Geochemical modeling of iron, sulfur, oxygen and carbon in a coastal plain aquifer

Fe(LII) reduction in the Magothy aquifer of Long Island, NY, results in high dissolved-iron concentrations that degrade water quality. Geochemical modeling was used to constrain iron-related geochemical processes and redox zonation along a flow path. The observed increase in dissolved inorganic carbon is consistent with the oxidation of sedimentary organic matter coupled to the reduction of O-2 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 rates of CO2 production calculated from dissolved inorganic carbon mass transfer (2.55 x 10(-4) to 48.6 x 10(-4) mmol l(-1) yr(-1)) generally were comparable to the calculated rates of CO2 production by the combined reduction of O-2, Fe(III) and SO (1.31 x 10(-4) to 15 x 10(-4) mmol l(-1) yr(-1)). The overall increase in SO(4)(2-)concentrations along the flow path, tooether with the results of massbalance calculations, and variations in delta S-34 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 groundwater data from well-water samples for geochemical modeling. (C) 2000 Elsevier Science B.V. All rights reserved.