Estimating Mixing Ratios for Source Waters in the Madison Aquifer Based on Water Chemistry, Wind Cave National Park, South Dakota

The U.S Geological Survey and the National Park Service are conducting a study of the Madison aquifer in Wind Cave National Park in the southern Black Hills of South Dakota. Wind Cave, which is within the karstic Madison Limestone, provides a unique insight into ground-water flow in the Madison aquifer because the cave contains underground lakes, drips, and flowing water. The purpose of the study is to better understand ground-water flow through the Madison aquifer and to characterize source-water areas for different sample locations based on water chemistry. This karst environment is a complex system to model because of fracture enlargement from dissolution reactions between ground water and the surrounding limestone. Therefore, to identify potential recharge areas or other source waters for the study area, an end member mixing model, utilizing principal component analysis, is used to estimate the water chemistry of probable end members (source waters). End-member chemistries are estimated using two different methods. The first method involves the estimation of end-member chemistry assuming that none of the samples represent any specific end member. The second method determines which samples most likely represent end members through statistical evaluation. Possible end members include local outcrop recharge and regional ground-water flow from west of the Black Hills. The mixing proportions of these end members are estimated for several sampling sites. Samples collected from 20 locations in 2007 were analyzed for major ions, arsenic, nitrate, and stable isotopes. Results of this study can be applied to better characterize ground-water flow through the complex karst environment and can provide insight about probable areas where the Madison aquifer is more susceptible to ground-water contamination in the southern Black Hills.