Kunio Furuno, Research Institute of Environmental Geology, Chiba (RIEGC) (Japan)
Takashi Kusuda, Research Institute of Environmental Geology, Chiba (RIEGC) (Japan)
Hisashi Nirei, (NPO)The Society of Geo-pollution Control, Japan (Japan)
Atsushi Kagawa, Research Institute of Environmental Geology, Chiba (RIEGC) (Japan)
Osamu Kazaoka, Research Institute of Environmental Geology, Chiba (RIEGC) (Japan)
The Kanto basin can be called the Kanto groundwater basin from the standpoint of the subsurface fluid resources. The bottom of the ground water basin corresponds to the base of the Kanto basin. The maximum depth of the groundwater basin is over 3,000m-2,500m below the surface. The lower part of it contains brine groundwater including natural gas and iodine. In the upper part of it, most of the aquifer layers composing the groundwater basin contain fresh water; it has the largest pumping up volume of groundwater. Over 38 million people live on the Kanto plain.
Changes of groundwater level have been measured in observation wells. The number of observation wells is about 500 in total as of 1990's. The amount of land subsidence has been measured by precise leveling which has been carried out once a year. The number of benchmarks and the total length of the leveling reached about 5,000 and over 7,300 km as of 1990's. It was in the early 1970's that the groundwater level fell to the lowest point. The level dropped to 60 m below sea level in the coastal industrial zone of Tokyo metropolis. After that, the recovery of the level was recognized according to the control of the pumping up volume. In the 1970's the regulations were strictly adhered to in the southern Kanto groundwater basin. Accordingly, in the years from 1975 to 1980, the level recovered 30 m to 40 m in the area of lowest groundwater. Thus the ground level showed a recovering tendency in the whole area. In recent years, in the central to northern part of the Kanto groundwater basin, the water level has shown a lowering tendency, that is, the area whole groundwater level is 20m - 30 m below sea level has increased. In other words, a center of groundwater depression moved up to the northern part as a result of the control of pumping volume in the southern Kanto groundwater basin. The lowest level is not the former 60 m but 20 m below sea level, and the area of the low value has spread out.
The groundwater level became lower due to the over pumping, resulted in the land subsidence. Consequently, the elevation of lowland Tokyo area subsided over 4.0m since the 1910s. Coincidentally, the areas below sea level were widely sprawled. These phenomena were recognized in the suburbs of Tokyo such as Funabashi, Gyotoku and Urayasu in western Chiba prefecture.
We need the periodical check of the groundwater basin by the simulation analysis and the monitoring system for the wise use of the groundwater. The land subsidence has been stabilized by the control of the pumping and stopped in certain areas. Moreover, the ground surface slightly uplifted by the recover of the groundwater level. It is a matter of great importance to the wise use of groundwater to decide the pumping volume in consideration of the change of the groundwater level and the ground movement with the law of dynamic equilibrium between man and nature due to the relations among human groups.