1:45 PM - 2:00 PM
[HCG32-06] Estimation of hydrogeological properties of a fault by geochemical analysis of groundwater and mass transport analysis
Keywords:groundwater, environmental tracer, mass transport analysis
Study area is the region around Mizunami Underground Research Laboratory of Japan Atomic Energy Agency (JAEA), Gifu prefecture, Japan. Main shaft of the laboratory is located adjacent to a fault referred to as Main Shaft Fault (MSF) with NW-SE strike and almost vertical dip. Groundwater samples were collected from 12 intervals of 5 boreholes on both sides of the MSF. Concentrations or isotope ratios of dominant dissolved ions, alkalinity, hydrogen and oxygen isotopes, sulfur hexafluoride, tritium were determined, and the spatial distribution and temporal change were considered by combining with geochemical monitoring data conducted by JAEA (Sai et al., 2011; Shingu et al., 2011; Shingu et al., 2012; Omori et al., 2013a; Omori et al., 2013b; Omori et al., 2014). In addition, a three dimensional hydrogeological model was developed, and groundwater flow analysis was carried out. Its results were used as boundary conditions in groundwater flow and mass transport analyses conducted using another model partly refined around the fault. The analytical results were compared with the actually measured values.
The sample analyses and past data revealed that spatial distributions of calcium, chloride, bromide ion concentrations, hydrogen and oxygen isotope ratios, and tritium concentration were different between both sides of the fault. Concentrations of calcium, chloride, and bromide ions similarly decreased with time only on the southwestern side of the fault. The results of the mass transport analysis considering the influence of the water drainage in the shaft excavation also showed decrease of chloride ion concentration only on the southwestern side of the fault. Head calculated in the groundwater flow analysis was different between both sides of the fault. In contrast, the mass transfer analysis of tritium showed infiltration of surficial young groundwater containing tritium along the damage zones on both sides of the fault. Estimated infiltration rate was larger on the southwestern side of the fault. These results indicated that the MSF acts as both of barrier and conduit.
This research was conducted under support of a grant from the Ministry of Economy, Trade and Industry (METI).
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