Over the last 30 years, the Mont Terri Underground Research Laboratory (URL) provided easy access to the Jurassic Opalinus Clay environment and thus presents a unique platform for scientific investigations related to radioactive waste repositories and CO₂ caprocks. One of the key topics was large-scale, in-situ diffusion and retention experiments using various tracers to understand their migration and to improve system-level understanding of transport processes of radiotracers in clay rocks . These experiments and their results have been summarised in several previous review and synthesis papers, which provide an important basis for evaluating long-term transport behaviour in clay-rich host rocks. As clay rocks are considered natural geological barriers, understanding their behaviour and the mechanisms that influence solute transport within these systems is essential for robust modelling and safety assessments. These aquitards exhibit extremely low permeabilities, and diffusion is typically the most important transport process. This is one of the reasons why numerous waste management organisations focus on argillaceous formations as prime host rocks for deep geological repositories. Moreover, sorption processes provide an important and often nuclide-specific retardation in clay rocks. The field investigations in the Mont Terri URL have been systematically complemented by an extensive body of other URL and laboratory diffusion and sorption studies conducted across multiple sites and institutes. Investigating diffusion and retention processes under controlled, large-scale, in-situ conditions is of particular importance, as this allows bridging small-scale laboratory investigations and borehole studies to repository-relevant spatial and temporal scales, thereby enabling validation of process understanding and underlying transport mechanisms. Building on the substantial contributions of diffusion research at the Mont Terri URL to fundamentally understand transport processes in clay rocks, this paper provides an overview of past field-scale diffusion experiments and synthesises their outcomes in the context of complementary laboratory studies and modelling developments. This document builds on previous summaries and focuses on methodological advances, model evolution, and the integration of diffusion data with deep borehole investigations, thereby highlighting the process-level insights gained from diffusion research across scales.
Raphaël et al. (Thu,) studied this question.