Mitigating methane emissions and air intrusion in heterogeneous landfills with a high permeability layer

Spatially variable refuse gas permeability and landfill gas (LFG) generation rate, cracking of the soil cover, and reduced refuse gas permeability because of liquid addition can all affect CH4 collection efficiency when intermediate landfill covers are installed. A new gas collection system that includes a near-surface high permeability layer beneath the landfill cover was evaluated for enhancing capture of LFG and mitigating CH4 emissions. Simulations of gas transport in two-dimensional domains demonstrated that the permeable layer reduces CH4 emissions up to a factor of 2 for particular spatially variable gas permeability fields. When individual macrocracks formed in the cover soil and the permeable layer was absent, CH4 emissions increased to as much as 24% of the total CH4 generated, double the emissions when the permeable layer was installed. CH4 oxidation in the cover soil was also much more uniform when the permeable layer was present: local percentages of CH4 oxidized varied between 94% and 100% across the soil cover with the permeable layer, but ranged from 10% to 100% without this layer for some test cases. However, the permeable layer had a minor effect on CH4 emissions and CH4 oxidation in the cover soil when the ratio of the gas permeability of the cover soil to the mean refuse gas permeability <= 0.05. The modeling approach employed in this study may be used to assess the utility of other LFG collection systems and management practices. (C) 2010 Elsevier Ltd. All rights reserved.