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

Major element concentrations and ^(87)Sr/^(86)Sr 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 Sr^(2+) in a carbonate groundwater system that is recharged by waters with high ^(87)Sr/^(86)Sr ratios draining Precambrian basement rocks and to establish the sources of Sr^(2+) 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 Sr^(2+) 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 ^(87)Sr/^(86)Sr ratios in the Madison aquifer decrease downgradient. Input waters draining the igneous Black Hills have ^(87)Sr/^(86)Sr ratios of ∼0.723, while highly evolved waters in the aquifer have ^(87)Sr/^(86)Sr ratios of ∼0.708. Dissolved Sr^(2+) 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 Sr^(2+), as both the dissolution rate and Sr concentration of dolomite are very low. Anhydrite is readily soluble in water, has a low ^(87)Sr/^(86)Sr 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 Sr^(2+) accompanies an ∼80-fold increase in dissolved SO_4^(2−). While anhydrite has a very low abundance in the aquifer rocks, the flow model indicates that anhydrite dissolution provides ∼300 times more Sr^(2+) per liter of water relative to dolomite dissolution. These findings suggest that anhydrite dissolution governs the Sr^(2+) 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.