ARSENIC OCCURRENCE, SOURCES, MOBILIZATION AND TREATMENT IN THE NEWARK BASIN IN NEW JERSEY

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. Strong statistical correlation's between REE (sum La to Lu) and As (r = 0.95), and moderate and weaker ones respectively between As and REE with Al₂O₃ (r = 0.80 for both) and Fe₂O₃ (r = 0.68 and 0.70) in 16 red mudstones and siltstones (4.5 to 14.8 mg/Kg As) from the middle and upper Passaic suggest a clay and hematite association for As. Abiotic leach experiments targeting the water-soluble fraction in red mudstone (14.8 mg/Kg As) demonstrated that the major As mobiliziation mechanism in the red beds is desorption, with maximum aqueous concentrations occurring at pH 8.0 and in the presence of competitive ions such as phosphate. The lithogeochemical setting affects water quality and water treatment system design. Arsenite is the dominant arsenic species in the Lockatong and arsenate in the Passaic formations. The NJDEP is field testing and evaluating arsenic water treatment systems to determine the most efficient, cost effective, user friendly, and environmentally sound technologies for removing arsenic from well water in New Jersey. Adsorptive media's have proven to be the preferred treatment technology.