DETERMINING THE CONTRIBUTING STREAMFLOW FROM CRYSTALLINE AND CARBONATE-KARST AQUIFERS IN RELATION TO HYPORHEIC FLOW IN SPRING FED BRIGHT ANGEL CREEK, GRAND CANYON NATIONAL PARK, AZ

The prevailing theory is that the majority of flow in tributaries to the Colorado River in the Grand Canyon, including Bright Angel Creek, is sourced from R-Aquifer springs. The R-aquifer is a karst aquifer with chemistry that is typically dominated by carbonate to dolomitic chemistry (Ca-HCO3 with varying amounts of Mg). As Bright Angel Creek approaches the river, it flows through the Vishnu Schist, a crystalline unit which has numerous small springs emerging from fractures. These springs and other potential springs buried beneath alluvial deposits have a possibility of contributing significantly to stream discharge. There is significant hyporheric exchange occurring as water moves in and out of gravel deposits over schist bedrock. Here we address the source of the added water to the stream: is this simply creek water reemerging or an addition of water emerging from the Vishnu schist. This study quantifies the overall contribution of water emerging from both the R-aquifer and Vishnu Schist formation through assessing variability in spring chemistry and utilizing an end-member mixing model to assess overall importance of each aquifer to the flow of Bright Angel Creek. The R-aquifer end member is a karst aquifer characterized by conduit and fracture flows, and contains gypsum, dolomite and sulphides. The Vishnu Schist end member aquifer is fracture dominated composed of calcite, dolomite and various pyrite (As-, Mo-, Pb-, Zn- and Cu-) sulphides. Study results contribute to the National Park Service initiative to further determine potential inputs to the stream versus accounting for gravel exchanged waters for determining environmental flows for federally listed endangered fishes native to the stream.