The Heterogeneity of the Sandy Facies of Opalinus Clay across Scales, from Seismic Surveys to Radionuclide Diffusion - an in-situ Test in the Swiss Rock Laboratory Mont Terri

Many countries consider clay rock formations as potential host rocks for high-level nuclear waste disposal. Clay rocks may exhibit heterogeneity on various scales, from the micro- to the facies-scale. In the Mont Terri rock laboratory, Switzerland, various experiments study properties and characteristics of the Jurassic Opalinus Clay, which is the target host rock for the Swiss nuclear waste repository but may also provide proxies for other considered clay rock formations. At Mont Terri, the Opalinus Clay mainly appears in a shaly and two sandy facies. So far, diffusion experiments at Mont Terri focussed on the relatively homogeneous shaly facies. The upper sandy facies (SF-OPA) exhibits a more pronounced internal – mineralogical and textural – heterogeneity. Clay rocks with comparable heterogeneity to SF-OPA may be present among the lower Cretaceous clay rocks of northern Germany, which are among the potential host rock candidates for a future German nuclear waste repository. Since 2020, seven institutions develop an in-situ diffusion experiment in SF-OPA, the so-called DR-D experiment, to explore the impact of rock heterogeneity on radionuclide diffusion in low permeability clay rocks.
So far, the DR-D experiment combined high-resolution seismic tomography, borehole logging, and detailed drill core analyses to characterize the heterogeneity of the selected SF-OPA area. The targeted rock zone exhibits a layer starting ca. 10 m below the gallery surface, which is characterized by relatively high seismic velocities. This layer is as well evident in the natural gamma- and the neutron backscattering logs. In the drill cores it stands out as whitish rock characterized by large concretions and traces of bioturbation in contrast to the dark layered clay-rock above and below with smaller concretions. Detailed analysis of seismic signals and drill-cores is still ongoing. In future, an in-situ diffusion test using various radioactive and non-radioactive tracers (e.g., HTO, 129I, 22Na) will be conducted targeting the evidently heterogeneous rock section 10 m below the gallery surface. The evolution of tracer concentrations in a synthetic porewater circulating in the diffusion interval will be monitored. A second seismic tomography survey is planned after the termination of the diffusion experiment. Finally, overcoring and post-mortem analysis of the rock affected by tracer diffusion will be used to determine the local variability of diffusion parameters.
In this contribution, we present the general concept, technical layout, and expected scientific impact of the DR-D experiment, as well as first results from field and related laboratory studies.