ARSENIC CONTAMINATION IN GROUNDWATER IN SOUTHWESTERN VERMONT

The Mettawee Community School serves the elementary-age schoolchildren of Pawlet and Rupert in southwestern Vermont. When the school was built in 1998, groundwater from two newly-drilled wells was in compliance with all state drinking water regulations. However, when the State of Vermont lowered its drinking water standard for arsenic from 50 ppb to 10 ppb, the school discovered it had an arsenic problem, with concentrations on the order of 30 - 40 ppb As. Forty wells within a ten-mile radius of the school were sampled for this study, in an effort to determine the extent and source of the arsenic contamination. Three other wells were found to have arsenic concentrations above 10 ppb, with one as high as 91 ppb. 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 are 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.

Because there is no obvious source of anthropogenic arsenic pollution (the region's main sources of income are dairy farming, slate quarrying, and tourism), bedrock is being targeted as the source of arsenic. The study area is in the Taconic Slate Belt. The dominant lithologies are Lower Cambrian and Lower-Middle Ordovician slate, phyllite and schist. The Cambrian St. Catherine Formation contains green phyllite, quartzite, purple and green slate, and chlorite schist. There are abundant quartz veins, sometimes including small, as-yet-unidentified masses of black, fine-grained, granular material. This unit is highly folded and faulted, and holds up the high ridges of the region. The Ordovician Pawlet Formation is a mixture of black slate and metagraywacke. The Ordovician Poultney Formation is dominantly gray and green slate, and is the leading quarried rock of the local area; a narrow north-south belt of this formation runs along the Vermont - New York border less than five miles west of the school. Pyrite is found is variable concentrations in all formations. Preliminary results suggest that elevated levels of arsenic in groundwater are related to the presence of the Saint Catherine Formation.