Clay-rich formations are internationally considered as ideal seals for underground disposal of radioactive waste [1, 2]. In the context of site exploration programs transport characteristics must - among other information – be locally quantified [3].

Here we present noble gas profiles from Hondingen, southern Germany [4], an area already disqualified as a siting region due to the shallow depth of the Opalinus Clay formation and an active fault zone. The sampling procedure generally followed guidelines for drill core sampling of the recent Nagra drilling campaign [5], refining procedures originally developed elsewhere [6 - 10].

We compare our data with analytical solutions [11] and numerical simulation results [12]. The ⁴He concentration profile has reached steady state. Within the clay barrier, the transport is diffusion dominated with the minimum porewater residence time calculating to some million years.

[1] M. J. Hendry et al. (2015) Geofluids 15(3) [2] Clay Club Catalogue (2022) NEA, OECD [3] A. Bath, et al. (2023) Appl. Geochem. 159 [4] K. Leu et al. (2023) J. Appl. Region. Geol. [5] D. Rufer and M. Stockhecke (2021) NAB 19-13(1) NAGRA [6] J. Lippmann et al., (1997) IAEA-SM-349/36; [7] K. Osenbrueck, et al. (1998) GCA 62(18) [8] A. P. Ruebel et al. (2002) GCA 66(8) [9] M. Mazurek et al. (2011) Appl. Geochem. 26(7) [10] D. Rufer, et al. (2017) Proc. Earth Planet. Sci., 17, [11] J. N. Andrews (1985) Chem. Geol., 49 [12] D. L. Parkhurst and C. A. J. Appelo (2013) Techniques and Methods, 6(A43), USGS, USA

The Opalinuston-Formation in Southern Germany comprises a thick (>100 m) sequence of Middle Jurassic claystones. In parts of Bavaria and Baden-Württemberg, the formation has been designated as a sub-area by the Federal company for radioactive waste disposal (BGE), indicating that favorable geological conditions for the final disposal of high-level radioactive waste can be expected there. However, coherent geological data for the complete formation is often lacking. Here we present new data on the geological variability of the Opalinuston-Formation based on four drill cores, each of which penetrated through the entire formation, and parts of the over- and underlying rock strata outside the sub-area. After completion of the drilling work, the cores were examined in high resolution for their petrophysical properties and element distribution using non-destructive analytical tools such as XRF core scanner and multi-sensor core logger. Furthermore, one sample per core meter was taken for a precise stratigraphic classification of the rocks and a geochemical-mineralogical and sedimentological characterization. Results exhibit notable geological variability at the investigated localities, particularly with respect to the Si/Al ratio. This is interpreted as changes in grain size due to relative sea-level fluctuations during deposition, and is thus useful to reconstruct stacking patterns of parasequences. In summary, this contribution highlights the geological variability of the Opalinuston-Formation in Southern Germany and discusses the possibilities of a sequence stratigraphic approach to identify the most suitable parts of sub-areas in claystone formations for the final disposal of high-level radioactive waste.

Ensuring the long-term safety of underground repositories for high-level radioactive waste requires a comprehensive understanding of overburden stability. In this BGE-funded project, we investigate past landscape evolution and erosion dynamics in the South German Scarplands utilizing a multidisciplinary approach. Through geomorphic analysis, stratigraphic reconstruction, thermochronology, cosmogenic nuclides, and landscape evolution modelling, we aim to quantify the intricate interplay of geological processes shaping overburden topography over a million to a thousand year timescales.

Our findings reveal that long-wavelength uplift pulses and graben formation control long-term erosion within the region. Notably, the latest post-15 Ma uplift pulse has led to a southeastward shift of the drainage divide between the Main/Neckar and the Danube rivers, accompanied by substantial river captures and localized pulses of erosion. Associated peak erosion rates can be up to several tens of meters per thousand years and create a cascading effect of erosion away from the original capture sites along the antecedent river valleys. The magnitude of these events is constrained by the accumulated differential uplift between the base-level of the Rhine to the west and the uplifted area to the east (i.e., the Swabian Alb).

Furthermore, we highlight the profound influence of the exposure of 'weak' rocks and associated drainage system reorganization on both the past and future evolution of the South German Scarplands. Our comprehensive quantitative analysis contributes valuable insights into the complex interplay of geological processes governing landscape evolution and surface erosion, which are crucial for assessing the long-term safety of underground repositories for radioactive waste.

To assess the present and future conditions of potential deep geological repository sites, understanding their evolution in the past is mandatory. Here, glaciation cycles strongly affected the long-term thermo-hydro-mechanical (THM) evolution of the geosystem.

The AREHS project studies the effects of time-dependent boundary conditions (BCs) on the long-term evolution of large-scale hydrogeological systems. The focus is on numerical modeling using the open-source multi-field finite element code OpenGeoSys with THM couplings. The impact of the glacial THM loading is taken into account using complex time-dependent THM BCs. A generic geological model for a clay host rock formation including predominantly sedimentary rock layers is applied. Assuming the plastic flow behavior of the sedimentary rocks to share qualitative features, the same generic material model is used for all layers: The elasto-plastic modified Cam clay model can describe qualitatively a range of relevant effects (dilatancy, contractancy, consolidation etc.) with few material parameters. Special emphasis is put on the specification of a suitable initial state: To this end, an initial simulation is carried out, where a reasonable plastic pre-consolidation is adjusted. Then, the thermodynamic state is transferred in full to the subsequent simulation of two glacial cycles. As a main result, the glacial cycles lead to persistent deviations in the subsurface, e. g. long-term pressure anomalies due to THM coupling. However, under the chosen assumptions, only the first glacial cycle leads to pronounced (contractant) plastic flow whereas the second cycle merely shows elastic rock behavior.

Funding: BASE Grant No. 4719F10402 (AREHS project)

The lowlands of northern Germany and neighbouring countries have been subject to glaciation several times in the pleistocene. These glacial periods have left behind several erosional features, of which tunnel valleys are the deepest and most remarkable. The current consensus of the respective field of science is that those valleys are formed by the flow of melt water below the melting glacier.

Within the site selection procedure in Germany for a high-level nuclear waste (HLW) repository, a safety assessment for 1 million years has to be conducted. During this time span repeated glaciations are likely to occur in northern Europe, and therefore repeated glacial erosion is to be expected.

However, beyond the safe inclusion in the selected host rock of a HLW repository, a sufficient stability of the overlaying overburden horizons must be ensured.

To address the numerous uncertainties regarding the glacier melting process and the conditions of the sedimentary layers eroded by melt water transport, we have developed a combined modelling approach. This method comprises a three-dimensional deterministic numerical modelling of the tunnel valley genesis with FLAC3D and a multivariate probabilistic modelling with GoldSim to account for the remarkable uncertainties over 1 Ma. The approach allows to evaluate the resistance of different overburden stratigraphies against deep glacial erosion processes and therefore can be a valuable contribution to the site selection process.

The research project presented here is funded by BASE under the grant number FKZ 4721F10401.