Impact of the landscape evolution on the hydraulic boundary conditions of the Callovo–Oxfordian formation

The Callovo–Oxfordian formation in the Eastern part of France was recognized as a potential nuclear waste repository layer. The Andra (National Agency for Nuclear Waste Management) has launched a few years ago a research program that aims to define the mechanisms of importance in the impact of the surface environment evolution on the site hydrogeology.Based on mapping and dating results, Andra has quantified the geomorphological evolution of the Meuse/Haute-Marne site in the past and has estimated the future evolution over 1 million years. The Callovo–Oxfordian boundary conditions depend on the hydraulic heads in the two surrounding aquifers, the Oxfordian limestone above, and the Dogger one, below. Both aquifer outcrops are modified over the next million years.For the present study, the geomorphologic evolution is considered independently of other processes and translated in the hydrogeological model in terms of changes in the hydraulic boundary conditions at the surface. The hydrodynamic simulations have been performed with the code Cast3M (implemented by CEA (Atomic Energy Commissariat)) using a mixed hybrid finite-element formulation. For these groundwater flow simulations, three modelled stages are presented: the Present, 500,000 years (500 Ky) and 106 years (1 Ma) in the future. The landscape evolution is merely considered through the use of three different topographies on which the boundary conditions are applied. According to Andra predictions, at the Meuse/Haute-Marne site, the valley incisions on the Bure plateau will locally reach the Oxfordian limestone. Thus, the Oxfordian aquifer exhibits more changes due to topographic evolution than the Dogger aquifer.Hydrodynamic simulations show a significant impact of the valley incisions on the groundwater flow by the creation of local outlets to the Oxfordian limestone aquifer in the North of the area for the 1 My topography. It induces local perturbations of the saturation level. The global erosion of the topography also pulls down the hydraulic heads on a regional basis. Changes induced by the geomorphologic evolution in the Dogger aquifer are located around 30 km East from the Underground Laboratory site, at the outcropping areas. Thus, at the Laboratory location, the piezometric surface does not show, for the future, significant modifications compared to the present state. The 20-m general lowering of the hydraulic heads in the Dogger is also globally less important than for the Oxfordian aquifer.The consequence on the Callovo–Oxfordian boundary conditions for the future is an increase of the internal upward vertical hydraulic gradient which, at present state, is well developed only immediately to the North of the Laboratory.