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This study presents the results of the coupled hydrogeochemical modeling of the geochemical compositions of spring and borehole waters from the Ringelbach catchment, which is located in the Vosges Mountains (France). This site has been equipped with 150-m-deep boreholes, facilitating the sampling of both rock and groundwater in the granitic bedrock. The data point to very contrasting chemical compositions between spring and borehole waters, which are discussed and explained in this study by the application of the coupled hydrogeochemical code KIRMAT. Using hydrological and geochemical data, simulations were performed through two different water pathways, which crossed different types of rocks within the Ringelbach massif: a subsurface and fast (>2.5 m H2O .yr −1 ) water flow, which is more or less parallel to the slope, for waters supplying the springs, and a rather vertical and slower flow (0.5–0.1 m H2O .yr −1 ) for the borehole waters. The KIRMAT simulations make it possible to account for not only the geochemical differences between the spring and borehole waters but also the geochemical variations observed in waters in both contexts. For borehole waters, the model confirms the importance of the dissolution of minor mineralogical phases that are present in the granite (here, carbonates/dolomites) on the chemical budget of waters. It also shows that the chemical differences between the waters collected in the two studied boreholes result from differences in the water flow in the granitic bedrock, i.e., the difference between water flow in a regular porous medium and water flow in a porous medium crossed by a fracture. This result likely highlights the role of geological inheritance on the hydrodynamical rock properties and the chemical compositions of waters circulating within the granitic bedrock. For spring waters, this model enabled us to constrain the nature of the rock in the pathway, which is neither saprolite nor fresh granite but is instead weathered granite with a weathering age of several tens of thousands of years. Spatial and seasonal variations in the chemical compositions of spring water can be explained as the result of the same circulation pattern for which the water-rock interaction time is determined by the length of the pathway and the water velocity. Especially in cases in which this interaction time is long enough, the precipitation of clay phases is enabled, which plays a major role in determining the chemical composition of the water. Despite the only one-dimensional approach and the uncertainties linked to the geochemical complexity and the associated kinetic data, the results obtained in this study demonstrate the effectiveness of using coupled hydrogeochemical modeling to better understand and quantify the weathering processes and the coupling that exists between water circulation dynamics and water-rock interactions at the catchment scale. [ABSTRACT FROM AUTHOR]

Abstract: Major element concentrations and U and Sr isotope ratios were determined in waters collected from the main springs located in the small (0.36km2) 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 150m depth and one down to a depth of 70m) 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+, Ca2+, Mg2+) 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. [Copyright &y& Elsevier]