Gas Transport Parameters in the Vadose Zone: Development and Tests of Power-Law Models for Air Permeability
2006
Kawamoto, Ken | Moldrup, Per | Schjønning, P. (Per) | Iversen, Bo V. | Komatsu, Toshiko | Rolston, Dennis E.
The soil-air permeability (k ₐ) and its dependency on air-filled porosity (ε) govern convective air and gas transport in soil. For example, accurate prediction of k ₐ(ε) 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/D ₀) measurements on the same soil samples suggested an analogy between how the two soil-gas transport parameters depend on ε. The exponent in a power-law model for k ₐ(ε) was typically smaller than for D ₚ(ε)/D ₀, however, probably due to the influence of soil structure and large-pore network being more pronounced for k ₐ than for D ₚ/D ₀ In analogy to recent gas diffusivity models and in line with capillary tube models for unsaturated hydraulic conductivity, two power-law k ₐ(ε) models were suggested. One k ₐ(ε) model is based on the Campbell pore-size distribution parameter b and the other on the content of larger pores (ε₁₀₀, 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 ε₁₀₀ were proposed but will cause at least one order of magnitude uncertainty in predicted k ₐ The k ₐ(ε) model based on only ε₁₀₀ 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.
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