INTEGRATION OF GROUNDWATER GEOCHEMICAL FINGERPRINTING WITH THE STRUCTURE OF A FRACTURED ROCK AQUIFER TO UNDERSTAND THE FATE AND TRANSPORT OF PFOA, BENNINGTON, VERMONT, USA

From 2016-2019, the Vermont Dept. of Environmental Conservation discovered that groundwater from 41% of ~700 wells (and springs) tested in the Town of Bennington and vicinity had PFOA concentrations >20 parts/trillion (Vermont action level). The distribution of contaminated wells was complicated, with wells < 100m apart exhibiting very different PFOA concentrations. PFOA was introduced into the environment by a factory that produced Teflon-coated fabrics from ~1978-2002. The complex fate and transport of PFOA involved emission from smokestacks at high temperature, wind transport in the atmosphere, deposition onto the ground surface, and downward eluviation through the vadose zone to the surficial and bedrock aquifers. We characterized the fractured rock aquifer in the contamination area through physical components (geologic mapping, spatial analysis of wells, and geophysical logging) to build a 3-D conceptual model and then mapped the distribution of chemical groundwater tracers (PFOA, major and trace elements, stable isotopes, and recharge ages), adding a temporal factor.

The field area is divided into 4 slices of carbonate and clastic metamorphic rock that are internally folded and bounded by east dipping thrust faults. The dominant fractures that overprint this infrastructure dip steeply, strike N-S and E-W, and control the topographic “grain”. Geophysical logging indicates that wells can generally be divided into those completed internal to or on the margins of thrust slices. Major and trace element analysis shows that groundwater can be spatially discriminated into carbonate (TDS, Ca, Mg, HCO₃), shaly (Sr, U, SO₄), and siliceous (K, Si) groups. Depleted stable isotope signatures characterize the siliceous group. Recharge-ages determined from CFC11, 12, and 113; SF6; and Tritium strongly correlate with geochemical group and were divided into “older” (~1953-73) and “younger” (1963-88) age brackets.

Our conceptual model has recharge from the New York highlands (west) and Green Mountains (east) flowing toward the Bennington basin where it is tapped by wells along thrust faults or fracture zones and has non-detect to low PFOA and older ages. Wells completed in the internal portions of thrust slices yield groundwater that is younger, which reflects mixing, and has medium - high PFOA levels.