ARSENIC MOBILIZATION AND TRANSPORT IN HYDROGEOCHEMICALLY DYNAMIC RECHARGE ZONES VERSUS STEADY-STATE DEEP WATER-BEARING CONDUITS IN THE LOCKATONG AND PASSAIC FORMATION AQUIFERS IN THE NEWARK BASIN, NEW JERSEY

Potable well water from Lockatong (pyritic black-argillite) and Passaic (red hematite-mudstone) Formation aquifers have arsenic concentrations up to 254 µg/l, and, dominant aqueous species of arsenite (H₃AsO₃⁰) and arsenate (HAsO₄^2-) respectively. This is consistent with As-enrichment in the cyclic chemically reducing (small pyritic samples, up to 240 mg/kg As), and, oxidized (hematite rich, up to 14.8 mg/kg As) strata. Questions concerning what portion of the flow-path is the major source of mobile As: geochemically aggressive and dynamic processes in recharge zones, versus, steady-state conditions in deeper downgradient conduits, exist? Results from a 12-week kinetic column-leach test (USEPA Method 1627), coupled with, well water chemistry, provides answers. Application of Method 1627, required specific PSDs (< 0.25 mm to ~10 mm) of black-argillite and red-mudstone, exposure to a humidified atmosphere (10% CO₂ + 90% air) at 1-Lpm for 6-days, followed by a 24-hour DIW-saturation, to simulate and accelerate, chemical weathering and precipitation flushing expected in recharge areas. Maximum dissolved arsenic in leachate occurred in pyritic argillite, ranging from 52 µg/l (weeks 1 to 4) to 7.0 µg/l by week 12. During weeks 5 to 8, a rapid increase in aqueous (SO₄) ^2-, and, formation of HFO on pyrite surfaces with coincident 70% arsenic adsorption, occurred. In red mudstone, initial concentrations up to 15.5 µg/l decreased to 2.4 µg/l by week 12. All leachate samples had a circum-neutral pH, due the ample carbonate phases. Twenty shallow Ambient Monitoring Network wells in these formations have a median arsenic concentration of 1.5 µg/l, versus, > 5 µg/l in multiple 2-square mile potable-well clusters. Therefore, sustainable arsenic mobilization and transport are most significant in deep water-bearing zones where chemical stability maintains optimum pH and redox, and, the continued formation of adsorbent reaction products is hindered due to depleted reactants such as O₂ and H⁺. Also, the hypothesis that near-surface pyrite oxidation is the major source of As in the Lockatong Formation aquifer is not supported. Alternatively, the hypothesis by Zhu et al. (2008, Geochemica et Cosmochimica Acta, 72:21), that aqueous S^- substitutes for As^- in pyrite, in the deeper sulfate reducing groundwater here, is possible.