This section addresses the characterisation of the hydrogeological system within which the repository will be constructed. The complete aquifer – aquitard system is described with a particular focus on the deep aquifers situated above and below the Opalinus Clay host rock and the surrounding confining units. These aquifers define the boundary conditions for transport in the low-permeability units between them and represent a potential connection to shallow aquifers and, ultimately, to the biosphere. A more detailed characterisation of transport phenomena and properties of the host rock and the confining units is given in Chapter 5. This section focuses on the present-day state and partly on the past evolution. Scenarios for future evolution of the aquifer – aquitard system are included in Section 6.5.

Structure of this section

  • Section 4.5.2 provides an overview of the relevant elements of the hydrogeological system, namely the topography, the river network, the hydrostratigraphic units and the main tectonic features.

  • The hydrostratigraphy section (Section 4.5.3) subdivides the sedimentary succession by hydrogeological criteria and presents the transport properties.

  • The analysis of the flow systems in the aquifers is presented in Section 4.5.4. The steady-state hydrodynamic model allows the consistency of the hydrogeological datasets to be checked, identification of recharge and discharge zones, and calculation of water fluxes and hydraulic gradients.

  • The hydrogeochemistry section (Section 4.5.5) provides information on the dynamics of the flow systems in the aquifers, notably on residence times. The chemical composition of the groundwater contains information on the units that the water has been in contact with and, therefore, may evidence interaction with other aquifers along faults (cross-formation flow). In addition, deep groundwater geochemistry provides the boundary conditions for the inter­pretation of natural tracer profiles in porewater (Section 4.7) and is used to check consistency with the flow fields computed with the hydrodynamic model.