VULNERABILITY MAPPING (1:24,000) OF THE MADISON AQUIFER NEAR RAPID CITY, SOUTH DAKOTA

Rapid City, South Dakota, is located within the Rapid Creek watershed in the east-central Black Hills. The City of Rapid City relies heavily on the Mississippian Madison aquifer for drinking-water supplies, utilizing several wells and springs. The Madison aquifer locally consists of limestone and dolomite and contains well-developed paleo-karst and recent karst features. Previous work indicates that surface water recharges the Madison aquifer from the adjacent Spring Creek watershed to the south and Boxelder Creek watershed to the north, with ground water converging on wells and springs in the Rapid City area several miles away. Spring Creek and Boxelder Creek lose all their flow to karst features in the Madison aquifer, except during periods of high discharge (greater than approximately 28 cubic feet per second for Spring Creek and 50 cubic feet per second for Boxelder Creek). Dye-tracer tests for this area indicate ground-water velocities are on the order of 300 meters per day (1,000 feet per day) and residence times can be less than 30 days. Based on this information, Rapid City’s water supply is extremely susceptible to contamination. Well logs, fractures, faults, geologic structures, water-quality data, dye-tracer tests, and human influences are being analyzed to develop a geologic model that will be used to better define local ground-water flow paths and characterize vulnerability zones. Inherent aquifer susceptibility will be combined with potential sources of anthropogenic contamination to develop a vulnerability map (1:24,000) of the Madison aquifer for the Rapid City area.