北海道幌延地域の新第三系珪藻質泥岩中の地下水の酸素・水素同位体比と塩化物イオン濃度

Abstract

Research and development on geological disposal of high-level radioactive waste have been carried out by Japan Atomic Energy Agency (JAEA, previous JNC) in Horonobe, the northern Hokkaido. The geology of Horonobe comprises the Neogene siliceous marine sediments which contain saline water. The origin of saline water is one of the most fundamental factors for evaluating geological environments. The synthesis of geo-scientific understanding contributes to develop a performance assessment methodology by providing a basis for setting parameters and constructing conceptual model. Geological environments are also important for exploration of oil fields. This study analysed the various data of JAEA reseach for hydro-geochemistry of Horonobe.The squeezed water and the lifted water from eight boreholes drilled in Horonobe are analyzed on oxygen and hydrogen isotopes and Cl- concentration by JAEA. Based on the analytical data, the groundwater is subdivided into three groups derived from the mixtures of meteoric water and different saline waters, which distributions are consistent with the regional geologic situations.δ18O, δD and Cl- of pore water during diagenesis of the siliceous sediments are calculated based on the assumption that water/rock interaction makes redistribution of the isotopes in rocks and pore water during the phase transformations from opal A through opal CT to quartz in open systems, then that water in biogenic silica is expelled to pore-space during the successive compaction in closed systems. The measured data points fall approximately on the mixing lines between the meteoric water and the pore waters calculated on the assumption of the phase transformations. The geological study on the diagenesis of the field shows that the silica phase transformations actually occur near the depth calculated from the porosity reduction, indicating that the transformation and the succeeding compaction contribute to the origin of the saline water.