STABLE ISOTOPE GEOCHEMISTRY OF RECHARGE IN AN ALPINE KARST AQUIFER, BEAR RIVER RANGE, UTAH

Karst aquifers comprise an important hydrologic resource, yet associated permeability enhancement precludes the use of traditional flow models. While dolomite and calcite dissolution along structures and bedding planes in lowland carbonate units typically result in multimodal flow regimes, little is known about karst permeability distribution in alpine environments. The Bear River Range of northern Utah comprises a classic alpine karst aquifer whose springs contribute significantly to anthropogenic uses in nearby Cache Valley. Stable isotope analysis of snow cores from varying elevations reveal an oxygen-18 altitude gradient of -0.28 (per mil/100m) and provides a baseline for determining recharge altitudes of local springs. Relative altitudes of catchments delineated by previous dye tracing (Spangler, 2004) align well with most groundwater oxygen-18 values, yet fail to describe one of the springs with the lowest, smallest catchment. As groundwater from this spring contains the most negative isotopic signature, this anomaly may represent a previously unmapped link with a much higher-altitude source. Groundwater oxygen and deuterium isotopes vary little across the seasons and adhere strongly to a local meteoric water line. Carbon-13 stable isotope values likewise differ minimally across seasons, possibly indicating only slight variations in residence time for slowest-moving (base flow) groundwater. In conjunction with calculated dissolution pathways of carbonate minerals, these data support a conceptual model of quick flow that occurs primarily through karstic solution conduits, rather than slower flow along smaller fractures or through interstitial pathways. Geochemical evidence for ubiquitously modern, quick-flowing groundwater in the Bear River Range yields implications for long-term behavior of alpine karst aquifers both in the United States and abroad.