Long-term effects of grass roots on gas permeability in unsaturated simulated landfill covers

Landfill cover is a barrier that can reduce landfill gas emission. Vegetation could affect soil hydraulic properties of landfill cover, while its effects on gas permeability is not well understood. This paper investigated the effects of plant roots on the evolution of gas permeability (k_g) in unsaturated landfill cover over a two-year period. One grass species (Cynodon dactylon) was selected for testing in the laboratory and there were six replications. Bare soil was used as a reference. k_g was determined after 6, 15 and 24 months of grass growth. The test results show that with the increase of suction, k_g increased linearly in log scale for both bare and grass-covered soils. k_g of grass-covered can be lower or higher than that of bare soil, depending on the plant age. After 6 months, k_g of grass-covered soil was smaller than that of bare soil by 85%, due to the root occupancy of soil pore space. However, after 15 and 24 months, k_g of grass-covered soil increased by 2-3 orders of magnitude within the root zone over the suction range considered (2-86 kPa). After 2 years, k_g of grass-covered soil was up to one order of magnitude higher than that in bare soil. It implies that mature vegetation is not beneficial for reducing gas emission in final landfill covers.