TRACING GROUNDWATER FLOW BY INORGANIC HYDROGEOCHEMISTRY: A TOOL TO UNDERSTANDING PFOA MIGRATION IN A FRACTURED ROCK AQUIFER

Groundwater in pelitic rocks tends to have high Na, Al, Fe, Sr, Cl and SO₄ compared to carbonate-dominated zones (high Ca, Mg, HCO₃). This has proven useful in groundwater flow analysis in Cambro-Ordovician clastic and carbonate aquifers, including the PFOA-contaminated fractured rock aquifer (FRA) in the Bennington area of southwestern Vermont. The contamination occurs in 3 thrust-bounded lithotectonic sheets ranging from shale (west) to carbonates (central) to mixed clastics and carbonates (east). Multi-element chemistry of groundwater by ICPMS was analyzed using traditional geology approaches (e.g. graphically by geochemical spider plots) and a by non-metric multidimensional scaling (NMDS) analysis. There are 3 main groundwater compositional zones corresponding to the lithotectonic sheets: (1) a shale signature west of the Breese Hollow thrust fault (BHT) imparted by black slates (Walloomsac Fm); (2) a carbonate signature in overthrusted dolostones and limestones in the central part of the study area; and (3) an intermediate shale and carbonate signature in overthrusted clastic-carbonate rocks. Two notable exceptions include a mixing zone of shale and carbonate signatures, and a series of wells with anomalously high K and very low Na, Cl and PFOA in the center of the 5-10 km radius area contaminated by PFOA air fallout. The mixing zone is in carbonate rocks in the hanging wall of the BHT – the groundwater in these carbonates has a shale groundwater signature plus many of the highest PFOA concentrations. In FRAs, faults can be barriers or conduits, depending on the permeability structure of the thrust and the hanging and foot wall rocks; in this case, low-permeability slates in the footwall impede downward recharge across the thrust. Mapping of the groundwater signature indicates flows that are controlled by geological structures: eastward along bedding and cleavage parallel to the thrust plane and southwestward down the plunge of folded layers. This interpretation would predict flow of PFOA eastward parallel to the plane of the BHT and SSW with plunge. The K-rich, PFOA-absent groundwater is compositionally distinct from surrounding carbonate or shale signatures; spatial coincidence with a N-S fracture zone and thrust fault suggests upwelling of (potentially) old groundwater, perhaps distally sourced.