Estimating hydraulic conductivity of the Opalinus Clay at the regional scale: Combined effect of desaturation and EDZ

A ventilation experiment (VE) was conducted in a non-lined microtunnel at the Mont Terri Rock Laboratory under well control conditions to evaluate in situ the consequences of desaturation induced by ventilation in consolidated Opalinus Clay rock. These investigations are carried out in the framework of the feasibility studies of radioactive waste underground repository. Specifically, from the point of view of long-term safety assessment in clay formations, it is crucial to estimate the evolution of the hydraulic conductivity. Yet, the combined effect of desaturation and the excavation damage zone is not well understood. We present the numerical hydrogeological modeling of the ventilation experiment based on measurements of water pressures and evaporation rates leaving the rock formation through the tunnel surface. It is found that, even though site characterization reveals that desaturation only occurs in a very small portion of the massif adjacent to the drift tunnel, nearfield water fluxes are still significantly driven by desaturation processes. In practice, this means that, even under minor desaturation conditions, routine characterization of effective saturated hydraulic conductivity (KS,eff) based on scoping calculations that consider the ventilation experiment as a large pumping well will largely overestimate KS,eff. An alternative conceptual model based on Kelvin’s equation was able to capture the physics of the experiment and provide a more accurate estimate of KS,eff.