Tokyo Bay is at the receiving end of the groundwater flow system of the Kanto Plain, Japan’s largest plain and groundwater basin. The groundwater flow, from mountains surrounding the Kanto Plain to Tokyo bay, is thought to run along the geological structure of the tectonic basin. Therefore, water discharged into the bay may contain information concerning deeper zones of the Kanto Plain.

Groundwater samples were collected around the bay from 25 to 2000m depth wells, and isotopes (Carbon, Oxygen and Hydrogen) and general water quality were analyzed. The deep zone water was determined to be composed, for the most part, of rainwater and fossil seawater. This suggests that rainwater filtered down to the deep zone on short term, that fossil seawater still exists in that zone, and further suggests that these characteristics of water reflect the geology (history) of the Kanto Plain. This study contributes to existing data of deep zone groundwater around the Tokyo Bay, and should serve further applied research or planning.

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.