Geochemical and isotopic (U, Sr) tracing of water pathways in the granitic Ringelbach catchment (Vosges Mountains, France)

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• 作者：Thi&eacute ; baud Schaffhauser^a ; Franç ; ois Chabaux^a ; ^{fchabaux@unistra.fr" class="auth_mail} ; Bruno Ambroise^a ; Yann Lucas^a ; Peter Stille^a ; Thierry Reuschl&eacute ; ^b ; Thierry Perrone^a ; Bertrand Fritz^a
• 关键词：Spring waters ; Major element concentrations ; Sr and U isotope ratios ; U modeling ; Granitic weathering
• 刊名：Chemical Geology
• 出版年：26 May, 2014
• 年：2014
• 卷：374-375
• 期：Complete
• 页码：117-127
• 全文大小：1119 K

文摘

Major element concentrations and U and Sr isotope ratios were determined in waters collected from the main springs located in the small (0.36 km²) Ringelbach catchment (Vosges, France), which has a basement consisting of Hercynian granite capped in its upper part by a residual cover of Triassic sandstones. The spring waters were sampled from 2001 to 2004 during highly contrasting hydrological conditions and monthly over two hydrological years October 2004-September 2006. Deep-water samples were also collected from three boreholes (two drilled down to a 150 m depth and one down to a depth of 70 m) and analyzed.

The data indicate systematic geochemical differences between the waters from sandstone and granite and important spatial variations in the chemical and isotopic compositions (Sr and U) between the spring waters on granitic lithology. For the granitic spring waters, the alkalinity, the cationic concentrations and the pH values increase with decreasing spring elevation. The (²³⁴U/²³⁸U) activity ratios of the granitic spring waters also increase with decreasing spring elevation, from approximately 1 in the upslope springs to 1.3 in the downslope springs. Waters of each spring have significant temporal variations in the elemental concentrations and elemental ratios but much less variations in their Sr isotopic ratios and U activity ratios. These spatial and temporal geochemical variations are unrelated to both the mixing scenarios between weathering and rainwater end-members and between granitic and sandstone weathering fluxes. Moreover, the comparison of the geochemical and isotopic compositions of Sr and U of the granitic spring waters with those of deep waters collected from boreholes demonstrates that the geochemical variations in the granitic spring waters cannot be explained by mixing between the surface water and these deep groundwaters. The spring waters are supplied by subsurface waters flowing through water paths disconnected from those of the borehole waters. Furthermore, the geochemical and Sr isotopic characteristics of the granitic spring waters suggest that the different granitic springs are most likely supplied by waters with relatively independent water pathways that are controlled by the geometry of the fracture network structure in the Ringelbach regolith. The increase in the alkalinity, the major element (Na⁺, Ca^2 +, Mg^2 +) concentrations and the U activity ratios of the granitic spring waters with decreasing spring elevation indicate that the main parameter that must be considered to explain the geochemical characteristics of these waters is the length of the water pathway within the bedrock, which controls the duration of the water-rock interactions. Based on this interpretation, the (²³⁴U/²³⁸U) activity ratios in the spring waters were modeled using a simple 1D reactive transport model that considers dissolution, precipitation and alpha recoil. By estimating the alpha recoil factor and the length of the water path for each spring, both the dissolution rate of the bedrock and the water residence time within the slope can be estimated.