Combining Hydrological Modeling (NIM) and Hydrogeochemical Modeling (KIRMAT) to Capture the Variability of Water Chemistry in an Elementary Watershed (Strengbach, France).

Understanding the variability of the chemical composition of surface waters is an important issue for the scientific community, especially given the prospect of significant environmental changes for the next decades. The study of concentration-discharge relationships has been intensively used to assess the spatio-temporal variability of the water chemistry at watershed scales, however, the lack of independent estimations of the water transit times within catchments limits our ability to model and predict the water chemistry with the only use of geochemical modeling approaches. This study demonstrates the interest of coupling mathematical hydrology and hydrogeochemical modeling to better understand the variability of the composition of water. Firstly, a hydrological and integrated depth model (NIM) has been used to evaluate the distribution of the flow lines that are feeding the springs. Secondly, hydrogeochemical simulations with the code KIRMAT have been performed to determine the evolution of the water chemistry along the flow lines. The results show that the concentrations in dissolved silica (H₄SiO₄) and in basic cations (Na⁺, K⁺, Mg²⁺, and Ca²⁺) in the spring waters are well reproduced with a straightforward integration along the flow lines. The results also indicate that the limited variability of the flow line distribution and of the flow velocity implies that the water transit times only vary from about 1.5 to 3 months from floods to drought events. These findings demonstrate that the chemostatic behavior of the spring chemistry is a direct consequence of a strong hydrological control on the water transit times within the watershed. This study also demonstrates that it is possible to correctly capture the chemistry of sub-surface waters with simply determined mineral reactive surfaces and standard kinetic constants determined through laboratory experiments. Overall, this work shows that the hydrogeochemical functioning of an elementary catchment such as the Strengbach is relatively simple. The acquisition of the water chemistry can be explained by water-rock interactions that are spatially relatively homogeneous within the catchment. All these results confirm the interpretations from previous studies concerning the functioning of this catchment (Ackerer et al., 2016; Ackerer et al., 2018).