Development of the investigation and evaluation method for geochemical condition of underground. In-situ redox conditions and its prediction at the Rokkasho site

For the safety assessment at the Rokkasho low level radioactive waste disposal site, it is necessary to evaluate the redox conditions of the natural barrier around facilities. In-situ rocks and groundwater around a redox front in the underground cavern were investigated and the alteration of redox environment was estimated and predicted. Water chemistry in the small holes drilled on the tunnel wall corresponds to the characteristics of the weathering zones and this method is effective as in-situ investigation of the groundwater. The oxygen consumed components in the rocks at the oxidation front are mainly pyrite. From the estimation of the amount of the components, it was presumed that about 600 times volume of dissolved oxygen in the pore water was necessary to oxidize the unit volume rock. Migration rate of the oxidation front was estimated about 0.2 m/y from the flow rate of groundwater. The oxidized zone (2.5-10m thickness) was formed during 10000-50000y under the constant hydraulic conditions. (author)