Technischer Bericht NTB 89-04

This report concerns the hydrogeological modelling of the potential waste repository site in the Oberbauenstock (Canton Uri). It includes a description of enhancements made to an existing model and presents the results obtained. The aim of the modelling is the simulation of groundwater flow conditions in the repository host rock and adjacent formations. The modelled region covers an area of approximately 32 km² in the neighbourhood of the overthrust of the Drusberg and Axen nappes.

Each stage of the enhancement of the model is discussed in the present report. The spatial layout of the relevant aquifers and aquitards, together with their hydraulic properties, is defined. The overthrusting structure of the nappes, steeply dipping fractures, the Seelisberg road tunnel and the repository excavation itself are included in the model as discrete structures.

The partial differential equation describing the hydraulic potential field in three dimensions is solved numerically using the finite element code FEM301, which calculates the stationary groundwater flow in a fully saturated porous medium.

This report expands the understanding of hydrogeological conditions in the Oberbauenstock obtained during an earlier phase of modelling performed as part of the "Project Gewähr". Parameter variations have been performed to ascertain the influence of some of the less well founded assumptions concerning hydraulic parameters, the spatial extent of the host rock and various boundary conditions. The modelling results yield values for the water flux through the repository and an indication of the flow trajectories and flow times from the repository caverns to the biosphere.

The results will contribute to the planning of the repository access tunnels as well as serving as the hydrodynamic basis for the preparation of an investigation programme to be performed during the forthcoming exploration phase.

The model described in this report is based on the most recent state of knowledge of the hydrogeology of the Oberbauenstock region. Available data allows independent tests to be made on certain model results. The planned investigations will yield new information that will permit a better definition of some parts of the current conceptual model, and modelling activities should therefore not be regarded as completed. However, this latest series of model calculations covers the possible extent of changes that previously could have inaccurately represented factors influencing the simulated flow conditions.