Hydrogeochemical Investigation of Groundwater in the Tono Area, Japan
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HYDROGEOCHEMICAL INVESTIGATION OF GROUNDWATER IN THE TONO AREA, JAPAN T. IWATSUKI, K. SATO, T. SEO AND K. HAMA Power Reactor and Nuclear Fuel Development Corporation (PNC), TonoGeoscience Center, 959-31, Jorinji, Toki, Gifu, 509-51, Japan. ABSTRACT Measurements of chemical compositions of groundwaters, carbon isotope analysis of dissolved inorganic carbon and batch experiments on water-rock interaction have been carried out in order to evaluate the chemical evolution of groundwater in the sedimentary rocks of the Tono area, Japan. The activities of Na and Ca ions in the groundwater plotted along 2the boundary between Na- and Ca-montmorillonite stability fields on a diagram of 2pH+log[Ca +] vs. pH+log[Na+], which indicates that the two mineral phases are in equilibrium. Groundwater samples, rich in Ca ions (14.8 ppm Ca ions and 16.3 ppm Na ions), were collected from the shallow zone of the Tono area, and were reacted with the sedimentary rocks in the batch experiments. The experiments have revealed that ion exchange occurs between Na ions in the rock samples and Ca ions in the groundwater samples, and the final solutions became rich in Na ions (1.2 ppm Ca ions and 35.2 ppm Na ions). The results of the batch experiments are in agreement with the variation of Na and Ca concentrations in the groundwater in the sedimentary rocks. The carbon isotope data showed that the fraction of HCO3 derived from the dissolution of calcite increases from 25% to 75% of the total HCO3 content with increasing depth. The molality of Na ions in the groundwater was measured to be twice that of HCO3 ions derived from calcite, which supports the Na and Ca ion exchange in the groundwater. The results suggest that the chemical evolution of the groundwater is characterized by the variation in Na, Ca and HCO3 concentrations, and that ion exchange and dissolution of calcite are dominant reactions in the evolution of groundwater in the Tono area. INTRODUCTION The chemical composition of groundwater is one of the most important factors influencing radionuclide migration in geological environments [1, 2]. For a better understanding of the chemical composition, the chemical evolution of groundwater must be evaluated with detailed
geochemical data. The Tono area is a favorable site to study the chemical evolution of groundwater because a large number of studies have been made on groundwater chemistry around the Tono uranium deposits. The purpose of this study is to evaluate the chemical evolution of groundwater in the sedimentary rocks of the Tono area. For this purpose, measurements of the chemical compositions of the groundwater, carbon isotope analysis of dissolved inorganic carbon and batch experiments on water-rock interaction have been carried out. OUTLINE OF THE STUDY AREA Geology The Tono area is located in central Japan. The dimensions of the study area are approximately 300x300x200m (Fig.l). In this area, Tertiary sedimentary rocks (Toki Lignite-bearing Formation, Akeyo Formation, Oidawara Formation in ascending order) overlie Cretaceous gra
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