Comparison and suggestion of indicators of concentrations associated with CO2 in seawater considering biological activity

Abstract Carbon dioxide capture and storage is expected to reduce large amount of CO2 emissions from pointwise large sources, such as coal power plants, by 2030 in Japan. For offshore CO2 storage, it is necessary to monitor concentrations associated with CO2 in seawater to ensure safe storage. To do so, it is also necessary to know the baseline concentrations in the target seas. However, such concentrations have large regional and seasonal variations, mainly because of biological activity. In this study, we compared various indicators of CO2 dissolved in seawater, using the seawater off Tomakomai, Japan, observed between 2013 and 2018. Among the indicators, we found very strong correlations between [nDIC + kDO] and T and between [DIC − 0.5TA + kDO] and T. These indicators are thought to be effective for investigating the possibility of unexpected leakage of CO2 stored in sub-seabed geological formations. However, data always contain a degree of scattering, even when their correlation is very high. In the case of the seawater off Tomakomai, the data that exceeded a criteria based on the residual standard deviation of some indicators were not necessarily the same as those of the other indicators. Therefore, it is suggested that two or more, different types of indicators should be used, such as pCO2-[DO saturation] and [nDIC + kDO]-T or [DIC − 0.5TA + kDO]-T.