Effects of dry bulk density and particle size fraction on gas transport parameters in variably saturated landfill cover soil

Landfill sites are emerging in climate change scenarios as a significant source of greenhouse gases. The compacted final soil cover at landfill sites plays a vital role for the emission, fate and transport of landfill gases. This study investigated the effects of dry bulk density, ρb, and particle size fraction on the main soil–gas transport parameters – soil–gas diffusivity (Dp/Do, ratio of gas diffusion coefficients in soil and free air) and air permeability (ka) – under variably-saturated moisture conditions. Soil samples were prepared by three different compaction methods (Standard and Modified Proctor compaction, and hand compaction) with resulting ρb values ranging from 1.40 to 2.10 g cm−3. Results showed that Dp and ka values for the ‘+gravel’ fraction (<35 mm) became larger than for the ‘−gravel’ fraction (<2 mm) under variably-saturated conditions for a given soil–air content (ε), likely due to enhanced gas diffusion and advection through less tortuous, large-pore networks. The effect of dry bulk density on Dp and ka was most pronounced for the ‘+gravel’ fraction. Normalized ratios were introduced for all soil–gas parameters: (i) for gas diffusivity Dp/Df, the ratio of measured Dp to Dp in total porosity (f), (ii) for air permeability ka/ka,pF4.1, the ratio of measured ka to ka at 1235 kPa matric potential (=pF 4.1), and (iii) for soil–air content, the ratio of soil–air content (ε) to total porosity (f) (air saturation). Based on the normalized parameters, predictive power-law models for Dp(ε/f) and ka(ε/f) models were developed based on a single parameter (water blockage factor M for Dp and P for ka). The water blockage factors, M and P, were found to be linearly correlated to ρb values, and the effects of dry bulk density on Dp and ka for both ‘+gravel’ and ‘−gravel’ fractions were well accounted for by the new models.

► The effects of soil physical properties on gas transport parameters were investigated.
► Higher values of Dp and ka exhibited in the ‘+gravel’ than the ‘−gravel’ fraction at same soil–air content (ε).
► Recent power law models for Dp (WLR) and ka (RPL) were modified.
► Model parameters were linearly related to easily measurable dry bulk density (ρb).