Some Factors Controlling Chemical Compositions of Groundwater in the. Shimousa Upland

Preliminary Case Study of the Southern Area of Noda City, Chiba

Abstract

Chemical compositions of thirteen g r o und and spring waters, and alteration minerals from aquifers above 100 m depth were analyzed to clarify the water-mineral interactions conducted to the chemical composition of the waters in the southern area of Noda city, Chiba, Japan. There are three aquifers (the first, second and third aquifers) whose water compositions are Ca-HCO₃ type. Ca²⁺ and HCO₃^- contents gradually increase with depth. Saturation indices of aragonite and calcite, and pH of the waters gradually increase with depth. Kaolinite and plagioclase are found in the aquifers. Based on these data and presence of shell fossils in the Kioroshi and Kamiiwahashi Formations, the increases of Ca²⁺ and HCO₃^- contents with depth are mainly interpreted by the following tw o mechanisms: (1) dissolutions of calcite and aragonite as the shell fossils, and (2)formation of kaolinite by weathering of plagioclase.