34S/32S Fractionation during Sulfate Reduction in Groundwater Treatment Systems: Reactive Transport Modeling

Isotope ratio measurements provide a tool for indicating the relative significance of biogeochemical reactions and for constraining estimates of the extent and rate of reactions in passive treatment systems. In this paper, the reactive transport model MIN3P is used to evaluate sulfur isotope fractionation in column experiments designed to simulate treatment of contaminated water by microbially mediated sulfate reduction occurring within organic carbon-based and iron and carbon-based permeable reactive barriers. A mass dependent fractionation model was used to determine reaction rates for S-32 and S-34 compounds during reduction, precipitation, and dissolution reactions and to track isotope-dependent mass transfer during SO4 removal. The delta S-34 values obtained from the MIN3P model were similar to those obtained from the Rayleigh equation, indicating that there was not a significant difference between the conceptual models. Differences between the MIN3P derived alpha value and the Rayleigh equation derived value were attributed to minor changes in the dissolution and precipitation rate of gypsum and mathematical differences in the fitting models. The results indicated that the prediction of delta S-34 was fairly insensitive to differences in the fractionation factor at the concentration ranges measured in the current study. However, more significant differences would be expected at low sulfate conditions.