PUMPING TEST AND ASSESSMENT OF THE DEADWOOD AQUIFER AT JEWEL CAVE NATIONAL MONUMENT, CUSTER COUNTY, SOUTH DAKOTA

Jewel Cave National Monument is 13 miles west of Custer South Dakota. The park has two public water-supply wells that draw water from the Deadwood aquifer approximately 550 feet below the land surface. Little is known about the characteristics of the Deadwood aquifer at Jewel Cave. A collaborative effort by South Dakota School of Mines and Technology and the National Park Service recognized the need for assessment to determine aquifer parameters and vulnerability. Two pumping test were conducted. The first measured an average discharge rate of 13.1 gal/min and a total drawdown of about 28 feet in the pumped well. The specific-capacity equation was used to determine a transmissivity of 800 gal/day-ft. Water-level measurements were taken with a non-invasive sonic water-level indicator and were analyzed manually and with curve-matching software. Techniques included the Theis method, Papadopulos-Cooper method, Hantush method, Moench method, and effects of casing storage. The average transmissivity was 600 gal/day-ft and the storage coefficient was about 10-3. Caution was taken when fitting the type curves to the drawdown values because of casing storage within the well. In the second aquifer test, one well was pumped and drawdown was measured in the second well, 800 feet away. The test confirmed a low transmissivity because little or no drawdown was measured in the second well. However, the well showed minor changes in water level with time. A plot of water levels and barometric data showed an inverse correlation. Calculated parameters from the pumping tests indicated a ground-water velocity of 0.1 ft/day, using an assumed effective porosity of 0.1. Assessment showed that the aquifer has a low to moderate vulnerability; however, continued monitoring of potential contamination sources is essential to protecting the ground-water supply because the confined aquifer is overlain by the karstic Madison Limestone in which Jewel Cave formed.