ARSENIC OCCURRENCE, SOURCES, MOBILIZATION AND TRANSPORT IN THE PASSAIC AND LOCKATONG FORMATIONS OF THE NEWARK BASIN

Ground water in bedrock aquifers of the Newark Basin in New Jersey contain up to 215 µg/L As. This basin is a Mesozoic aged half graben filled with non-marine strata and intrusive and extrusive basaltic rocks. Arsenic > 10 µg/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 in black shale is hypothesized to mobilize As into ground water in the Lockatong Formation. The sum of REE (La to Lu) 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 the degree of water-rock contact, 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. Many red beds in the Passaic Formation contain strata-bound dissolution zones where secondary, sparry, carbonate and sulfate minerals are dissolved out. These zones are likely sources of As to wells tapping them because they buffer and control aqueous pH, provide maximum surface exposure to As-bearing hematite coatings on matrix clays, and are sites of elevated hydraulic conductivity and yields.