Time-intensive discharge and chemical data for two alpine streams in the Loch Vale watershed, Colorado, were used to identify sources of runoff, flow paths, and important biogeochemical processes during the 1992 snowmelt runoff season. In spite of the paucity of soil cover the chemical composition of the streams is regulated much as in typical forested watersheds. Soils and other shallow groundwater matrices such as boulder fields appear to be more important in controlling surface-water chemistry than their abundance would indicate. The chemical composition of the major source waters (usually thought of as end-members whose chemical composition is relatively constant over time) changes at the same time that their mixing ratio in streams changes, confounding use of end-member mixing models to describe stream-water chemistry. Changes in the chemical composition of these source waters are caused by the ionic pulse of solutes from the snowpack and the small size of the shallow groundwater reservoir compared to the volume of snowmelt passing through it. The brief hydrologic residence time in the shallow groundwater indicates that concentrations of most dissolved constituents of stream water were controlled by fast geochemical processes that occurred on timescales of hours to days, rather than slower processes such as weathering of primary minerals. Differences in the timing of snowmelt-related processes between different areas of the watershed also affect the stream-water chemical composition. Cirque lakes affect discharge and chemical composition of one of the streams; seasonal control on stream-water NO 3 and SiO 2 concentrations by diatom uptake in the lakes was inferred. Elution of acidic waters from the snowpack, along with dilution of base cations originating in shallow groundwater, caused episodes of decreased acid-neutralizing capacity in the streams, but the streams did not become acidic. This paper is not subject to U.S. copyright. Published in 1995 by the American Geophysical Union. Paper number 95WR02037. is not well known [Wigington et al., 1990]. Concentrations of acid anions (NO3 and SO4) in the snowpack along the Continental Divide in northern Colorado were found to be twice the regional background level in 1991 and 1992 [Turk et al., 1992], indicating strong potential for acidic meltwater in this area. Snowmelt also dilutes base cations which come from weathering, making these streams particularly susceptible to episodic acidification during the snowmelt runoff period. Episodic acidification of lakes and streams is well documented in areas known to be affected by acid deposition such as Scandinavia and the northeastern United States [Wigington et al., 1990]. Although deposition rates are much lower in the western United States, many high-altitude lakes and streams have little natural buffering capacity and are at risk to acidification from even small amounts of acid deposition. The risk of episodic acidification during snowmelt is exacerbated by the ionic pulse effect that causes elevated concentratio...
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