Analysis of Groundwater Quality in a Fractured Rock Aquifer Influenced by Black Shales in the Central Champlain Valley, Western Vermont

Residents of rural regions tend to rely on a largely unregulated water source, private groundwater wells. The focus of this study is the central Champlain Valley of northwestern Vermont, with wells penetrating a... [ view full abstract ]

Residents of rural regions tend to rely on a largely unregulated water source, private groundwater wells. The focus of this study is the central Champlain Valley of northwestern Vermont, with wells penetrating a shale-dominated aquifer. Black shales tend to contain elevated trace element concentrations, particularly of redox-sensitive elements like arsenic and uranium, and recent studies in the northern Appalachians have identified black shales as a source of elevated arsenic in bedrock aquifers. Accordingly, this study examines the inorganic chemistry of black shales as well as groundwater produced from black shales in the hanging wall and foot wall of the Champlain thrust. Preliminary results from 29 shale-dominated wells in the Stony Point Formation of the footwall document relatively low levels of arsenic – only one well exceeded 10 ppb As, although ~17 % exceeded 5 ppb As. Preliminary data from wells sampling the Hortonville Formation in the hanging wall indicate that ~16 % of wells tested thus far contain > 10 ppb As. Additional groundwater analyses paired with XRD, XRF, ICP-MS and SEM-EDS analysis of shale chemistry will provide insight into controls exerted by black shales on groundwater, including potential control exerted by paleo-redox environment present at the time of deposition.