Convective-Dispersive Transport of Soil Gas in Laboratory Experiments.

1. Introduction       Air is often injected into soil with the flow rate of more than 50L/min in a case of bioremediation using bioventing. With this flow rate, the gas transport caused by mechanical dispersion may occur. A detailed estimate of the soil gas transport including the convective-dispersive flow is required for a success of bioremediation. The purpose of this study is to evaluate the aerodynamic dispersion coefficient, D, on soil-gas transport by a displacement experiment.

2. Methods and Materials     The experiments were conducted using soil columns 0.1m in height. The soil sample was packed into an acrylic ring 49.5mm in diameter. Coastal sand (2mm<) was used as the soil material. Its particle density and bulk density are 2.65 and 1.59 Mg/m³, respectively. Air that CO₂ concentration is about 500 ppm was used as the carrier gas and the gas that CO₂ concentration is controlled at about 3000 ppm was used as the tracer gas. CO₂ concentration was measured with an infrared gas analyzer.

3. Results and Discussion     D was obtained by the analysis similar to that used for the solute movement in soil liquid phase. The increase of flow rate of the carrier gas from 0.1 to 1.0 L/min resulted in the increase of D from 4.2×10^-5 to 3.2×10^-4 m²/s at the volume fraction of soil gas of 0.21, and from 9.6×10^-6 to 4.3×10^-5 m²/s at the volume fraction of soil gas of 0.40. The intrinsic permeability of soil gas was determined to be 2.4×10^-11 and 7.6×10^-11 m² at the volume fraction of soil gas of 0.21 and 0.40, respectively. These results confirmed that the soil gas transport caused by the mechanical dispersion occurred at the flow rate of the carrier gas of 1L/min or less.