Stable isotope and hydrochemical data for rock glacier meltwater and streamwater in the Uinta and La Sal Mountains, Utah, USA

Water draining from rock glaciers in the Uinta and La Sal Mountains of Utah (USA) was analyzed and compared with samples of stream water. Samples were collected during the melt season of 2022 using five automated ISCO samplers. Two samplers were installed at springs discharging from the termini of rock glaciers in the Uinta Mountains, and a third at a rock glacier in the La Sal Mountains. A fourth sampler was deployed in the La Sal Mountain downstream from the studied rock glacier. The fifth sampler was deployed in the La Sal Mountains along a stream in a high elevation catchment lacking rock glaciers. Composite samples of summer and fall rain were also collected, as well as grab samples of snow. Samples were analyzed for their stable isotopic composition using a Los Gatos 45-EP Triple Liquid Water Isotope Analyzer at Middlebury College. Samples were run against a bracketing set of 4 standards and calibrated with a cubic spline function. Each sample was analyzed 10 times, with the first 2 injections discarded to minimize cross-over. Accuracy of the instrument is 0.4‰ for δD and 0.1‰ for δ18O. Hydrochemical characterizations were made with a Thermo iCap ICP-MS at Middlebury College. Samples were run against a set of standards derived from NIST Standard Reference Material 1643f “Trace Elements in Water”. An in-house standard was used to determine the abundance of Si and Ti, which are not present in 1643f. The NIST standard and the in-house standards were run after every 10 unknowns and a linear correction was applied to compensate for instrument drift. Interpretation focused on elements that consistently exhibited concentrations >1 ppb. Results support the interpretation that rock glaciers discharge water derived from snowmelt in the early summer, but as the snowpack is exhausted, water derived from the melting of internal ice becomes dominant. This later summer water is distinguished by higher values of δ18O, δD, and d-excess, along with elevated electric conductivity and greater abundances of cations including Ca and Mg. The pattern of shifting water sources in 2022 was nearly identical to that noted at the same Uinta Mountain rock glaciers in 2021, indicating that the system is consistent from year to year. Values of EC and hydrochemical parameters are similar for the two Uinta sites and the rock glacier-influenced sites in the La Sal Mountains, but are strikingly different between the two mountain ranges. This discrepancy is attributed to the dominance of quartzite bedrock in the Uintas versus igneous trachyte in the La Sals. Finally, the three sites considered in the La Sals reveal that the hydrochemical characteristics of rock glacier meltwater are transmitted downstream along the main drainage in a watershed containing rock glaciers, establishing notable contrasts in stream water chemistry with a nearby drainage lacking rock glaciers. Collectively, the results of this study underscore the significant role played by rock glaciers in controlling the hydrochemistry of high-elevation watersheds, and provide further motivation to better understand these features as they rapidly adjust to changing climatic conditions in mountain environments.