Hydrological control of stream water chemistry in a glacial catchment (Damma Glacier, Switzerland)

The temporal and spatial controls of stream water chemistry in a small, granitic, glacial catchment were investigated in conjunction with high-resolution hydrological and meteorological measurements. Significant systematic seasonal and diurnal variations were observed in the stream water chemistry, which were not caused by the mixing of water draining different lithologies. A hydrological model (ALPINE3D, Lehning et al., 2006) was used to calculate the relative contributions of the principal water sources, snow melt and ice melt, throughout the period of this study. Pronounced seasonal minima in δ18O and 87Sr/86Sr were attributed to spring snow melt. Clear changes in X/Si ratios (X = Ca, Mg, K, Na, Sr) between summer and winter were observed. These changes are interpreted to reflect seasonal changes in the average residence time of water in the sub-glacial drainage network with short residence times in summer, when the discharge was greatest, leading to high X/Si ratios, and long residence times in winter, when the discharge was lowest, resulting in low X/Si ratios. This study shows that the time dependent stoichiometry of cation to Si ratios in glacial stream water (and likely all catchments) strongly depends on the hydrological state of the catchment at the time of sampling. Annual fluxes based on spot samples varied by a factor of six depending on the time of year in which the sample was collected, highlighting the importance of long-term catchment monitoring. An improved understanding of the spatial and temporal controls acting on stream water chemistry will allow silicate weathering processes to be more precisely quantified.

Research highlights
► Marked temporal variation in chemical composition observed in a granitic catchment.
► Combination of hydrological modelling and stream geochemistry.
► Hydrological control of stream water chemical composition.
► Spring decrease in 87Sr/86Sr and δ18O can be explained by snowmelt.