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 sourc es and geochemical controls on the composition of surface water in the Emerald Lake watershed (ELW), a high-altitude basin located in the so uthern Sierra Nevada. The solute composition of stream waters at the E LW can be divided into three periods: snowpack runoff, a transition pe riod in summer as snowpack runoff decreases and little precipitation o ccurs, and a low-flow period from late summer through winter. Each per iod has different geochemical controls on the solute composition of su rface waters. During snowpack runoff approximately 50% of stream flow was from direct surface runoff and approximately 50% of stream flow wa s return flow from subsurface reservoirs. Hydrologic residence time of subsurface water at maximum snowpack runoff was measured directly wit h a (LiBr)-Li-6 tracer and varied from 9 to 20 h. Three independent me asurements show that the acidity in snowpack runoff was neutralized by cation exchange in soils and talus. Discharge from soil reservoirs wa s the primary source of stream flow during the summer transition perio d when the composition of stream flow was congruent with the stoichiom etry of plagioclase weathering. Processes occurring below the soil zon e exerted the dominant geochemical controls on the composition of stre am waters during the period of low flow, with preferential weathering of the anorthite component of plagioclase in subsurface rock and furth er weathering of kaolinite to gibbsite.