Geochemical Modeling of the Madison Aquifer in Parts of Montana, Wyoming, and South Dakota

TLDR: In this article, stable isotope data for dissolved carbonate, sulfate, and sulfide were combined with water compo- sition data to construct geochemical reaction models along eight flow paths in the Madison aquifer in parts of Wyoming, Montana, and South Dakota.

Abstract: Stable isotope data for dissolved carbonate, sulfate, and sulfide are combined with water compo­ sition data to construct geochemical reaction models along eight flow paths in the Madison aquifer in parts of Wyoming, Montana, and South Dakota. The sulfur isotope data are treated as an isotope dilution problem, whereas the carbon isotope data are treated as Rayleigh distillations. All reaction models reproduce the observed chemical and carbon and sulfur isotopic composition of the final waters and are partially validated by predicting the observed carbon and sulfur isotopic compositions of dolomite and anhydrite from the Madison Limestone. The geochemical reaction models indicate that the dominant groundwater reaction in the Madison aquifer is dedolomitization ~calcite precipita­ tion and dolomite dissolution driven by anhydrite dissolution). Sulfate reduction, [Ca + + Mg2+ ]INa+ cation exchange, and halite dissolution are locally important, particularly in central Montana. The groundwater system is treated as closed to C02 gas from external sources such as the soil zone or cross-formational leakage but open to C02 from oxidation of organic matter coupled with sulfate reduction and other redox processes occurring within the aquifer. The computed mineral mass transfers and modeled sulfur isotopic composition of Madison anhydrites are mapped throughout the study area. Carbon 14 groundwater ages, adjusted for the modeled carbon mass transfer, range from modem to about 23,000 years B.P. and indicate flow velocities of 7~7 ftlyr (2.1-26.5 rnlyr). Most horizontal hydraulic conductivities calculated from Darcy's Law using the average 14 C flow velocities are within a factor of 5 of those based on digital simulation. The calculated mineral mass transfer and adjusted 14 C groundwater ages permit determination of apparent rates of reaction in the aquifer. The apparent rate of organic matter oxidation is typically 0.12 JLIDOl/Uyr. Sulfate and, to a lesser extent, ferric iron are the predominant electron acceptors. The (kinetic) biochemical fractionation of 34 S between sulfate and hydrogen sulfide is approximately -44%o at 25•c, with a temperature variation of