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ORIGINAL RESEARCH

Gas Transport Parameters in the Vadose Zone: Development and Tests of Power-Law Models for Air Permeability

^a Graduate School of Science and Engineering, Saitama Univ., 225 Shimo-okubo, Sakura-ku, Saitama, 338-8570, Japan
^b Environmental Engineering Section, Dep. of Biotechnology, Chemistry, and Environmental Engineering, Aalborg Univ., Sohngaardsholmsvej 57, DK-9000 Aalborg, Denmark
^c Dep. of Agroecology, Danish Inst. of Agricultural Sciences, Research Centre Foulum, P.O. Box 50, DK-8830 Tjele, Denmark
^d Dep. of Land, Air, and Water Resources, Univ. of California, Davis, CA 95616
^* Corresponding author (kawamoto{at}post.saitama-u.ac.jp)

The soil-air permeability (k_a) and its dependency on air-filled porosity () govern convective air and gas transport in soil. For example, accurate prediction of k_a() is a prerequisite for optimizing soil vapor extraction systems for cleanup of soils polluted with volatile organic chemicals. In this study, we measured k_a at different soil-water matric potentials down to 5.6-m depth, totaling 25 differently textured soil layers. Comparing k_a and soil-gas diffusivity (D_p/D₀) measurements on the same soil samples suggested an analogy between how the two soil-gas transport parameters depend on . The exponent in a power-law model for k_a() was typically smaller than for D_p()/D₀, however, probably due to the influence of soil structure and large-pore network being more pronounced for k_a than for D_p/D₀. In analogy to recent gas diffusivity models and in line with capillary tube models for unsaturated hydraulic conductivity, two power-law k_a() models were suggested. One k_a() model is based on the Campbell pore-size distribution parameter b and the other on the content of larger pores (₁₀₀, corresponding to the air-filled porosity at –100 cm H₂O of soil-water matric potential). Both new models require measured k_a at –100 cm H₂O (k_a,100) as a reference point to obtain reasonably accurate predictions. If k_a,100 is not known, two expressions for predicting k_a,100 from ₁₀₀ were proposed but will cause at least one order of magnitude uncertainty in predicted k_a. The k_a() model based on only ₁₀₀ performed well in the model tests and is recommended together with a similar model for gas diffusivity for predicting variations in soil-gas transport parameters in the vadose zone.