Characterisation of the Geological Environment

The safety case rests to a large extent on the very favourable properties of the geological environment of the Zürcher Weinland, and especially the Opalinus Clay stratum. Accordingly, the IRT has taken considerable interest in the geological characterisation, which is summarised in the Safety Report (Nagra, 2002a) and described in detail in the Geosynthesis Report (Nagra, 2002b).

The geological framework for the current study was derived from a systematic host rock study for the whole of Switzerland. Beginning in 1978, the geological studies involved:

• selection of tectonically inactive and simply structured areas;

• analysis of the flow regimes and groundwater chemistry;

• understanding of important groundwater recharge and discharge areas.

The study of crystalline rocks was the subject of Project Gewähr (Nagra, 1985), which was reassessed from a safety perspective in the Kristallin-I report (Nagra, 1994).

Initially, the study of sedimentary deposits concentrated on the identification and a general assessment of suitable host rocks with low permeability and sufficient thickness in northern Switzerland. Subsequently, the

characteristics of the potential host rocks were determined and assessed from an engineering, hydrodynamic and safety viewpoint.

The relevant information for the performance assessment of the Opalinus Clay was derived from the investigations carried out in Nagra boreholes (1982-1988), shallow boreholes (1989-1994), Mont Terri (from 1996 on) and the Benken borehole (1998-2001). The investigations have included:

• mineralogy and porosity data from Benken, Weiach, Riniken and Schafisheim boreholes, and from Mont Terri;

• rock mechanical data from Benken and Mont Terri;

• hydraulic property data and other information from in situ tests at Benken, Riniken, Schafisheim, Weiach, Mont Terri, and from laboratory tests on cores from Benken and Mont Terri, from 10 tunnels in the Folded Jura and 40 shallow boreholes in horizontally bedded Opalinus Clay;

• gas transport properties from Benken in situ tests and cores, and from Mont Terri in situ tests and cores;

• porewater geochemistry data from Benken cores and Mont Terri in situ tests and cores;

• diffusion data from Benken cores, Mont Terri in situ tests and cores, and natural tracer profiles from Benken and Mont Terri.

The IRT considers that the selection of the Zürcher Weinland as the priority area for local explorations in the Entsorgungsnachweis project is geologically transparent since it is the largest tectonically undisturbed area with Opalinus Clay at a suitable depth. Since 1991, the investigations in the Zürcher Weinland have included;

• one deep borehole (Benken) involving an extensive testing and logging programme;

• shallow boreholes (two piezometers, various uphole measurement boreholes for 3D seismics);

• about 60 km of 2D seismics;

• about 50 km² of 3D seismics, interpreted using modern evaluation tools; and

• assessment of hydrogeological boundary conditions from the regional survey.

The methodology and the objectives of the geological investigations are in line with international “geo”-practice and IAEA and NEA recommendations (IAEA, 1999; IAEA, 2001b; IAEA, 2003; NEA, 2001c; NEA, 2003; ITC, 2003;

Witherspoon et al., 2001). The understanding of the geological conditions in the investigation area is based on the high quality of the 2D and 3D seismic campaigns as well as the cored and intensively tested Benken borehole (Nagra, 1995; Nagra, 2001a; Nagra, 2001c; Nagra, 2001d). The core material from the Benken borehole was carefully logged and selected cores were subjected to detailed stratigraphic, mineralogical and petrographic studies (Nagra, 2001b;

Nagra, 2001c; Nagra, 2001d). A comprehensive geophysical programme was undertaken, including petrophysical and structural investigations and seismic measurements (Nagra, 2001c; Nagra, 2001d). Neotectonic observations extending over several years were also undertaken (Nagra, 2002f). Hydraulic investigations were carried out in the Benken borehole (Nagra, 2001c; Nagra, 2001d).

In terms of the safety of a deep repository, the most important results of the neotectonic observations can be summarised as follows (Nagra, 2002f):

• In large areas of northern Switzerland, and particularly in the Zürcher Weinland, the Opalinus Clay has remained largely undisturbed since its formation some 180 million years ago.

• Seismic analysis shows that there is only minor seismic activity in the Zürcher Weinland.

• No safety-relevant impacts (no mechanical damage to the barrier system) of earthquakes on a deep geological repository in Opalinus Clay (650 m below the surface) are expected, even in the unlikely case of a large earthquake.

• Based on data from different sources (geomorphology, burial and uplift history, geodesy), it is assumed that linear erosion keeps pace with the long-term uplift.

The IRT took particular interest in the results and interpretation of the chemical and isotopic profiles in the porewater within the Opalinus Clay and adjacent layers. Nagra has modelled the data for isotopes of hydrogen and oxygen, and argued that they demonstrate that diffusion rather than advection has been the dominant transport process in the Opalinus Clay for periods of the order of 1 million years up to the present time. The chlorine isotope data is less consistent but does not contradict this conclusion (Gimmi and Waber, 2003).

The IRT was impressed by the strength of this argument because of the long

timescales involved and because it complements other evidence based on permeability.

From the viewpoint of performance assessment, the IRT notes that, with respect to the Zürcher Weinland:

• The system geometry, including the depth, thickness of the host rock and the surrounding formations is, in general, sufficiently well known.

• Most safety-relevant properties of the Opalinus Clay are specific to the rock type and data from various locations show that these properties show little spatial variability. This makes an extrapolation of information from the Benken borehole to the 3D seismic area acceptable.

• Some safety functions of the Opalinus Clay, including its capacity to self-seal, depend on the rock stress and are thus related to the thickness of the overburden. The depth of the Opalinus Clay in the Zürcher Weinland, and thus the thickness of the overburden, are clearly indicated in the boreholes and in the seismic studies.

• The assessment of the properties of the host rock relevant to safety assessment and engineering feasibility are based not only on investigations in the Benken borehole, but also on a large number of other investigations in Opalinus Clay, such as deep boreholes in the near and far vicinity of the Zürcher Weinland, tunnels, the Mont Terri URL, shallow boreholes and clay pits.

• For this stage of the project, the confidence in the characterisation of Opalinus Clay is adequate for safety analysis. Although it is possible that undetected geological features (tectonic heterogeneities) might be found in more extensive investigations, they are unlikely to be a source of major uncertainty. The environmental tracers in porewater as well as the hydraulic overpressures, indicate that permeability is very low over the scales of interest, in space and time, for transport through the Opalinus Clay, which is consistent with measurements in situ and in the laboratory.

In summary, the IRT has reached the following conclusions with respect to the geological environment in the studied area:

• the geometric model of the Opalinus Clay of the Zürcher Weinland is well founded,

• the techniques and methodologies used by Nagra to characterise the geological setting are consistent with accepted geological practice and the conclusions are supported by multiple lines of evidence,

• Nagra has presented substantial evidence for the homogeneity of the safety-relevant properties of the Opalinus Clay within the Zürcher Weinland and evidence that these properties can be extrapolated over a wide region,

• the characterisation of the underground environment involves uncertainties, including the possibility of permeability variations and faults. These uncertainties are adequately taken into account in the safety assessment,

• the site characterisation work is in line with current international standards, e.g. IAEA (2004).

Within this context, the IRT considers that Nagra has good reason to focus its future efforts on the Opalinus Clay of the Zürcher Weinland. The IRT concurs with Nagra’s stated intention to drill one or more additional boreholes in the Opalinus Clay of the Zürcher Weinland before going underground at this site, should a decision eventually be made to do so.