Robust geological site selection for deep geological repositories relies on an interrelated understanding of stratigraphy, structure, and hydrogeology, particularly the capacity of low-permeability formations to contain fluids and solutes over geological timescales. Argillaceous formations such as the Opalinus Clay are therefore intensively studied as potential host rocks for radioactive waste disposal. A key issue concerns the holistic hydrogeochemical characterization of containment providing rock zones and their effectiveness in preventing radionuclide migration across formation boundaries. In this context, this study explores how the role of such units can be inferred within the wider hydro-geological framework of entire sedimentary successions, particularly when interpretations rely on limited subsurface data and in structurally complex settings compared to more extensively characterized, sub-horizontal successions. The Mont Terri Project in Switzerland, with three decades of multidisciplinary research on the Opalinus Clay, provides a unique natural laboratory and infrastructure to investigate and analyse the hydrogeological, geochemical, and rock mechanical properties of argillaceous formations (Bossart et al. 2017). The DEBORAH (DEep BOrehole to Resolve the Mont Terri Anticline Hydrogeology) project — the deepest drilling project to date at the Mont Terri site — offers the opportunity to complement previous extensive local studies on the regional scale, encompassing the full stratigraphic succession of the Mont Terri anticline. DEBORAH aims to systematically sample and quantitatively characterize the geological system in and around the Opalinus Clay. To this end, we present three complementary experiments that together explore different aspects of the system. The project integrates: (1) an approximately 800 m deep, fully cored underground borehole from the St-Ursanne Formation (Upper Jurassic) down to the Schinznach Formation (Middle Triassic), to enable a detailed characterization of the Opalinus Clay across contrasting structural settings, including its lateral extent beyond the anticline, and to assess associated hydrogeological and hydrogeochemical variability; (2) seismic reflection and tomography studies combining surface, tunnel, and downhole acquisition geometries to image the geological structure of the Mont Terri anticline; and (3) hydrogravimetric monitoring of natural fluid migration in aquifers above the Opalinus Clay. The resulting datasets will support 3D geological, hydrogeological, and reactive transport modelling, enabling improved quantification of hydraulic connectivity and containment within and across the system. Beyond Mont Terri, DEBORAH seeks to develop transferable in-situ investigation workflows to support future site selection procedures for radioactive waste disposal in Germany and elsewhere. In particular, the realization of a deep borehole in the exceptionally well-characterized geological setting at Mont Terri is likely unique worldwide and will provide critical insights into what can be learned about repository site selection from a single exploration borehole. This paper provides an overview of the general project design, outlines its three related subprojects, and their current status.
Kästner et al. (Thu,) studied this question.