CHARACTERIZING THE GROUNDWATER-SURFACE WATER SYSTEM IN A PFOA-CONTAMINATED FRACTURED ROCK AQUIFER USING RADON AND STABLE ISOTOPES

Perfluorooctanoic acid (PFOA) contamination in the Walloomsac River watershed in southwestern Vermont has been identified in surface water, overburden sediments, and groundwater from both the overburden and the underlying fractured rock aquifers. In order to characterize transport pathways for PFOA in the system, a conceptual model for the site-specific hydrologic system, including groundwater-surface water interactions, is being developed. Field work from the team in July and August 2017 yielded 51 samples for radon (n=12) and stable isotopic (δ¹⁸O-H₂O and δ²H-H₂O) analyses (n=51) of stream water, spring water, streambed seepage, and groundwater samples from till, gravel, and bedrock aquifers. Samples ranged in electrical conductivity from 225 – 803 (μS/cm). Radon results show a strong signal differentiating surface water and groundwater. All samples possess deuterium excess of > 10, typical for this region, and range from -11 to -7.5 in δ¹⁸O-H₂O per mil. Surface water shows the most variability of sample type and likely reflects a colder and higher elevation source of the Walloomsac River in the Green Mountains. The enriched samples are all from Paran Creek, a smaller lower elevation catchment that feeds into the Walloomsac River. Local streambed seepage (identified through cold anomalies on FLIR cameras) sampled with drivepoints were always more isotopically enriched than the streamwater and likely reflect that local streambed seepage is a distinct source of water. The till, gravel, and spring waters fall between the most depleted and enriched stream waters. The bedrock waters don’t follow this pattern and 5 of the 6 samples from deep bedrock wells have the most depleted compositions that were analyzed. The single bedrock well sample that does not follow this pattern is from a shallower depth. The 3 deep bedrock samples also have a distinct d-excess value. δ¹⁸O-H₂O and δ²H-H₂O isotope sampling are shown to be useful to differentiate groundwater and surface water, and results suggest a distinctive isotopic signature related to different groundwater regions of the study area. This sampling work is part of an ongoing multi-component research project to study the fate and transport of PFOA through the groundwater-surface water system.