MAPPING PFAS, MAJOR AND TRACE ELEMENTS, STABLE ISOTOPES, AND RECHARGE-AGES IN 3-DIMENSIONS IN THE FRACTURED ROCK AQUIFER BENEATH THE RUTLAND SOUTHERN VERMONT REGIONAL AIRPORT

Contamination of drinking water by per- and polyfluoroalkyl substances (PFAS) is a growing concern to scientists, citizens, and governments globally. In Clarendon, VT, state officials have determined that the use of aqueous film-forming foams (AFFFs) since the 1980s at the Rutland Southern Vermont Regional Airport (RSVRA) is the primary source of PFAS contamination in local groundwater. The field area is rural, with numerous private water wells producing from the regional fractured rock aquifer (FRA) system. The bedrock comprising this FRA is Cambro-Ordovician carbonates folded in a NNE-plunging syncline, with Proterozoic gneisses and schists located a few km to the east in the Green Mountain uplands. Structural mapping indicates that bedding and prominent NNE-SSW and ENE-WSW fractures strongly influence groundwater and PFAS flow in the FRA.

Our main objective is to investigate subsurface processes governing the transport of PFAS from the RSVRA, and to integrate PFAS with other tracers to map the subsurface. The study uses PFAS data from 48 wells and a series of geochemical measurements for 25 wells, including major and trace elements, CFC and tritium age dating, and hydrogen and oxygen isotopes for tracing groundwater recharge. Spatial mapping of individual PFAS compounds indicates (A) that SSW flow is influenced by fractures and folded bedding, and (B) delineation of 4 potentially distinct PFAS groups, including, from source to downgradient: (1) proximal FTS-enriched waters; (2) water enriched in C4-C7 PFAS compounds; (3) water relatively enriched in C8 and C9 (PFOA, PFOS and PFNA); and (4) distal waters with below detection (BD) PFAS. Preliminary hydrogeochemical analysis indicates varied water types that may correspond to PFAS categories, from Ca-Mg-HCO₃ waters with high PFAS to waters relatively enriched in K and SiO₂ that have low or ND PFAS and are likely derived from distant recharge in the metamorphic rocks of the Green Mountains to the east. This is similar to the hydrogeology of PFOA contamination in a FRA in Bennington, VT, where upwelling K-Si-rich, PFAS-absent groundwater associated with intersecting fracture zones and thrust faults is compositionally distinct from the dominant carbonate signature. Mapping such heterogeneity is critical for predicting PFAS migration in the subsurface.