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Published January 25, 2005 | public
Journal Article

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 SO₄²⁻. While anhydrite has a very low abundance in the aquifer rocks, the flow model indicates that anhydrite dissolution provides ∼300 times more Sr²⁺ per liter of water relative to dolomite dissolution. These findings suggest that anhydrite dissolution governs the Sr²⁺ geochemistry of Madison aquifer groundwaters, whereas dolomite dissolution and calcite precipitation control the bulk chemistry. Lastly, it is shown that the calculation of net mineral masses transferred to solution requires very high relative proportions of anhydrite and clay to dolomite. Mineral masses obtained by this approach actually represent bulk contributions without consideration of the intrinsic reaction rates of the phases or their modal abundances in the aquifer rocks. However, consideration of these parameters in a transport equation for flow through porous media shows that the high apparent abundances are not a real requirement. This latter approach is consistent with laboratory experiments and indicates that as little as 0.04 wt.% of very soluble anhydrite and 0.3 wt.% of exchangeable clay in the aquifer rock are sufficient to produce the observed groundwater chemistry in most of the flow path.

Additional Information

We thank N. Plummer (USGS, Reston), who critically aided our data interpretation and offered much insight into the hydrology and geochemistry of the Madison aquifer. We also thank L. Putnam (USGS Rapid City) and K. Buhler (South Dakota Department of Environment and Natural Resources) who guided the selection of sample sites and provided invaluable support in the field. M. Wilson (geologic consultant, Denver) collected Madison anhydrite samples from cores at the USGS Core Library in Denver, CO. L. Benson (USGS, Boulder), A. Davis (South Dakota School of Mines and Technology), T. Hayes (USGS, Tucson), A. Long (USGS, Rapid City), and F. Phillips (New Mexico Institute of Mining and Technology) are also thanked for sound guidance. We are grateful to the private citizens and municipal water managers in South Dakota who granted access to wells sampled in this study. S. Trimble (Wayne State University) significantly aided field work, and H. Ngo and D. Papanastassiou (Caltech) provided much help and guidance in the laboratory. We would like to acknowledge an anonymous reviewer whose meticulous and critical evaluation of our work greatly improved the manuscript in print. This work was supported by DOE DE-FG03-88ER13851. Caltech Division contribution No. 8910(1112). [LW]

Additional details

August 22, 2023
October 24, 2023