PREDICTIVE-DESCRIPTIVE MODELS FOR GAS AND SOLUTE DIFFUSION COEFFICIENTS IN VARIABLY SATURATED POROUS MEDIA COUPLED TO PORE-SIZE DISTRIBUTION: III. INACTIVE PORE SPACE INTERPRETATIONS OF GAS DIFFUSIVITY.

Accurate description of the soil-gas diffusion coefficient (DP) as a function of air-filled ([epsilon]) and total ([PHI]) porosities is required for studies of gas transport and fate processes. After presenting predictive models for DP in repacked and undisturbed soils (Part I and II), this third paper takes a more descriptive approach allowing for the inclusion of inactive air-filled pore space, [epsilon]in. Three model-based interpretations of [epsilon]in are presented: (1) a simple power-law model (labeled Millington-Call) with the exponent (V) taken from Millington (1959; Science 130:100-102), and expanded with a constant [epsilon]in term (= 0.1 m^3 m^-3), (2) a model (SOLA) based on analogy with solute diffusion and assuming a linear increase in pore continuity from zero at the threshold air-filled porosity where gas diffusion ceases ([epsilon]th) to a maximum at [epsilon] = [PHI], (3) a power-law model (VIPS) assuming variable [epsilon]in that linearly decreases from a maximum at [epsilon] = [epsilon]th to zero at [epsilon] = [PHI]. Assuming [epsilon]th = 0.1 m3 m-3, all three models satisfactorily predicted DP in 18 repacked soils. The difference between the three models is mainly pronounced for higher-[PHI] soils, and each model has its own advantage. The SOLA model together with similar models for solute diffusivity allows a direct comparison of pore continuity in the soil gaseous and liquid phases, suggesting large differences in tortuosity and inactive fluid-phase between the two phases. The low-parameter Millington-Call model could account for variability in measured DP along a field transect (Yolo, California) by varying [epsilon]in with (plus-minus)0.03 m^3 m^-3 and is applicable for stochastic gas transport simulations at field scale. The mathematically flexible VIPS model highly accurately fitted DP([epsilon]) data for undisturbed soil, illustrating the large possible variations in [epsilon]th and V. The VIPS model is coupled with the van Genuchten (vG) soil-water characteristic model, yielding a closed-form expression for DP as a function of soil-water matric potential. The VIPS-vG model is useful to illustrate the combined effects of pore size distribution and inactive pore space on soil-gas diffusivity.