Hydrochemistry of the Groundwaters in the Izu Collision Zone and Its Adjacent Eastern Area, Central Japan
Geochemical Journal, Vol. 45, pp. 309 to 321, 2011 Hydrochemistry of the groundwaters in the Izu collision zone and its adjacent eastern area, central Japan YOICHI MURAMATSU,1* YUTA NAKAMURA,2 JITSURO SASAKI3 and AMANE WASEDA4 1Department of Liberal Arts, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan 2Department of Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan 3Department of Pure and Industrial Chemistry, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan 4Japan Petroleum Exploration Co., Ltd., Research Center, 1-2-1 Hamada, Mihama-ku, Chiba 261-0025, Japan (Received April 8, 2010; Accepted March 6, 2011) Chemical and stable isotopic (δD, δ18O, δ34S) compositions of rivers and groundwaters, mineral constituents of rock samples from wells, and δ34S values of anhydrite in the Izu collision zone and its adjacent eastern area, southern Kanto Plain, central Japan, were analyzed to constrain the water-rock reactions and flow systems of the groundwaters. Inside the accreted Izu–Bonin–Mariana (IBM) basin, a two-dimensional map of the geothermal gradient calculated roughly using the discharge groundwater temperatures and the borehole temperature logging data confirms that the aqui- fer is recharged by the local meteoric water (LMW) and the high density seawater in the area. The oxygen and hydrogen isotopic compositions reveal that the Ca·Na–SO4 groundwaters in the Tanzawa Mts. and the high Na–Cl groundwaters in the coastal area are of meteoric water and weakly altered fossil seawater origins, respectively.
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