Geochemistry of groundwater in the Shimousa Group, Chiba Prefecture

Abstract

We investigated chemical compositions of twenty-two groundwater samples from aquifers in the Shimousa Group and alteration mineral assemblage of middle to upper Pleistocene sediments and shell fossil obtained by drilling of a test well (bottom of 44 m depth) at the Noda city, northwestern part of Chiba Prefecture. Moreover, we discussed water-rock interactions conducted to the chemical composition of the groundwaters. There are four aquifers (the first surface, second, third and fourth aquifers in descending order) which are predominantly Ca-HCO₃ type. Ca²⁺ and HCO₃^- concentrations and pH in the groundwaters gradually increase with depth. The groundwat er changes gradually to be saturated with aragonite and calcite with depth. Quartz and plagioclase as primary minerals, and kaolinite and montmorillonite as secondary minerals are found in the first aquifer. Based on these data and presence of shell fossils in the Kioroshi and Kami-iwahashi Formations, the increases of Ca²⁺ and HCO₃^- concentrations in the groundwaters with depth are interpreted by the following two mechanisms: (1) dissolutions of aragonite and calcite as the shell fossils, and (2) formations of kaolinite and Ca-montmorillonite by weathering of plagioclase.