Abstract

The hydrogeochemical and isotopic on deep groundwater (below a 550 m depth from the ground surface) in the Munkyeong are, Kyeongbuk province were carried out. Two types of deep groundwater (CO₂-rich groundwater and alkali groundwater) occur together in the Munkyeong area. CO₂-rich groundwater (Ca-HCO₃ type) is characterized by low pH (5.8~6.5) and high TDS (up to 2,682 mg/L), while alkali groundwater (Na-HCO₃ type) shows a high pH (9.1~10.4) and relatively low TDS (72~116 mg/L). CO₂-rich water may have evolved by CO₂ added at depth during groundwater circulation. This process leads to the dissolution of surrounding rocks and Ca, Na, Mg, K and HCO₃ concentrations are eniched. The low P_co2 (10-6.4 atm) of alkali groundwaters seems to result from the dissolution of silicate minerals without a supply of CO₂. The δ¹⁸O and δD values and tritium data indicate that two types of deep groundwater were both derived from pre-thermonuclear meteoric water and have evolved through prolonged water-rock interaction. The carbon isotope data show that dissolved carbon in the CO₂-rich water was possibly derived from deep-seated CO₂ gas, although further studies are needed. The δ₃₄S values of dissolved sulfate show that sulfate reduction occurred at great depths. The application of various chemical geothermometers on CO2-rich groundwater shows that the calculated deep reservoir temperature is about 130~175°C. Based on the geological setting, water chemistry and environmental isotope data, each of the two types of deep groundwater represent distinct hydrologic and hydrogeochemical evolution at depth and their movement is controlled by the local fracture system.

Keywords

geochemistry, isotope, groundwater, CO2-rich water, water-rock interaction