Study on groundwater flow system in a sedimentary rock area; Case study for the Yoro river basin, Chiba Prefecture

Sakai, Ryutaro; Munakata, Masahiro ; Kimura, Hideo

Japan Atomic Energy Agency (JAEA) has started to investigate a sedimentary rock area in the Yoro river basin, in Chiba Prefecture. Hydro-chemical conditions of the regional groundwater were discussed based on temperature, chemical compositions, isotopic ratios of hydrogen and oxygen, and the isotopic age of radioactive carbon for water samples collected from wells, rivers and springs in the Yoro river basin. It was found that the groundwater system in this basin consists of types of water: Ca-HCO type water, Na-HCO type water and NaCl type water. The Ca-HCO type water is meteoric water cultivated several thousand years or after, the Na-HCO type water is meteoric water cultivated under cold climates several to twenty thousand years ago. The NaCl type water is fossil brine water formed twenty thousand years ago. It was also observed that the Na-HCO type water upwelled at the surface originates from GL-200m to -400m.