Hydrogeochemical processes, reaction rates and effect of spatial scales in carbonate critical zone observatories: insights from the reactive-transport modeling of C-Q relations in four mountain watersheds

This study investigates hydrogeochemical processes, reaction rates and the effect of spatial scales in mountain carbonate watersheds. A reactive-transport model was deployed to capture the concentration-discharge relations (C-Q relations) of major solute species measured in the river waters of two critical zone observatories located in the south of France (southern Alps and Ardèche). The specific control of evaporite, carbonate and clay minerals on river water geochemistry is identified by the reactive-transport modeling. The dissolution of carbonate and evaporitic minerals strongly controls C-Q relations in Na⁺, Ca²⁺, Mg²⁺, SO4^2- and Cl^- solute species. The key role of evaporite dissolution cannot be neglected in the studied Alpine watersheds. The dissolution/precipitation of clay minerals and the surface remobilization is more important for shaping C-Q relations in H₄SiO₄ and K⁺. Concerning the coupling between hydrological and geochemical processes, the chemostatic behavior of rivers and the observation of overland flow events during high discharge periods can be reconciled by considering the evolved overland flow observed on hillslopes and characterized by high solute concentrations. Overland flow may lead to an overestimation of water transit time at high flow if elevated solute concentrations from overland flow sources are attributed to time-dependent weathering reactions. The C-Q relations in Ca²⁺ and Mg²⁺ are overestimated by our reactive-transport model, and the reactive surface area of calcite and dolomite must be reduced by 4–5 orders of magnitude in the input of the model to capture the measured data. We interpret this discrepancy as a result of probable subsurface heterogeneity in carbonate watersheds, since macroporosities and fractures can strongly decrease the apparent mineral reactive surfaces in the field. For the effect of spatial scales, shifting from elementary (<5 km²) to mesoscale watersheds (20–50 km²) has little impact on the geochemical composition of river waters while differences in hydrological functioning are observed. This absence of geochemical contrast cannot be only explained by equilibrium concentrations, but must also imply a geomorphological control on water transit times. Finally, our work highlights the importance of multi-observatory investigations. It also demonstrates that a thorough knowledge of the regional geology is key to understand the critical zone architecture, its mineralogical composition and the main hydrogeochemical processes.