Analysis and evaluation of hydraulic impacts of the shafts and the tunnel access to the future radwaste repository on the Meuse/Haute-Marne site by the use of finite element variably saturated flow model

Document available in extended abstract form only. The French National Agency for Nuclear Waste Management (Andra) conducted a site investigations program within the project of a deep geological disposal of intermediate and high-level long lived radioactive waste in the Meuse/Haute-Marne region. The construction of the shafts to the underground Laboratory in the Callovo-Oxfordian clay host formation provides through monitoring boreholes hydrodynamic data of the hydraulic perturbation of the Oxfordian aquifer. These data are hydraulic pressures measurement in the macro-pore chambers between packers and discharge rates of the groundwater drained into the shafts. The understanding of the local flow behaviour and impacts in the Oxfordian aquifer will provide confidence on the hydraulic perturbation predictions of the access structures (shafts and tunnel) to the future repository located at a couple of kilometres north of the underground Laboratory. The objective of the modelling work is to establish a transient flow model derived from the integrated region-sector flow model developed by Andra and integrating the monitoring data collected from the construction and the exploitation of the underground Laboratory since year 2000. Based on the geological and hydrogeological data (hydraulic head time series and discharge flux time series), the transient flow modelling aims to establish a basis for future environmental impact assessment studies and addresses the following issues: - The hydraulic relationship between the macro-pore zones (HP1-4, HP5, HP6 and HP7). - The extension of the unsaturated zone around the underground Laboratory shafts. - Estimate the hydraulic depression in the layers of the Oxfordian aquifer. - Prediction of the local hydraulic behaviours in the upper aquifers during the shafts and the tunnel construction and also during the repository operational period. - Investigate any potential impact on the groundwater resources. - Estimate the impacts of the repository closure during the re-saturation time period. A variably saturated flow model of a local multi-layered aquifer system covers an area of 4700 km2 and integrates the Oxfordian aquifer, the Kimmeridgian aquitard and the Portlandian aquifer. The spatial discretization of the aquifer system into finite element mesh (1.6 Million nodes and 3.2 Million elements) concerns the geological objects (layers, macropore zones, faults and diffusive fracture zone) and the shafts. The variably-saturated flow Richard's equation is solved with the finite element simulator GroundWater previously used to develop the integrated regional-local hydrogeological model which provides the lateral and bottom boundary conditions of Dirichlet type. Boundary conditions of Neumann type are specified on the top surface representing rainwater infiltration of approximately 240 mm/year. The first step consists on reproducing the head and the discharge rate time series recorded in the monitoring boreholes and in the underground Laboratory shafts during the last 12 years. Then prediction of the shafts and tunnel access to the future repository impacts will be carried out using the calibrated flow model as initial situation. The modelling work is on-going and the scoping calculations show that the van Genuchten model (van Genuchten 1980) of saturation and upstream weighting of the relative permeability together with a saturation control on simulation time allows obtaining stable solutions of the Richards' equation. Simulations results show there is a time shift in the de-saturation/saturation of the modelled units depending on their parameters. (authors)