Evolution of geological setting .1 Introduction

As discussed in Section 4.2.5, overpressures are observed in the Opalinus Clay. Their evolution is discussed in Section 5.2.2.2 below. In the very long run, uplift and erosion will – if they continue – become more important. The effects of continuing uplift on the geological situation are discussed in Section 5.2.2.3. Besides these slow and continuous geologic processes, the likelihood and effects of unlikely (rare) geological events need to be discussed. This is done in Section 5.2.2.4.

5.2.2.2 Compaction of Opalinus Clay and evolution of hydraulic overpressures

The Opalinus Clay is overpressured (see Section 4.2.5), even though the overlying sediments are currently being eroded. It is assumed that these overpressures are the result of an earlier period of rapid burial or, alternatively, ongoing lateral thrust, during which drainage and compaction of the Opalinus Clay has not yet resulted in a state of pressure equilibrium. Thus, the process of compaction and dissipation of overpressures due to porewater drainage is expected to continue (if the hypothesis of a threshold gradient holds then overpressures will be maintained). This will also be affected by reduction of the total stress imposed (erosion of overburden) or increases in stress (loading of ice during glaciation).

Expected evolution

From the safety point of view, porewater drainage is important because of possible advective radionuclide transport through the Opalinus Clay (and/or through the access tunnel system of the repository – for the effects due to the presence of the repository see Section 5.5.3).

Calculations show that very low hydraulic conductivities (10^-15 m s^-1 or less) and/or the exis-tence of a threshold gradient are required to reproduce the observed current overpressures of the Opalinus Clay (see Section 4.2.5 and Nagra 2002a). If no threshold gradient exists, the time for complete overpressure dissipation is expected to be in the order of one million years and is characterised by extremely low flow rates – for the case with a threshold gradient the over-pressures will not completely dissipate.

As stated in Section 5.2.1, it is expected that the glacial/interglacial climate will continue to be dominant for the next one million years. As a result, several glaciations are expected to take place during this time period. Within the Opalinus Clay, the additional (to lithostatic) load temporarily imposed by the presence of 200 – 400 m of ice (equivalent to a load increase of about 20 – 30 %) will initially be carried predominantly by the clay porewaters. It will be transferred only gradually to the grain-to-grain contacts of the solid matrix (the rock frame) as a result of drainage and compaction. Due to the extremely low hydraulic conductivity of the Opalinus Clay, the relaxation time of glaciation-induced overpressures is expected to be much longer than the actual duration of the glaciation (tens of thousands of years). The palaeohydro-geological evidence for the stability of the Opalinus Clay environment (Section 4.2.5) indicates that the six previous Quaternary cycles of glaciation have had no perceptible impact on the transport processes of solutes in the host formation.

Thus, while it could be expected that the rates of water movement in the two regional aquifers will be affected, as will the composition of shallow recharge waters, the response time of the Opalinus Clay hydrogeological system is so slow that the relatively short-term (tens of thou-sands of years) cyclic changes in external hydraulic boundary conditions will be substantially smoothed out within the proposed repository rock volume.

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Possible deviations from expected evolution

Due to the possibility that the measured overpressures are only artefacts and have dissipated much faster than indicated above, it cannot be ruled out that for future loading the transient involving compaction of the Opalinus Clay takes place much faster, leading to higher specific water flow rates.

5.2.2.3 The effect of uplift and erosion on the properties of the host rock and on the hydrogeological situation

As discussed in Section 4.2.1, northern Switzerland and the Zürcher Weinland are considered to be influenced by both the alpine orogeny and the updoming of the Black Forest and thus, for the time being, continuous uplift can be expected to occur as it has for the past few millions of years.

Long-term evolution of the topography (rivers and relief) in the case of uplift is characterised by a dynamic equilibrium between uplift and erosion. According to geological and geomorpho-logical data, the evolution of the landscape of northern Switzerland including the Zürcher Weinland can thus be described as a steady state process for the past several millions of years.

The river Rhine (or its equivalent in the long term) acts as the (local) level of erosion and cuts down into the bedrock at a rate similar to that of uplift.

In addition to this linear erosion, the local relief will also develop with time. The evolution of the local relief (e.g. drainage pattern, steepness of hillslopes) depends upon the climate, the bedrock and – since historic times – upon anthropogenic effects (farming, deforestation etc.).

Furthermore, the effects of glacial erosion need to be considered as a major factor of landscape and relief development. Future glacial excavation along valleys is expected to be of a magnitude similar to that of excavations in past ice ages and is thus of limited extent in the Zürcher Weinland.

Expected evolution

The geological record of northern Switzerland provides a good data base for estimating the long-term regional uplift. Information from geomorphological studies, basin modelling and also from high-precision leveling gives a consistent picture and indicates that the uplift is in the order of 0.1 mm a^-1.

Taking into account the evolution of both the base level of erosion and the local relief, a maximum reduction of repository overburden of 200 m after one million years is possible (Nagra 2002a), with the expected regional uplift in the order of 0.1 mm a^-1, resulting in 100 m erosion. Additionally, a one-time down cutting of the base level of erosion of about 100 m has been assumed, which may be caused both by back-erosion of the Rhine Falls and more importantly by the relief adjustment because of the Bodensee (see Fig. 8.3-1 of Nagra 2002a).

With a remaining overburden of 450 m or more for the period of primary interest of about one million years, no changes are expected in the hydraulic properties of the Opalinus Clay.

Erosion will change the outcrop areas of the regional aquifers (Malm, Muschelkalk) and of the smaller aquifers in the confining units (Wedelsandstein Formation, Stubensandstein Formation) and will thus give rise to some changes in flow patterns and location of discharge areas.

Although changes in the location of discharge are possible, they are still expected to be in the Rhine valley or its future equivalent.

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Possible deviations from expected evolution

Although there is some uncertainty in the future evolution of the uplift rates, significant changes to the geological environment of the repository can be ruled out for at least a few million years.

The estimates referred to above are based on a pessimistic interpretation of the available infor-mation.

The long-term changes in the regional hydrogeological circulation system cannot be predicted in all details; in particular, it cannot be excluded that discharge takes place in a tributary to the Rhine or its future equivalent.

Possible evolution for a time period exceeding one million years

If uplift continues, erosion will continue to cut down into the bedrock and eventually, after more than a few million years, the repository may be exhumed. During the very gradual uplift and erosion, the repository overburden would be reduced to about 200 m after about 4 million years, if present uplift rates continue. As the overburden is reduced further, the permeability of the Opalinus Clay would slowly increase with time, although it would remain relatively low and redox conditions would remain reducing. Calculations assuming an increased permeability and a reduced overburden indicate that doses would continue to be well below the regulatory guide-line. After in excess of 5 million years, erosion can be expected to expose some tunnels, but the large area of the repository and the development of topographic relief due to erosion would lead to only localised exposure of repository tunnels. Chemical conditions would remain reducing in the vicinity of the decayed wastes until erosion reduces the overburden to approximately 10 – 20 m. The radiotoxicity of the spent fuel, the most hazardous of the wastes, will have declined by this time to a value similar to that of the uranium ore from which the fuel was produced (see Section 2.5.4). No rigorous assessment of doses for times beyond several million years has been carried out, but some illustrative calculations of releases have been performed that indicate that sufficient safety is maintained even after millions of years.

5.2.2.4 Potential effects of infrequent geological events Expected evolution

The potential role in repository evolution of various infrequent geological events (e.g. earth-quakes, neotectonic movements) needs to be assessed. Such an assessment includes, on the one hand, an evaluation of whether or not such events can occur at all in the immediate surroundings of the repository and on the other hand, an assessment of the potential impact of such events.

The events considered are (Nagra 2002a, Chapter 8.4):

• re-activation of existing or formation of new discontinuities;

• effects of earthquakes;

• magma intrusion.

The formation of significant new fracture zones is considered to be very unlikely because neo-tectonic movements will occur along pre-existing structures. Therefore, a respect distance will be kept between the repository and already existing larger-scale fracture zones such as the Neuhausen Fault. In the current design, it is also envisaged to stay away from the Wildensbuch Flexure, the inactive fault zone delimiting the crystalline and Permo-carboniferous basement and some minor faults in the Opalinus Clay identified by 3 D seismics, although these are not

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expected to be no-go areas. The precautionary decision to avoid these features is due to the limited information available on them. There is no indication of new fault systems currently being developed away from the above-mentioned tectonic features. Based on current know-ledge, as in the past few million years, negligible neotectonic activity is expected in the Zürcher Weinland within the next one million years.

Small fracture zones are expected in the vicinity of the repository, these are currently not water-conducting and it is expected that their hydraulic properties will not be affected by any neotec-tonic events. In particular due to the self-sealing capacity of the Opalinus Clay, such fractures will not have enhanced transmissivity.

Seismic analysis shows that there is only minor seismic activity in the Zürcher Weinland (Nagra 2002a). Furthermore, once the repository is closed, no mechanical damage to the barrier system is to be expected, even in the unlikely case of a large earthquake. In some areas with permeable geological formations and earthquakes of large magnitudes, there are indications of episodic, earthquake-induced fluid migration, especially in the vicinity of vertical/sub-vertical faults (e.g.

Muir-Wood & King 1993). Similar effects can be excluded in the case of the Opalinus Clay, because there are no indications either of active water-conducting features or of any significant fluid movements in the past.

The possibility of magmatic intrusion can be excluded by geologic reasoning and by the geo-thermal maps, the latter indicating that the magma is deep enough to be of no concern.

Possible deviations from expected evolution

Based on current scientific understanding, the existence of preferential pathways due to small fracture zones affected by neotectonic events can be excluded. However, small fracture zones with an enhanced transmissivity (10^-10 m² s^-1) are included in the safety analysis as a "what if?"

case (see Section 3.7.4), to test the effect of such enhanced transmissivities on system performance.

5.3 Evolution of the SF / HLW near field