Simultaneous Measurements of The Soil-Gas Dispersion Coefficient, Gas Permeability, and Gas Diffusivity in Repacked, Structureless and Aggregated Soils.

It is important to understand soil-gas transport phenomena and parameters when predicting transport of gaseous contaminants such as volatile organic chemicals at polluted soil sites or investigating the exchange of greenhouse gases at the soil-atmosphere interface. It is well known that the transport of gaseous compounds in soil is mainly caused by gas diffusion, advection, and dispersion. Of these three processes, the gas dispersion is the least understood, especially how it is linked to soil type, moisture conditions, and gas permeability and diffusivity. In this study, a novel experimental set-up that enables the simultaneous measurement of soil-gas dispersion coefficient, soil-gas permeability and soil-gas diffusivity is presented. One main objective is to investigate the gas dispersion phenomena, with emphasis on the effects of soil structure, particle size and scale, and the links to soil-gas permeability and diffusivity. One-dimensional column experiments were conducted by using non-aggregated  (sandy) and aggregated soils (volcanic ash soil) with different particle size fractions. Soil was sieved and packed into columns with three different dimensions (i.d. 5-cm inner diameter and 30-cm length,5-cm i.d. and 60-cm length,14-cm i.d. and 100-cm length) with given bulk density. The repacked soil columns were flushed with nitrogen gas initially, and then air was injected into the columns. The oxygen (21%) in the air was used as a tracer gas. The air pressure fluctuation and the oxygen  concentration within the soil columns were also measured. The measured oxygen breakthrough curves were used to determine the gas dispersion coefficient as a function of pore-gas velocity. In addition, the gas diffusion coefficient and air permeability for each soil were calculated from the oxygen concentration and air pressure data. The effects of soil (pore) structure, particle (aggregate) size and measurement (column) scale on gas transport parameters (gas diffusivity, air permeability, gas dispersivity) and possible links between the three gas transport parameters are discussed.