Geochemical evolution of water in the Madison Aquifer in parts of Montana, South Dakota, and Wyoming

Abstract

The geochemical models BALANCE and WATEQF have been used to deduce the major chemical reactions controlling the ground-water chemistry in the Madison aquifer in parts of Montana, South Dakota, and Wyoming. Although the Madison aquifer extends into North Dakota, wells were not available in North Dakota from which an uncontaminated sample of water could be collected. The chemical system was assumed to be closed to mass transfer of carbon dioxide. The dominant reactions along each of the eight selected flow paths were found to be dedolomitization caused by the irreversible dissolution of anhydrite and the bacterially mediated reduction of sulfate. Some cation (calcium/sodium) exchange and halite dissolution were determined to occur along flow paths in central Montana.