teap DISTRIBUTION AND Geochemical modeling of
redox-active constituents in THE MAGOTHY AQUIFER, LONG ISLAND, NEW YORK
by
Craig J. Brown[1], Martin A.A. Schoonen[2], John D. Coates[3]
ABSTRACT
Fe(III) reduction in the Magothy aquifer of Long Island, N.Y., results in high dissolved-iron concentrations that degrade water quality. Analytical results of aquifer geochemistry, terminal electron-accepting process (TEAP) assays, and geochemical modeling were used to constrain iron-related geochemical processes and redox zonation along a flow path. The observed increase in dissolved inorganic carbon is consistent with lignite oxidation coupled to the reduction of O2 and SO42- in the aerobic zone, and to the reduction of SO42- in the anaerobic zone; estimated rates of CO2 production through reduction of Fe(III) were relatively minor by comparison. The overall increase in SO42- concentrations along the flow path, together with the results of mass-balance calculations, and variations in d34S values along the flow path, indicate that SO42- loss through microbial reduction is exceeded by SO42- gain through diffusion from sediments and through the oxidation of FeS2. Geochemical and microbial data on cores indicate that Fe(III) oxyhydroxide coatings on sediment grains in local, organic carbon- and SO42--rich zones have been depleted by microbial reduction and resulted in localized SO42--reducing zones in which the formation of iron disulfides decreases dissolved iron concentrations. These localized zones of SO42- reduction, which are important for assessing zones of low dissolved iron for water-supply development, could be overlooked by aquifer studies that rely only on ground-water data from well-water samples for geochemical modeling.