Geochemical Approaches to Understanding a Shallow Groundwater Flow in the Kanamaru Uranium Mineralization Area (Japan)
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0893-JJ09-07.1
Geochemical Approaches to Understanding a Shallow Groundwater Flow in the Kanamaru Uranium Mineralization Area (Japan) Regis Bros, Yoji Seki, Atsushi Kamei, Yutaka Kanai, Koichi Okuzawa and Yoshio Watanabe AIST, Research Center for Deep Geological Environments, Tsukuba, Japan ABSTRACT Predicting the behaviour of radioactive wastes can be facilitated by comparison with the evolution of natural groundwater systems. During a study of the Kanamaru U mineralization (Japan), geochemical approaches for understanding a shallow (0-50 m) fresh groundwater flow system are being assessed. Deep granitic waters are Ca-HCO3-dominated and slightly acidic to slightly alkaline. Shallow waters within sediments display lower pH and they are more dilute. Halide concentrations suggest the existence of a non marine Br-rich and Cl-depleted deep groundwater in the basement. 234U/238U and 230Th/234U activity ratios in the mineralized sedimentary rocks indicate that U mobilization took place within the last 350,000 years. U dissolution currently continues and it is controled by lateral groundwater flow whereas vertical diffusion appears negligible. Dissolved alkaline earths concentrations and the 87Sr/86Sr ratio indicate that solutes exchanges take place through uppermost low permeable granite followed by mixing with more dilute and Cl-type shallow groundwater.
INTRODUCTION Deep underground geological burial is one possible method for the disposal of radioactive wastes. Existing information supports the belief that appropriate geological media in conjunction with engineered barriers will be required to isolate wastes from the biosphere. Studies of natural aquifer systems such as uranium orebodies may provide convincing evidence for radionuclide transport and retardation although no global comparison with a repository site is legitimate [1]. Groundwater flow investigations aim to model past flow patterns over long time-scales (1 Ma or more). These models can be extrapolated to the future, although the current flow system may differ from past states and may not be at equilibrium with present driving forces. Numerical flow simulation are better constrained using geochemical informations from groundwater and bed rocks in addition to hydrogeological data such as hydraulic conductivity and groundwater heads [2]. As a study of relevance to the assessment of radionuclides migrations in the far-field of radioactive waste repositories, hydrogeochemical investigations are conducted in the Kanamaru area (Japan) in order 1) to improve the conceptualization of a shallow groundwater flow system, 2) to characterize the geochemical controls on solute transport, 3) to test and validate the used geochemical methods and 4) to obtain informations of relevance to the performance assessment of the long-term evolution of repository conditions.
SETTING The field of studies is located in northeastern Japan (fig. 1). It consists of a Cretaceous granitic basement overlain by Neogene sedimentary rocks where surface survey and drilling
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