Abstract

The soil-air permeability (ka) and its dependency on air-filled porosity (ε) govern convective air and gas transport in soil. For example, accurate prediction of ka(ε) is a prerequisite for optimizing soil vapor extraction systems for cleanup of soils polluted with volatile organic chemicals. In this study, we measured ka at different soil-water matric potentials down to 5.6-m depth, totaling 25 differently textured soil layers. Comparing ka and soil-gas diffusivity (Dp/D0) 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 ka(ε) was typically smaller than for Dp(ε)/D0, however, probably due to the influence of soil structure and large-pore network being more pronounced for ka than for Dp/D0. In analogy to recent gas diffusivity models and in line with capillary tube models for unsaturated hydraulic conductivity, two power-law ka(ε) models were suggested. One ka(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 ka at −100 cm H2O (ka,100) as a reference point to obtain reasonably accurate predictions. If ka,100 is not known, two expressions for predicting ka,100 from ε100 were proposed but will cause at least one order of magnitude uncertainty in predicted ka. The ka(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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