Effects of Gas Flow Rate and Soil Properties On Soil-Gas Dispersion.

Recently, advective transport of gas-phase contaminants in unsaturated porous media has become a subject of research. Dispersion is one of the characteristics of gaseous flow through porous media. It is assumed that total dispersion represents both molecular diffusion and mechanical dispersion. For remediation processes, information regarding soil-gas dispersion with respect to various soil properties is necessary for improved modeling and prediction capabilities. The objective of this study is to investigate the dependence of gas dispersion coefficients on different gas flow rates and soil properties such as water content and soil particle diameter using one-dimensional column displacement experiments. The experiments were conducted using soil columns (height, 0.15 m; diameter, 49.5 mm). Coastal sand (particle density, 2.65 Mg/m³) was used as the soil material. Air was used as the carrier gas, and CO₂ was used as the tracer gas. The CO₂ concentration was measured using an infrared adsorption spectrum meter. The breakthrough curves of the measured CO₂ concentration were used to determine the gas dispersion coefficient and the retardation factor; these were expressed as a function of the average gas flow rate. The effective diffusion coefficient and dispersivity were calculated on the basis of the relationship between the gas dispersion coefficient and the average gas flow rate. The gas dispersion coefficients that were determined from experiments conducted using different soil-water content columns were expressed as a function of the Peclet number.