Deep Groundwater in the Kanto Plain

抄録

Kanto Plain is the largest plain and also it is the largest groundwater basin in Japan. It has been believed that the deep groundwater is recharged by meteoric water (Hayashi, 2003). However, the basin is underlain by marine deposits that contain fossil water, as well as, in some parts, natural gas and fossil seawater. This is a review work about deep groundwater mainly from two previous studies (Marui et al., 2001 and Seki et al., 2001).

Deep groundwaters from deep drill holes were analyzed to determine chemical characteristics and modes of occurrences, and to understand deep groundwater environment. Deep groundwater in the Kanto Plain was divided into two types: one with low EC and of Na-HCO₃ type, and another with high EC and of Na-Cl type, from the analyseis of dissolved ions and isotopic compositions. It was concluded that the former was groundwater originated from meteoric water, and the latter formed by mixing of meteoric water and fossil seawater from the viewpoint of the isotopic compositions. At the depths of more than 1,000 m in coastal areas, the Cl^- concentrations of groundwater were lower than those of seawater. This suggests existence of upward deep groundwater flow system.