A Two-dimensional Model for the Gas Transport in the Unsaturated Zone with the Soil Aeration

To develop methods based on integrating the two-phase (liquid and gas) flow and free flow in the porous media to optimize the operation of subsurface gas venting, we developed a two-dimension soil aeration model based on the coupling of two-phase flow (liquid and gas) in the porous media with the single-phase flow (methane, CH4) in the free-flow domain under homogeneous, isotropic, and isothermal conditions. The dissolution, bioreaction, and thermal diffusion of CH4 are not included in the model. Numerical experiments were conducted with diverse near-surface meteorological conditions, soil properties (e.g., porosity, soil layering, air permeability, and soil moisture), and the deployment of venting bar holes to study the effects of environmental conditions and venting system designs on the gas flow in the subsurface. Simulation results not only demonstrated the capability of the soil aeration model on the prediction of the migration of the residual CH4 concentration in the subsurface due to the venting but also highlighted the influence of soil permeability, deployment of venting bar holes, and the venting pressure on the change in residual gas concentration in the unsaturated zone. During the soil aeration, the low soil permeability impacted the migration of advective air flow by venting in the soil and prolonged the operation time of the soil aeration. Furthermore, the Peclet number of the gas migration significantly decreased from the center of the venting bar hole with the decrease in soil permeability and venting pressure. The variation of venting pressure is more sensitive to the development of venting flow rates than that of the number of venting bar holes. The proposed 2D soil aeration model and approaches of evaluation of soil aeration in this study provide insights to investigate the multiphase flow in the subsurface due to soil aeration operation under various environmental conditions and venting strategies.