Unveiling the Hydrogeology of the Mont Terri Anticline – Insights from the DEBORAH Deep Drilling Project

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. However, their role within the wider hydrogeological framework of entire sedimentary successions remains insufficiently constrained. A key issue concerns the holistic hydrogeochemical characterization of containment zones and their effectiveness in preventing radionuclide migration across formation boundaries.The Mont Terri Underground Research Laboratory in Switzerland, with over three decades of multidisciplinary research on the Opalinus Clay, provides a unique natural laboratory and infrastructure to investigate these questions. 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. The project integrates: (A) an approximately 800 m deep, fully cored underground borehole from the St-Ursanne Formation down to the Schinznach Formation, including dedicated hydrogeochemical porewater sampling, in-situ downhole testing, and geophysical core and downhole logging; (B) seismic reflection and tomography studies combining surface, tunnel, and downhole acquisition geometries to image the geological structure of the Mont Terri anticline; and (C) 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, thereby advancing best practices in scientific continental drilling and subsurface safety assessment. 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.