GEOCHEMICAL AND HYDROLOGIC CONTROLS ON THE COMPOSITION OF SURFACE-WATER IN A HIGH-ELEVATION BASIN, SIERRA-NEVADA, CALIFORNIA

Hydrologic, mineralogic, and soil data are used to determine the sources and geochemical controls on the composition of surface water in the Emerald Lake watershed (ELW), a high-altitude basin located in the southern Sierra Nevada. The solute composition of stream waters at the ELW can be divided into three periods: snowpack runoff, a transition period in summer as snowpack runoff decreases and little precipitation occurs, and a low-flow period from late summer through winter. Each period has different geochemical controls on the solute composition of surface waters. During snowpack runoff approximately 50% of stream flow was from direct surface runoff and approximately 50% of stream flow was return flow from subsurface reservoirs. Hydrologic residence time of subsurface water at maximum snowpack runoff was measured directly with a (LiBr)-Li-6 tracer and varied from 9 to 20 h. Three independent measurements show that the acidity in snowpack runoff was neutralized by cation exchange in soils and talus. Discharge from soil reservoirs was the primary source of stream flow during the summer transition period when the composition of stream flow was congruent with the stoichiometry of plagioclase weathering. Processes occurring below the soil zone exerted the dominant geochemical controls on the composition of stream waters during the period of low flow, with preferential weathering of the anorthite component of plagioclase in subsurface rock and further weathering of kaolinite to gibbsite.