Arsenic occurrence, sources, mobilization, transport and prediction in the major bedrock aquifers of the Newark Basin

Ground water in bedrock aquifers of the Newark Basin in New Jersey contain up to 215 mug/L As. This basin is a Mesozoic aged half graben filled with non-marine strata and intrusive and extrusive basaltic rocks. Arsenic > 10 mug/L in ground water mainly occur in deep lucustrine organic rich black and gray shale of the Lockatong Formation and shallow lake/playa derived red mudstone and siltstone of the younger Passaic Formation. Analyses of black shale, gray shale, and red mudstone show maximum As concentrations of 240, 50 and 14.8 mg/Kg respectively (crustal average, 1.8 mg/Kg). Electron microanalyses of pyrite in black shale confirmed that it is the major mineral source of As containing up to 4 weight percent. The oxidation of pyrite may mobilize As into ground water. The sum of REE in 16 red mudstones and siltstones (4.5 to 14.8 mg/Kg As) from the middle and upper Passaic Formation strongly correlate with As (r = 0.95) and moderately with Ti (r = 0.79) suggesting a clay association for the REE and therefore As. The congruent mobilization of As and REE from red mudstone leached using hot oxalic acid indicates that hematite is also a source of these elements. Abiotic leach experiments targeting the water-soluble fraction in red mudstone (14.8 mg/Kg As) show that As concentrations directly correlate with pulverized rock surface area, duration of water-rock contact and As concentrations in the source rock. Also, competitive adsorption with phosphate and a pH of 8.0 maximize As concentrations in solution indicating that desorption and not dissolution is the main mobilization mechanism. Therefore, the desorption of As from clay minerals coated with early hematite is the dominant mechanism for As mobilization during water-rock contact in the red strata. Dissolution type water-bearing zones found in some red strata can supply much lithogenic As to wells intersecting them because they buffer and control aqueous pH as carbonate minerals dissolve, provide maximum surface exposure to matrix clays, and have high hydraulic conductivity and yields. The spatial distribution of arsenic concentrations in wells can be associated with identifiable geologic strata and extrapolated along strike to other areas.