Analytical solution for one-dimensional steady-state VOCs diffusion in simplified capillary cover systems

The analytical solution for a one-dimensional and steady-state model is proposed to investigate the diffusion of VOCs within landfill capillary cover systems under unsaturated water flow and stable conditions. Examples of coupled water flow and VOCs diffusion are evaluated by the proposed analytical solution to discuss the effects of the evaporation rate and clay thickness. The proficiency of the unsaturated cover system in minimising VOC emissions was also investigated. It was found that the non-uniform distribution of water content can cause one order of magnitude reductions in the effective diffusion coefficient of VOCs, resulting in the attenuation of VOCs in the cover system. The concentration profile of VOCs is lower for the case with a larger evaporation rate. Ignoring the non-uniform distribution of water content induced by water flow would underestimate the surface flux of VOCs by a factor of 1.8. The water content and VOC concentration in the cover system can be effectively diminished with the thickness of the clayey layer increasing. The surface flux of VOCs is considerably decreased with increasing thickness of clayey layer. Increasing the thickness of clayey layer from 0.2 to 1 m leads to a reduction in the surface flux of VOCs by a factor of 2.9. The sensitivity analysis provides further insights into the critical role of water flow in affecting VOC transport through the unsaturated cover system. The outcomes highlight that the impacts of unsaturated water flow within the covering system should be considered for assessment of VOC emissions.