END-MEMBER MIXING ANALYSIS APPLIED TO THE KARSTIC MADISON AQUIFER USING WATER CHEMISTRY IN THE SOUTHERN BLACK HILLS, SOUTH DAKOTA

The U.S. Geological Survey and the National Park Service are studying the Madison aquifer in Wind Cave National Park in the southern Black Hills of South Dakota. The purpose of the study is to better understand complex groundwater flow through the karstic Madison aquifer in and around Wind Cave National Park and to estimate source-water areas for different sample locations based on water chemistry. Samples collected from 19 locations in 2007 were analyzed for major ions, nitrate, and stable isotopes. To identify 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 source waters, or end-members. End-member chemistries are estimated using two different methods. The first method assumes that none of the samples necessarily represents any specific end-member. The second method determines which samples most likely represent end-members through statistical evaluation. Possible end-members include source water from local outcrop recharge, recharge on the outcrop on the western side of the Black Hills, and regional groundwater flow from west of the Black Hills. The mixing proportions of these end-members are estimated for several sampling sites. Results of this study can be applied to better characterize groundwater flow through the complex karst environment and can provide insight about which areas are more important to protect from contamination in relation to a particular well or spring in the southern Black Hills.