The Kanto Plain is the largest groundwater basin and the largest groundwater used region in Japan. There is a groundwater area with high Cl concentration (from 10 to 150 mg/l) in the depth of GL-100m to -300m of the central part of the plain. This groundwater area is thought to be made by regional groundwater flow, from the viewpoint of three-dimensional observations of groundwater quality, stable isotopes, and subsurface temperature. On the other hand, it has been pointed out the reduction of the high Cl groundwater area since 1960’s. However, the tendency and the process of the reduction have not been clarified yet. In this study, secular change of high Cl groundwater area and distribution of Cl concentration in 40 years was studied. Also, process of the change was considered by comparison with secular change of distribution of hydraulic head and land subsidence area.
From the result, reduction of the high Cl groundwater area and decrease of Cl concentration in the groundwater area was clarified, and these changes were remarkable in the southern part of the high Cl groundwater area. The reduction and the decrease was caused by secular change of groundwater flow according to depression of hydraulic head in depths of GL-100m to -300m, and supply of low Cl groundwater from the shallower part, the deeper part, and the surroundings to the high Cl groundwater area.