Data from the Ambient Ground Water Quality Network, public water supplies and other studies reveal that arsenic concentrations in ground water are highest in certain bedrock aquifers of the Newark Basin than other aquifers in New Jersey. The Newark Basin here is mainly comprised of red, gray and black (organic rich) shale and sandstone that are inter-layered and with basic igneous intrusions. Random sampling of 94 domestic wells in a 200 square mile study area within the basin indicate that 15 percent of the wells have arsenic concentrations exceeding 10 parts per billion with a maximum of 57 ppb. Well water with arsenic concentrations greater than 10 ppb are characterized by having low dissolved oxygen concentrations (DO < 3 mg/L) and have pH values ranging from 7.5 to 8.0. Arsenic concentrations greater than 40 ppb are associated with anoxic (DO < 1.0 mg/L) or nearly anoxic ground water.  Subsequent sampling in the basin has identified localized areas where well water contains over 200 ppb arsenic.

Initial whole rock geochemical analysis of red (maximum: 11 ppm As), gray (maximum, 50 ppm As) and black shale (maximum: 130 ppm As) from several members of the Passaic Formation, indicate that black shale has the highest arsenic concentrations. Electron microprobe analysis of the black shale identified pyrite (FeS₂) as the major source of arsenic. In two separate black bed members from the lower part of the Passaic Formation the maximum arsenic concentrations measured in pyrite were 40,000 and 3000 ppm. A domestic well near the black bed with 40,000 ppm As contained 48 ppb As. A municipal well intersecting but cased off from the bed with 3000 ppm As had a concentration of 44 ppb As.

One conceptual model for arsenic mobilization and transport in the Newark Basin involves the oxidation of pyrite, and possibly other sulfide minerals, in the recharge area as the release mechanism with subsequent transport along the ground-water flow path. Another is desorption of arsenic from hematite and possibly other oxide minerals in the red shale under moderately reducing chemical conditions. More than one mechanism for the release of natural arsenic to ground water in the basin is likely to exist. The solubility, and therefore mobilization and transport, of arsenic are enhanced along ground-water flow paths with reduced dissolved oxygen content and alkaline environments

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