Tracing and modeling of weathering processes on a small catchment scale: the Ringelbach (Vosges, France)

The main goal of the present thesis is to better constrain the parameters that control weathering processes at a small catchment scale, focusing specifically on the role of hydrology. For this purpose, temporal and spatial variations of the chemical and isotopic (U, Sr) water composition of the Ringelbach catchment are studied. Several springs of this catchment whose basement is mainly composed of granite more or less fractured located along an altitudinal profile were monthly sampled over a period of two years. The additional interest of this site is that three deep boreholes (up to 150 meters deep) allow the sampling of both deep rocks and waters. The connectivity of the different hydrological compartments is evaluated based on the geochemical interpretation of water samples. A schematic hydrological functioning is proposed based on a good knowledge of the geological context. Finally, a weathering rate and a water flux are estimated from the modeling of the uranium activity ratio along a water path. The originality of this study lays also in combination of a geochemical and modeling approaches using the software KIRMAT (Kinetic Reactions and Mass transport) that integrates geochemical reactions (dissolution/precipitation) and 1D mass transport equations. It allow to simulate the reactive transport of a fluid through a rock along a given water pathway. This modeling is based on the characterization of the mineralogical and physical properties of the rock, sampled along the boreholes and the geochemical interpretation of the waters. Thus, the modeling of the chemical composition of the spring waters and borehole waters permit to improve the understanding of weathering processes including the role of precipitated secondary phases. It also allows to better understand the interplay of parameters that that control the water chemical signature at the catchment scale. (author)