International Geologiical Congress - Oslo 2008


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EGC-01 General contributions to environmental geochemistry - Part 1


Chemical erosion in high Alpine catchments: Water-rock interaction controlling the composition of surface water in Zermatt-Matterhorn area, Switzerland


Wei Zhou, University of Freiburg (Germany)
Ingrid Stober, University of Freiburg (Germany)
Kurt Bucher, University of Freiburg (Germany)


The hydrochemistry of surface runoff has been extensively used to infer hydrological flow paths and the mechanisms of solute acquisition in open system environments. In this study we concentrate on the chemical evolution of surface waters and dissolution of solids by waters in geologically distinct Alpine catchments of the Zermatt-Matterhorn area, Switzerland.
Fifty surface water samples were collected in autumn 2007, from three glacier localities and three major river systems, representing the geographical and lithological diversity. Reaction rates of water - catchment rock interaction strongly relate to lithology, soil cover and vegetation. To avoid direct anthropogenic or organic contibutions, waters were collected from small water bodies at high elevation. The altitude of sampling area is around 1600m to 3200m. The waters gain their solutes predominantly by direct water-rock interaction.
Sample temperature, pH, and electric conductivity (EC) were determined on site. The cations were measured with atomic absorption spectrometry and HCO3 by titration. All other anions were analyzed by ion chromatography. The electroneutrality of the analyses were all less than 8%. The total of the dissolved solids (TDS) is relatively low and varies from 6 to 244 mg/l. The dominant solutes in the samples are Ca, HCO3, SO4 and minor components are Mg, Na, K, NO3, Cl and Si, while F and B occur in traces only.
A hydro-geochemical inverse modeling has been set up to estimates the contributions of the predominant various rock-forming minerals to the water samples' chemical composition. Multivariate analyses and mass balance modeling using PHREEQC permitted to group genetically related waters.
Multivariate analysis of the samples divided the data into two types of chemical correlation coordinates. Cluster analyses (K-technique) show that Ca-HCO3-SO4 and Na-K-Cl are the strong correlation groups, while, Mg relates preferentially to Na-K-Cl rather than to Ca-HCO3-SO4. The modeling with PHREEQC shows that surface waters are undersaturated with respect to all minerals of the chemical system considered: Calcite, aragonite, chalcedony, gypsum, dolomite show high to moderate SI, the SI of sepiolite, chrysotile, talc and other silicates are always very low.
Three types of water, reflecting the three main catchment types, can be distinguished. Type 1 water comprises 12 samples with Mg-Na-SO4 as main solutes. The catchments are dominated by ophiolites comprising serpentinites and eclogites. Type 2 water groups 23 samples with Ca-HCO3 signature. The composition is controlled by the dissolution of calcite from metasediments including calcareous micaschists and marbles. Type 3 water includes 15 samples mostly from larger rivers in the main valleys showing characteristic mixing patterns.


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