Estimation of mechanical dispersion and dispersivity in a soil–gas system by column experiments and the dusty gas model

In a previous study, column experiments were carried out with Toyoura sand (permeability 2.05 × 10− 11 m2) and Toyoura sand mixed with bentonite (permeability 9.96 × 10− 13 m2) to obtain the molecular diffusion coefficient, the Knudsen diffusion coefficient, the tortuosity for the molecular diffusion coefficient, and the mechanical dispersion coefficient of soil–gas systems. In this study, we conducted column experiments with field soil (permeability 2.0 × 10− 13 m2) and showed that the above parameters can be obtained for both less-permeable and more-permeable soils by using the proposed method for obtaining the parameters and performing column experiments. We then estimated dispersivity from the mechanical dispersion coefficients obtained by the column experiments. We found that the dispersivity depended on the mole fraction of the tracer gas and could be represented by a quadratic equation.

► Column experiments were conducted for nitrogen or oxygen and carbon dioxide gas systems. ► The molecular diffusion and Knudsen diffusion coefficients with tortuosity could be obtained. ► The relation between the molar fraction could be fitted by a quadratic equation.