Effect of Large Flow CO2 on Indexes of Water Quality and LD50 of Crucian Carp

Abstract

CCS (Carbon dioxide capture and storage) is an effective way to reduce carbon dioxide levels of atmosphere in the near future. However, due to the geological problems in storage areas and the technical problems of CO2 injection, there is still a risk of CO2 leakage during the implementation of CCS projects. Once the stored CO2 leaks through water body, the water quality, aquatic ecosystem and fisheries in the leakage areas will be affected. Therefore, the research on impacts of CO2 on water quality and aquatic ecosystem is significant for the promotion of CCS project.

The effects of different CO2 flow rates on indexes of water quality and crucian carp mortality were investigated by injecting CO2 into aquarium. In the impact experiment of CO2 to water quality, three indicators which included pH, DO (dissolved oxygen) and DCO2 (dissolved carbon dioxide) were chosen, and four types of experiments were designed in the research which included blank water, fish-farming water, water with injecting CO2 and fish-farming water with injecting CO2. The acute toxicity test included preliminary experiment and formal experiment. The 24-hour all-dead flow rate (LD100) and the 96-hour non-dead flow rate of CO2 (LD0) were determined by preliminary experiment. The equal logarithmic interval method was used to set the groups of CO2 flow rates based on the above flow rates. The survival number and mortality of experimental fish in each group were recorded and calculated at 24h, 48h, 72h and 96h. At last, the LD50 of CO2 flow rate at each node of time was calculated according to the improved Kärber method. The main conclusions were obtained as follows:

(1) The average background values of pH, DO and DCO2 in blank water were 7.69, 8.58mg/L and 10.05mg/L respectively. The average values of pH, DO and DCO2 in fish-farming water were 7.26, 7.31mg/L and 43.78mg/L respectively. Comparing the above conclusion, pH and DO in blank water were slightly higher than that in fish-farming water, while the DCO2 was lower obviously.

(2) When CO2 was injected into the non-fish farming water, pH and DO decreased and DCO2 increased with time, and all indexes balanced quickly. The larger the CO2 flow rate, the shorter the equilibrium time. The equilibrium values of pH and DO decreased with CO2 flow rate, whereas the equilibrium concentration of DCO2 increased with CO2 flow rate. When CO2 was injected into the fish farming water, all indexes in water had the similar rule with the non-fish farming water. Comparing with the indexes of water quality in non-fish-farming water with injected CO2, pH and DCO2 in fish-farming water with injecting CO2 were higher, while the DO was lower. The reason may be the dissolution of CO2, fish's respiratory consumption, and neutralization of ammonium ions formed by manure decomposition.

(3) According to preliminary experiment of acute toxicity, the 24-hour all-dead flow rate of CO2 (LD100) was 800 mL/min, and the 96-hour non-dead flow rate of CO2 (LD0) was 100 mL/min. The acute toxicity test showed that the mortality of crucian carp increased with time and CO2 flow rates. According to the semi-lethal experiment of the modified Kärber method, the LD50 of CO2 to crucian carp at 24h, 48h, 72h and 96h was 1490±326 mL/m3﹒min, 799±181 mL/m3﹒min, 503±66 mL/m3﹒min, 427±0 mL/m3﹒min, respectively.

(4) According to above results, the CO2 leakage of the CCS project can be evaluated by monitoring of surface water quality and crucian carp's mortality rate in the future.

Keywords: CCS; Crucian carp; Indexes of water quality; LD50