RADIONUCLIDES, GROUNDWATER GEOCHEMISTRY, AND HYDROGEOLOGY ABOVE, BELOW, AND THROUGH THE HINESBURG THRUST: NW VERMONT

The discovery of elevated naturally-occurring radioactivity in a bedrock public water supply well in 1999 in the Town of St. George prompted the VT Geological Survey to evaluate the geological context of this well. Geological data showed that the well was drilled through the Hinesburg Thrust (HT), a major tectonic boundary between metamorphic rocks of Green Mountains and sedimentary rocks of the Champlain Valley. At the time, it was not known whether the source of the radioactivity was upper plate metasedimentary rocks, lower plate carbonate and clastic rocks, or the fault zone itself. Publicity associated with the elevated radioactivity finding prompted water testing by many well owners in neighboring towns, and collation of this data led us to suspect rocks of the upper plate.

Groundwater from 54 domestic wells was tested for radioactivity, metals, anions, alkalinity, and hardness. The groundwater geochemistry was integrated with well logs, 1:24K maps, and cross-sections. Wells with the highest gross alpha (GA) levels were completed in the upper plate of the HT in the Pinnacle (33 + 34 pCi/L) and Cheshire fms (19 + 2 pCi/L); some isolated high values occurred in the Fairfield Pond Fm (8.7 + 13 pCi/L). Wells completed in the lower plate away from the HT front consistently had lower GA (7.6 + 5.7 pCi/L) than the upper plate, whereas GA values of wells completed in lower plate rocks producing below the HT (17 + 12 pCi/L) were < upper plate and > lower plate, suggesting that radioactivity may leak from the upper plate through the fault into the lower plate. Wells in the upper plate consistently have higher levels of Ca and Ba.

To evaluate whether there is natural communication between the aquifers in the upper and lower plates, we used logs from wells drilled through the HT in the field area (n=19). The average well was completed in the lower plate 85 m below the HT, suggesting that there is little water associated with the thrust zone and that it is generally a barrier to downward flow. This is further supported by a specific well log that shows that the fault zone is dry and harder to penetrate than rocks on either side. Only 4 wells were either completed or had water producing zones near the thrust. Wells that have producing zones in the upper and lower plates may blend groundwater from both aquifers. Future use of CFC and 3H dating methods is planned.