[H-SC05] CCUS (Carbon Dioxide Capture, Utilization, and Storage) for Climate Mitigation

[HSC05-P08] Characteristics of pCO₂ variability in Osaka Bay 2: Factors that cause pCO₂ fluctuations

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

Keywords:CO2 leakage, offshore CO2 storage, pCO2

The relationship between CO₂ partial pressure (pCO₂) and dissolved oxygen saturation (DO%) has been proposed as a threshold for anomalously high values of dissolved CO₂ in seawater due to CO₂ leakage in the sub-seabed CO₂ geological storage. Various factors are known to affect the relationship between pCO₂ and DO%, for example water temperature, total alkalinity etc. In order to elucidate the characteristics of this threshold method, we carried out the continuous survey of pCO₂, DO% and the relevant items at two points, off Kobe Port (KOBE) and off Kansai International Airport (KIX), in Osaka Bay from July 2018 to October 2019. The seasonal variability of covariance between pCO₂ and DO% is presented in Part 1. In this study (Part 2), we analyzed the data to shed light on factors that cause upward pCO₂ deviations from a regression line between pCO₂ and DO%. In our surveys, two groups of the deviations were observed. One group was observed from summer to autumn at both KOBE and KIX. The other was seen in summer only at KOBE. The deviations from summer to autumn are considered to be related with the difference in the gas equilibrium rate between CO₂ and O₂. From summer to autumn, the vertical structure of the water column shifts from the stratification phase to the mixed phase, and bottom water with high pCO₂ and low DO% is vertically mixed with surface water. When the effects of gas exchange between the atmosphere and the sea surface extend to the entire water column, the DO% of the bottom layer rises, pCO₂ decreases, and DO% and pCO₂ in the bottom layer tend to equilibrate with the atmosphere. However, the gas equilibrium rate of CO₂ between the atmosphere and seawater is 1/20 of the rate of O₂. Therefore, when O₂ in the water mass may reach the equilibrium with the atmosphere and becomes saturated (DO% = 100), CO₂ cannot reach the equilibrium. In this situation, pCO₂ deviates upward from the regression line on the DO%-pCO₂ plane. The deviations in summer at KOBE are, on the other hand, related with anoxic water and river runoff. In summer anoxic water is often generated in bottom layer such as KOBE. The pCO₂ in lower layer becomes higher if CO₂ produced by the anaerobic respiration in anoxic sediment layer diffuses into the water column. Moreover, pCO₂ at KOBE in summer were higher in 2018 than 2019. There was a large amount of flood from Yodo River in early July and, consequently, decrease of salinity in bottom water was occurred in summer 2018 at KOBE. The decrease of salinity makes pCO₂ higher even if the concentration of dissolved inorganic carbon (DIC) remains unchanged. It is thought that pCO₂ at KOBE usually increases significantly in summer due to the hypoxia.; in addition, lower salinity led to higher pCO₂ in 2018 than usual. The pCO₂ fluctuates by various factors such as salinity, temperature and so on. Therefore, even if anomalously high pCO₂ is obtained, it is not always resulted from CO₂ leakage; it is important to examine physical, chemical and biological background data.

Acknowledgement: This presentation is based on results obtained from a project (JPNP18006) commissioned by the New Energy and Industrial Technology Development Organization (NEDO).