PYRITE AS THE SOURCE OF GROUNDWATER ARSENIC IN TACONIC SLATES, SOUTHWESTERN VERMONT

Arsenic concentrations above the EPA and Vermont state drinking water standard (0.01 mg/L) in domestic and public drinking water supplies has been found in more than 10% of wells tested in southwestern Vermont. The host rocks of these wells are slates and phyllites of the Taconic range. The slate occurs in black, green, purple, gray and red forms. The phyllite is dominantly silvery-gray with variable quartz content. All rocks are variably deformed, and quartz veins are common throughout the region. It appears that the arsenic, at least in part, is derived from arsenic-bearing sulfides, predominantly pyrite. Pyrite occurs in Taconic slates in a variety of forms: (1) As isolated crystals and masses growing within the foliation, unrelated to bedding or to veins, suggesting growth during metamorphism as metals and sulfur were scavenged from the immediate surrounding region, perhaps due to the presence of isolated organic material in the original sediment; (2) As layers in bedding (at an angle to foliation), suggesting organic-rich layers in the original sediment; (3) As crystals and small crystalline masses associated with quartz veins, suggesting association with metamorphic hydrothermal fluids carrying metals and sulfur, with deposition of sulfides occurring due to changes in pressure or temperature or chemical conditions within the hydrothermal system; and (4) as large (3-5 cm) framboids within foliation and rimmed by pressure shadows filled with fibrous quartz, suggesting that the pyrite is at least in part syngenetic or early diagenetic. The highest arsenic concentrations are found in pyrite associated with black, often graphitic, slate, as layers parallel to bedding (over 1000 ppm As) and in framboids (over 200 ppm As). Lower concentrations are found in pyrite in green slate. This suggests that As-bearing pyrite grows during lithification and/or metamorphism of originally organic-rich sediment. This study attempts to identify the role of As-bearing pyrite in the occurrence of arsenic in groundwater, and to determine the physical and chemical pathways of dissolution and transport of arsenic by groundwater in the region.