USING MULTIPLE GROUNDWATER TRACERS TO ASSESS THE FATE AND TRANSPORT OF PFOA IN A SURFICIAL- FRACTURED ROCK AQUIFER SYSTEM: BENNINGTON, VERMONT

The fate and transport of PFOA – a highly soluble industrial compound and emerging contaminant – is complex, involving A) smokestack emission at high temperature, B) wind transport in the lower atmosphere, C) deposition on the ground surface, D) eluviation through the vadose zone, and E) flow in surficial and bedrock aquifers. In Bennington, this multi-stage process operated for ~25 years and D and E continue today. Approximately 50% of 600 wells over a 20 km² area had PFOA levels above the Vermont action level (20 ppt).

The field area is composed of faulted and folded bedrock overlain by heterogeneous surficial deposits. Aquifer characterization integrates physical (geologic mapping, spatial analysis of domestic well logs, geophysical logging) and chemical (major + trace element, stable isotope, age-tracer, and PFOA analysis of groundwater) hydrogeology datasets to construct a Conceptual Site Model (CSM).

The Rice Lane Low (RLL) PFOA zone occurs along the Green Mt. Frontal Thrust (GMFT) and is intersected by a N-S fracture zone filled with thick overburden. Shallow wells (<61m) in this zone draw from fractures below the GMFT and the groundwater has unique properties, including (1) no PFOA, (2) isotopically distinct (low dD and d18O), (3) geochemically distinctive (high Si and K, very low NO₃, Cl, low Ca-Mg-HCO₃), and (4) old recharge ages (>70 years). The anomalies are similar to those in a nearby public water supply bedrock spring ~3 km to the east. This signature also occurs 1 km west of the RLL in a PFOA-free private well surrounded by many others with high PFOA; the PFOA-free well in this example produced water from depths >162 m, significantly deeper than the other wells, all of which have dominantly young water (recharge ages ~20 yrs).

Wells initiated in the hanging wall (carbonates) of the Breese Hollow Thrust (BHT) and drilled into the foot wall (phyllites) produce water from the thrust surface. Groundwater here has no PFOA, intermediate stable isotope levels, hybrid major and trace element composition, and an age of ~55- 70 years. Wells completed along the eastern splay of the BHT have similar tracer signatures, but high PFOA.

Our multi-disciplinary methodology was effective in characterizing this heterogeneous and anisotropic aquifer system, where thrust faults and fracture zones strongly influence groundwater flow.