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An isotopic study on the Cl^--rich groundwater in central Kanto plain, Japan(Proceedings of the 19^<th> Symposium on Chronological Studies at the Nagoya University Center for Chronological Research in 2006,Part 2)

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  • 関東平野中央部の高Cl^-濃度地下水に関する同位体的研究(第 19回名古屋大学年代測定総合研究センターシンポジウム平成18(2006)年度報告,第2部)

Abstract

In the Kanto plain, the largest Quaternary groundwater basin in Japan, water samples were obtained from 144 water supply boreholes with pumps installed to characterize the groundwater system. The spatial variation in chloride (Cl^-) concentration indicates that in its central part there exists groundwater with Cl^- concentrations of more than 100 mg/l (up to 216 mg/l) between 150 m and 430 m depth below ground surface. This 8 km wide, 35 km long Cl^--rich area, spreading from the northwest to the southeast, corresponds with the so-called Motoarakawa tectonic zone bounded by the faults on its long sides. We have found the Cl^--rich groundwater is also characterized by low δD, low δ^<18>O, and high δ^<13>C values. Both the chemical and isotopic evidence strongly suggests the Motoarakawa tectonic zone divides the regional groundwater system in the Kanto plain into three distinct hydrologic subareas. Two faults bordering the Motoarakawa tectonic zone act as barriers to the southward and northeastward regional movements of groundwater. Radiocarbon (^<14>C) data suggest that the retention time of groundwater in the tectonic zone is long in comparison with that outside the tectonic zone. In conclusion, the Motoarakawa tectonic zone groundwater is assumed to have been recharged by either precipitation under cooler climate condition than the present, or groundwater and/or river water originated from precipitation on high altitude areas around the Kanto plain. It had been put under an isolated hydrologic environment for a long period of time with respect to the regional groundwater system, resulting in highly-evolved water chemistry with an elevated Cl^- concentration. Contribution of pore water (formation water) squeezed out of the adjacent aquitards and/or deep-seated groundwater can not be ruled out as another possible source of water and Cl^- for the tectonic zone groundwater.

第19回名古屋大学年代測定総合研究センターシンポジウム平成18(2006)年度報告<第2部> Proceedings of the 19th symposiumon on Chronological Studies at the Nagoya University Center for Chronological Research\in 2006 日時:平成19 (2007)年1月15日(月)~17日(水) 会場:名古屋大学シンポジオン Date:January15th-17th, 2007 Venue:Nagoya Uhiversity Symposion Hall

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