SOURCE AND TRANSPORT OF ARSENIC IN GROUNDWATER IN SOUTHWESTERN VERMONT

Drinking water from some wells in the southwestern Vermont towns of Pawlet, Rupert and Wells is contaminated with arsenic. Of 80 wells tested in the three towns, eight contain more than 10 ppb arsenic (the drinking water standard for the State of Vermont and the EPA). Most wells are drilled in bedrock. Temperature, pH, total dissolved solids, conductivity and oxidation-reduction potential were also measured at each well. pH ranges from 6.3 to 9.1; higher arsenic concentrations tend to be associated with higher pH values. There is also a correlation between arsenic concentration and oxidation-reduction potential, with higher levels of arsenic associated with lower ORP values, although not with the most reducing values. The study area is in the Taconic Slate Belt. The dominant lithologies are Lower Cambrian and Lower-Middle Ordovician slate, phyllite and schist. Slaty cleavage is oriented in a dominantly northerly direction, dipping uniformly to the east. The bedrock is well fractured, with fractures occurring in a great variety of orientations. Quartz and quartz-calcite veins and veinlets are somewhat more clustered in a general northerly direction. These veins are often iron-stained, indicating the likely presence of pyrite. A narrow brecciated zone found in one slate quarry contains quartz, calcite, and minor sulfides, including pyrite and chalcopyrite, and perhaps other sulfides (too fine to identify in hand sample). Many of the slate formations of the region contain pyrite that is not associated with veins. This pyrite occurs as the locus of reduction spots and as larger clots of fine-grained material flattened along the foliation. If arsenic is being leached by groundwater from sulfides in the bedrock, determining whether it might occur in an individual well is complicated by the variable orientation of the fractures through which the groundwater is flowing.