Résumé
One of the current scientific and societal challenges concerning the characterization of the spatial and temporal evolution of the Earth continental surfaces is to define all the active water circulations and water-rock interactions within the critical zone. The vertical extension of the critical zone, beyond the few meters classically studied through the analysis of weathering profiles, is still a very open question. The understanding of the deep critical zone functioning will certainly be improved by the information obtained from the semi-deep boreholes (10 m to 100 m deep) drilled in the substratum of several critical zone observatories. This is the case for the Strengbach watershed, one of the French environment observatory labeled "Service National d'Observation" by the INSU-CNRS, France, and integrated to the French Research Infrastructure OZCAR. The Strengbach catchment has been equipped with 6 boreholes from 50 to 100 m deep, drilled along the two slopes of the catchment, in its upper part. Some of them have been cored. Several piezometers ( approximately 15 m deep) have been also drilled. The petro-geochemical analysis of the drill cores, the loggings of the wells, the monitoring of water levels, along with the geochemical characteristics of the borehole waters allow us to give a conceptual diagram for the localization and the nature of the water circulations and the related weathering processes in the Strengbach basin. Comparing these data with the hydrogeochemical data of the springs emerging on the Strengbach watershed makes it possible to quantify the part of the deep hydrogeochemical fluxes relative to the sub-surface ones. The results are also important to better understand the role of the deep critical zones in the constitution of the water resources in such granitic catchments.