Proposal of a New Standard of CO2 Concentration in Seawater

CO2 capture and storage (CCS) is one of promising means to mitigate global warming. CCS operators who inject and store CO2 into subseabed geological formations may be legally required to monitor CO2 leakage into seawater, even though such events are very unlikely. However, the base concentration of CO2 in sweater is far from constant and it is difficult to distinguish the increase caused by a leakage. An approximate linear correlation between partial pressure of CO2 (pCO2) and dissolved oxygen saturation (DO%) is used as a standard CO2 concentration in the context of the Japan’s Marine Protection Act. However, the sea off Tomakomai, Japan, where an offshore CO2 storage demonstration is now carried out, is known as a mixing area of a warm current separated from the Kuroshio Current and the Oyashio Current from the Okhotsk Sea. Therefore, the linear correlation between pCO2 and DO% scatters very much. Therefore, it is necessary to derive another standard with a certain accuracy. Here, we proposed a linear relationship between dissolved inorganic carbon (DIC) + αDO and temperature. In 2016, ten out of 400 monitoring data (above the upper limit of 95% confidence interval) were regarded as possible CO2 seepage off Tomakomai. The CCS operator was forced to stop CO2 injection and to seek any hint of CO2 bubble seepage in vein. This costed a tremendous amount of money and was incredibly time-consuming. If both of this method and our proposed one are used, the probability will shrink to the cases above the 98% confidence interval. In addition, we applied this standard to other seawaters to investigate its universality.