SOURCE OF ARSENIC IN DRINKING WATER AND GROUNDWATER FLOW PATHWAYS IN SOUTHWESTERN VERMONT

Drinking water from some wells in the Taconic slate belt of southwestern Vermont is contaminated with arsenic. The source of the arsenic appears to be sulfides in the host rocks. Pyrite is fairly abundant, occurring in a variety of forms, including as individual grains growing within the foliation, as aggregates of finer-grained crystals with quartz also growing within the foliation, and as crystals in foliation-parallel and cross-cutting quartz veins. Three samples of pyrite were analyzed for a suite of elements, including arsenic. One large individual crystal not associated with any quartz contains 182 ppm As. An aggregate of fine-grained pyrite and quartz growing within the foliation contains 531 ppm As, and pyrite associated with a foliation-parallel quartz vein contains 993 ppm As. A very small grain of arsenopyrite has been identified by SEM in quartz in a narrow brecciated zone that contains other sulfides (dominantly pyrite). Reflected light microscopy work suggests a few tiny discrete grains of arsenopyrite from the foliation-parallel quartz vein described above.

In several cases, a well with arsenic concentrations above the EPA drinking-water standard of 10 ppb is located within 50 m of an uncontaminated well. Most wells are drilled into bedrock, and groundwater flow through bedrock is fracture-controlled. The presence or absence of arsenic in a well that intersects a particular fracture depends on whether water in that fracture has come in contact with arsenic source material (and if the geochemical parameters were right for the arsenic to be mobilized). Additionally, a fracture may contain arsenic-contaminated groundwater, but this will not be a drinking-water problem unless a well happens to intersect that fracture. This fracture control on groundwater flow can explain why neighboring wells could have different arsenic content. Fracture orientations in outcrops (including road cuts and quarry walls) were measured to help determine groundwater flow direction. Plotting 296 fracture orientations on a rose diagram shows two dominant fracture directions: 010° to 030° (parallel to regional structural trends) and 100° to 110°. Further analysis will determine if the distribution of arsenic can be correlated to these structural trends.