Analytical solution for coupled water-gas transport in landfill cover

The transport of water and gas is the most important process for the performance evaluation of landfill cover. However, the transport behaviors of these two components have been calculated separately (i.e., ignoring their coupling effect) in analytical solutions due to the complexity, although this coupling effect has been widely demonstrated in numerical studies, laboratory tests, and field tests. This study presents an analytical solution for coupled water-gas transport in a single-layer landfill cover, which can consider transient diffusive-advective transport of gas under steady-state water transport. The proposed analytical solution is verified against the field data, laboratory data, and numerical simulations. Parametric study is conducted to investigate the effects of four important parameters (i.e., infiltration rate, evaporation rate, desaturation coefficient, and saturated coefficient of water permeability) on the coupled water-gas transport behaviors, and the results indicate that these parameters all have a significant impact on the coupled water-gas transport (e.g., the gas emission flux for the case with desaturation coefficient alpha = 0.9 m(-1) is 11.9 times greater than that with alpha = 0.3 m(-1)). For the simulations considered, ignoring the coupling effect would underestimate the steady-state gas emission flux by about 10.9- 54.2%, due to the changes of gas transport properties induced by water transport. The magnitude of this underestimation (i.e., error induced by ignoring the coupling effect) increases with decreasing infiltration rate, increasing evaporation rate, increasing desaturation coefficient, and decreasing saturated coefficient of water permeability.