PROCESSES GOVERNING DISTRIBUTION OF AFFF-DERIVED PFAS IN A FRACTURED CARBONATE AQUIFER, WEST-CENTRAL VERMONT, USA

Contamination of groundwater by aqueous film-forming foams (AFFFs) containing per- and polyfluoroalkyl substances (PFAS) is a growing concern globally. The use of AFFFs since the early 1980s at the Southern Vermont Regional Airport (SVRA) is the primary source of PFAS contamination in local groundwater. SVRA is surrounded by numerous private water wells producing from the regional fractured rock aquifer (FRA) comprised of Cambrian-Ordovician carbonates with Precambrian gneisses and schists located up gradient within 2 km to the east. Hydrogeological processes governing the transport of PFAS from SVRA were studied by mapping spatial trends in major and trace elements, concentrations of 21 PFAS compounds, groundwater recharge ages (CFC and tritium), field geological mapping, drone mapping and geophysical analysis of well boreholes.

Mapping of groundwater composition indicates 3 general categories: (1) proximal to source fluorotelomer sulfonate-enriched waters (up to 600 ng/L PFAS); (2) farther down gradient (0.5 – 2 km) groundwater relatively enriched in C4-C9 carboxylic acid PFAS (e.g. PFBA, PFHxA, PFOA, PFOS and PFNA and others up to 73 ng/L PFAS);(3) distal waters (> 2.5 km) with no detectable PFAS (< 2 ng/L). Within these categories, PFAS occurrence is highly heterogeneous, suggesting transport along bedrock structures (e.g. bedding and fracture planes). Locally-upwelling groundwater derived from the Green Mountains – with anomalous chemical signatures (low Ca-Mg-HCO₃) and relatively old recharge ages (> 45 y) – provides PFAS-free water to localized compartments of the carbonate aquifer. Concentrations of the carboxylic acid PFAS compounds co-vary in the aquifer system whereas sulfonate PFAS compounds do not correlate with other sulfonates, nor with carboxylic acid PFAS. Extent of PFAS distribution suggests groundwater flow ~ 50-70 m/yr.

Surface mapping, drone photogrammetry, and geophysical logging reveal that bedding, foliations, fractures, and fold structures influence PFAS migration, individually or in combination. Conspicuous, steeply dipping, ~ N-S and E-W striking fracture zones that intersect could explain the apparent southwest stepping migration of PFAS from the point source area. Ongoing work will integrate the chemical and bedrock data into a 3D conceptual model.