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SOIL PHYSICS

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

^a Saitama Univ., Simo-okubo, Sakura-ku, Saitama, Japan
^b Dep. of Biotech. Chemistry and Environ.Eng., Aalborg Univ., Sohngaardsholmsvej 57, Aalborg, DK9000 Denmark
^c Land, Air, and Water Resources, Univ. of California, Davis, CA 95616-8627

^* Corresponding author (s07de004{at}mail.saitama-u.ac.up)

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 (D_p), air permeability (k_a) and soil-gas dispersion coefficient (D_H), respectively. Of three gas transport parameters, D_H is the least understood, especially how it is correlated to soil type, moisture conditions, and other transport parameters (i.e., D_p and k_a). In this study, a unified measurement system (UMS) that enables sequential measurement of D_p, k_a, and D_H on the same soil core was developed. The experimental sequence is based on a two-chamber measurement of D_H and k_a, followed by a one-chamber measurement of D_p. 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, D_H linearly increased with increasing pore-air velocity. For Toyoura sand, gas dispersivity ( = D_H/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 D_p) and the gas dispersivity () was observed.

Abbreviations: APR, Arterial Pore Region • UMS, Unified Measurement System • WLR, Water-induced Linear Reduction