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

[HSC06-08] Seasonal difference of increase in DIC and pCO₂ of seawater due to CO₂ leakage

*Keisuke Uchimoto^1,2, Yuji Watanabe^1,2, Kazuhiro Misumi³, Tsubono Takaki³, Daisuke Tsumune³, Jiro Suekuni^1,2, Ziqiu Xue^1,2 (1.Geological Carbon dioxide Storage Technology Research Association, 2.Research Institute of Innovative Technology for the Earth, 3.Central Research Institute of Electric Power Industry)

Keywords:offshore CO2 geological storage, CO2 leakage, seasonal variability, biological impact

Leakage of CO₂ into the sea increases values of carbonate variables of the seawater such as partial pressure of CO₂ (pCO₂) and dissolved inorganic carbon (DIC), so that monitoring a carbonate variable of the seawater at offshore CO₂ geological storage sites is mandatory in Japan. The monitoring is required to conduct once a year at an appropriate time that should be set considering the seasonal variability of carbonate variables. In this paper, we discuss which season is appropriate to monitor a carbonate variable, based on an ocean simulation, and also estimate biological impacts due to CO₂ leakage based on the simulation results, compiling data of biological impacts due to pCO₂ increase. In the simulation, a passive tracer regarded as the increase of DIC due to CO₂ leakage was continuously released at a constant rate from the bottom grid point off Tomakomai. The result shows that the concentration of the tracer is the highest in summer and the lowest in winter, and so suggests that summer is the most appropriate season to monitor a carbonate variable. While the tracer tends to remain near the bottom in summer because of the stratification, it tends to be well dispersed vertically due to the vertical mixing in winter. In addition, the horizontal flow is stronger in winter than summer at the release point. Thus, both horizontal and vertical flows make the tracer concentration higher in summer than winter. The difference between summer and winter becomes more striking in pCO₂ increase than DIC increase because higher temperature leads to higher pCO₂ when other conditions including DIC are the same. This implies that impacts on marine organisms would be greater in summer than in winter in the event of CO₂ leakage. To assess biological impacts, the increase in pCO₂ is estimated for leakage rate of 1,000, 10,000, and 100,000 tonnes/year, assuming the seasonally constant in situ pCO₂. It is only in the case of 100,000 tonnes/year that the thresholds for biological impacts are exceeded, although it should be noted that the resolution of the model is relatively coarse (1/120^°).