Unified Measurement System for the Gas Dispersion Coefficient, Air Permeability, and Gas Diffusion Coefficient in Variably Saturated Soil

Abstract

The transport of gaseous compounds in soil takes place by gas diffusion, advection, and dispersion. Gas transport processes are influenced by the soil‐gas diffusion coefficient (Dp), air permeability (ka) and soil‐gas dispersion coefficient (DH), respectively. Of three gas transport parameters, DH is the least understood, especially how it is correlated to soil type, moisture conditions, and other transport parameters (i.e., Dp and ka). In this study, a unified measurement system (UMS) that enables sequential measurement of Dp, ka, and DH on the same soil core was developed. The experimental sequence is based on a two‐chamber measurement of DH and ka, followed by a one‐chamber measurement of Dp Gaseous oxygen concentration and air pressure sensors are located in inlet and outlet chambers as well as at multiple points along the soil column. Using different particle‐size fractions of non‐aggregated (Toyoura sand) and aggregated (Nishi‐Tokyo loam) soils, the effects of soil structure, particle (aggregate) size, and column scale (5‐cm i.d. and 30‐cm or 60‐cm length) on the three gas transport parameters were investigated. For both soils, DH linearly increased with increasing pore‐air velocity. For Toyoura sand, gas dispersivity (λ = DH/u0) decreased with increasing soil‐air content, while for Nishi‐Tokyo loam, gas dispersivity decreased with increasing soil‐air content to a minimum value when inter‐aggregate pores were drained and increased again when the pores inside the soil aggregates started to act as tortuous air‐filled pathways. In the arterial pore region (corresponding to the total pore volume for Narita sand and the inter‐aggregate pore volume for Nishi‐Tokyo loam), a linear relation between tortuosity of the air‐filled pore network (T, calculated from Dp) and the gas dispersivity (λ) was observed.