RADIONUCLIDE-ENRICHED GROUND WATER, KNOX MOUNTAIN PLUTON, VERMONT: OCCURRENCE AND LITHOLOGIC CONTROLS

Since 2003, two public and five private groundwater wells in the towns of Marshfield and Peacham, VT have demonstrated elevated uranium concentrations exceeding EPA and VT Department of Health maximum contaminant levels (i.e. [U]=28 to 300 ppb). Due to the prevalence of unregulated private wells in the region, the extent of uranium contamination is unknown, posing the potential for a major public health concern. All seven contaminated wells are completed deep within fractured bedrock aquifers of the Knox Mountain Pluton, implicating lithologies within this Devonian intrusion as a likely natural source. Thus, evaluating the extent and specific origins of the naturally-occurring uranium in groundwater within the Knox Mountain Pluton is of both social and geologic significance.

This study combines groundwater and bulk rock geochemistry to assess, respectively: (1) the distribution of uranium-enriched groundwater in the study area; and (2) the potential of differing lithologies within the pluton as sources of uranium and associated radionuclides in the groundwater. Approximately 30 water samples, primarily from bedrock wells within the Knox Mountain Pluton, will be analyzed for major and trace element geochemistry, with particular focus on concentrations of uranium and associated radionuclides, as well as gross alpha levels. Bedrock samples from various lithologies within the pluton will be evaluated using a combination of optical, SEM, and ICP methods for mineralogy, major and minor bulk rock geochemistry, and leaching potential of uranium.

This work should ultimately provide Marshfield, Peacham, and other neighboring towns, as well as the VT Geological Survey and the VT Department of Health, with a greater understanding of radionuclide groundwater contamination in the area of the Knox Mountain Pluton, raise awareness of private well-owners, and allow for safer and more cost-effective siting of wells in the future. The study may also have regional implications for other areas of northeastern Vermont and New Hampshire underlain by similar plutons of the New Hampshire Magma Series.