Modeling carbon dioxide transport in unsaturated soils

Transport of carbon dioxide in the unsaturated zone is important to soil geochemistry and vegetation health. In addition, CO₂ monitoring in unsaturated soils may shed light on various human-instigated activities involving manipulation of the carbon cycle. Examples of human activities that can impact the unsaturated zone include imposed changes in vegetation type and distribution, and deliberate injection of CO₂ into deep subsurface formations. Both manipulation of vegetation and injection of CO₂ are potential options for mitigation of the greenhouse gas and global warming problems. Changes in plant type and density, and leakage of CO₂ from deep to shallow subsurface zones, can impact the unsaturated zone. Because CO₂ is denser than air, transport in the unsaturated zone may show significant density dependence. In addition, careful analysis shows that a newly derived 'slip coupling' term might also be important in these gaseous systems. In this work, we present the general governing equations for CO₂ migration in unsaturated soils for a variety of boundary and initial conditions. A one-dimensional numerical model is used to investigate CO₂ migration under different leakage scenarios, where the leakage is assumed to be associated with large-scale injection operations as part of a deep-aquifer carbon storage strategy. We demonstrate how CO₂ migrates in the unsaturated zone, how the resulting distribution compares to natural background distributions, and what the results may imply both for monitoring strategies and for vegetation and ecosystem health.