Groundwater flow analysis of Horonobe underground research laboratory project

This report shows the process and the result of the research on the hydrogeological modeling and groundwater flow (GW) analyses of Horonobe Underground Research Laboratory (URL) Project until fiscal year 2004. In the report, Surface-based Investigation (Phase 1) of the project was divided into two phases; the investigation phase based on literature review and the borehole investigation phase. In the investigation phase based on literature review, the sensitivity analyses were carried out with the hydrogeological model built based on the considerable information from the literature review. The results showed that the sensitivities of recharge rate and hydraulic conductivity of the surface layer on hydraulic head and those of hydraulic conductivities of both Wakkanai formation and Koetoi formation on GW migration time were high. The results also showed that the GW flow system in and around the URL area was governed by the Shimizu river and Penke-ebekorobetsu river in the shallower zone (local GW flow system) and the Teshio river in the deeper zone (regional GW flow system). 'Regional scale' analytical domain was defined based on the results. In the borehole investigation phase, the results of both the in-situ hydraulic tests and the laboratory hydraulic tests were summarized as follows: The deeper it was, the lower the hydraulic conductivity was in the same layer, especially in Wakkanai formation. The hydraulic conductivity of the upper layer was lower than that of the lower layer in the same depth. The distribution of hydraulic conductivity was strongly related to that of fracture zone. The GW flow analyses based on newly developed geological model and the results of the hydraulic tests described above were performed. The GW flow analyses were verified by comparing with the measured hydraulic pressures in the boreholes. Besides, the behavior of saline GW was examined. Furthermore, the excavation of the URL shafts and tunnels was simulated. From the results, remarkable drops of the hydraulic pressures in HDB-3 and 6 were predicted while the pressures in the other boreholes were almost stable. (author)