Characteristics of soil CO₂ under different conditions and its influence on water chemical composition: an experimental and modeling study in the laboratory

The water quality of the rainwater is obviously different from that of the groundwater after it reaches the surface and passes through the vadose zone. This study aims to further evaluate and analyze how soil CO₂ changes under natural conditions, to provide a theoretical background for the establishment of CO₂ geological storage, and to provide a basis for the detection of CO₂ leakage in order to provide insights for expanding CO₂ research. The changes in soil CO₂ concentration under different conditions were simulated and analyzed by laboratory experiments, and the effects of CO₂ on water chemical composition were also analyzed. In this study, two experimental groups (vegetation group (V) and high-temperature sterilization group (R)) and one blank group (B) were established. The results showed that the CO₂ concentration in column R was the lowest, while that in column V was the highest. With rainfall infiltration, soil CO₂ concentration gradually increased. When the rainwater infiltrated to 215 cm, the CO₂ concentrations in the columns V, B, and R were 5100 mg·m⁻³, 4450 mg·m⁻³, and 32,000 mg·m⁻³, respectively. At infiltration depths from 5 to 215 cm, the Na⁺, Ca²⁺, Mg²⁺, SO₄²⁻, and HCO₃⁻ concentrations in columns V and B decreased, whereas they increased in soil column R. The simulation revealed that the CO₂ concentration ranged from 560 mg·m⁻³ to 50,000 mg·m⁻³; pH value decreased; NO₃⁻ and Cl⁻ remained stable; SO₄²⁻ decreased; and fCO₂, HCO₃⁻, Ca²⁺ and Mg²⁺ increased.