ENVIRONMENTAL TRACERS TO ASSESS GROUND-WATER FLOW IN THE MADISON AQUIFER, WIND CAVE NATIONAL PARK, SOUTH DAKOTA

Wind Cave National Park, located in the southern Black Hills of South Dakota, is home to one of the world's most extensive and complex cave systems. Underground lakes within the cave system are a unique feature in the Black Hills and offer a rare view of ground-water flow in the Madison aquifer. Recent proposals to increase ground-water withdrawals from the Madison aquifer in the vicinity of the park are of increasing concern. An immediate need exists to characterize source-water areas, discharge areas, transit times, and ground-water flow in the vicinity of the park. Therefore, a comprehensive analysis using multiple tracers is being undertaken to address this need. Sources of recharge to the Madison aquifer are sinking streams and infiltrating precipitation. Discharge occurs from water table springs in the outcrop area and artesian springs downgradient from the park. Water samples were collected from sinking streams, cave drip water, underground lakes, wells and springs in the vicinity of the park on a seasonal basis. Samples were analyzed for major ions, arsenic, stable isotopes of water, tritium and chloroflourocarbons (CFCs). Preliminary results suggest that cave sites and nearby wells are similar in concentration and may represent locally recharged water. Artesian springs have a different geochemical signature and may be influenced by regional flow. Enriched Ca, Mg and SO4 concentrations in artesian springs likely reflect dissolution of anhydrite in the overlying Minnelusa formation. CFC concentrations indicate that ground-water age at several sites, including one cave site, is likely a mixture of modern water and water that is decades old or more.