Hard rock aquifer architecture and water circulation levels in the Strengbach critical zone observatory (France)

We present an integrated petrological, petrophysical, and hydrogeological study of the critical zone (CZ) developed in the Hercynian granitic basement of the Strengbach watershed ( Vosges Massif, France) to characterize its deep architecture and water circulation levels. For this purpose, six boreholes (50-120mdepth), from which three are cored, and three piezometers (10-15mdepth) were drilled to define the vertical extension and lateral variability of the main CZ horizons. The Strengbach watershed is composed of a topsoil horizon of limited vertical extension (0.81.2 m), a mobile saprolite level, and an in-place fractured bedrock. The latter is subdivided into a few meters thick saprock horizon, defined by open sub-horizontal fractures and a deeper fractured bedrock horizon with steeply dipping fractures (>50 degrees). In the north-facing slope, the vertical extension of the mobile saprolite horizon increases from approximate to 1-2 m at the top of the slope to approximate to 9 m downstream, close to the valley bottom. In contrast, the south-facing and more easterly slope shows a mobile saprolite horizon with limited vertical extension (approximate to 2-3 m thick). Such a difference is associated with the existence of a knickpoint in the river bed, separating a downstream zone marked by currently active erosion from an upstream one, less prone to erosion, with preserved reliefs formed around 20 ka ago. The water circulation scheme within the Strengbach watershed involves two different systems: a subsurface circulation within the shallow aquifer, corresponding to the mobile saprolite horizon and the saprock, and a deeper circulation in the fractured bedrock. The water circulation in the fractured bedrock is controlled by fractures of regional orientations, linked to the Vosges massif and the Rhine Graben Tertiary tectonics, and partly to reactivated Hercynian fracture zones. The unaltered bedrock was not reached by any of the three cores. These results from the Strengbach CZ demonstrate theimportance of integrating geological history of the watershed, either the long-termgeological bedrock evolution or the Quaternary erosion patterns, to better understand and model the CZ hydrological functioning at the watershed scale.