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

Abstract

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, ρ b , and particle size fraction on the main soil–gas transport parameters – soil–gas diffusivity ( D p / D o , ratio of gas diffusion coefficients in soil and free air) and air permeability ( k a ) – under variably-saturated moisture conditions. Soil samples were prepared by three different compaction methods (Standard and Modified Proctor compaction, and hand compaction) with resulting ρ b values ranging from 1.40 to 2.10 g cm −3. Results showed that D p and k a 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 D p and k a was most pronounced for the ‘+gravel’ fraction. Normalized ratios were introduced for all soil–gas parameters: (i) for gas diffusivity D p / D f , the ratio of measured D p to D p in total porosity ( f), (ii) for air permeability k a / k a ,pF4.1, the ratio of measured k a to k a 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 D p ( ε/ f) and k a ( ε/ f) models were developed based on a single parameter (water blockage factor M for D p and P for k a ). The water blockage factors, M and P, were found to be linearly correlated to ρ b values, and the effects of dry bulk density on D p and k a for both ‘+gravel’ and ‘−gravel’ fractions were well accounted for by the new models.