4D CHARACTERIZATION OF A FRACTURED BEDROCK AQUIFER CONTAMINATED WITH PFOA: BENNINGTON, VERMONT

During 2016, elevated levels of perfluorooctanoic acid (PFOA) were discovered by the Vermont DEC in groundwater from numerous wells in Bennington. The dominant source of contamination was a former factory where PFOA was used to manufacture Teflon-coated waterproof fabrics until 2002. As a waste product from this process, PFOA was released into the air through smokestacks at high temperature, transported by wind, deposited on the ground surface, and eluviated downward through the vadose and saturated zones in soil/surficial material to the fractured rock aquifer (FRA). The fate and transport of PFOA is complex and requires a multi-disciplinary team of state, academic, and federal partners and a comprehensive aquifer characterization process. The process includes: 1) bedrock and surficial mapping; 2) correlation of well locations with driller’s logs, 3) construction of watershed-scale physical hydrogeology maps; 4) geochemical analysis of groundwater and surface water, including H and O isotopes and radon; 5) geophysical logging; 6) groundwater recharge ages (tracers), and 7) conceptual model development. This presentation summarizes tasks 1-3.

Lithologic heterogeneity and structural anisotropy must be considered when assessing the transmission of groundwater and PFOA downward from the surficial aquifer or internal to the FRA. The contamination area is comprised by 3 fault-bounded, weakly-metamorphosed, shallowly east-dipping, thrust slices (west to east): A) New York (NY)- phyllites and limestones; B) North Bennington (NB)- dolostones and limestones; and C) Route 7 (R7)- dolostones and quartzites. All slices were folded into map-scale, south-plunging anticlines and synclines. Whereas bedding is the dominant planar structure in the NB and R7 slices, phyllitic cleavage overprinted bedding in the NY slice. Solution-enlarged thrust faults, bedding, and fractures were observed in geophysical and camera logs from some carbonate wells.

The most abundant fracture sets in the field area dip steeply and strike ~N-S and ~E-W and control the rectilinear course of the Walloomsac River. A NNE-SSW striking fracture subset may be folding-related. N-S striking fracture zones form both exhumed and buried bedrock valleys. Thick surficial deposits follow the eroded hanging wall side of the eastern thrust fault.