A Gas Diffusivity Model Based on Air-, Solid-, and Water-Phase Resistance in Variably Saturated Soil

Gas diffusion in soil is governed by the gas diffusion coefficient (D-p) and its variation with air-filled porosity (epsilon). Accurate or an upper-limit (risk assessment standpoint) prediction of D-p(epsilon) is essential when carrying out gas transport and fate calculations. We developed a D-p(epsilon) model for relatively unstructured soil separating the individual resistance of soil air, solids, and moisture to D-p. Assuming the total soil resistance to gas diffusion can be described by three power-law terms representing air-content reduction, solids-induced tortuosity,and water-induced disconnectivity yields the socalled Soil Air Phase Individual Resistances (SAPHIR) model. The SAPHIR model predicts D-p as a function of the actual epsilon, a particle shape factor (p), the volumetric soil water content (theta), and a water-blockage factor (w). The D-p(epsilon) was measured at different theta on repacked and undisturbed soil samples. The new D-p data combined with literature data implied values of p in the interval 0 to 1 and w in the interval 1 to 7, depending on particle diameter, fine-particle content, and compaction. Tested against 810 measurements of D-p on undisturbed soils, SAPHIR with average values of p = 0.6 and w = 3 performed equally well or better than traditional models; however, the test implied a need for different parameter values for more sandy soils (lower p and higher w), as well as for more compacted soils (lower p).