Air Permeability in Undisturbed Volcanic Ash Soils

Abstract

Soil air permeability (ka) governs convective air and gas transport in soil. The increased use of soil venting systems during vadose zone remediation at polluted soil sites has created a renewed interest in ka and its dependency on soil type and soil air‐filled porosity (ε). Predictive ka(ε) models have only been tested within limited ranges of pore‐size distribution and total porosity. Andisols (volcanic ash soils) exhibit unusually high porosities and water retention properties. In this study, measurements of ka(ε) on 16 undisturbed Andisols from three locations in Japan were carried out in the soil matric potential interval from −10 cm H2O (near water saturation) to −15000 cm H2O (wilting point). Two simple power‐function ka(ε) models, both with measured ka at −100 cm H2O as a reference point, gave similar and good predictions of ka(ε) between −10 and −1000 cm H2O. For one location comprising finely textured and humic Andisols, both models largely underpredicted ka(ε) in dry soil (<−3000 cm H2O), suggesting a sudden occurrence of highly connected air‐filled pore networks during drainage. For the two other locations, the models satisfactorily predicted ka also in dry soil. Using recently published data for gas diffusivity and soil‐water retention together with the ka data in the Millington and Quirk (1964) fluid flow model, a plot of equivalent pore diameter as a function of soil matric potential was made for each soil. This plot, labeled a soil structure fingerprint (SSF), proved useful for illustrating effects of soil cultivation and high organic matter content on soil structure.