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

The soil-air permeability (k(a)) and its dependency on air-filled porosity (epsilon) govern convective air and gas transport in soil. For example, accurate prediction of k(a)(epsilon) 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 epsilon. The exponent in a power-law model for k(a)(epsilon) was typically smaller than for D(p)(epsilon)/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)(epsilon) models were suggested. One k(a)(epsilon) model is based on the Campbell pore-size distribution parameter b and the other on the content of larger pores (epsilon100, 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)(epsilon) 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.