Vertical Hydrochemical Stratification of Groundwater in a Monitoring Well: Implications for Groundwater Monitoring on CO₂ Leakage in Geologic Storage Sites

In this study, we examined the groundwater monitoring data from the national groundwater monitoring station in Andong at the southeastern part of South Korea. The monitoring station is located in Cretaceous Gyeongsang sedimentary basin which has been considered for geologic CO₂ storage in South Korea. Groundwater at the station is characteristically CO₂-rich and typically shows a vertical stratification of water chemistry with depth. Groundwater at shallow depths (< 7 m deep) has low electrical conductivity (EC) values, ca. 280 μS/cm, while deeper groundwater (down to 70 m below the land surface) has higher EC values (up to 3,900 μS/cm). The EC logging in the well also shows the significant stepwise variation of the EC values with depth. The results of the well logging of physicochemical data in conjunction with the borehole imaging using a nano camera and the hydrochemical analysis of groundwater indicate that the observed vertical stratification in the well is the result of physicochemical changes of CO₂-rich groundwater with high EC values via CO₂ outgassing and associated precipitation of minerals during ascending. The results of the present study demonstrate that, provided there is vertical stratification of EC in a monitoring well or in a storage site, geochemical monitoring at shallow depths may lead to a bias in the monitoring results. Our results suggest the following implications: 1) careful baseline data analysis at several depths is required for precise groundwater monitoring in CO₂ storage sites and 2) based on the good understanding of hydrogeological condition, proper sites and depths of geochemical monitoring should be selected.