Implications of Simulation of CO₂ Dispersion on Marine Potential Environmental Impact Assessment and Monitoring at CO₂ Storage Sites

When carbon dioxide (CO2) is stored in sub-seabed geological formations, the environmental impact assessment assuming CO2 leakage and monitoring CO2 concentration in the sea are mandatory in Japan. The marine environment is not impacted by CO2 storage itself but by the unlikely event of leakage. Thus, it is referred to as potential environmental impact assessment (PEIA). We conducted an ocean simulation for Hidaka Bay off Hokkaido, releasing passive tracers regarded as leaked CO2. Biological impact data were newly compiled, and 4 thresholds for biological impacts depending on both the increase in partial pressure of CO2 (pCO2) and exposure time were set. The increase in pCO2 estimated in the simulation, assuming CO2 leak rates of 1,000 tonnes/y and 10,000 tonnes/y, exceeded no thresholds. The tracer concentration became almost equilibrium within about a week after the commencement of the release, and the increase in pCO2 was much larger in summer than winter. These results suggest that the simulation of leaked CO2 for PEIA be run for about a month in summer, and that monitoring CO2 concentration be also conducted in summer. It is also implied that monitoring pCO2 could detect leakage at O(104) tonnes/y or larger than it.