Geochemical Study on CO2-rich Waters of Daepyeong Area, Korea: Monitoring Implication for CO2 Geological Storage

Geochemical monitoring was performed on dilute CO2-rich water as a natural analogue study of CO2 geologic storage. Field measurement and sampling for dissolved ions analyses and isotope analysis were carried out from July of 2009 to November of 2010. Spring and groundwater samples were classified into diluted CO2-rich water (average pH=4.6, EC=151 S/cm), concentrated CO2-rich water (average pH=6.1, EC=1,301), and ordinary groundwater (average pH=6.6, EC=201). pH, EC and dissolved major ions were not changed noticeably during monitoring period which indicates that reservoir of CO2-rich water was not affected by changes of surface condition and recharged groundwater. pH and EC of diluted CO2-rich water from this study area is lowest value among the CO2-rich waters from South Korea. On the other hand, concentrated CO2-rich water was also observed in 5 km north of diluted CO2 - rich water of Daepyeong area. Carbon isotope ratio (δ13 C) of concentrated CO2-rich water is -7.4% and average δ13 C of diluted CO2 rich water is -7.2% which indicates that the CO2 source of concentrated and diluted CO2-rich water could be closely related. The result of simple mixing model revealed that low pH and high TDIC (Total Dissolved Inorganic Carbon) concentration of diluted CO2-rich water was not explained by simple mixing between groundwater and concentrated CO2-rich water. Therefore, inflow of excess CO2 and/or secondary accumulated CO2 should be applied CO2-rich water system in Daepyeong area and reacted with host rock. A batch kinetic model was applied for simulating the ion concentration, pH and TDIC of diluted CO2-rich water after reacting with groundwater, rock and high CO2 condition (50.2 mmol/L). As a result of batch kinetic model, Na+ and K+ concentration of DPw-4 sample was reached in about 20 years. Conclusively, low EC, dissolved ions and high TDIC contents of dilute CO2-rich water indicate that water-rock interaction has been took place in short period of time after input of CO2. The result of this study implies that shallow groundwater quality except pH could not change significantly in the early stage of leakage of CO2. The water-rock reaction rate might not be fast enough in surface environment.