
Field data were collected in 2021 and 2022 as part of a Remedial Investigation (RI) at a former United States Department of Defense (DoD) site where aqueous film forming foam (AFFF) containing per- and polyfluoroalkyl substances (PFAS) was used. The data were collected to support vadose source zone leaching to groundwater input to a transient numerical groundwater transport model. Data collection included a combination of soil samples for PFAS analysis and Leaching Environmental Assessment Framework (LEAF) testing, and lysimeter porewater sampling for PFAS analysis. The resulting leaching to groundwater concentrations were implemented in the numerical model which was then used to predict future groundwater concentrations associated with the source zones.
Approach/Activities
A combination of modeling and field data was used to determine the source zone PFAS loading to groundwater for implementation in a predictive numerical groundwater flow model. Vadose zone soil and porewater investigations were performed at five source areas identified at the former DoD site where significant groundwater PFAS contamination exists. The investigation included the collection of vadose zone soils for PFAS analysis to delineate the source areas, the installation of porous cup suction lysimeters for collection of vadose zone porewater samples from the source areas for PFAS analysis, and collection of unsaturated soil samples collocated with lysimeters and submitted for PFAS analysis to determine “apparent” distribution coefficients (Kd) and used in leaching tests to determine declining source zone leaching values and associated time frames. The LEAF up-flow percolation column procedure, United Stated Environmental Protection Agency (EPA) Method 1314, was used for the leaching tests. The recharge rate for the source zones was combined with four quarters of porewater concentrations from lysimeter samples to calculate a current annual mass flux to groundwater. The LEAF testing results were used to determine a time frame for source zone depletion and to assign declining concentrations for the source zones in the numerical groundwater transport model. Recharge rates were determined from literature, numerical model calibration, and supported by vadose zone modeling using Hydrus-1D.
Results/Lessons Learned
The results of the field sampling and laboratory analyses provided the data to determine the transient source zone PFAS leaching rates to groundwater from the vadose zone source areas. The PFAS LEAF testing results were assessed in combination with porewater concentrations from the lysimeters and used to develop transient recharge concentration input for the numerical groundwater model allowing the prediction of future groundwater PFAS concentrations. An “apparent” Kd was calculated from the lysimeter porewater and soil pairs and used in a first approximation to assess soil to groundwater leaching using the EPA soil screening levels (SSL) method. However, this simplistic calculation may over predict transport and a more robust model for vadose zone transport can be obtained using a combination of LEAF soil and lysimeter porewater data.