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. In Vermont, 40 % of the state’s population obtains drinking water from private wells that are rarely tested for inorganic or organic contaminants. The focus of this study is the central Champlain Valley of northwestern Vermont, where a large portion of the population relies on private wells that produce drinking water from a shale-dominated aquifer. The Champlain Valley is comprised of Cambrian-Ordovician passive margin sediments with black shale as a common lithology in some areas. The shales are the youngest units in the sequence and were deposited in a downwarping, deepening basin related to convergence associated with the Taconian Orogeny. The Champlain thrust fault divides the bedrock aquifer system, placing allochthonous Hortonville Formation black shales structurally above parautochthonous black shales of the Stony Point Formation. 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 and black slates as sources of elevated arsenic in bedrock aquifers. Accordingly, this study is designed to examine 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, and also from wells that originate (at the surface) in the hanging wall and are completed below the thrust in the footwall. 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 the EPA MCL of 10 ppb As, although 5/29 (~17 %) exceeded 5 ppb As. Preliminary data from wells producing from black shales of the Hortonville Formation in the hanging wall of the Champlain thrust indicate that 2 out of 12 wells tested thus far contain > 10 ppb As. Additional groundwater analyses paired with XRF, ICPMS 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.