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ORIGINAL RESEARCH

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

^a Environmental Engineering Section, Dep. of Biotechnology, Chemistry and Environmental Engineering, Aalborg Univ., Sohngaardsholmsvej 57, DK-9000 Aalborg, Denmark
^b Graduate School of Science and Engineering, Saitama Univ., 225 Shimo-okubo, Sakura-ku, Saitama, 338-8570, Japan
^c Dep. of Land, Air, and Water Resources, Univ. of California, Davis, CA 95616
^* Corresponding author (pm{at}bio.aau.dk).

Received 4 February 2008.

Gas diffusion in soil is governed by the gas diffusion coefficient (D_p) and its variation with air-filled porosity (). Accurate or an upper-limit (risk assessment standpoint) prediction of D_p() is essential when carrying out gas transport and fate calculations. We developed a D_p() 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 so-called Soil Air Phase Individual Resistances (SAPHIR) model. The SAPHIR model predicts D_p as a function of the actual , a particle shape factor (p), the volumetric soil water content (), and a water-blockage factor (w). The D_p() was measured at different 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).

Abbreviations: WLR, water-induced linear reduction