EVALUATION OF GEOLOGIC CONTROLS ON ELEVATED NATURALLY-OCCURRING RADIOACTIVITY IN BEDROCK GROUND WATER WELLS, NW VERMONT

Between 1999 and 2003, elevated levels of naturally-occurring radioactivity were discovered in numerous bedrock ground water wells in the towns of St. George and Hinesburg. This area straddles the east-dipping Hinesburg Thrust (HT), which separates Proterozoic-Cambrian metamorphic rocks to the east from Cambrian-Ordovician sedimentary rocks to the west. The HT transported the metamorphic rocks westward over the sedimentary rocks in the Ordovician. Since elevated radioactivity in ground water poses a chronic health risk to humans, the goal of this study is to identify the geologic factors contributing to the problem in the Hinesburg and St. George area. Ground water radioactivity may be controlled by lithologic, structural, geochemical, and hydrologic factors. This multidisciplinary study concentrates on ground water geochemistry (including radioactivity testing) of wells penetrating each formation and fault and bulk rock geochemistry from each formation and fault.

By integrating geologic maps with well driller logs, we found that eight of eleven bedrock wells with elevated radioactivity were completed in metamorphic rocks above the HT (Fairfield Pond, Cheshire, and Pinnacle fms.), whereas the other three wells penetrated the HT into carbonate rocks (e.g. Bascom Fm.). Because this is a high growth residential area, there are many other wells completed above and below the HT that have not yet been tested for radioactivity. We will evaluate whether there is consistent lithologic and/or structural control on elevated radionuclide levels in ground water by sampling representative wells above and below the HT and running gross alpha tests and major and trace element geochemical analyses.

Preliminary bulk rock geochemical analyses of the Cheshire and Fairfield Pond fms. indicate elevated Zr (17-1210 ppm), Ba (66-1840 ppm), V (29-241 ppm), and Ti (0.03-2.25 ppm), whereas the Bascom Fm. limestone contains background concentrations of these elements. Elevated levels of Ba are associated with elevated levels of Ra. Zr, V, and Ti positively correlate with Ba in the Cheshire and Fairfield Pond rocks. The dissolution of radiogenic trace minerals (e.g. zircon) containing these elements is likely a primary source of radioactivity for bedrock ground water. We are currently analyzing the geochemistry of fault zone rocks.