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Mieko Uchiyama, Tsuru University (Japan)
Akihiko Inamura, Geological Survey of Japan, AIST (Japan)
Toshimichi Nakanishi, Public Works Research Institute (Japan)
Katsumi Kimura, Geological Survey of Japan, AIST (Japan)
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Kanto Plain is the largest coastal plain and the most human activity in Japan. Alluvial deposits are distributed in the Tokyo Bay area and along the rivers, and Plio-Pleistocene deposits are mainly distributed around the Alluvial deposits. The lowlands consisted in the Alluvial deposits were suffered damaged seriously because of the land subsidence. The land subsidence was occurred by excess pumping groundwater using for industrial water, and the volume of clay contracted caused by squeeze. After the regulation law of pumping groundwater established in 1956, hydraulic head had returned and the land subsidence was stopped. The pore water quality existing within Alluvial marine clay is investigated from the GS-MHI-1 core in this study.
GS-MHI-1 core was drilling at Misato City, Saitama, which is located at the south area of Nakagawa Lowland. This site is estimated that the deepest point of the buried valley named Paleo-Naka River, and located in about 25km far from the Tokyo Bay site. Paleo hydraulic quality of GS-MHI-1 core is estimated from shell and diatom fossils ; GL (ground level) -53.2 to -35.8 m was fresh water, GL -35.8 to -8.5 m was brackish to marine water, GL -8.5 to -2 m was fresh water.
The sediments of brackish to marine water are presumed with transgressive and regressive formation from radiocarbon dates and its depositional facies. Although transgression and regression environments at the depositional time, present pore water quality is displaced to the meteoric water. The concentrations of Cl-ion in the pore water are 100 - 600 mg/l. That is very low than marine water which is 19.0 g/kg. In addition to Cl-ion concentration, the values of stable hydraulic isotope (delta-D) are supporting. The delta-D of meteolic water (such like presipitation, shallow groundwater and origin of river water) around the Nakagawa Lowland are about -50 to -60 per-million, and around the margin of plain are about -60 to -70 per-million (Yasuhara, 2002). The delta-D of pore water within the marine clay of GS-MHI-1 core show about -60 to -65 per-million (Fig.2).
Confined groundwater within the basal gravel (GL -53.2 to -50.2 m) is considered that permeated at the margin of plain. The pore water is rather supplied from confined groundwater than permeated from precipitation depend on the delta-D. And that might be supplied as return the hydraulic head caused by stopped the pumping.
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