Technical Report NTB 93-01

Geology and Hydrogeology of the Crystalline Basement of Northern Switzerland

This report summarizes the results of regional geological investigations that were carried out by Nagra (Swiss National Cooperative for the Disposal of Radioactive Waste) in Northern Switzerland. These investigations assessed the suitability of the crystalline basement as a host rock for a repository for high-level and long-lived intermediate-level radioactive waste. The investigation programme was conducted during 1981 -1993 and included the following:

  • a deep drilling campaign that consisted of seven boreholes, predominantly cored, with depths that range from 1306 to 2482 m (5900 meters in the crystalline basement and 6400 meters in the overlying sediments) and an extensive programme of logging, packer tests and water sampling
  • geophysical investigations that consisted of 400 km of reflection seismics and 230 km of refraction seismics, gravimetry and aeromagnetic surveys
  • long-term monitoring of hydraulic pressures
  • hydrogeological modeling
  • hydrochemical investigations
  • geological mapping and compilation of geological maps
  • geological studies of outcrops of the crystalline basement in the Southern Black Forest (Germany)
  • neotectonic studies, including installation of a network of seven stations for measuring microearthquakes, geodetic measurements, geomorphological studies and stress measurements.

Data from the investigation programme were analysed in detail and the resulting information is contained in more than 100 Nagra Technical Reports. This report synthesises all of those results.

The present-day geological setting of Central Northern Switzerland is the product of a geological evolution that can be traced back more than 400 million years. The most significant events occurred during the Paleozoic Era and Tertiary Period. During the Variscan orogeny, the crystalline basement underwent strong structural and metamorphic/hydrothermal overprinting, which was accompanied by plutonic and volcanic activity. Since then, the lithology and structure of the basement have not changed significantly. The Permo-Carboniferous Trough of Northern Switzerland formed during the late phase of the Variscan orogeny, about 245 million years ago. During the Tertiary Period, after a time of relative tectonic inactivity that lasted about 200 million years, further changes occurred in the region, namely the formation of the Upper Rhine Graben, the updoming of the Black Forest and the formation of the Folded Jura.

The tectonic-structural regime in the crystalline basement cannot be mapped accurately due to the presence of the sedimentary overburden and because current exploration techniques are inadequate. A deterministic characterization of the conditions in the basement is thus not feasible. For this reason, data from surface­based investigations of the neighboring Southern Black Forest and from boreholes and seismic investigations in Northern Switzerland were used to construct a schematic fault model, which then served as a basis for developing the local hydrogeological model and deriving the explorability study.

On the basis of a detailed analysis of drillcores in the vicinity of water-inflow points, water flow in the crystalline basement correlates with the following structural and lithological discontinuities (water-conducting features):

  • cataclastic zones
  • fractured zones with open joints
  • aplites and aplitic gneisses with brittle deformation

Major water-conducting faults consist of local concentrations of cataclastic zones accompanied by open joints. Due to extensive hydrothermal alteration, the rock in the immediate vicinity of water-conducting features generally has an enhanced microporosity and clay minerals that formed under hydrothermal conditions usually are present; this situation is favorable for the retention of radionuclides.

From a hydrogeological viewpoint, the crystalline basement consists of (1) an upper, higher-permeability domain that is several hundred meters thick and that has a mean hydraulic conductivity of about K=1 E-7 m/s; and (2) a lower, low-permeability domain with K.

On the basis of chemical and isotopic compositions, groundwater of the crystalline basement is subdivided into four groups: recent calcium-bicarbonate water with low mineralization (dissolved solids < 0.2 g/l) in the Black Forest; sodium-bicarbonatesulphate water (dissolved solids about 0.5 g/l) in the area around the Siblingen borehole, which infiltrated during a cold climatic period at least 10,000 years ago; sodium-sulfate water (dissolved solids 0.9-1.4 gil) in the region Zurzach-Leuggern-­Böttstein-Kaisten, which infiltrated before the last ice age, i.e., at least 70,000 years ago; and highly-mineralized, probably in part very old saline waters in low-permeability areas of the crystalline basement (Böttstein, Leuggern, Weiach) or in the vicinity of the Permo-Carboniferous Trough (Säckingen, Schafisheim). The low-mineralized groundwater and some saline groundwater probably evolved entirely within the crystalline basement (Weiach, Leuggern), whereas other saline water contains a component of sedimentary origin (Schafisheim, Säckingen, possibly Böttstein). The understanding of the groundwater flow regime in the basement that is derived from hydrochemical considerations is generally consistent with the understanding derived from hydrogeological modeling.

The long-term geological and climatic evolution of Northern Switzerland was evaluated by considering a range of plausible scenarios. These scenarios were formulated on the basis of data derived from the tectonic history of the region and from neotectonic studies. The updoming of the Southern Black Forest is expected to continue. This process will result in the Rhine river being displaced southwards, in some places as much as 2 km in 1 million years. Fluvial erosion of the Rhine, which is associated with uplift, is estimated to be a maximum of 200 m in the Koblenz region. Differential movements along first order faults (e.g. the Vorwald fault) are predicted to be a maximum of 100 m per million years. Movements along smaller faults (cataclastic zones) will probably be less than 1 m for the same time period.

The geological dataset for the performance assessment model chain was compiled from the results of the regional investigation programme and is presented in the form of conceptual models with a range of parameters; uncertainties are also discussed. The dataset includes hydrogeological information, such as water flow through a repository, distribution of flow among individual water-conducting features and dilution potential of near-surface aquifers and rivers; conceptual models of water-conducting features, with relevant data on their geometry, mineralogy and porosity; an estimate of in situ hydrochemical conditions (reference water); data on concentrations and compositions of colloids in groundwater and geotechnical parameters for studies of repository design and layout.

As part of an explorability study, an investigation concept was formulated that will be used to identify the locations of subvertical, layout-determining water-conducting faults and the extent of suitable blocks of low-permeability crystalline basement. The concept includes reflection seismic surveys and a vertical borehole with a "star" array of four inclined boreholes, all drilled from the same site. Hydraulic testing will be carried out in the boreholes and seismic tomography measurements will be made among the boreholes and between the boreholes and the earth's surface. Statistical analyses indicate that, if the schematic fault model for Northern Switzerland reflects the actual conditions in the crystalline basement of the region, then there is a good chance that at least one sufficiently large block that is suitable for hosting part of a repository can be identified using the above concept. It may be necessary, in a later investigation phase, to drill additional boreholes before underground characterization of the crystalline basement can be done using a shaft and exploratory drifts.

The geological synthesis indicates that potentially suitable crystalline rock at appropriate depths occurs in two regions of Northern Switzerland. The most promising siting options are in a strip of land that is a few kilometers wide south of the Rhine river (area West: Kaisten-Leuggern-Böttstein). A second-priority potential siting region is in Canton Schaffhausen (area East: Siblingen). Area East is less well-characterized and, based on current results, it is recommended that for the time being no further investigations be carried out in this area. Taking into consideration the project-specific boundary conditions set by the Swiss high-level waste-disposal programme, it is proposed that the next investigation phase should include 3D seismics in the Böttstein-­Leuggern region and drilling of a "star" of inclined boreholes at the former deep drilling site at Leuggern or Böttstein. These two deep boreholes have already revealed the presence of large sections of low-permeability crystalline basement that are considered suitable for hosting a repository.