GEOCHEMICAL AND ISOTOPIC CHARACTERISTICS OF THE DEEP BOREHOLE GROUNDWATER IN THE YUSEONG AREA, KOREA

The Korea Atomic Energy Research Institute is conducting the research program of the high level radioactive waste disposal in Korea.  To understand the deep geological environment of the crystalline rocks, 6 boreholes (76mm) with depth of 200~500m were drilled in the Yuseong area (E.L. 82m), mainly consisting of Jurassic granite.  The multi-packer system was installed in the borehole with depth of 500m, which was isolated to 14 sections.  Before the MP installation, the grouting activity was carried out with Portland cement around GL-115m of borehole, due to collapse after the drilling and hydraulic testing.  For monitoring of geochemistry of borehole groundwater, field measurements and chemical and isotopic analysis were carried out.

The thermal gradient is obtained to 0.26^oC/100m.  pH of groundwater at upper section shows about 7 and the pH below GL-200m reaches almost constant value as 9.9~10.3, whereas the groundwater affected by grouting has abnormally high pH (<12.7).  Eh of groundwater varies with depth and more negative values were recognized in deep groundwater.  Except the section affected by grouting materials, the concentrations of Na, Ca, Mg, K, Cl, HCO₃, and SO₄ of groundwater are constant below GL-200m.  F concentration in deep groundwater is high (<12.7 mg/L) and tends to increase with depth.  Surface water and shallow groundwater shows Ca-HCO₃ type, whereas deep groundwater below GL-200m belongs to typical Na-HCO₃ type.  The variations of geochemical data of groundwater with time show that the groundwater was stabilized within about 6 months after the installation of MP system.  The d¹⁸O and dD values are nearly plotted along the worldwide meteoric water line, indicating that groundwater was recharged from local meteoric waters under present climate conditions.  It is noteworthy that isotopic compositions of deep groundwater are lighter than those of shallow groundwater in the borehole of the Yuseong area. Tritium contents close to zero are observed in the deep groundwater, confirming a long residence time of deep groundwater. The d¹³C value of groundwater show the contribution of carbon from either microbial oxidation of organic matter or carbon dioxide from plant respiration.  The high d³⁴S_SO4 value of groundwater indicates that the sulfate reduction might be occurred in the deep environment.