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Atsushi Kagawa, Research Institute of Environmental Geology, Chiba (Japan)
Kunio Furuno, Research Institute of Environmental Geology, Chiba (Japan)
Hisashi Nirei, RIEGC (Japan)
Takashi Kusuda, Research Institute of Environmental Geology, Chiba (Japan)
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The Kanto Plain is called the Kanto Groundwater Basin. The maximum depth of the basin is over 3000m - 2500m below surface. The groundwater basin is composed of Plio-Pleistocene marine sediments. Chiba Prefecture is located in south-east part of the Kanto groundwater basin, and monitoring the land subsidence by 146 observation wells and 1185 benchmarks in total as of 2007. The mechanism of land subsidence has been made clear by totally and continuous monitoring. The lower aquifer contains brine groundwater (fossil sea water) including natural gas and iodine. Maximum land subsidence of 247mm/year in 1971 was due to the natural gas production. Then regulations were strictly adhered to in Tokyo Bay area. Consequently, the recovery of the groundwater level was recognized and the surface has been slightly uplifted. The upper aquifer has the largest pumping up volume of groundwater for aqueduct, agricultural, industrial, and building use. This aquifer had the largest pumpage volume of water in the Kanto groundwater basin. The groundwater level fell to its lowest in the early 1970s. Many local governments regulated groundwater pumpage, and surface water source supplied for the industrial district. Accordingly, the groundwater level recovered 30m to 40m in the southern Kanto groundwater basin. Recently, the groundwater level has been stabilized from the standpoint of the long time scale. The most upper part of the basin, a characteristic compaction of the Alluvial deposits has been recognized on deltaic low land area. We could almost succeed the pumping up of groundwater without land subsidence. Lately, GPS and SAR based surveys are being increasingly used for the monitoring land subsidence. Moreover, we have to control groundwater pumpage and expand monitoring in the groundwater basin for sustainable groundwater using.
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