Hydrogeological characterization based on the results of long term monitoring in the Mizunami Underground Research Laboratory Project

The Mizunami Underground Research Laboratory (MIU) is now under construction by the Japan Atomic Energy Agency in the Cretaceous Toki granite in the Tono area of central Japan. Conceptual design of the MIU consists of two 1,000 m shafts (the Main Shaft and the Ventilation Shaft) and horizontal research galleries. The MIU project is a broad scientific study of the deep geological environment as a base for the research and development for geological disposal of nuclear wastes. One of the main goals is to establish comprehensive techniques for investigation, analysis and assessment of the deep geological environment in fractured crystalline rock. The project is being 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, surface investigations were carried out in a stepwise manner in order to obtain information of the geological environment at the site scale (about 2km square). Geological modeling and simulations of various kinds had been carried out in order to synthesize these investigation results. Two NNW-trending faults, which are important for hydrogeological characterization, are included in the model. One of the faults (fault A) strikes through the site in the immediate vicinity of the Main Shaft and another fault (fault B) strikes through the southern part of the MIU construction site. In Phase II, field investigations have been carried out in and around the MIU construction site. For hydrogeological characterization, long term monitoring of hydraulic pressure, surface tilt and self-potential have been carried out on surface and in the research galleries to obtain information on changes of groundwater flow due to shaft excavation. The results of the long term monitoring focused on fault A are as follows: - The hydraulic pressure responses are observed in surface boreholes in and around the MIU construction site and the galleries. The drawdown in the east side of fault A due to the shaft excavation is smaller than the drawdown in the west side of fault A. - An inverse analysis to predict the change of groundwater volume are conducted using surface tilt data obtained by high precision tiltmeters on surface. The results indicate that the change of groundwater volume due to the shaft excavation occurred mainly in the area bounded by faults A and B. - Self Potential (SP) have been monitored using a grid array of electrodes on surface around the MIU construction site. Local variation in SP (ΔSP) occurred as shaft excavation progressed. The ΔSP changes to positive (suggesting inflow with time) on the east side of fault A and changes to negative (suggesting outflow with time) on the west side of fault A. - The results of three different monitorings show that fault A should act as a hydraulic barrier. - Multiple lines of evidence derived from the long term monitoring during Phase II support the validity of the hydrogeological conceptual model, especially the hydrogeological characterization of fault A, obtained during the Phase I investigations.