AN ANALYSIS OF RADIONUCLIDES IN THE CLARENDON SPRINGS FORMATION: IMPLICATIONS FOR GROUNDWATER COMPOSITION IN NORTHWESTERN VERMONT

Groundwater produced from fractured bedrock of the Clarendon Springs Formation (CSF) is enriched in uranium and alpha radiation in some areas of northwestern Vermont, but this issue has not been examined in all areas where the CSF occurs. Previous research by the Vermont Geological Survey in Chittenden County has indicated that the surface radioactivity in the CSF is associated with ‘black-chip’ breccia horizons that were speculated to be rich in U-bearing phosphorite minerals or organic matter; however, the CSF farther south in Addison County generally does not contain elevated radionuclides, indicating a possible depositional or diagenetic control on uranium distribution in rock and groundwater. Airborne NURE surveys from the 1970s identified U anomalies associated with the CSF in northern Chittenden County of northwestern-most Vermont, but no systematic study of bedrock and well water chemistry has been performed in this area. Accordingly, the purpose of this study is to determine if the concentration and distribution of radioactivity in the Highgate area is confined to the CSF breccia horizons and whether or not the radioactivity is being mobilized into drinking water, resulting in elevated U and/or gross alpha in private bedrock wells. 50 bedrock samples have been collected across the Highgate area of the CSF as well as other formations. Using both ICP-AES and ICP-MS, preliminary geochemical data indicates a correlation between U concentration and phosphorus content, particularly in CSF black chip breccias near the Vermont-Quebec border. For example, preliminary data from bedrock in this area indicates 0.7 to 2.4 % P₂O₅ and 210 to 1130 mg/kg U₃O₈. 30 domestic water samples will be collected and thus far, of the first 16 samples collected, 25 % contain elevated gross alpha (between 5 to 19 pCi/L). Each of the wells with elevated gross alpha is situated in or adjacent to the CSF. We speculate that syndepositional tectonics that produced the down-dropped Franklin Basin in late Cambrian time led to diagenetic formation of U-rich phosphorite minerals that occur in the black chip breccias. Current research is examining mineralogy, mineral textures and chemistry by a combination of XRD, SEM-WDS and ICP-AES/MS in order to better understand U speciation and mechanisms by which U is mobilized in the bedrock aquifer system.