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

2006; Soil Science Society of America; Volume: 5; Issue: 4 Linguagem: Inglês

10.2136/vzj2006.0030

1539-1663

Ken Kawamoto, Per Møldrup, Per Schjønning, Bo V. Iversen, Toshiko Komatsu, Dennis E. Rolston,

Landfill Environmental Impact Studies

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 (ε 100 , corresponding to the air‐filled porosity at −100 cm H 2 O of soil‐water matric potential). Both new models require measured k a at −100 cm H 2 O ( 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 ε 100 were proposed but will cause at least one order of magnitude uncertainty in predicted k a The k a (ε) model based on only ε 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.