MULTIVARIATE STATISTICAL ANALYSES OF MAJOR IONS AND TRACE METALS REVEAL ROCK-WATER INTERACTION PATTERNS IN AN ALPINE WATERSHED

Geochemistry is commonly used as a tool in hydrogeology to delineate groundwater flowpaths. In this study we test the utility of trace metals to identify rock-water interaction pathways sourcing perennial springs. Our field site is the well-studied Sagehen Creek basin, an alpine catchment in the Sierra Nevada northwest of Lake Tahoe. Spring water samples were analyzed for 24 major ions and trace metals to identify the primary weathering reactions and sources of trace metals in groundwater. We identified patterns in the geochemistry of the spring waters by utilizing Principal Component Analysis (PCA), Hierarchical Cluster Analysis (HCA), and pending PHREEQC modeling from three separate sampling events (Oct. 2021, June 2022, and Sept. 2022). After reducing the dimensionality of the datasets using PCA, multiple HCAs utilizing different data types grouped springs into clusters. The analytes exerting the most control on the multivariate analyses are calcium, boron, and vanadium. In general, increases in major ion and select trace metal concentrations are related to indicators of rock-water interaction (e.g., discharge temperature and groundwater age). However, clustering analysis results reveal that the most geochemically similar springs tend to emerge from similar underlying geology. For example, according to the major ion HCAs, springs Diversion and Yum, two high elevation springs in the basin, are closely related. Diversion emerges from semi-consolidated, heavily weathered granitic boulder deposits and Yum between glacial till, largely unweathered granite boulders, and glacial drift. Springs Kiln, Leopold, and Turnaround consistently cluster together in the trace metals HCAs from multiple sampling events. Most of the remaining springs cluster together in one group. Overall, these analyses indicate that subtle differences in the underlying geology affect spring geochemistry and that trace metals are an effective tool for tracing rock-water interaction.