| Popis: |
Accurate description of the soil-gas diffusion coefficient (Dp) as a function of air-filled (e) and total (Φ) 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, e in . Three model-based interpretations of e 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 e 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 (e th ) to a maximum at e = Φ, (3) a power-law model (VIPS) assuming variable e in that linearly decreases from a maximum at e = e th to zero at e = Φ. Assuming e th = 0.1 m 3 m -3 , all three models satisfactorily predicted Dp in 18 repacked soils. The difference between the three models is mainly pronounced for higher-Φ 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 e in with ±0.03 m 3 m -3 3 and is applicable for stochastic gas transport simulations at field scale. The mathematically flexible VIPS model highly accurately fitted D P (e) data for undisturbed soil, illustrating the large possible variations in e 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. |
