MODELING DIFFUSION AND REACTION IN SOILS: IX. THE BUCKINGHAM-BURDINE-CAMPBELL EQUATION FOR GAS DIFFUSIVITY IN UNDISTURBED SOIL

Abstract

Accurate description of gas diffusivity (ratio of gas diffusion coefficients in soil and free air, D_S/D₀) in undisturbed soils is a prerequisite for predicting in situ transport and fate of volatile organic chemicals and greenhouse gases. Reference point gas diffusivities (R_p) in completely dry soil were estimated for 20 undisturbed soils by assuming a power function relation between gas diffusivity and air-filled porosity (ε). Among the classical gas diffusivity models, the Buckingham (1904) expression, equal to the soil total porosity squared, best described R_p. Inasmuch as our previous works (Parts III, VII, VIII) implied a soil-type dependency of D_S/D₀(ε) in undisturbed soils, the Buckingham R_p expression was inserted in two soil- type-dependent D_S/D₀(ε) models. One D_S/D₀(ε) model is a function of pore-size distribution (the Campbell water retention parameter used in a modified Burdine capillary tube model), and the other is a calibrated, empirical function of soil texture (silt + sand fraction). Both the Buckingham-Burdine-Campbell (BBC) and the Buckingham/soil texture-based D_S/D₀(ε) models described well the observed soil type effects on gas diffusivity and gave improved predictions compared with soil type independent models when tested against an independent data set for six undisturbed surface soils (11-46% clay). This study emphasizes that simple but soil-type-dependent power function D_S/D₀(ε) models can adequately describe and predict gas diffusivity in undisturbed soil. We recommend the new BBC model as basis for modeling gas transport and reactions in undisturbed soil systems.