Compaction Effect on the Gas Diffusion Coefficient in Soils

Abstract

Gas diffusion is important in determining the aeration status of soils for crop production and in providing estimates of transport of volatile chemicals at waste disposal or chemical spill sites. This study investigated the effects of compaction on the gas diffusion coefficient in four soils. The experiment involved equilibrating bulk loose soil to a known matric potential in a pressure chamber, compressing the equilibrated moist soil in metal cores at a given applied stress, and measuring the concentration of N₂ diffusing through these cores into a diffusion chamber. The diffusion coefficient was calculated by fitting an analytical solution of the transient diffusion equation to the measured N₂ concentration in the diffusion chamber as a function of time. The diffusion coefficient of N₂ in four soils at four water contents and four applied loads varied exponentially as a function of air‐filled porosity and was nearly the same function for all four soils. At an air‐filled porosity of ≈ 10%, the diffusion coefficient was close to zero, reflecting a discontinuity in the pathways at an air‐filled pore space of 10% or lower. A model for predicting the diffusion coefficient of gases in soils based on the diffusion coefficient of individual soil constituents was tested with the data set. For known shape factors for soil solid and soil water, the model overpredicted the diffusion coefficient for all four soils. A reduction factor suggested in the literature to account for blocked air slightly improved predictions. The ratio of the measured to predicted diffusion coefficient vs. degree of air‐filled saturation suggests a second‐degree correction factor to account for blocked air in soils.