PFAS Fate and Transport During Aquifer Storage and Recovery: Effects of Aquifer Mineralogy and Conditioning

Abstract Per‐ and polyfluoroalkyl substances (PFAS) are increasingly detected in groundwater and may affect aquifer storage and recovery (ASR) operations. This study evaluated PFAS fate and transport during ASR with emphasis on the effects of aquifer mineralogy and conditioning on recovered water quality. A mineral‐component adsorption database was developed that used log K OC for organic matter, empirical chain‐length regressions for clays, and a CD‐MUSIC surface complexation model for iron oxyhydroxides. The database was coupled to a reactive transport model (RTM) to simulate 18 PFAS over repeated ASR cycles. When PFAS were assumed to be initially present in native groundwater, the model predicted that clay minerals were the primary sorbing component for many short‐chain PFAS, with iron oxyhydroxides providing a secondary contribution. By contrast, organic matter was more important for long‐chain PFAS. During successive ASR cycles, sediment‐associated PFAS in the near‐well aquifer became progressively depleted through aquifer conditioning, causing recovered water concentrations to decline over time. For initial groundwater concentrations of 10 ng/L or less, the model predicted that recovered perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) concentrations decreased to near or below the federal maximum contaminant level (MCL) within two ASR cycles. By contrast, when PFAS were initially present in recharge water, multiple pore volumes conditioned the near‐well aquifer toward elevated recharge concentrations, even during the first ASR cycle. The model results indicate that ASR may be feasible under aquifer conditions such as those evaluated when PFAS are initially present in native groundwater; however, when recharge‐water PFOA or PFOS concentrations exceed their MCLs, adsorption alone may be insufficient to maintain recovered water below the MCLs.