Geochemical insights into groundwater movement in alpine karst, Bear River Range, Utah, USA

Abstract

Alpine karst aquifers control the availability and longevity of some water resources, but are not well understood. A conceptual model of the alpine karst aquifer system in the Bear River Range of northern Utah (USA) has been developed by geochemical analysis (major ions, δ18O, δ2H and δ13C values) of seasonal water samples from seven perennial springs, and residence-time assessment (3H and CFCs) of two low- and two high-discharge springs. All spring data can be explained by reaction paths dominated by the dissolution of calcian dolomite. The δ13C values align well with reaction paths for open-system dissolution. Saturation indices and low Ca:Mg molar ratios indicate that incongruent dissolution exerts a strong control on water–rock interactions, complicating interpretation of natural solute tracers. Values of δ18O and δ2H in springs follow the Utah meteoric water line. Snow δ18O values correlate with elevation, but not with increasing rainout distance, providing qualitative estimates of recharge elevation that generally align with previous dye-traces to five of the seven springs. Concentrations of 3H and CFCs likely are best described by binary mixing of subannual recharge with 60–65-year-old groundwater, suggesting that the alpine karst aquifer system in the Bear River Range is best represented by a double-porosity model. Subannual recharge documented by dye traces implies that caverns are the primary flowpaths to the springs, but the presence of decadal-age water may indicate that lower permeability flowpaths dominate during baseflow. No evidence was found for a longer-residing flow component, suggesting high sensitivity to future climate variability.