Anhydrite and the Sr isotope evolution of groundwater in a carbonate aquifer

Major element concentrations and 87Sr/86Sr ratios were measured in groundwaters and bedrock from the Madison aquifer in western South Dakota. In this region, the Madison aquifer is primarily comprised of dolomite belonging to the Madison Limestone Group. The purpose of the study was to investigate controls on the downgradient evolution of dissolved Sr2+ in a carbonate groundwater system that is recharged by waters with high 87Sr/86Sr ratios draining Precambrian basement rocks and to establish the sources of Sr2+ added to the waters by reaction with the aquifer lithology. A mass-balance model following previous workers was used to calculate amounts and effective rates of mineral dissolution and precipitation during groundwater transport along a ∼240-km flow path. Both the calculated reaction rates and data for the Sr isotope geochemistry of the reacting phases were then used to develop a self-consistent and quantitative description of the concentration and isotopic composition of dissolved Sr2+ in the aquifer waters. The major ion chemistry of Madison aquifer groundwater is known to evolve according to dolomite dissolution, anhydrite dissolution, calcite precipitation, and ion-exchange with clay minerals. Dissolved 87Sr/86Sr ratios in the Madison aquifer decrease downgradient. Input waters draining the igneous Black Hills have 87Sr/86Sr ratios of ∼0.723, while highly evolved waters in the aquifer have 87Sr/86Sr ratios of ∼0.708. Dissolved Sr2+ concentrations undergo a concurrent increase from ∼1200 to 66,000 nmol/l. Model results indicate that dolomite dissolution exerts a critical control on the major ion chemistry but is not the primary source of nonradiogenic Sr2+, as both the dissolution rate and Sr concentration of dolomite are very low. Anhydrite is readily soluble in water, has a low 87Sr/86Sr ratio (∼0.708), and a very high Sr concentration (∼50,000 nmol/g). Anhydrite is also greatly undersaturated in the groundwaters, and the downgradient evolution of Sr2+ accompanies an ∼80-fold increase in dissolved S