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

In the safety assessment for a geological disposal of long-lived radioactive waste such as high-level radioactive waste and TRU waste etc, it is important to estimate radionuclide migration to human society associated with groundwater flow. Groundwater flow systems for many domestic areas including Tono Mine, Kamaishi Mine and Horonobe district have been studied, but deep groundwater flow circumstances, and mixing between deep groundwater and shallow groundwater flow system are not well understood. Japan Atomic Energy Agency (JAEA) has started to investigate a sedimentary rock area in the Yoro river basin, in Chiba Prefecture, where the topographic and geological features are relatively simple for mathematical modeling, and hydraulic data as well as data from river and well water are available. 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-HCO3 type water, Na-HCO3 type water and NaCl type water. The Ca-HCO3 type water is meteoric water cultivated several thousand years or after, the Na-HCO3 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-HCO3 type water upwelled at the surface originates from GL-200m to -400m. This observation indicates that the Na-HCO3 type water upwelled through the Ca-HCO3 type water area with the both waters partially mixed. (author)