Hydrogeological Characterization in the Mizunami Underground Research Laboratory Project

The Mizunami Underground Research Laboratory (MIU) is now under construction by Japan Atomic Energy Agency in the Cretaceous Toki granite in the Tono area of central Japan. One of the main goals of the MIU project, which is a broad scientific study of the deep geological environment as a basis of research and development for geological disposal of nuclear wastes, is to establish comprehensive techniques for the investigation, analysis and assessment of the deep geological environment in fractured crystalline rock. The conceptual design of the MIU consists of two 1,000 m shafts and horizontal research galleries. The project is implemented in three overlapping phases: Surface-based Investigation (Phase I), Construction (Phase II) and Operation (Phase III), with a total duration of 20 years. In Phase I, hydrogeological investigations have been carried out in a stepwise manner in order to obtain information on important parameters such as, water balance at the surface, location of water conducting features, hydraulic parameters, hydraulic connectivity and groundwater pressure distribution. Hydrogeological modeling and groundwater flow simulations, which are: 1) for evaluation of undisturbed hydrogeological and groundwater flow conditions before excavation of the URL and 2) for estimations of volume of inflow into the shafts and galleries and changes of water table and hydraulic pressure, have been carried out in order to synthesize these investigation results, to evaluate the uncertainty of the hydrogeological model and to identify the main issues for further investigations. In Phase II, long-term hydraulic monitoring which is for mainly determination and assessment of hydraulic responses during excavation of the shafts and galleries, geological mapping and hydraulic testing using boreholes drilled from underground galleries have been also carried out. Based on the results of these investigations, hydrogeological model constructed in Phase I has been tested and revised. Lessons learned through the hydrogeological characterization are as follows; - Hydrogeological characterization combining field investigations and hydrogeological modeling iteratively is quite efficient. - Data of hydraulic transient change is useful to calibrate the hydrogeological model and to constrain the uncertainties of characteristics of groundwater flow. - It has possibility that the existence of unknown hydrogeological structures can be predicted by combining the hydraulic monitoring with calibration of hydrogeological models. The study to establish comprehensive techniques for investigation, analysis and assessment of hydrogeological environments will be continued in Phase II and Phase III of the MIU project.