Alkalinity dynamics in an unacidified alpine lake, Sierra Nevada, California1

Gem Lake, a small, unacidified, high elevation lake in the Sierra Nevada, undergoes an annual cycle of low alkalinity (between 30 and 40 µcq liter−1) during the ice‐free season and high alkalinity (>240 µeq liter−1) in fall and winter. Both pH and alkalinity decrease drastically (up to one unit for pH and 200 µeq liter 1 for alkalinity) during spring snowmelt, but chemistry of the snow, snowmelt, and runoff indicate that the declines in alkalinity are due to dilution rather than acidification. The hydraulic residence time in the lake increases from about 135 d in winter to <7 d during snowmelt; high alkalinity water is simply flushed from the lake and replaced by water with an ionic composition very near that of the snowmelt runoff. Increases in alkalinity in fall and winter can be accounted for by the rates of weathering in the watershed. A mass balance approach is used to reconstruct weathering reactions in the basin; it suggests that the lake receives both surface runoff, with the weathering products of the hydrolysis of granodiorite minerals to kaolinite, and deep water from subterranean circulation, with the products of granodiorite weathering to both kaolinite and smectite. The relative importances of low alkalinity surface runoff and high alkalinity groundwater to the recharge of the lake are proposed as factors controlling the seasonal bicarbonate concentration of the lake.