Modeling the effects of gas-phase CO₂ intrusion on the biogeochemistry of variably saturated soils

The transport of gas-phase carbon dioxide through unsaturated soils has the potential to significantly alter the soil biogeochemistry. Leakage of CO₂ from deep reservoirs, either naturally occurring or anthropogenically emplaced, may radically alter the redox conditions of a soil. Evidence of such alterations has been seen at several sites at Mammoth Mountain, California where natural leakage has led to soil-gas CO₂ concentrations of over 90 percent. This in turn has pointed potential risks to ecosystem system health due to leakage from sites of geologically sequestered CO₂.A two-dimensional numerical model has been developed to explore the effects of gaseous CO₂ leakage on the biogeochemistry of a variably saturated porous media. The model describes the sequential degradation of organic carbon by microorganisms using a series of terminal electron acceptors. Gas-phase CO₂ intrusion results in changes in redox conditions and pH of the soil water both of which may lead to alteration of the biogeochemistry of the soil. Alteration of the biogeochemical profile of a representative field site is also explored with the numerical model.