Predicting the gas diffusion coefficient in undisturbed soil from soil water characteristics

The gas diffusion coefficient in soil (D-P), and its dependency on soil phy sical characteristics, governs the diffusive transport of oxygen, greenhous e gases, fumigants, and volatile organic pollutants in agricultural, forest , and urban soils. Accurate models for predicting D-P as a function of air- filled porosity (epsilon) in natural, undisturbed soil are needed for reali stic gas transport and fate simulations. Using data from 126 undisturbed so il layers, we obtained a high correlation (r(2) = 0.97) for a simple, nonli near expression describing D-P at -100 cm H2O of soil water potential (D-P, D-100) as a function of the corresponding air-filled porosity (epsilon(100) ), equal to the volume of soil pores with an equivalent pore diameter >30 m u m, A new D-P(epsilon) model was developed by combining the D-P,D-100(epsi lon(100)) expression with the Burdine relative hydraulic conductivity model , the latter modified to predict relative gas diffusivity in unsaturated so il, The D-P,D-100 and Burdine terms in the D-P(epsilon) model are both rela ted to the soil water characteristic (SWC) curve and, thus, the actual pore -size distribution within the water content range considered. The D-P(epsil on) model requires knowledge of the soil's air-filled and total porosities and a minimum of two points on the SWC curve, including a measurement at -1 00 cm H2O. When tested against independent gas diffusivity data for 21 diff erently textured and undisturbed soils, the SWC-dependent D-P(epsilon) mode l accurately predicted measured data and gave a reduction in root mean squa re error of prediction between 58 and 83% compared to the classical, soil t ype-independent Penman and Millington-Quirk models. To further test the new D-P(epsilon) model, gas diffusivity and SWC measurements on undisturbed so il cores from three 0.4-m soil horizons (sandy clay loam, sandy loam, and l oamy sand) within the 4 to 7 m depth below an industrially polluted soil si te were carried out. For these deep subsurface soils the SWC-dependent mode l best predicted the measured gas diffusivities.