Development of the Hydrogeological Survey Method Using the Natural Dissolved Noble Gases for the Site-Characterization f
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DEVELOPMENT OF THE HYDROGEOLOGICAL SURVEY METHOD USING THE NATURAL DISSOLVED NOBLE GASES FOR THE SITE-CHARACTERIZATION FOR RADWASTES DISPOSAL
YASUNORI MAHARA*
* Central Research Institute of Electric Power Industry, 1646 Abiko, Abiko-city, Chiba,
270-11, Japan ABSTRACT
This study focused on developing the tracer method using the dissolved natural noble
gases. The method measures precisely helium and neon dissolved in groundwater for dating. Since the dissolved helium consists of three different components (Atmospheric He, Radiogenic He and Mantle He ), if it is possible to separate total dissolved helium into
each component, the mixing rate between young groundwater and ancient will be predicted
in in-situ groundwater. Then, dating of young groundwater is determined by measuring tritium and tritiogenic 3He in a groundwater sample. On the other hand, the dating method for old groundwater is not still verified. In this study, the old groundwater age is predicted by the accumulating rate of radiogenic helium through the alpha decay of uranium and thorium contained in rock. This tracing method was applied to a confined aquifer in the Kumamoto plain (10kmi x20km ), where the groundwater flow and the geohydrological formation of aquifer was investigated in detail in the past. The groundwater in the area consists of mixture of young groundwater (less than 20 yrs old ) and ancient isolated groundwater (more than 1000 yrs old). Furthermore, the drastic change in the helium isotopic ratio revealed unexpectedly the existence of active faults covered in groundwater basins. Consequently, the method using the dissolved noble gases is practicably available for investigating the geohydrologic
formation of aquifer at the early stage of the site characterization. INTRODUCTION
The most important criteria for site characterization for radwaste disposal are immobile groundwater and a long-term geological stability. At the early stage of the characterization, it is economically very important to investigate these two key terms. However, there is still
no way to simultaneously investigate both terms. In this study, the tracer method using dissolved noble gases was applied for surveying groundwater flow and for searching active
faults covered in a deep stratum. The dissolved noble gases inform to us the residence time and origin of groundwater, and various geochemical signals from active faults. For verifing
the availability of the method, it is applied to the volcanic aquifer in the Kumamoto Plain, where there are a number of active faults covered in deep strata. Futhermore, since the aquifer was investigated in the past by many researchers, it is helpful in the verification of the method. STUDY AREA
The Kumamoto Plain is on the west side of Mt. Aso, outside the caldera. The basement rocks, which are impermeable, consist of Mifune layers which are metamorphic rocks, PreAso volcanic rock, and unclassified Diluvium. Four pyroclastic flow deposits, called Aso1, Aso-2, Aso-3 and Aso-4 in chronological order from the bottom to gr
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