Gas Diffusivity and Thermal Properties of Compost-mixed Soils under Variable Water Saturation

Abstract

Gas and heat transport through compostmixed landfill cover soils affect the emission of toxic gases and methane oxidization processes. In this study, we mixed soils with three different composts in the ratio of either 1:5 or 1:10 (compost:soil) to understand the effect of compost mixing for gas diffusivity and thermal properties. The gas diffusion coefficient (Dp), thermal conductivity ( ), and heat capacity (HC) were measured for soils, composts, and compost-mixed soils at different soil-water matric potentials ( ) starting from nearly saturated to  = -10,000 cm H2O and dry conditions. Data were fitted to the Brooks-Corey soil-water retention curve model to estimate the bubbling pressure ( b). For all materials, Dp increased linearly with increased air content ( ), and the Penman-Call linear Dp( ) model with the model slope (C) and threshold soil-air content ( th) fitted the data well. The  th values increased with increasing compost content, relating non-linearly to the Brooks-Corey  b but highly linearly to the soil macroporosity. Analogous to the Dp( ) model, Penman-Call type linear  ( ), and HC( ) models with slopes (C′ and C′′) and intercepts ( 0 and HC0, thermal conductivity and heat capacity at a volumetric water content of  = 0) captured reasonably well the data measured from dry to wet conditions. The C′ for varied depending on the compost ratio and decreased with increasing compost ratio. The C′′ for HC, on the other hand, had less effect on the compost mix. The thermal properties under the dry condition,  0 and HC0, were well correlated to the volumetric solid content. The results from this study will be helpful towards designing compost-mixed landfill cover soils with optimal heat and gas transport characteristics.