2001 年 31 巻 3 号 p. 3_11-3_24
The Kanto Plain is Japan’s largest plain, and is also that country’s largest groundwater basin. The basin is constituted by marine layers that contain fossil water, as well as, in some parts, natural gas and fossil seawater. Past research has led one to estimate that the groundwater was recharged by meteoric water in the deep part of the layers.
In this study, groundwater taken from hot spring wells was analyzed in order to clarify the chemical characteristics, origin, and flow system of deep groundwater, and chemical analysis of major dissolved ion and isotopic composition of oxygen and hydrogen was carried out. As a result, deep groundwater in the Kanto Plain was divided into two types: one with low EC and Na-HCO3 water quality, and one with high EC and Na-Cl water quality. The isotopic composition leads one to estimate that the former was groundwater originated from meteoric water, and that the latter has been formed by the mixing of meteoric water and fossil seawater. In deeper parts, more than 1,000m in coastal areas, the Cl concentration of groundwater was lower than that of seawater. This result suggests the existence of a deep groundwater flow system.