GROUND-WATER VULNERABILITY OF THE KARSTIC MADISON AQUIFER IN THE EASTERN BLACK HILLS

The karstic Madison aquifer is western South Dakota’s major subsurface reservoir. It is the primary water source for the City of Rapid City and several other communities in the eastern Black Hills. As a result of karst development, the Madison aquifer is extremely heterogeneous and anisotropic. It varies in thickness between 60 and 155 meters in the Rapid City area because of paleotopographic relief from erosion during late Mississippian time. Field mapping indicates that ground-water flow paths are influenced by fractures, joints, faults, folds, stratigraphic variations, and a Laramide-age, graben-like structural depression in the Rapid City area. The Madison aquifer is highly sensitive and vulnerable to contamination because of extremely fast ground-water velocities and little filtering of contaminants. Parts of the recharge area that are believed to display the greatest vulnerability contain sinkholes, disappearing streams, and other karst features, along with highways and concentrations of on-site wastewater systems in areas of residential and urban development. Current vulnerability mapping involves modifications of methods such as DRASTIC and KARSTIC. This vulnerability mapping is part of an effort involving nine contiguous 1:24,000-scale quadrangles in the Rapid City area.