Localization, characterization and dating of water circulations in thesoil-saprolite system of the Strengbach watershed: petrological,hydro-geophysical and geochemical evidences.

International audienceThe characterization of the critical zone along depth profiles remains a major scientific issue for understandingand modelling the response of continental surfaces to climatic, tectonic and anthropogenic forcings. Besides char-acterization it requires the modelling of the water circulations within the substratum of the critical zone. A seriesof boreholes drilled along the north and the south slopes of the Strengbach watershed makes it possible to charac-terize the critical zone to depths of100 to 150 m within this critical zone observatory. In this study we attemptto combine mineralogical and petrological observations of the cores recovered through the drilling with chemicaldata of waters collected in each of these wells and hydro-geophysical data in order to characterize processes ofwater-rock interactions, visualize the water arrivals within the boreholes and bring new information on the deepwater circulations within the watershed.Mineralogical, petrological and hydrogeophysical data suggest that deepwater circulation in the watershed likelyoccurs along fractures, concentrated in relatively narrow areas, several centimeters wide, interspersed with areaswhere the granite is much less fractured. This points to the occurrence of deep waters circulating in a networkof more or less independent conduits, which could extend over several tens to hundreds of meters deep. The hy-drochemical data from the boreholes, show contrasting characteristics for surface waters collected at 10 to 15 mdepth and the deeper waters collected between 50 to 80m depth; the surface waters are very similar to those of thespring waters collected in the watershed (Pierret et al., 2014), and the deeper waters collected between 50 to 80mdepth. The residence times of the circulating waters are also very variable, with ages of up to a few months forsurface and subsurface waters and ages exceeding several decades for the deep waters. These differences suggestthat the subsurface circulation systems are quite different from the deeper circulation ones. They also point to theimportance to focus future studies on deep-water circulations in order to properly characterize the functioning ofthe critical zone in watersheds, especially in mountainous areas, such as the Strengbach watershe