Hydrochemistry of the groundwaters in the Izu collision zone and its adjacent eastern area, central Japan

Abstract

Chemical and stable isotopic (δD, δ 18 O, δ 34 S) compositions of rivers and groundwaters, mineral constituents of rock samples from wells, and δ 34 S 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 aquifer 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. Sulfur in the SO4 rich groundwaters is derived from anhydrite and gypsum based on the sulfur isotopic compositions. The sulfate-type groundwaters were produced by the following process: the LMW infiltrated downward with dissolution of the sulfate minerals from hydrothermal veins in the Tanzawa Group, produced the Ca‐SO4 groundwater as a result of Ca 2+ exchange partly on Na‐smectite layer of mixed-layer chlorite‐smectite in the sedimentary rocks of the Tanzawa Group. The Ca‐Cl groundwaters in the eastern margin of the Tanzawa Mts. were produced by mixing of LMW with fossil seawater recharged from the surface of the coastal area, and Ca 2+ exchange of the mixed-layer mineral in pyroclastic rocks of the Tanzawa Group. Outside the accreted IBM basin, the Na‐HCO3 groundwaters in the shallow aquifer were formed by dissolution of authigenic calcite with LMW, and Na + exchange in the Kazusa Group. The moderate Na‐Cl groundwaters in the deep aquifer were formed by mixing of the deep seated fossil seawater with the Na‐HCO 3 waters in permeable sandstone and conglomerate of the Kazusa Group.