MANGANESE CONTAMINATION OF BEDROCK WELLS ASSOCIATED WITH NEW DEVELOPMENTS IN CONNECTICUT

High density developments are being constructed in Connecticut having bedrock wells for their sole source water supply. The Connecticut Department of Environmental Protection (DEP) has identified developments in 14 towns in metamorphic highland areas where concentrations of dissolved manganese in bedrock wells are high, ranging up to about 25 mg/l. In some cases pre-existing homes, which had not experienced high manganese levels, began to exhibit high levels following nearby development. Manganese is a neurotoxin and nuisance for clothes and fixtures. This on-going study is directed at evaluating whether the high levels of manganese are naturally occurring or related to development. A GIS based study was conducted using water quality data from 1,200 wells sampled during National Uranium Resource Evaluation project of the late 70s. The mean manganese level was about 0.1 mg/l, with the highest level being less than 1 mg/l. This mean value is similar to that observed in bedrock groundwater studies conducted in other New England states and clearly reinforces the anomalous nature of the high manganese levels. Based on manganese geochemistry and its abundance in rocks and soils in the State, the high concentrations observed may be attributed to the lowering of pH or Eh conditions which are conducive to manganese reduction (+4 to +2) and increased solubility. These conditions can potentially result from rock-blasting related activities or site excavation and construction activities. Eh reduction can be due to: incomplete combustion or spillage of common blasting agents (ANFO); decomposition of blasting combustion products; burial of wood and plant debris; leaching of fertilizer in establishing lawns; and septic field startup. The reduction in pH can be due to: oxidation of carbon monoxide generated from blasting to carbon dioxide and subsequent carbon dioxide dissolution; exposure of fractured crystalline rock at the surface to low pH precipitation; and increased groundwater recharge following bedrock excavation and exposure along new and preexisting fracture pathways. In some but not all cases the developments are spatially associated with pyrite bearing schists. The oxidation of the pyrite following rock exposure or burial of rock fragments could also generate low pH recharge.