Tuesday, November 14, 2006 - 3:10 PM

Individual Phase Resistances to Gas Diffusivity in Unsaturated Soil.

Per Moldrup, DENMARK,Aalborg Univ., Dept. of Biotech. Chem. and Environ. Engineering, Aalborg Univ Sohngaardsholmsvej 57 D-building, Aalborg, DK9000, DENMARK, Anne Thorbjørn, Danish Environmental Consulting, Fanøgade 17, Jerslev, DK9740, Denmark, Torben Olesen, Aalborg Municipality, City and Environment Section, Stigsborg Brygge 5, Nørresundby, DK9400, Denmark, Seiko Yoshikawa, National Agricultural Research Centre, Ikano 2575, Zentsuji, Kagawa, 7650053, Japan, Toshiko Komatsu, JAPAN, Saitama University, Saitama University, Shimo-okubo Sakura-ku, Saitama, 338-8570, JAPAN, and Dennis Rolston, University of California Davis, Dept. of Land, Air, and Water Resources, 1 Shields Avenue, Davis, CA 95616-8627.

The soil-gas diffusion coefficient (gas diffusivity) and its variations with soil type and water content govern a multitude of gas transport and fate processes in the soil vadose zone, including soil uptake or emission of greenhouse gases (carbon dioxide, methane, and nitrous oxide). In this study, we isolate the individual effects (resistances) from each of the three main soil phases (solids, water, and air) in a simple model for gas diffusivity as a function of the air-filled and water-filled porosities. The new gas diffusivity model (TRIUS) includes two soil-type dependent parameters, (i) a particle-shape factor quantifying the additional length of air-filled pathways in completely dry soil caused by irregular particle shape and heterogeneous particle distribution, and (ii) a water-lock factor quantifying the blocking of air-filled pathways caused by inter-particle water. With constant values of the two parameters, TRIUS accurately predicts measured gas diffusivities for repacked soils across a wide range of soil textures and total porosities. Fitting the TRIUS model to gas diffusivity data for undisturbed soils, however, suggests that the particle-shape and water-lock factors both vary with soil structure and compaction. Lastly, we link each of the three individual gas diffusivity resistances to the soil pore-size distribution as described by the van Genuchten soil-water characteristic model. The combined TRIUS-van Genuchten model is applied to measured data and appears useful to understand which of the three resistances to soil-gas diffusivity will dominate in different soil types and at different levels of soil-water matric potential.