MONITORING OF HYDRAULIC AND GEOCHEMICAL PROPERTIES OF THE BOREHOLE GROUNDWATER IN THE YUSEONG AREA, KOREA

As a part of the radioactive waste disposal research program in Korea, the Korea Atomic Energy Research Institute drilled 9 boreholes with depth of 200~500m in the Yuseong area, mainly consisting of Jurassic granite. Fracture system (orientation, frequency and aperture size) was investigated by core logging and BHTV scanning, and hydraulic conductivities was estimated by hydraulic tests such as injection and pulse tests. Also, the groundwater of 3 boreholes was isolated to more than 10 sections by multi-packer systems according to depth and fractures, and hydraulic and geochemical properties of groundwater including environmental isotopes have been monitored in each section since Sep. 2001.

The profile of potential head of groundwater in the borehole with depth of 500m shows the lowest value around 250m below surface (82m asl) and the highest values >400m below surface. The potential head monitored is almost constant, but during rainy season, the potential head up to 300m below surface tend to slightly increase with rain event. Also, the potential head below GL-350m of borehole drilled first highly varies with continuing drilling of adjacent borehole. The geochemical properties of groundwater sampled with depth in open borehole before installation of MP system was almost constant due to upflowing by the high potential head in depth below GL-400m. After installation of MP system, shallow groundwater shows Ca-HCO₃ type (pH ~7), whereas deep groundwater below GL-200m belongs to typical Na-HCO₃ type (pH ~10). The chemistry of groundwater from each section of borehole was stabilized within about 6 months after the installation of MP system. The chemical and isotope data indicate that the groundwater has been geochemically evolved by simple water/granite interaction.