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

Rapid City, South Dakota, is within the Rapid Creek watershed in the east-central Black Hills. The city and surrounding developments rely heavily on the Madison aquifer for drinking-water supplies, utilizing several wells and springs. Within the Black Hills, the aquifer consists of Mississippian limestone and dolostone of the Pahasapa Limestone and contains well-developed Mississippian-Pennsylvanian and Early Tertiary paleo-karst and possibly recent karst features. Water chemistry analyses and dye-tracer test results reported by the U.S. Geological Survey indicate that surface water recharge from the Spring Creek watershed to the south and the Boxelder Creek watershed to the north converge on wells and springs in the Rapid City area eight to ten km (five to six miles) away. Spring Creek and Boxelder Creek lose all their flow to karst features in the Madison aquifer, except during periods of very high discharge (greater than 25 to 30 ft³/sec).

Published and unpublished dye-tracer tests for this area indicate that ground-water velocities are on the order of 300 meters per day (1,000 feet per day) or faster, and residence times can be less than 30 days. Based on this information, the water supply for Rapid City and surrounding areas is extremely vulnerable to natural and anthropogenic contamination.

Geologic structures, fractures, stratigraphy, structural contour maps, water-quality data, dye-tracer tests, and human influences are being analyzed to better define local ground-water flow paths and to define vulnerability zones for Rapid City’s water supply. Current findings indicate that structural controls (e.g., strongly developed systematic fractures, Laramide folds, and fracture traces) play a major role in ground-water flow.