The Opalinus Clay represents an excellent natural transport barrier and provides favourable geochemical conditions for the stability of the engineered barriers

In Northern Switzerland, the Opalinus Clay lies in a tectonically stable, large area with low uplift and deformation rates and limited structural complexity.

The thickness of the Opalinus Clay in the three siting regions is comparable, ranging between about 100 and 120 m. The variability of properties is low, and the clay-mineral content is high (usually well above 40 wt-%). This results from the specific depositional environment of the Opalinus Clay, a shallow epicontinental sea with a high influx of clay-rich sediments.

Hydraulic packer tests conducted in the deep boreholes and analyses on laboratory samples show a very low hydraulic conductivity, which can be attributed to the high clay-mineral content. There is no evidence of significant preferential flow paths in the Opalinus Clay. This is explained by the partitioning of deformation in the comparatively soft rock in combination with the high self-sealing capacity.

The high clay-mineral content of the Opalinus Clay also leads to a high sorption capacity and thus to a low mobility of most radionuclides. Regarding the stability of the engineered barriers, the geochemical conditions are reducing, the pH neutral and salinity is moderate. These conditions are very stable as a result of the efficient buffering provided by the mineralogy of the host rock.

Low-permeability confining units above and below the host rock contribute to the barrier function; the overall thickness of the aquitard sequence is greatest in NL

In all the siting regions, the Opalinus Clay is bounded above and below by low-permeability confining units. These exhibit greater variability than the Opalinus Clay and contain thin layers (< few metres) with a low clay-mineral content (hard beds). Evidence from the deep boreholes shows that the hard beds do not represent continuous flow paths. Also, a several-decametre-thick biohermal limestone («Herrenwis Unit») occurring in the eastern part of the NL siting region is part of the aquitard series around the host rock. This is evidenced by the findings in the deep boreholes (low hydraulic permeability and fluid underpressures) and is supported by the fact that the unit is embedded in clay-rich rocks.

In NL and ZNO the low-permeability confining rock units are bounded at the top by the Malm aquifer and in JO by the Hauptrogenstein aquifer. The distance between the centre of the Opalinus Clay and the overlying aquifer is greatest in NL and smallest in JO.

The next regional aquifer below the host rock is represented by the Muschelkalk aquifer, which represents the most active deep groundwater system in all the siting regions. Between the Opalinus Clay and the Muschelkalk aquifer, the local, internally complex Keuper aquifer defines the bottom of the low-permeability confining units. The distance between this and the centre of the Opalinus Clay is comparable in all three siting regions

In contrast to the other siting regions, the enhanced permeability zones in the Keuper Group in NL are limited to sandstone channels. These are found on a local scale only and their connectivity is assumed to be limited. Where sandstone channels are absent or are hydraulically isolated, the underlying units down to the Muschelkalk aquifer also contribute to the barrier function.

Independent evidence demonstrates the effective barrier function of the host rock and confining rock units over long time periods in the past; the hydrogeological situation is the most stable in NL

The slow, diffusion-dominated transport processes in the host rock and confining rock units are supported by various independent lines of evidence. The four long-term hydraulic monitoring systems installed in deep boreholes in the Opalinus Clay indicate underpressures, which are considered a transient phenomenon caused by a vertical stress decrease in response to the melting of glacial ice and/or to erosion. The long-term preservation of these underpressures can only be explained by very low hydraulic conductivities, which prevent fast pore pressure re-equilibration.

A second important independent line of evidence for the very low hydraulic permeabilities is the chemical and isotopic composition of the porewaters. The change in the composition across the host rock, low-permeability confining units and neighbouring aquifers is the result of transport processes that took place over the last 10⁴ – 10⁷ years. The shape of these profiles of natural tracers in the porewater can best be explained in all the siting regions by very slow mass transport, dominated by molecular diffusion, in the host rock and in the neighbouring confining rock units.

The availability of numerous tracer profiles in all the siting regions allow the identification of regional trends and differences in palaeohydrogeological evolution. Comparing the stable water isotope composition (δ¹⁸O values vs. δ²H values), it can be shown that, in NL, the Opalinus Clay porewater has a comparatively large proportion of old porewater and is less affected by meteoric waters. Overall, the chemical composition of the porewaters of the host rock and the confining rock units as well as the composition and residence times of the groundwaters (⁸¹Kr model age) from the neighbouring deep aquifers indicate that the aquitard – aquifer system in NL provides the most stable conditions.

With regard to the placement of a deep geological repository, all the siting regions include areas without seismically mappable faults; in NL, this area is largest

The fault pattern was mapped in detail using 3D seismic reflection data. All three siting regions show areas with no seismically mappable faults, offering flexibility in the location and design of the repository. The greatest flexibility is offered by NL, where clearly defined regional fault zones bounding the area to the north and south absorbed a large proportion of the deformation in the past. Furthermore, the influence of the Internal Jura is smaller in NL compared to the JO siting region lying further to the west, and the Hegau – Bodensee Graben has less impact on NL than on ZNO.

In the period under consideration, climatic and geodynamic processes have no relevant impact on the barrier function of the host rock and confining rock units; protection against future erosion is greatest in NL

The recent tectonic deformation rates in Northern Switzerland are low. This is expected to be the case for the next one million years. As in the past, future deformation will be preferentially located at larger-scale faults. These have been mapped using 3D seismic reflection data and will be taken into account when planning the location of the disposal areas. Subseismic faults and potential new faults in the Opalinus Clay are expected to be segmented and short in length and offset. The deformation and swelling behaviour of the Opalinus Clay also ensure reliable self-sealing of tectonic faults in the case of a rock overburden thickness > 200 m.

As in the more recent geological past, the climate is expected to go through glacial/interglacial cycles. However, because of anthropogenic CO₂ emissions, the next major glaciation of the Alpine Foreland may not be expected for at least 200'000 years. Future erosion rates are likely to be comparable to the rates observed over the last 1 – 2 million years. The erosion rates will probably be even lower, as no major regional river reorganisation or smaller glaciers are expected in the next one million years. In addition, layers of limestone with low erodibility are situated above the host rock and confining geological units.

The erosion assessment shows that, with the geological evolution expected for the period under consideration, the overburden thickness will remain large enough to ensure robust self-sealing within the Opalinus Clay. Regarding less likely erosive events, the NL siting region is the most robust, particularly because of the greater depth of the repository. JO is the least robust, especially towards possible future local river diversions.

The argumentation is robust and relevant uncertainties are captured

The improved knowledge of the geological barrier in the three siting regions is based on comprehensive field investigations and laboratory studies using data from around 200 km² of 3D seismic reflection surveys and nine new deep boreholes. A supplementary investigation pro­gramme served to reconstruct erosion over the past 1 – 2 million years with the objective of specifying potential future scenarios.

Uncertainties associated with the 3D seismic reflection data and its interpretation were captured by the use and comparison of different seismic processing vintages and by superimposing inter­pretations from different interpreters. A reference processing product with consistent workflows and processing input parameters across the siting regions and a rigorously structured inter­pretation approach applying established criteria ensured comparability among the siting regions.

To ensure a robust database and argumentation, key properties were measured with high frequency in multiple boreholes per siting region. Where available, multiple independent analytical and experimental approaches were applied and compared. The borehole measurements carried out at different scales and with different methods are consistent. The results of the laboratory and downhole measurements are consistent with the tunnel-scale observations at the Mont Terri rock laboratory and in other tunnels. Conceptual models to explain the observed relationships between key parameters are in place. The observations and key processes suggested for the Opalinus Clay are consistent with international experience from research on clay-rich rocks.

Scenarios for the future long-term geological evolution are based on knowledge of past geological evolution, but, where appropriate, also consider process understanding and rates derived from other areas. The climate scenarios include both a natural evolution as well as a number of scenarios with different anthropogenic CO₂ emissions. To address the particularly large uncertainties of future erosion, a newly developed hybrid-probabilistic method was used, allowing a robust assessment.

With regard to the post-closure safety assessment, a reference long-term geological evolution is described. In addition, the erosion assessment includes the results of less likely evolutions. For numerical analyses of the post-closure safety, parameter bandwidths taking into account uncertainties are specified in a separate report (Nagra 2024m).

In summary, the significantly expanded knowledge base confirmed the safety-relevant properties of the Opalinus Clay and its long-term stability, which had already been shown in earlier stages of the siting process. Furthermore, it demonstrates key differences between the siting regions. The main conclusions are supported by multiple lines of arguments. Important results and lines of argumentation were reviewed by recognised experts from various specialist areas. Remaining uncertainties are captured and are treated further in the post-closure safety assessment.