Conference Agenda

In 2024, Nagra submitted the general licence application for a deep geological repository in the Opalinus Clay of Northern Switzerland. The local geological conditions and the rock properties play a central role in long-term repository safety. Thanks to its high clay-mineral content, the Opalinus Clay represents a transport barrier characterised by extremely low hydraulic conductivities, a high radionuclide sorption capacity, and an efficient self-sealing behaviour. These favourable barrier properties are documented by numerous field and laboratory measurements. To demonstrate the robustness of these properties against future climatic, erosive and tectonic processes, the reconstruction of past landscape evolution, deformation and transport processes in the deep subsurface plays a central role. The presentation shows how the performance of the Opalinus Clay and surrounding units as a transport barrier over time scales of up to 10⁶ years can be demonstrated using various geological archives and natural tracers.

The pore water (PW) composition in very low-permeability clay rocks – some of which are potential host rocks for radioactive waste repositories – has evolved significantly since sediment deposition and compaction. On geological timescales, various processes have altered the PW composition, which was originally seawater in contact with the atmosphere. Characterizing a clay host rock should therefore aim to identify the processes responsible for its specific PW characteristics. Here, we focus on an observed noble gas abundance pattern in clay pore water obtained from a recent drill core in southern Germany, where we observe that noble gas abundance correlates strongly and linearly with the square root of their atomic mass.

Pores that have been depleted of certain system components no longer serve as effective transport pathways for them. Consequently, quantifying the remaining effective porosity is crucial for radionuclide transport modelling. While it is well-established that the exclusion of anions from interlayer-equivalent pores is driven by their negative charge (Tournassat et al. 2026), the mechanism for dissolved noble gases remains a subject of debate. Torgersen et al. (2004) suggest a plausible sequential diffusive depletion of nanosized pores during compaction, whereas Gadikota et al. (2017) propose element-specific solvation of noble gases within clay interlayer nanopores.

In a large-scale, homogeneous, and diffusion-dominated formation, we suggest using the atmospheric neon concentration as a proxy for the effective porosity.

Crystallographic clay mineralogical tools combined with cation exchange capacity (CEC) provided new insights into the geochemical variability and sequence stratigraphic interpretation of the mudstone-dominated Lower Cretaceous succession in the eastern Lower Saxony Basin, Northern Germany.

Rather than relying on indirect proxies such as XRD peak broadening to describe illite–smectite development during diagenesis, we applied the Rietveld method for quantitative phase analysis of the < 2 µm fraction [1-2]. This approach allowed a detailed assessment of interstratified clay minerals, verified through chemical composition and further constrained by CEC analyses using the Cu-triethylenetetramine method. Since smectitic 2:1 (TOT) layers show much higher CEC than illitic, chloritic, or kaolinitic layers, CEC variability principally reflects changes in smectitic layer proportions.

CEC data from a ≈ 200 m deep borehole (1 sample/m) showed that in several intervals, high CEC values could not be fully explained by illite–smectite content alone, suggesting the presence of an additional high-CEC phase. Subsequent investigations of finer fractions (< 0.2 µm) confirmed the occurrence of pure smectite as a physical mixture, previously undetected in the bulk < 2 µm analyses. Updated quantitative clay mineral compositions from improved Rietveld refinements now correlate well with measured CEC values even in these anomalous sections.

These integrated mineralogical and geochemical tools significantly enhance understanding of the Lower Cretaceous succession—particularly the Valanginian Weissert Event—and refine methods for identifying high sorption capacity lithologies in mudstone-dominated systems.

[1] Ufer K et al. (2012a). https://doi.org/10.1346/CCMN.2012.0600507

[2] Ufer K et al. (2012b). https://doi.org/10.1346/CCMN.2012.0600508