Transport of Per- and Polyfluoroalkyl Substances (PFAS) at a Managed Aquifer Recharge Site

Abstract

In response to aquifer overuse and its related issues in eastern Virginia, the Hampton Roads Sanitation District (HRSD) is conducting a managed aquifer recharge (MAR) project called the Sustainable Water Initiative For Tomorrow (SWIFT). The objective of the SWIFT program is to treat wastewater to drinking water standards before injecting it into the underlying Potomac Aquifer System (PAS). SWIFT is an aquifer long-term replenishment (ALTR) designed to increase the storage and potentiometric surface of the PAS. Previous work used simple analytical models to simulate the transport of conservative constituents present in SWIFT Water and delivered to the PAS at the HRSD SWIFT Research Center (SRC). Using the previously calibrated model, this study describes adaptations to the model for simulating the transport and attenuation of selected per- and polyfluoroalkyl substances (PFAS) in groundwater at the SRC. The transport of seven PFAS detected at the SRC is examined in this study: perfluorobutanoic acid (PFBA), perfluorobutane sulfonic acid (PFBS), perfluoropentanoic acid (PFPeA), perfluorohexanoic acid (PFHxA), perfluorohexane sulfonic acid (PFHxS), perfluoroheptanoic acid (PFHpA), and perfluorooctanoic acid (PFOA). Among these PFAS, five are short-chain compounds (PFBA, PFBS, PFPeA, PFHxA, and PFHpA) and two are long-chain compounds (PFHxS and PFOA). The fluorocarbon chain length among these compounds ranges from four (PFBA) to seven (PFOA). Two compounds contain a sulfonic acid functional group (PFBS and PFHxS) and five compounds contain a carboxylic acid functional group (PFBA, PFPeA, PFHxA, PFHpA, and PFOA). Sorption is modeled at the SRC using retardation factors, which were calibrated for each PFAS to best fit the observed concentrations at a PAS monitoring well. Calibrated retardation factors are then used to calculate PFAS-specific partition coefficients (Kd) and organic carbon partition coefficients (Koc) and compared to literature values. In general, best-fit retardation factors increase with PFAS chain length, with the exception of PFHxS, whose calibration is challenged by lack of data and low levels of detection. Calculated Kd values fall largely within the range of representative values reported in the literature, with the exception of PFBA. Some calculated Koc values approach or exceed the upper ranges of values observed in the literature, suggesting that sorption to natural organic carbon in aquifer sediment does not fully account for PFAS sorption in the PAS. Overall, this study presents a unique data set and novel analysis of PFAS transport and attenuation in an oligotrophic aquifer under complex field conditions.