CHEMICAL EVOLUTION AND HYDROLOGY OF NORTHERN SIERRA NEVADAN MINERAL SPRINGS

Mineral springs in the Sierra Nevada of California provide small but unique montane habitats, which support rare biota. The geochemical evolution and hydrologic flow paths of groundwater discharging into these springs provides insight into the stability of these rare environments in a changing climate. General geochemical, stable isotope, and chlorofluorocarbon age tracer measurements were collected from thirteen springs in the Northern Sierra Nevada to understand these unique environments. A GIS database was created to study the geologic substrate and hydrology, and NETPATH was utilized to perform mass balance calculations. Five sites are located in Yosemite National Park, and eight are in the headwaters of the North Fork of the American River (NFA) within Placer County. All samples exhibited high levels of conductivity, which reflects their mineralized state. All springs are bicarbonate waters, but the dominant cation varied between sites. Mineral waters of Yosemite are of sodium-potassium type, but also contained high levels of calcium characteristic of its granodiorite setting. NFA waters are dominated by calcium, with an isolated two-spring subset exhibiting the highest values of calcium, sodium, and chloride. Although these two springs are associated with the surrounding granitic geology, a yet unidentified source of chloride and sodium influences their geochemistry. Mineral spring waters range in apparent ages from 40 to 60 years. Oxygen and hydrogen isotopes suggest a meteoric origin for all spring waters sampled. Yosemite waters deviate from the global meteoric water line along a trend indicative of evaporative enrichment. The Yosemite spring waters are also more isotopically depleted than the Placer County waters, likely due to a higher recharge elevation. Stable isotope values negatively correlated with geochemical age at each of the sites suggesting changing environmental conditions. This study provides insight on an example of a slowly circulating hydrologic system that is responsible for producing important microhabitats.