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International audience; The 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 watershed

International audience; Une des étapes du projet « SILURES Bretagne » (Système d’Information pour la Localisation et l’Utilisation des Ressources en Eau Souterraine), mené de 2002 à 2008, a consisté à modéliser les hydrogrammes des rivières bretonnes afin de connaître la participation des eaux souterraines au débit de ces rivières. En s’appuyant sur le schéma conceptuel des aquifères de socle [1], 70 modélisations globales pluie-débit ont été réalisées au pas de temps journalier avec le logiciel BRGM Gardénia [2].Sur 12 des 70 bassins versants étudiés en zone de socle (l’Elorn, l’Aulne, le Yar, l’Horn, le Coët-Dan, le Dourduff, la Maudouve, la Noë sèche, l’Oust, l’Yvel, le Nançon et l’Aron), les volumes d’eau souterraine ont été calculés sur les 50 premiers mètres du sous-sol (dans le cadre de 5 études menées entre 2003 et 2014). A partir d’une modélisation des épaisseurs des aquifères (altérites et milieu fissuré) dans les roches constituant le sous-sol du bassin versant, et des teneurs en eau libre mesurées grâce à des sondages de Résonance Magnétique Protonique, le volume d’eau souterraine est approché [1].Connaissant le débit souterrain moyen annuel sortant d’un bassin versant et le volume d’eau souterraine, ainsi que la vitesse moyenne que met une goutte d’eau à passer du sol à la nappe [3], des temps moyens de résidence des eaux souterraines ont été calculés (temps moyen que met une goutte d’eau à s’infiltrer du sol à la nappe, puis de la nappe à la rivière). Ces temps sont très variables (de 2 à 8 ans) selon la géologie du bassin et des pluies efficaces l’alimentant.Ces temps de résidence ne correspondent pas à des temps de reconquête de la qualité de l’eau en raison des phénomènes de mélange qui se déroulent entre le sol et la nappe, dans les altérites et dans le milieu fissuré. Par contre, ce sont des temps minimums en deçà desquels il est utopique d’attendre des résultats tangibles sur la qualité des nappes et ensuite sur les cours d’eau, en réponse à une action de surface.

International audience; « SILURES Bretagne » is a scientific program (Information system for the Localization and the Use of groundwater resources), which aims to realize decision-making tools for a policy of management of the water (better consideration of groundwater, definition of priorities in the actions of restoration quality of Breton superficial waters). One of the stages of this project consisted in modelling the hydrograms of rivers to know the contribution that groundwater makes to the flows of Breton rivers and estimate the slowness of groundwater physical environment. By basing on the conceptual diagram of hard-rocks aquifer platforms (R. Wyns and al. 2004, B. Dewandel and al. 2006), 70 global modellings rains/river flow were realized in the step of daily time with the software BRGM Gardenia.On 11 of 70 catchments studied (Elorn, Aulne, Yar, Horn, Coët-Dan, Dourduff, Maudouve, Noë sèche, Oust, Yvel and Nançon), the volumes of groundwater in the first 50 metres of basement were calculated. By modelling the thickness of the aquifer layers (regolith and fissured horizon) in the rocks that form the bed of a catchment, and the content of free water in rocks measure with Protonic Magnetic Resonance system, the volume of groundwater present in the zone can be estimated (R. Wyns and al. 2004).Knowing the amount of groundwater flow exiting a river catchment, as well as the average speed a water droplet takes to reach the water table from the ground, (according to the bibliography, this speed varies between 1.75 and 2.5 m/yr.), response times of groundwater can be estimated (the time taken for a drop of water to seep from the ground to the water table and then from the water table to the river). These times are very varied (between 2 and 8 years), depends very much on the geology of the catchment and also on the amount of rainfall they have each experienced.These times do not correspond to the amount of time taken for water quality to be recovered. This is for a mixture of reasons based around the ground, the water table, the regolith and the fissured horizon. However, these are minimum times and it is imperative to obtain tangible results on the quality of the water tables and consequently the water system, which can then be followed by action on the surface.

International audience; Pumping tests carried out in the fissured layer of a granitic hard-rock aquifer, interpreted at the observation wells by means of the analytical solution of Neuman and at the pumping wells using that one of Gringarten show the vertical anisotropy of this layer of the aquifer; the horizontal permeability being clearly and systematically higher than the vertical permeability. These results perfectly agree with the geological observations, the fissured layer of the weathered granite profile showing the existence of many sub-horizontal fractures. It confirms dominating role, within the fissured layer, of the permeability of fractures due to the weathering process over that one of fissures with a tectonic origin.

International audience; Pumping tests carried out in the fissured layer of a granitic hard-rock aquifer, interpreted at the observation wells by means of the analytical solution of Neuman and at the pumping wells with that of Gringarten show the existence of a strong vertical anisotropy of this layer of the aquifer; the horizontal permeability is clearly and systematically higher than the vertical permeability. These results agree perfectly with the geological observations, the fissured layer of the weathered granite profile showing the existence of many sub-horizontal fissures. It confirms that, within the fissured layer, the permeability of sub-horizontal fissures due to the weathering process dominates over that of sub-vertical fissures of tectonic origin.