POTENTIAL ULTRAMAFIC-DERIVED ARSENIC CONTAMINATION IN BEDROCK WATER WELLS IN NORTH-CENTRAL VERMONT

Analysis of 30 bedrock ground water wells in the vicinity of Stowe, Vermont reveals three wells with arsenic concentrations that exceed the EPA MCL of 10 ppb, with two of the wells producing water containing 86 and 275 ppb As. Seven additional wells contained between 1.5 and 7.0 ppb As. The wells are located in the Stowe, Ottaquechee, Moretown, and Hazens Notch Formations, and possibly penetrate isolated serpentinites like the Barnes Hill body. Geochemical analysis of the Stowe, Ottaquechee, Moretown and Hazens Notch Formations (Anderson, 2006; Morris, 2006) document low concentrations of arsenic in schists and quartzites (mean = 3.3 ppm) and greenstones (mean = 0.7 ppm). However, recent analysis of five rock samples from the Barnes Hill serpentinite body in northeast Waterbury document arsenic concentrations between 21 and 449 ppm (mean = 124 ppm). The use of SEM on the thin sections from the Barnes Hill body did not identify a particular trace mineral that contained arsenic; instead, it appears that the arsenic is disseminated throughout the serpentine, possibly substituted into tetrahedral layers in serpentine (Hattori et al., 2005).

The elevated concentrations of arsenic within the rocks from the Barnes Hill Formation, and the lack of elevated arsenic in nearby rock formations, suggest that the ultramafic rocks are the source of the high arsenic concentrations in the ground water. To date, we have found no correlation between arsenic and iron or other elements expected of a sulfide source (e.g. Cu, Ni, Pb, Zn, Mg) in either bedrock or ground water. The next steps in this project will be to analyze more ground water wells in transects across ultramafic bodies, and conduct additional field and geochemical analysis of a larger ultramafic sample suite. Sequential chemical extractions will be performed to examine mineralogical associations of arsenic in the serpentinites.