Long-term column experiments for evaluation of geochemical changes in various geologic media due to CO2 leakage

Carbon capture and storage (CCS) is a technology that can reduce atmospheric CO₂ by storing it in an underground reservoir. However, the stored CO₂ can leak into the surrounding aquifer, which may cause undesirable negative impacts on groundwater geochemistry. Previous studies have identified changes of groundwater geochemistry in a particular geologic material; however, there have been limited studies that evaluate such changes comparably in different geologic media. Also, monitoring and evaluation of geochemical parameters are mostly focused on short-term changes. In this study, the changes of groundwater composition in various geologic media due to CO₂ leakage were evaluated systematically through laboratory column experiments for about 1 year. The soil collected from the Environmental Impact evaluation Test (EIT) facility of the Korea CO₂ Storage Environmental Management (K-COSEM) test site, clean sand, sand and limestone mixture, and Quaternary alluvium sediment were used to construct the columns. In each column, temporal and spatial changes of pH, Eh, electrical conductivity (EC), alkalinity and cation concentrations were monitored. The results showed that, as aquifer materials were reacted with the dissolved CO₂, generally pH decreased and EC, alkalinity, and cation concentrations increased. However, each column showed different geochemical characteristics upon contact with CO₂. For example, in the EIT and sand columns, the concentrations of Mn and Co increased particularly high. In the limestone column, Ca and alkalinity concentrations were very high compared to the other columns. On the other hand, Quaternary alluvium column showed the rapid change of the geochemical parameters at the beginning of the experiment. These differences may be attributed to the changes in the pH buffer capacities of different geologic media. The trends in the increases in EC, alkalinity, and cation concentrations were declined over time. The results of this study can be useful in monitoring potential CO₂ leakage in various geological environments and managing the impacts on groundwater geochemistry. This research was supported by the "R&D Project on Environmental Management of Geologic CO₂ Storage" from the KEITI (Project Number: 2018001810002).