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

Fe(III) 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 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 rates of CO2 production calculated from dissolved inorganic carbon mass transfer (2.55×10−4 to 48.6×10−4 mmol l−1 yr−1) generally were comparable to the calculated rates of CO2 production by the combined reduction of O2, Fe(III) and SO42− (1.31×10−4 to 15×10−4 mmol l−1 yr−1). The overall increase in SO42− concentrations along the flow path, together with the results of mass-balance calculations, and variations in δ34S 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.