Temporal and spatial variability of cation and silica export in an alpine watershed, Emerald Lake, California

A reaction set of possible mineral weathering reactions is proposed to explain observed cation and silica export for the Emerald Lake watershed, a small Sierra Nevada, California catchment. The reaction set was calculated through a stoichiometric mole-balance method, using a multiyear record of stream flow and snowpack chemical analyses and site-specific mineral compositions. Reaction-set calculations were intended to explore how the processes controlling stream cation and silica export depend on differing bedrock mineralogy across the catchment as snowmelt and runoff patterns change over the year. Different regions within the watershed can be differentiated by lake inflow subdrainages, each exhibiting different stream-flow chemistry and calculated weathering stoichiometry, indicating that different silica and cation generation processes are dominant in wet steep portions of the catchment. Short-term differences in stream concentrations were assumed to reflect ion exchange equilibria and rapid biological processes, whereas long-term persistent stream concentration differences in different areas of the catchment were assumed to reflect spatial variability in mineral weathering stoichiometry. Mineralogical analyses of rock samples from the watershed provided site-specific chemical compositions of major mineral species for reaction calculations. Reaction sets were evaluated by linear regression of calculated versus observed differences between snowmelt and stream-flow chemistry and by a combined measure. Initially, single weathering reactions were balanced and evaluated to determine the reactions that best explained observed stream chemical export. Next, reactions were combined, using mineral compositions from different rock types to estimate the dependence of ion fluxes on lithology. The seasonal variability of major solute calculated fluxes is low, approximately one order of magnitude, relative to the observed three orders of magnitude variability in basin discharge. Reaction sets using basin-averaged lithology and Aplite lithologies gave superior explanations of stream chemical composition.