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This study focuses on 20 years of hydrochemical monitoring of the small springs that emerge in the experimental granitic catchment of Strengbach (OHGE, France) and the simulation of these data using the KIRMAT code. The data indicate that the Strengbach springs display chemostatic behavior; that is, limited temporal variations were noted in the concentrations of dissolved silica (H 4 SiO 4 ) and most of the basic cations during the studied period (1987–2010), resulting in relative stability of the global weathering fluxes exported by the springs. Only the Ca 2+ concentrations reflect a significant decrease in all the Strengbach springs since 1987, and the variations differ from one spring to another. The modeling results show that the decrease in Ca 2+ in the Strengbach springs is due to the response of the water-rock interactions within the bedrock to the variations in the chemical composition of the soil solutions, which were characterized by a significant increase in pH and a decrease in Ca 2+ concentrations between 1987 and 2010. The decrease in Ca 2+ concentrations seen in the Strengbach springs is controlled by changes in the apatite dissolution rate and the compositions of clay minerals induced by the soil solution changes. The differences observed between the Ca 2+ trends of the springs are related to changes in the residence time of the water supplying the different springs. The weak impact of the soil solution modifications on the dissolution rates of other primary minerals and on the bulk precipitation rates of the clay minerals explains the relative stability over time of the concentrations of the other cations and dissolved silica in the water derived from the Strengbach springs. Further, the hydrochemical simulations suggest that the chemostatic behavior of the Strengbach springs cannot be explained by the mobilization of waters that are close to chemical equilibrium, but rather by a hydrological control of the spring water residence times. Finally, a comparison of current and long-term weathering rates determined from the spring water monitoring and a regolith profile shows that the modern chemical fluxes of Ca 2+ are higher than the long-term ones, whereas the weathering fluxes of H 4 SiO 4 and Na + have likely been much more stable over time. All of these results indicate that the silicate weathering processes are characterized by weak spatial and temporal variability in the Strengbach catchment, while the chemical elements such as Ca 2+ , for which the budget in the spring waters is controlled by the dynamic behavior of clay minerals and minor minerals such as apatite, are significantly affected by Quaternary climatic variations and decennial environmental changes. [ABSTRACT FROM AUTHOR]

U–Th–Ra disequilibria, cosmogenic in situ 10 Be concentrations and major and trace element concentrations have been analyzed in a 2 m-deep weathering profile sampled at the summit of the granitic Strengbach catchment (France). The data have been used to independently estimate both the long-term regolith production and denudation rates and the weathering and erosion rates. Modeling of the 238 U– 234 U– 230 Th– 226 Ra disequilibrium variations in the lower part of the profile yields a regolith production rate of 12 ± 4 mm/kyr (30 ± 10 T/km 2 /yr), while modeling of the high-resolution 10 Be concentration profile leads to an exposure age of 19.7 ± 2.2 kyr , an inherited concentration of 15,000 ± 1,000 at/g in quartz and a mean denudation rate of 22 ± 10 mm/kyr (37 ± 15 T/km 2 /yr). The consistency between production and denudation rates suggests that, on a millennial timescale, the regolith mass balance at the summit of the catchment is close to a steady state, even if the watershed may have been impacted by Quaternary climatic changes and by recent anthropogenic perturbations (e.g., 20th century acid rain and recent afforestation efforts). The results also indicate that physical erosion is likely the dominant long-term process of regolith denudation in the catchment. Furthermore, the comparison of the long-term production and denudation rates and of weathering and erosion rates determined from the depth profile analyses with the current weathering and erosion rates estimated at the outlet of the watershed based on monitoring of the water chemistry and sediment fluxes suggests that physical erosion may have varied more than the chemical weathering flux during the last 150 kyr. Although very few other sites with U-series, in situ 10 Be and stream monitoring data are available for comparison, the current data suggest that (1) the mass balance steady state of regolith might be commonly achieved in soil mantled landscapes, and (2) physical erosion has varied much more than chemical weathering in mid-mountain catchments over the last 10–150 kyr. These results highlight the importance of the combined analysis of U-series nuclides and in situ 10 Be in the same weathering profile for the determination of key geomorphic parameters, which are important to constraining landscape stability and the responses of landscapes to natural or anthropogenic forcing. [ABSTRACT FROM AUTHOR]

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]