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Estimating historical concentrations of per- and polyfluoroalkyl substances with groundwater flow and transport models and a Monte Carlo analysis
Engers, Aaron James
Engers, Aaron James
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Singha, Kamini
McCray, John E.
McCray, John E.
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2021
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Abstract
Here we present a numerical modeling effort to estimate historical groundwater concentrations of perfluorooctanoate (PFOA), perfluorooctane sulfonate (PFOS), and perfluorohexane sulfonate (PFHxS) resulting from releases of aqueous film-forming foam (AFFF) near Colorado Springs, Colorado, U.S.A. AFFF was used during firefightier training at the Peterson Air Force Base upgradient of the towns of Security, Widefield, and Fountain, CO from 1970 to 2017. All three municipalities’ water supply systems relied heavily on groundwater during this time period. We developed one Modflow6 flow model and three MT3D-USGS transport models, one for each PFAS, to help evaluate human exposure to these compounds via drinking water. A Modflow6 model was calibrated using PEST. Locations and timing of potential source zones were established from Air Force documentation; the masses of contaminants introduced to the water table were estimated using inverse methods to match data collected during a 2018 sampling event of the municipal water wells. Concentrations estimated by our models provided reasonable approximations of historical exposures to PFOS, PFOA, and PFHxS in the Security-Widefield area, but not in Fountain. We conducted a Monte Carlo analysis to quantify uncertainty of the model results given available data. The average range between upper and lower 95\% confidence limits at our calibration targets was 174 ng/L, 108 ng/L, and 238 ng/L for PFOS, PFOA, and PFHxS, respectively. A mass balance between the PFASs assumed to be applied on the surface the calibrated mass loading to the water table indicated that 99.5\% of PFOS, 50\% of PFOA, and 88\% of PFHxS remained unaccounted for, which were assumed to be retained in the vadose zone near the source areas and along the transport pathway. The framework employed here may be suitable for a variety of poly- and perfluoroalkyl substance (PFAS) modeling problems where saturated-zone transport is the primary concern. However, a complete picture of PFAS fate and transport that accounts for all released mass is likely not feasible with traditional control-volume finite-difference simulators like Modflow6/MT3D-USGS; this will hopefully be better achieved when the analytical solutions and PFAS-specific simulators currently under development become more readily available.
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