TRANSPORT AND TRANSFORMATION OF PFAS AT AN AFFF-IMPACTED FRACTURED ROCK AQUIFER IN VERMONT, USA

Per- and polyfluoroalkyl substances (PFAS) are components of aqueous film-forming foams (AFFFs) used to extinguish jet fuel fires. To develop a detailed understanding of the spatiotemporal behaviors of these toxic compounds in groundwater, the transport, sorption and transformation of PFAS compounds were analyzed in a fractured rock aquifer (FRA) located hydrologically downgradient from an airport where dozens of AFFF applications (by FAA mandated spray-testing) occurred over four decades (late 1970s to 2016). Shallow monitoring wells in the vadose zone adjacent to the AFFF spray-test site contain 6:2 FTS as the dominant PFAS compound (12,000 ppt) followed by PFPeA (3600 ppt), PFHxA (1700 ppt), PFHpA and PFOS (1100 ppt), PFOA (640 ppt), PFBA (370 ppt) and PFHxS (360 ppt). These high concentrations are from spray-testing that introduced PFAS to the vadose zone between 8-40 years ago, indicating (1) the vadose zone here will facilitate PFAS infiltration to the FRA decades into the future, and (2) sorption of PFAS to solids or air-water interface is significant. Presence of PFPeA, PFHxA and PFHpA in the vadose zone may be due to transformation from 6:2 FTS. In the saturated zone (FRA), PFAS concentrations in 49 drinking-water wells determined quarterly-to-annually from 2018-23 indicate changing ratios of PFAS compounds over time – notably a significant correlation between increasing PFHxA and decreasing 6:2 FTS – in groundwater wells hydrologically downgradient from AFFF spray-tests. Furthermore, the presence of 6:2 FTS in wells 150-300 m deep immediately downgradient of the AFFF spray-test site indicates persistence of FTS for at least 8 years in this aquifer system. FTS absence downgradient is attributed to transformation of FTS to PFHxA and other PFCAs of similar alkyl chain length. The transformation of FTS to PFCAs observed herein indicates (1) FTS transformation to end-member PFCAs occurs over decades, (2) increasing amounts of PFCAs highlights that FTS breakdown simply results in formation of terminal end-products (e.g. PFHxA, PFOA) that are unlikely to decompose any further, and (3) testing and regulation of precursor PFAS compounds (e.g. FTS) in drinking water is necessary for a systematic understanding of groundwater quality. These findings are currently being integrated into a 3D Conceptual Site Model.