Characteristics analysis of CO₂ environmental background baseline for potential carbon sequestration sites in Yellow River Delta

The large-scale sequestration of CO₂ may result in leakage, which could intrude into the shallow subsurface and cause environmental impacts. Due to the complex changes of natural processes, accurately determining baseline values and distributions is crucial for monitoring and controlling near-surface leakage. This study utilized portable devices along scheduled path to conduct mobile and field monitoring and sampling laboratory test, and monitored and analyzed atmospheric and soil gas components and CO₂ concentrations at potential sequestration sites in Yellow River Delta to determine baseline values and distributions. The results indicate that: ① in the monitoring maps of three different layers, CO₂ concentration data from the portable monitoring method are consistent with the continuous monitoring data from the fixed monitoring points, indicating the periodic fluctuation characteristics of the monitored CO₂ concentration. Additionally, the statistically significant relationship between \delta^13 \mathrmC_\mathrmCO_2 and CO₂ and O₂ content confirms the reliability of the portable monitoring method.② Two environmental background baseline values are established. The first is \delta^13 \mathrmC_\mathrmCO_2 and δ¹³C_OC baseline, with the soil \delta^13 \mathrmC_\mathrmCO_2 isotope values of -14.35‰~-20.95‰, atmospheric \delta^13 \mathrmC_\mathrmCO_2 values of -16.92‰~-19.89‰, and soil δ¹³C_OC values of -20.59‰~-24.79‰. These values revealed the range of carbon isotope values produced by biological activity in temperate climates. The second is atmospheric and soil CO₂ concentration baseline, with the atmospheric CO₂ concentration of 219.20~416.4 ppm, soil CO₂ concentrations at 30 cm depth of 1 002.793~14 808.13 ppm, and soil CO₂ concentrations at 50 cm depth of 1 497.26~19 658.59 ppm. The environmental CO₂ concentration and isotope baseline of shallow surface in potential carbon sequestration sites of Yellow River Delta were confirmed in this research, providing reference benchmarks and monitoring scheme recommendations for future surface monitoring after large-scale CO₂ sequestration.