GEOLOGIC CONTEXT OF ELEVATED RADIONUCLIDE OCCURRENCES IN NW VERMONT

Elevated naturally-occurring radionuclide levels have been found in bedrock water wells from Milton/Colchester, and St. George, Vermont. The geologic “landscape” of the Milton/Colchester radionuclide problem is significantly different than that of St. George.

A cooperative study between the Dept. of Health, Geological Survey, and towns of Milton and Colchester was initiated in response to the discovery of elevated gross alpha levels in private bedrock wells in a Milton housing subdivision. Since previous radiometric studies in the Milton/Colchester area had identified general areas of elevated radioactivity within the Clarendon Springs Formation (CSF), additional radiometric analyses were conducted on wells within/in close proximity to the CSF; eight well clusters were identified that exceed EPA gross alpha standards. Secondary testing implicated the 238U decay series.

U-bearing minerals in the Cambro-Ordovician CSF dolomites have primary and secondary origins. Detrital zircon, apatite, monazite, and sphene can be found disseminated throughout the dolomite in certain locations whereas secondary sphalerite, galena, and pyrite are found associated with fractures;the highest radiometric readings are related to secondary U-bearing minerals(Parker, 1988; Whitten, 1988). The Hinesburg Synclinorium folds the CSF and is truncated by the Champlain and Hinesburg thrusts.

The St. George Trailer Park (SGTP) lies near the convergence of the Paleozoic Hinesburg Thrust and the Mesozoic St. George Normal Fault. These faults juxtapose Cambrian metasedimentary rocks (Fairfield Pond Fm and Cheshire Qtzite) with Ordovician Limestones (Bascom Fm). The primary water supply for the SGTP was found to have elevated gross alpha levels in 1999; this bedrock well penetrated an outlier of the Hinesburg Thrust. Reconnaissance bulk-rock geochemistry of lithologies surrounding the SGTP demonstrate that the Fairfield Pond Fm phyllite and Cheshire Quartzite have Th abundances that significantly exceed those of average continental crust. Syn and post-tectonic fluid migration along these major fault zones is inferred to be responsible for concentrating radionuclides in some bedrock wells.