[H-SC06] CCUS (Carbon Dioxide Capture, Utilization, and Storage) for Climate Mitigation

[HSC06-P05] Consideration on baseline surveys for dissolved CO₂ in a perturbed coastal area with warm and cold water systems

*Yuji Watanabe^1,2, Keisuke Uchimoto^1,2, Saeko Mito^1,2, Ziqiu Xue^1,2 (1.Geological Carbon dioxide Storage Technology Research Association, 2.Research Institute of Innovative Technology for the Earth)

Keywords:offshore CO2 storage, coastal baseline survey, pCO2

In the marine environmental monitoring survey for sub-seabed CO₂ geological storage, it is required to confirm whether the CO₂ composition in seawater is within the range of natural variation. To determine the natural fluctuation range, it is important to conduct a baseline survey appropriately. Around Japan, there are warm currents such as the Kuroshio Current and cold currents such as the Oyashio Current. The area where two currents meet is called a perturbed area, and the prevailing current often change with the seasons. There are several perturbed areas around the coast of Japan. For example, Hidaka Bay, where the Tomakomai CCS demonstration project was conducted, is one such area. It is strongly influenced by the Tsugaru Warm Current in summer and the Coastal Oyashio in winter. In the Tomakomai demonstration project, the correlation between the partial pressure of CO₂ (pCO₂) and the dissolved oxygen saturation (DO%) was used as an index of CO₂ condition in seawater, and when the observed value exceeds the prediction interval, it is determined to be an anomaly. It is known that pCO₂ in seawater changes depending on the properties of the seawater such as water temperature, alkalinity etc. In the perturbed areas, pCO₂ may be affected, as the water temperature and salinity may change depending on the prevailing water system which differs from season to season. Therefore, we examined the influence of the characteristics of the perturbed area on pCO₂ using the data observed in the baseline and the monitoring surveys of the Tomakomai demonstration project. The data are available on the Ministry of the Environment website. In addition, to obtain the characteristics of water systems of Hidaka Bay, we examined the data of water temperature and salinity (2010-2019) along the coast of Hidaka Bay from the marine database (Marine Net Hokkaido) conducted by the Hokkaido Research Organization. In Hidaka Bay, water masses from Oyashio and Coastal Oyashio water systems prevail in winter, resulting in low temperature and low salinity. From spring to summer, the water temperature increases while the salinity remained unchanged. In the summer, the salinity increases due to the shift to the Tsugaru Warm Current water system. In the baseline surveys conducted in four seasons on the Tomakomai demonstration project, low temperature and low salinity belonging to Oyashio and Coastal Oyashio water systems were observed in winter and spring surveys. In summer and autumn surveys, high temperature and high salinity belonging to the Tsugaru Warm Current system were observed. As a result, data for the transition period from spring to summer, when pCO₂ is likely to increase, was not available in the baseline surveys. It suggests that the initial criteria may have been set low. On the other hand, in the monitoring surveys, some surveys were conducted during the transition period when water temperature rose. Under such conditions, the observed values may be more likely to exceed the judgment criteria. In order to estimate the natural variability of pCO₂ in the perturbed areas by the baseline survey, it is important to conduct the survey at an appropriate time based on the characteristics of water system using water temperature and salinity as indicators.