Extreme Compaction Effects on Gas Transport Parameters and Estimated Climate Gas Exchange for a Landfill Final Cover Soil

Landfill sites have been implicated in greenhouse warming scenarios as a significant source of atmospheric methane. In this study, the effects of extreme compaction on the two main soil-gas transport parameters, the gas diffusion coefficient (D-p) and the intrinsic air permeability (k(a)), and the cumulative methane oxidation rate in a landfill cover soil were investigated. Extremely compacted landfill cover soil exhibited negligible inactive soil-air contents for both D-p and k(a). In addition, greater D-p and k(a) were observed as compared with normal compacted soils at the same soil-air content (epsilon), likely because of reduced water-blockage effects under extreme compaction. These phenomena are not included in existing predictive models for D-p(epsilon) and k(a)(epsilon). On the basis of the measured data, new predictive models for D-p(epsilon) and k(a)(epsilon) were developed with model parameters (representing air-filled pore connectivity and water-blockage effects) expressed as functions of dry density (rho(b)). The developed D-p(epsilon) and k(a)(epsilon) models together with soil-water retention data for soils at normal and extreme compaction (rho(b)=1.44 and 1.85 g cm(-3)) implied that extremely compacted soils will exhibit lower D-p and k(a) at natural field-water content (-100 cm H2O of soil-water matric potential) because of much lower soil-air content. Numerical simulations of methane gas transport, including a first-order methane oxidation rate, were performed for differently compacted soils by using the new predictive D-p(epsilon) model. Model results showed that compaction-induced difference in soil-air content at a given soil-water matric potential condition is likely the most important parameter governing methane oxidation rates in extremely compacted landfill cover soil.DOI: 10.1061/(ASCE)GT.1943-5606.0000459. (C) 2011 American Society of Civil Engineers.