Availability and quality of water from the bedrock aquifers in the Rapid City area, South Dakota

An evaluation made in cooperation with the U.S. Bureau of Reclamation of the availability and quality of water from the bedrock aquifers in the Rapid City area, South Dakota, concluded that Madison aquifer has the greatest potential for additional development of the three major aquifers investigated (the Inyan Kara, the Minnelusa, and the Madison). Ground-water availability and quality were evaluated on the basis of unit thickness and depth, potentiometric-surface altitudes and gradients, estimated recharge and discharge rates, estimated aquifer transmissivities and storage coefficients, reported yields of existing wells, and concentrations of ions in the water that may affect its use as a community supply. The Inyan Kara aquifer has the least potential for additional development because of reported small well yields , the proximity of the outcrop, and concentration of radium-226 exceeding 5 picocuries per liter. The Minnelusa aquifer is unsuitable for development in the eastern two-thirds of the study area because the concentrations of dissolved solids and sulfate commonly exceed the recommended maximum level for community water supplies. The Madison aquifer has the greatest potential for additional development because it has the greatest recharge rate, has areas with significant fracture permeability, yields as much as 500 gallons per minute to wells, and has satisifactory water quality, though it is hard (hardness 120 to 180 milligrams per liter) to very hard (hardness greater than 180 milligrams per liter). (USGS)