The safety case for a deep geological repository in Switzerland: site and geological barrier characteristics

1 Introduction

The “geological barrier” of the deep geological repository for HLW and L/ILW in Switzerland is centred around the Opalinus Clay host rock characterised by favourable properties, including extremely low hydraulic conductivity, high sorption capacity, self-sealing behaviour, and stable geochemical conditions (Nagra, 2024f). The entire geological low permeability barrier section includes the Opalinus Clay and the adjacent, over- and underlying low-permeability confining units (i.e., Keuper, Lias, Dogger Groups), and is termed the containment-providing rock zone (CRZ). The CRZ forms the framework for evaluating long-term hydrogeological stability, the suitability of the subsurface for safe repository construction, and the overall robustness of the site, which is assessed through various modelling and other safety-relevant geological assessments in the post-closure safety case for the general licence application (Nagra, 2024h). For the Swiss repository, the safety case relies on a system of multiple complementary technical and geological barriers that provide passive safety over geological timescales (Smith, 2026). Because hydrogeological and geochemical changes in deep, aquitard units occur slowly over hundred thousand to millions of years, radioactive waste can be safely isolated from the human environment for the long-term.

Nagra’s characterisation of the Opalinus Clay host rock began more than 30 years ago with Project Opalinus Clay (Nagra, 2002). As in other waste management organisations, host rock characterisation relies on multi-phase geological investigations, hydrogeological testing and safety evaluations across broad spatial and temporal scales (e.g., crystalline rock types: Sweden, SKB, 2022; SKB, 2023; Canada, NWMO, 2023a, Finland, Posiva 2012; sedimentary host rocks: Canada, NWMO 2023b, France, ANDRA, 2006). Switzerland’s site selection followed a three stage procedure defined in the Sectoral Plan for Deep Geological Repositories (ENSI, 2020), that moved from nationwide screening (Stage 1) to narrowing down suitable regions (Stage 2) and finally to detailed investigations and site comparison (Stage 3). During Stage 3, Nagra significantly expanded the geoscientific basis through 3D seismic acquisition and nine deep boreholes across the three siting regions that remained under investigation at that time, namely, Jura Ost, Nördlich Lägern, and Zürich Nordost. This provided highly resolved lithological, hydrogeological, geomechanical, and geochemical data across the Opalinus Clay and the CRZ (Nagra, 2024f).

The characterisation of the geological barrier in Switzerland is based on a comprehensive, multi-scale investigation programme (Nagra, 2024f). Nagra’s workflow integrated key data sources, including 3D seismic reflection surveys for structural and stratigraphic mapping (Nagra, 2024a), deep boreholes for lithological controls (e.g., Jordan et al., 2022; Schürch et al., 2022), direct sampling, geophysical downhole logging, in-situ testing (e.g., Jordan et al., 2022; Schürch et al., 2022; Nagra, 2024g, etc.), and laboratory analyses of core samples to determine key hydraulic (Nagra, 2025), mechanical (stress-strain, swelling) (Crisci et al., 2022; Crisci et al., 2024), and geochemical rock properties and states (sorption, porewater geochemistry, diffusion, etc.) (Mäder and Wersin, 2023; Marques Fernandes et al., 2024a; Van Loon et al., 2024). These investigations focused on the characterisation of the entire CRZ and were complemented by previous exploratory campaign information (e.g., Nagra, 2002; Nagra, 2008; Nagra, 2014a) and decades of research in the Opalinus Clay of the Mont Terri underground rock laboratory in Switzerland (e.g., Marschall et al., 2017; Leupin et al., 2017).

This paper follows the structure of the supporting documentation of the post-closure safety case for the Nördlich Lägern siting region (Nagra, 2024f; Nagra, 2024e; Nagra, 2024h). Similarly to Andra’s Dossier 2005 Argile (ANDRA, 2005), the characterisation of the geological barrier in the Swiss safety case provides both, a source of model parameters and a structured assessment basis that integrates site data and phenomenological understanding for the safety case (e.g., Nagra, 2024h, Nagra 2024j). Consequently, the geological assessment basis (i.e., Nagra, 2024f) with its geoscientific observations of, for example, formation-scale continuity, self-sealing, or long-term geochemical stability, is embedded in the argumentation used in the safety case (i.e., Nagra, 2024h) and underlies the demonstration of fulfilment of key safety functions over the long time period for assessment. This paper therefore demonstrates how a systematic and integrated geological investigation significantly reduces uncertainties and builds confidence in long-term safety assessments. The study highlights relevant geoscientific characteristics and their important role within a safety case and may be regarded as a relevant reference framework for other waste management organisations developing safety cases for repositories in clay mineral-rich formations or similar low-permeability host rocks.

2 Geological setting and CRZ architecture

This section provides an overview of the geology of central Northern Switzerland, establishing the large-scale structural framework that controls the long-term Alpine Foreland basin evolution. Because the Opalinus Clay was identified as an ideal host rock for deep disposal (Nagra, 2002), regional investigations focused on areas in which this formation lies at a technically and geologically favourable depth (approximately 300–1,000 m, see Figure 1a). New 3D seismic datasets and deep borehole information from Stage 3 of the siting programme has significantly improved the structural model and its spatial constraints on bedding geometry, fault distribution and formation continuity.

2.1 Regional setting

The regional geological setting provides the boundary condition for the assessment of the long-term stability of a site with respect to erosional, climatic, and tectonic processes. The study area in northeastern Switzerland represents a region with relatively low seismicity, low uplift rates, and a simple structural configuration, compared with large parts of Switzerland (Figures 1a–d). The selected siting region is located between the Upper Rhine Graben to the west, the Hegau-Bodensee Graben to the northeast, and the internal part of the Jura Fold-and-Thrust Belt to the southwest (Figure 1b). Hence, the siting region is placed outside the major tectonic elements that dominate the most recent deformation (Alps, internal Jura Fold-and-Thrust-Belt, European Cenozoic rift system). As a result, the region is relatively structurally simple and contains large area devoid of seismically mappable faults. Such areas are ideal for the placement of the deep geological repository, thanks to the simple, flat-lying host rock geometry and the high stability regarding future tectonic movements.

Over the last ∼700 ka, global climate variability has been dominated by the 100-ka glacial-interglacial cycle that resulted in successive glaciations in Switzerland, which formed pronounced glacially overdeepened valleys in parts of the landscape. However, although Nördlich Lägern was reached by several glaciations during these climate oscillations and left behind glacial overdeepenings a significant overburden above the Opalinus Clay still remains (the Opalinus Clay lies at a depth of 800–900 m, Figure 1a). It is, however, important to understand the past landscape evolution and the included processes and rates to assess possible future developments of the site and its surroundings from a long-term safety assessment perspective.

Uplift is considered a first-order driver of long-term landscape evolution in Northern Switzerland, with strong modulation by glacial-interglacial cycles and contribution from phases of regional drainage re-organisation (Nagra, 2024c). It is driven by a combination of tectonic shortening due to ongoing convergence between the African and Eurasian Plates, buoyancy-related mantle processes, and isostatic responses to erosion and glacial unloading (e.g., Sternai et al., 2019). These mechanisms generate a well-defined uplift gradient perpendicular to the Alpine front, with rates steadily decreasing toward the north-northwest into the Alpine foreland (Sánchez et al., 2018; Figure 1d). This pattern is also consistently observed in thermochronological indicators (Fox et al., 2015), regional denudation and incision datasets (Nagra, 2024c), and modern geodetic measurements (Sánchez et al., 2018). Within the Molasse Basin and Jura Mountains (Figure 1b), vertical motions are markedly lower than in the Alpine range (Figure 1d).

Quaternary river incision in Northern Switzerland is influenced by both foreland uplift and regional baselevel changes associated with drainage re-organisation and episodic subsidence in the Upper Rhine Graben (Nagra, 2024c). Overall, modern fluvial incision rates are on the order of tenths of a millimetre per year, consistent with the subdued tectonic regime of the foreland (Nagra, 2024f).

Seismically, the Alpine Foreland is significantly less active than the Alpine range or the adjacent rift systems of the Upper Rhine Graben and the Hegau-Bodensee Graben (Figure 1b). Importantly, no evidence of differential horizontal movement across regional fault zones in horizontal geodetic velocities has been observed across Northern Switzerland (Nagra, 2024c), supporting the low deformation rates in the region.

In summary, the combination of low uplift rates, limited seismicity, structural simplicity, and the absence of major active faults demonstrates that the study area provides a geodynamically stable setting compared with large parts of the country. These characteristics (Figures 1a–d) support the suitability for a deep geological repository, directly strengthen the safety case, and ensure long-term predictability of the geological boundary conditions relevant for repository performance.

2.2 Host rock and CRZ

In Switzerland, the Middle Jurassic Opalinus Clay has been selected as host rock for the deep geological repository (Nagra, 2014b). At Nördlich Lägern, the Opalinus Clay occurs at a depth of 800–900 m (Figure 1a) and dips gently southwards towards the Alps. Together with the overlying and underlying low permeability formations, the Opalinus Clay forms the containment-providing rock zone (CRZ) (Figure 2). The CRZ is defined as the entire low-permeability sequence that hydraulically separates the two main regional aquifers of the Malm Group above and the Muschelkalk Group below (Nagra, 2024e). Within this sequence, a local Keuper aquifer, which was not identified in all boreholes of Nördlich Lägern, is also considered in this delimitation (Nagra, 2024f).

Above the Opalinus Clay, the CRZ includes the clay-mineral-rich Upper Dogger Group units consisting mainly of marls and claystones. In the eastern part of the Nördlich Lägern siting region, a carbonate-rich, spatially restricted reefal platform, the «Herrenwis Unit», occurs within this interval, situated few decametres above the top of the Opalinus Clay (Figure 2). Where present, it is overlain by further clay-rich Dogger units, followed by the 50 to 100 m-thick Wildegg-Formation (Malm Group), a sequence dominated by calcareous marls. On the top, the Wildegg-Formation is overlain by limestones, which hosts the regional upper deep aquifer (Malm Aquifer; Figure 2).

Below the Opalinus Clay, the lower confining units comprise the clay-mineral-rich formations of the Lias Group, the upper Keuper (upper Klettgau Formation), and the anhydritic-argillaceous-dolomitic units of the lower Keuper Group (Figure 2). The underlying dolomitic Muschelkalk Group, hosting the regional Muschelkalk aquifer, marks the lower boundary of the CRZ. Where present within the upper Keuper Group, the locally developed Keuper aquifer may also contribute to defining the hydraulically relevant lower limit of the barrier system (Nagra, 2024e, Javanmard, 2026).

The resulting CRZ at Nördlich Lägern constitutes an exceptionally thick and laterally consistent low-permeability sequence. Among the three siting regions, it has both the greatest CRZ thickness and the greatest distance between the host rock and the nearest bounding aquifers among the three siting regions (Nagra, 2024f). In addition, extensive areas free of seismically mappable faults (Nagra, 2024a) further support the integrity of the CRZ, ensuring long-term geological stability as well as favourable flexibility with respect to repository position and layout (Figure 2).

2.3 Abstraction of the stratigraphy

Geological abstraction (Figure 2) provides the essential link between the detailed site characterisation and the modelling framework used in safety assessment. The complex stratigraphy of the Mesozoic sequence is simplified into units with comparable clay mineral content, porosity, hydraulic conductivity, diffusion and sorption properties (see Nagra, 2024e). The systematic grouping of lithostratigraphic formations into transport relevant modelling units results in safety relevant geological features being preserved while removing detail that does not affect system behaviour at the scale relevant for performance and safety assessment modelling. The resulting “layer-cake” abstraction then allows consistent parameter allocations for a transparent and reproducible modelling workflow, including sorption properties or diffusion parameters of radionuclides (Miron et al., 2024; Glaus et al., 2024a). The abstraction also provides the foundation for developing and evaluating hydraulic activity or preferred pathways, along compositionally different units (i.e., clay-poor), such as the «Herrenwis Unit», hard beds, CM, CD, SCD, etc. (see Figure 2d) to explore bounding advective transport behaviour, for safety assessments. Furthermore, the extent of the CRZ can be evaluated through assessments of effects and impacts of activity/inactivity of the local Keuper aquifer (Figure 2). Since each abstracted unit has specific reference and bounding parameter values (for transport, physical, hydraulic, etc properties), statistical, including deterministic and probabilistic calculations, can also be determined for uncertainty assessments (Nagra, 2024f), which forms a transparent and reproducible link between geological characterisation and safety assessment.

2.4 Depositional environment and large-scale continuity of the Opalinus Clay

The Opalinus Clay was deposited in a shallow marine epicontinental environment during the Middle Jurassic Epoch (Aalenian Stage). At that time, the opening of the Piemont - Liguria Ocean in Central Europe due to continued N-S extension led to subsidence and created accommodation space for the deposition of clay mineral-rich strata (Wetzel et al., 2003). Hence, the Opalinus Clay is a regionally continuous formation with a thickness of approximately 100–120 m, showing only small thickness variations and no evidence of abrupt thinning or pinch-out within the siting regions. The clay mineral input and sedimentation rates were high. This depositional setting, together with the absence of proximal land masses, resulted in clay mineral-rich successions with rare silt/sand laminae which become more frequent towards the top of the Opalinus Clay. The fine-grained and clay mineral-rich microfabric appears as a massive, homogenous sedimentary package with low lithological variability and therefore well-predictable petrophysical and transport properties (Wohlwend et al., 2024; Zimmerli et al., 2025; Mazurek et al., 2023a).

Three-dimensional seismic data tied to the deep borehole data demonstrate that the Opalinus Clay forms large, coherent rock volumes with subhorizontal to gently inclined bedding, and that tectonic deformation has not disrupted its regional continuity at the site (Figure 3). The existence of a broad zone devoid of seismically mappable faults in the central part of Nördlich Lägern is one of the key arguments for selecting this site for the general licence application. This structural simplicity offers significant flexibility for optimising the placement and layout of the repository (see Figure 2). Furthermore, neither seismic data nor core data indicate that the Opalinus Clay acted as a décollement horizon within Nördlich Lägern (e.g., Bond et al., 2025). Consequently, these characteristics infer that the site represents a robust and predictable geometric framework for repository construction within a rock volume characterised by favourable radionuclide retention and transport properties.

3 Key geological barrier properties

3.1 Key properties of the CRZ (including Opalinus Clay)

The Opalinus Clay is characterised by a high clay-mineral content (typically 50–70 wt%, average ∼60 wt%) (see Figure 4), limited lithological variability, and very low hydraulic conductivity (Nagra, 2024f; Mazurek et al., 2023a). The clay fraction is dominated by illite and illite/smectite mixed-layer minerals with variable amounts of kaolinite and subordinate chlorite and a high external surface area of 25–35 m² g^-1 on average. The composition of the clay fraction remains consistent within the Opalinus Clay, both vertically and laterally, and although the four end-member clay minerals are also present in the clay-rich units above and below the Opalinus Clay, these units have distinct higher illite/kaolinite ratios compared to the Opalinus Clay (Mazurek et al., 2023a). However, various transport and retention characteristics are uniformly related to clay-mineral content which then allows reliable predictions of rock properties to be made (Marques Fernandes et al., 2024a; Van Loon et al., 2023), which are critical for thorough assessments of transport, geochemical buffering, and self-sealing behaviour.

The porosity of the Opalinus Clay is dominated by the fine-grained, clay mineral-dominated rock matrix that dictates the pore network. Water-loss porosity is in the range of 0.112 ± 0.013 with effective pore space characterised by nanometre-scale pores with narrow pore throats (3.08 ± 0.2 nm) (Mazurek et al., 2023a) and high tortuosity. The latter was determined from the interpretation of diffusion experiments where an electrical double layer model was applied, which leads to selective accessibility for charged species (Glaus et al., 2024b). Anion exclusion is observed for conservative anions such as Cl⁻, while cations such as Na⁺ experience surface-enhanced diffusion. These effects are well understood (Glaus et al., 2024b) and can be robustly quantified using experimentally validated diffusion models based on the electrical double layer and Donnan concepts (Van Loon et al., 2024), that provide realistic representations of radionuclide-specific transport behaviour.

3.2 Hydraulic isolation of the host rock and the CRZ

From a hydraulic perspective, the Opalinus Clay and the other upper and lower confining units have very low permeability. At repository depth, rock mass scale hydraulic conductivities in the Opalinus Clay range between 10⁻¹³ and 10⁻¹⁴ m s^-1 (Figure 4) based on laboratory measurements and in-situ hydraulic packer tests (Nagra, 2024f). These values are several orders of magnitude lower than those of typical aquifers and are consistent across the three siting regions. The extremely low hydraulic conductivity, the low hydraulic gradient and the well-known effective diffusion coefficients result in diffusion-dominated transport (Peclet number well below 1) (Nagra, 2024f). The results of numerous hydraulic packer tests indicate that the effective transmissivity of fractures, subseismic faults and potentially fractured, mechanically more competent layers within the CRZ (calcareous “hard beds” or the «Herrenwis Unit»), which are embedded in clay-mineral rich sedimentary units, is very limited. The dominant clay mineral-rich-units are characterised by high self-sealing capacity, thanks to the presence of swelling clay minerals, the effective stress conditions at depth and the characteristic deformation behaviour (Nagra, 2024f). Therefore, porewater flow within the Opalinus Clay and the CRZ is negligible under present-day conditions and remains very limited under foreseeable future evolutions because of the self-sealing properties. Moreover, anomalous hydraulic heads observed in the Opalinus Clay in several boreholes in Northern Switzerland provide additional confirmation of very low formation-scale hydraulic conductivities (Nagra, 2024f). These observed underpressures can only be maintained when hydraulic conductivity (i.e., hydraulic conductivity/specific storage coefficient) is sufficiently low for pressure signals to remain decoupled from surrounding formations over geological timescales and are probably explained by changes in vertical load (e.g., erosion, melting of ice cover).

3.3 Diffusion properties in the host rock and across the CRZ

Comprehensive measurements of effective diffusion coefficients (D _e) for HTO, ³⁶Cl⁻ and ²²Na⁺ show consistently low values within the Opalinus Clay and comparably low values (though with some scatter) in the over- and underlying confining units (Van Loon et al., 2024; Van Loon et al., 2023).

For HTO (Figure 4), which is treated as a conservative reference tracer, the Opalinus Clay shows unique quasi-homogeneous diffusion properties throughout Nördlich Lägern, with D _e ≈ (0.7–1.2) × 10⁻¹¹ m² s^-1 and porosities of about 0.11–0.13 (Van Loon et al., 2024). The surrounding formations of the upper and lower confining units have broadly comparable, though more variable, HTO diffusion coefficients due to their more heterogenous lithologies. Across all units of the CRZ, diffusion coefficients strongly correlate with clay mineral content, resulting in a predictive, mineralogy-based framework or dependency (i.e., rock compositions) (Van Loon et al., 2024) that is directly used in diffusion look-up tables for safety assessment. Equally important, the core data show effective diffusion values that are quasi-identical to those measured in rocks from the Mont Terri underground rock laboratory (Leupin et al., 2017) and previous borehole studies. This demonstrates that the Opalinus Clay, despite differing tectonic settings and varying amounts of overburden, can be considered laterally (regionally) homogeneous with respect to diffusive transport.

Charged species have a systematic deviation from HTO behaviour. For example, chloride diffusion is much slower than HTO diffusion due to anion-exclusion effects arising from the permanently negatively charged clay mineral surfaces (Van Loon et al., 2024). In Nördlich Lägern, the effective diffusion coefficients of ³⁶Cl⁻ in the Opalinus Clay are typically in the order of 10⁻¹² m² s^-1 with an anion accessible porosity of about 0.05–0.07. The magnitude of anion exclusion is influenced by the composition and ionic strength of the porewater (Van Loon et al., 2024). Across the confining units above and below the Opalinus Clay, the diffusion coefficients are more variable, reflecting the lithological heterogeneity. Carbonate-rich and siliciclastic units have generally higher D _*e* values for anionic species than the Opalinus Clay, although the values are within the same order of magnitude with variabilities related to speciation charge, clay-mineral content, pore space architecture and microfabric. For the deterministic radionuclide transport modelling, a depth-related, site-specific temperature correction (Arrhenius correction factors) was also applied to the effective diffusion coefficients (Li et al., 2026).

3.4 Natural tracer profiles as independent evidence for long-term isolation

The slow, diffusion-dominated transport regime is independently supported by natural porewater tracer profiles of the CRZ (e.g., stable water isotopes, chloride, and noble gases) (Nagra, 2024f). In the boreholes of Nördlich Lägern, the δD (Figure 4) and δ¹⁸O values show smooth, laterally coherent gradients across the Opalinus Clay and adjacent clay-mineral-rich confining units, with no abrupt shifts or excursions that would indicate advective transport. Such profiles strongly provide independent evidence that molecular diffusion is the dominant transport mechanism between the bounding aquifers. This is further supported by quantitative modelling. The models demonstrate that the observed δD distributions require diffusion-dominated transport over timescales of hundreds of thousands to millions of years to develop (Nagra, 2024f; Ma et al., 2026; Gimmi et al., 2024). The findings for the water isotope tracers are consistent with independently derived chloride and noble-gas tracer profiles (Nagra, 2024f).

The tracer profile data interpretation is supported by the geochemical groundwater signatures of the bounding aquifers. In the Malm Group, the geochemistry points towards quasi-stagnant old groundwater with ⁸¹Kr model ages in the order of at least several hundred thousand years (Nagra, 2024f; Ma et al., 2026). Below the Opalinus Clay, the Keuper aquifer waters in Nördlich Lägern also have high residence times (0.4–0.9 Ma). Interestingly, the porewaters in the Opalinus Clay of Nördlich Lägern have the highest fraction of old formation waters and only a minor contribution of meteoric water compared with the other siting regions. This further supports a particularly conservative retention behaviour and favourable long-term hydraulic isolation at Nördlich Lägern.

3.5 Sorption and retentive rock properties of the CRZ

Beside the diffusion dominated transport regime, the Opalinus Clay has an effective, high and predictable sorption capacity because of the large external surface area and the negative charge of the abundant 2:1 clay minerals (i.e., illite, smectite, chlorite). The cation-exchange capacity (CEC) correlates closely with the clay mineral content across the Opalinus Clay and the confining units (Figure 4) (Marques Fernandes et al., 2024b). Measured Ni-en CEC values typically range from several tens to more than 100 meq kg^-1 in clay mineral-rich lithologies, which represents a robust proxy for the radionuclide sorption capacity (Baeyens and Marques Fernandes, 2022), that form the basis of the radionuclide retention parameters used in the safety assessment (Nagra, 2024i). Sorption experiments and predictive modelling using the bottom-up approach (Marques Fernandes and Baeyens, 2021) demonstrate that radionuclide retention (e.g., for Cs, Ni, Eu, Th and U) is well constrained and dominated by sorption on illite and illite-smectite mixed-layer clays. As a result, the radionuclide species that can influence dose rates are predominantly anions, given that cations and most neutral species are efficiently retained by the sorption-dominated behaviour of the clay host rock.

3.6 Evidence and support for effective self-sealing capacity within the Opalinus Clay

The drill cores retrieved from Nagra’s most recent exploration campaign were systematically investigated for structural discontinuities (Nagra, 2024f; Akker et al., 2023). In the Opalinus Clay, hydraulic conductivities measured in packer test intervals including small-scale faults are systematically low (<1 × 10⁻¹³ m s^-1) and are comparable to test intervals without structural discontinuities (Figure 5). Hence, unlike what is typically observed in fractured geological media with lower clay-mineral contents, fractures in the Opalinus Clay in the siting region under present-day stress conditions have low transmissivity and are not considered to be preferential flow paths (Nagra, 2025; Nagra, 2024f). Additional evidence of effective self-sealing at rock-mass scale after fault reactivation in the Opalinus Clay was also demonstrated in stepped-constant differential head injection tests at approximately 600 m depth in the borehole Rheinau 1-1 just a few kilometres east of Nördlich Lägern (Figure 2) (Nagra, 2024d).

Extensive evidence of effective self-sealing behaviour of the Opalinus Clay has also been collected from the Mont Terri underground rock laboratory, where fault architectures tend to be complex. Studies of artificially induced fractures along the excavation-damaged zone (EDZ) and artificially activated fault zones show rapid resealing and return to low transmissivities once the fractures are resaturated and in-situ stress and pore pressure conditions are restored (Gautschi, 2017; Gautschi, 2001; Marschall et al., 2017; Guglielmi et al., 2025; Barakat et al., 2024; Bock et al., 2010; Williams, 2021; Orellana et al., 2022). For example, the EDZs of different tunnel sections were investigated for EDZ evolution and recovery up to 20 years after excavation, showing recovered seismic velocities, stabilised water contents, and reduced hydraulic transmissivities, which all indicate a progressive self-sealing at the rock-mass scale. Repeated in-situ hydraulic testing showed that fracture transmissivities in the EDZ decrease over time by orders of magnitude, approaching values comparable to those of the undisturbed rock mass as mechanical equilibrium is re-established (Marschall et al., 2017; Williams, 2021). Similarly, gas-migration experiments in-situ (at Mont Terri) and in the laboratory showed that gas transport occurred via a transient, pressure-dependent pathway opening which relatively quickly ceased once pressure decreased, without any permanent increase in host-rock permeability (Keller et al., 2026; Marschall et al., 2017; Lanyon et al., 2014).

In summary, investigations of various types and scales, including decades of in-situ observations, illustrate that self-sealing in the Opalinus Clay is strongly influenced by hydro-mechanical-chemical processes, including fracture closure due to effective stress, swelling, and creep (Crisci et al., 2024; Lanyon et al., 2014). These properties are expected to be maintained for as long as the Opalinus Clay remains at sufficient depth (i.e. 200 m, Mazurek et al., 2023b; Nagra, 2024f). These observations demonstrate that both natural fractures and those created by excavation within the host rock tend to rapidly self-seal over time, allowing transmissivity and rock-mass hydraulic properties to approach values similar to those of the intact Opalinus Clay. Geological long-term evidence for persistent low permeability even after regional tectonic events is further supported by the continuous natural tracer profiles discussed above and by natural tracer profiles across the main fault in the Mont Terri underground rock lab (Gautschi, 2017; Gimmi et al., 2014).

A further line of evidence for the long-term persistence of extremely low bulk permeability in the Opalinus Clay comes from regions where the formation has experienced significant tectonic overprint. In the Mont Russelin anticline, porewaters sampled from highly deformed Opalinus Clay show strong mineralisation despite intense faulting and folding (Mazurek and de Haller, 2017). This indicates that, even under substantial tectonic strain, low hydraulic conductivities persisted over geological timescales, limiting advective exchange with external aquifers (Nagra, 2024f). Together, these independent observations reinforce the conclusion that the Opalinus Clay maintains its sealing capacity even in structurally complex settings, further supporting the robustness of the safety-relevant hydrogeological barrier properties.

4 Special geological features within the CRZ: isolated carbonate platform and decimetre-to-metre scale hard beds and their relevance for safety assessments

Within the CRZ of Nördlich Lägern, several, mechanically more competent clay-poor units occur above and below the Opalinus Clay, which are intercalated in clay-mineral-rich units. These include decimetre- to metre-scale hard bed intervals within the Dogger Group above Opalinus Clay, and the more prominent «Herrenwis Unit» in the eastern part of the siting region. These units differ mineralogically and mechanically from the clay-mineral-rich host rock and confining units (e.g., reduced self-sealing potential). The field investigations do not give indications for hydraulic conductivities comparable to the neighbouring aquitards and the porewater geochemistry (profiles of the natural tracers) do not provide evidence for relevant advective flow in these units. These observations and the fact that both hard beds and «Herrenwis Unit» are embedded within the overall thick, clay-mineral-dominated aquitard sequence suggest that these units do not represent relevant flowpaths. However, to cover the remaining uncertainties (enhanced transmissivity related to fracturing, enhanced matrix permeability) these units are further analysed in safety assessment (Javanmard, 2026; Li et al., 2026; Papafotiou et al., 2026).

4.1 «Herrenwis Unit»

The isolated Herrenwis carbonate platform, measuring 4.5 by 8 km, is the most distinct feature within the CRZ and occurs about 40 m above the Opalinus Clay in the eastern part of Nördlich Lägern (Figure 6) (Nagra, 2024f; Ruchat et al., 2026; Ruchat et al., 2024). Seismic and core data illustrate that this ∼40 m thick reefal carbonate body transitions laterally into the clay-mineral-rich Dogger Group strata. Despite its heterogenous composition and diagenetic history and thus texture, reflected by interbedded clay laminae, marly layers, reefal debris, preserved in-situ corals with local vuggy and open porosity, and stylolithic features, the dominant rock volume has very low porosity and permeability due to compaction and diagenetic history. Hydraulic testing confirmed the low effective porosity and low hydraulic conductivity measured in boreholes, consistent with the presence of an effectively tight and hydraulically isolated carbonate body embedded within clay-rich units (Nagra, 2024f).

Multiple lines of evidence support this hydraulic isolation. Geochemical tracer profiles (stable water isotopes, Cl, Br,) indicate a diffusion-dominated exchange with no measurable advective signature (e.g., Nagra, 2024f; Ma et al., 2026), and hydraulic heads are lower than the potential discharge area would suggest. This indicates that the carbonate platform has been effectively hydraulic isolated within the clay-mineral-rich strata.

Despite its reefal character (Ruchat et al., 2026), the «Herrenwis Unit» at Nördlich Lägern shows strong hydraulic isolation. Comparable isolated reefal platforms embedded in low-permeability basinal strata are known from other Paleozoic and Mesozoic settings, illustrating that the presence of reefal carbonates does not necessarily infer advective, enhanced transmissivities or aquifer behaviour.

4.2 Hard-beds and clay-poor intervals within the CRZ

Beside the «Herrenwis Unit», thin, up to few metres thick, hard beds are present within the Dogger Group. In Nördlich Lägern, these beds are rather rare and occur within a unit with overall high-clay-mineral content. Hydraulic packer tests and laboratory measurements indicate low hydraulic conductivities (Nagra, 2024f). The limited bed-thickness, the absence of anomalies in natural tracer profiles and the low hydraulic conductivities (Figure 5) across these units confirm that they do not act as hydraulically active pathways. Although parts of these hard beds have elevated carbonate contents, they differ significantly from the regionally continuous and thick limestones, such as the Malm or Muschelkalk Groups (Nagra, 2024f). Across the CRZ, the combination of restricted lateral extent, diagenetic cementation, small fracture inventory, low effective porosity and hydraulic conductivity, and, most importantly, the embedment within thick clay-mineral-rich (diffusion-dominated) confining units prevents the development of connected fracture system and explains the hydraulic isolation from the bounding aquifers.

For safety assessment, the hard beds (clay-mineral-poor intervals) are, due to the diagenetic cementation, low matrix permeability, and embedment within thick sequences of clay-mineral-rich confining units, to be considered hydraulically isolated. Nonetheless, the effect of increased hydraulic conductivities are investigated during safety assessment (Javanmard, 2026; Li et al., 2026).

5 Future geological and climate evolution

Over the next one million years, long-term geological evolution in the Alpine Foreland will be influenced by tectonic shortening, uplift, erosion, and cyclic climatic variations. Based on different climate development scenarios, three to five large foreland-reaching glaciations are assumed to occur during this period and shape the landscape (shown in Figure 7 by dark-green boxes highlighting potential times of large foreland glaciations). Current climate trends are likely to delay the onset of the next glacial inception, with no major glaciation expected within the next 200,000 years (Talento and Ganopolski, 2021) (see also Nagra, 2024c).

Future landscape evolution models based on extensive regional Quaternary investigations and a probabilistic framework (Nagra, 2024c) indicate that fluvial incision will be the dominant long-term driver of erosion with plausible vertical incision ranging between 80 and 380 m within the next 1 Ma (Figure 7) at the Aare-Rhine confluence near Nördlich Lägern, consistent with observed Quaternary incision rates in the region. The present location of overdeepened glacial valleys and the distribution of current local topography suggests that during future foreland glaciations, deep glacial erosion may preferentially occur in the eastern part of Nördlich Lägern. Ice thicknesses may reach 400 m during a future large glaciation, which may last a few thousand years (Nagra, 2024b). Cumulative glacial erosion during these phases is most likely to reach ∼70 m below the future local erosion base (Nagra, 2024c). In less likely but still plausible cases, overdeepened glacial troughs could reach the Malm Group because the regional base level may already have been lowered by fluvial incision (Figures 2, 7). However, as the Malm Group is composed of mechanically competent limestones with limited erodibility, further erosional incision would be subdued, with most likely rates estimated to be about half those affecting the more easily erodible sand/siltstones of the overlying Molasse. Consequently, adverse effects due to glaciation induced overdeepening are unlikely, and erosional processes are not expected to reach the host rock. Given these incision estimates, a remaining repository overburden of at least ∼550 m is most likely after the period of assessment of 1 Ma (Nagra, 2024c). Because of the large remaining overburden, the barrier properties are not expected to be significantly affected by any external processes or undergo significant changes in hydraulic conductivity, porewater salinity or diffusion properties (Nagra, 2024f; Mazurek et al., 2023a).

Future glaciations or significant erosion may temporarily modify hydraulic gradients or heads or lead to permanently increased aquifer hydraulics. Because the maximum ice thickness at the repository site of future glaciations is expected to be rather small (300–400 m), the resulting changes in hydraulic gradients in the aquitard sequence are expected to be limited. Furthermore, full-scale glaciation events and their effects are short-lived compared with the time period for assessment. Therefore, only minor impacts of future glaciations are expected on transport in the host rock. This is supported by the profiles of natural tracers in the porewater, which, despite ice coverage during past glaciations, do not show indications for significant advective tracer fluxes in the CRZ. Therefore, little impact for post-closure safety and barrier performance is expected (Nagra, 2024f).

Another effect of the climatic cycles is the formation of permafrost during the coldest phases. Depending on climatic conditions, permafrost depths of 100–200 m could be expected (Nagra, 2024b). This deep-reaching permafrost may reduce groundwater fluxes in deep aquifers, affecting flow systems primarily through hydraulic conductivity changes in near-surface layers. However, since the remaining overburden above the top of the Opalinus Clay is expected to remain at least ∼550 m thick, permafrost is not expected to play a role and freeze/thaw processes should thus not impact barrier integrity.

In summary, future climatic and geological evolution in the Alpine Foreland will modify the surface landscape and hydrogeological conditions but will not compromise the long-term integrity of the barrier system of the Opalinus Clay in Nördlich Lägern. This is also expected to be the case for future evolution involving cyclic but limited perturbations of the hydrogeological system during glaciations and permafrost episodes. These processes are temporary and will not lead to significant changes in the barrier function of the Opalinus Clay as supported by the tracer profiles suggesting consistent and long-lasting isolation of the Opalinus Clay porewaters from the over- and underlying aquifers.

6 Conclusion

The low hydraulic conductivity, well-defined diffusive transport properties, and high sorption capacity under stable reducing geochemical conditions provide broad and well supported evidence for the effectiveness of the Opalinus Clay and the CRZ at Nördlich Lägern as a long-term geological barrier (Nagra, 2024f). The geological barrier assessment (Nagra, 2024f) for the Swiss safety case demonstrates that key safety-relevant properties of the CRZ are laterally continuous and predictable at formation scale. Clay-mineral content and continuity are particularly high in the Opalinus Clay (average ∼60 wt%), whereas the adjacent confining units, although overall clay-mineral-rich, exhibit greater heterogeneity. These sections of the CRZ include units with reduced clay-mineral contents that are up to a few metre thick hard beds, as well as the up to 40 m-thick, laterally confined reefal carbonate platform of the «Herrenwis-Unit». Although parameter uncertainties are less constrained, no evidence, throughout the extensive investigations, indicates that they form significant laterally connected flow paths.

The Nördlich Lägern siting region provides conditions that are stable in the long term and strongly limits the migration of any radionuclides that are released from the repository. The combination of low deformation rates, the absence of seismically mappable faults in the proposed repository area, the high self-sealing capacity of the clay-mineral-rich formations, and the significant overburden supports the assumption that the host rock will remain intact against perturbations over the time periods for assessment of up to 1 Ma. The probabilistic framework taking into account a broad range of likely and less likely future evolution scenarios further demonstrate that long-term climatic, geodynamic, and erosional processes do not critically affect host rock properties or the stress, hydraulic, and geochemical conditions required for sustained barrier performance (Nagra, 2024c; Nagra, 2024b; Nagra, 2024d).

The characterisation of the Opalinus Clay and the CRZ provides transport parameters required for deterministic and probabilistic safety assessment modelling and a comprehensive geological barrier evidence base that reduces the risk of a systematic bias within the safety case. For example, laboratory diffusion experiments (HTO, Cl⁻, Na⁺) consistently demonstrate low effective diffusive behaviour of the Opalinus Clay and other CRZ units and these findings are independently supported by detailed formation-scale natural tracer profiles of the rock porewaters (δD, δ¹⁸O, Cl⁻, noble gases). These porewater signatures illustrate that diffusion was the dominant transport mechanism in the CRZ over hundreds of thousands to millions of years. There are no indications for significant advective pathways.

These findings form the basis of the geosphere transport concept and the lines of arguments used in the safety assessment (Nagra, 2024h; Nagra, 2024f, Nagra, 2024j). Core observations, laboratory experiments, data from boreholes, and long-term in-situ studies at Mont Terri underground rock laboratory demonstrate that fractures and excavation-damage zones in the Opalinus Clay exhibit effective self-sealing. As hydraulic and mechanical equilibrium is obtained, transmissivities decrease rapidly and converge towards the hydraulic properties of the intact host rock (Marschall et al., 2017). These lab and field-scale findings demonstrate that short-term perturbations of the geological barriers do not compromise the long-term geological barrier efficiency.

Overall, the geosynthesis (i.e., Nagra, 2024f) integrates the regional- to site-scale geological, hydrogeological, geochemical, and geomechanical evidence into a comprehensive framework for the Swiss deep geological repository. It provides the reference geological basis for the Safety Case (Nagra, 2024h), and allows the abstraction and parameterisation of the geosphere transport concepts that allow the post-closure safety assessment (Nagra, 2024i; Li et al., 2026). More importantly, it represents a fundamental and traceable geological basis for current and future regulatory evaluation for the licensing processes.

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Footnotes

Edited by: Paul Smith, Safety Assessment Management GmbH, Switzerland

Reviewed by: Jens Eckel, Bundesamt für die Sicherheit der nuklearen Entsorgung, Germany

Reinhard Fink, Federal Company for Radioactive Waste Disposal (BGE), Germany