(1) Knowledge obtained by the RHS Project

The following knowledge is obtained by the RHS Project carried out in the area including the Shobasama site.
River water collected at 29 locations is classified as Ca²⁺-Na⁺-HCO₃^- -type⁶⁰⁾. The fact indicates that this type of groundwater is expected in the Seto Group and Mizunami Group and the shallow part of the Toki Granite as one of the candidates of water chemistry. It suggests that the groundwater of the RHS Project area is of rainfall-type⁶⁰⁾.
Chemical analyses of groundwater collected at 20 points in 6 boreholes and measurements of physicochemical parameters are carried out. The results indicate that groundwater in the shallow part (< 300m in depth) in the granite distribution area is of Ca²⁺-Na⁺-HCO₃^- -type, neutral (pH 7) and oxidizing (Eh>0 mV). On the other hand, the groundwater in the deep part (> 300m in depth) is Na+-HCO3- -type, alkalescent (pH 9) and reducing (Eh<-300mV) (Figs.4.59 and 4.60)⁵⁹⁾.
The following knowledge on microbes in groundwater is obtained by counting the number of total bacteria and analysis of specific bacteria (sulfate reducing bacteria, iron oxidizing bacteria) in groundwater in the Toki Granite and Mizunami Group.
The total number of bacteria ranges from 10⁶10⁷cell/ml, regardless of rock varieties or depth.
There are depth ranges that contain no sulfate reducing bacteria or some 10³CFU/ml of them.
The number of iron oxidizing bacteria ranges from 10⁴10⁵cell/ml throughout the entire depth.
Chemistry of groundwater in the Toki Granite is formed by dissolution of pyrite and calcite, argillization of feldspars, and ion exchange between groundwater and clay minerals⁵⁹⁾.
Results of measurement of hydrogen-oxygen isotope ratios indicate that groundwater in the Toki Granite is of meteoric water origin. Results of the measurement of ¹⁴C suggest the retention time of some ten thousand years for groundwater around of 1,000m in depth¹⁵⁾.

(2) Current status of investigations in the MIU Project

Simultaneous data acquisition related to various study fields in the same borehole is not only time-saving but advantageous to comparative examinations on the data in different study fields. In borehole investigations in the initial stage of the RHS Project, physical logging, hydraulic tests and pumping tests are carried out in this order after borehole excavation using fresh water. However, such incidents as borehole collapse at a large-scale fructure zone and water leakage causing percolation of a lot of drilling water into the rock mass, renders it nearly impossible to collect groundwater.

Based on these experiences, groundwater collection in the MIU-13 is planned to be carried out after geophysical logging, hydraulic tests, installation of the MP system in the boreholes and a long-term drainage. Thus, the MP systems are installed in the MIU-1 and MIU-2, while it is planned to be installed in the MIU-3 in 2000 FY. However, groundwater collection in the MIU-1 and MIU-2 could interfere in other simultaneous borehole investigations (including hydraulic tests, etc.). Therefore it has yet to be carried out. The timing of water collection will be decided based on the progress of borehole investigations.

Based on the experience of failure in efficient water collection in the MIU-13, change
in investigation procedure is planned for MIU-4 investigation (planned in 2000 FY). According to this change, water collection is carried out in combination with pumping tests subsequent to immediate suspension of drilling if water leakage occurs⁴³⁾. In addition, a definite concentration of dye is added with drilling water to maintain the data quality on hydrochemical properties of groundwater by identifying the residual amount of drilling water and the groundwater infiltrated from other irrelevant zones. Also, different dyes are applied to the sedimentary rocks, and the hanging wall and the footwall of the Tsukiyoshi Fault on the assumption that these zones might have different hydrochemical properties. Pumping tests carried out as part of hydraulic tests are utilized as a preparatory water collection to remove drilling water. The removal of drilling water is decided by concentrations of mixed dye and physicochemical parameters obtained by continuous monitoring of groundwater.

Furthermore, hydrochemical data is obtained as part of study on the evolution of groundwater chemistry using rock core. Measurements of ratio between Fe³⁺ and Fe²⁺ in the granite indicate that Fe³⁺ is more than Fe²⁺ in the shallow part (< 300m in depth), whereas Fe²⁺ is more than Fe³⁺ in the deeper part (> 300m in depth). This probably indicates that oxidation-reduction environment in the granite changes at around 300m in depth.