PREDICTIVE-DESCRIPTIVE MODELS FOR GAS AND SOLUTE DIFFUSION COEFFICIENTS IN VARIABLY SATURATED POROUS MEDIA COUPLED TO PORE-SIZE DISTRIBUTION: II. GAS DIFFUSIVITY IN UNDISTURBED SOIL

Abstract

The soil-gas diffusion coefficient (D_P) and its dependency on soil type and air-filled porosity (ε) controls a multitude of gas transport and fate processes in natural soils, including soil aeration, emission of greenhouse gases, and spreading of volatile organic chemicals. Recent studies show that D_p(ε) often differs between repacked and undisturbed soils. This paper revisits two models for predicting D_P(ε) in undisturbed soil, the three-porosity model (TPM) and the Buckingham-Burdine-Campbell (BBC) model. The TPM is modified by a new expression for predicting D_P as a function of ε at −100 cm H₂O of matric potential (reference point). In the hereby obtained three-porosity-encased (3POE) model, the nonlinearity in gaseous phase pore continuity is assumed to be monotonously increasing with the volumetric fraction of larger pores (drained at −100 cm H₂O). To make the model more generally applicable, reference-point D_P(ε) expressions for other matric potentials than −100 cm H₂O are developed from data for 44 soils and can be used in cases where ε at −100 cm H₂O is not known. Measurements of D_P at eight different matric potentials were made on intact soil samples from two fields with volcanic ash soil. The 3POE and BBC models were tested against the measured data and data from literature, comprising 65 soils, and gave similar and accurate predictions. The 3POE model is coupled with the widely used van Genuchten soil-water characteristic (SWC) function to obtain a closed-form model (3POE-vG) that directly relates gas diffusivity in undisturbed soil to pore size distribution. As a low-parameter alternative, the BBC gas diffusivity model is coupled with the Campbell SWC function. Both SWC-coupled gas diffusivity models can assist in analyzing soil type and management effects on gas diffusivity-matric potential relations in undisturbed soils, for example, in relation to improving soil aeration and reducing soil emissions of volatile chemicals.