CHARACTERIZING THE DEPENDENCE OF GAS-DIFFUSION COEFFICIENT ON SOIL PROPERTIES

Knowledge of the gaseous diffusion coefficient is necessary to properl y model gas movement in porous media. In this study, a nonreactive tra cer (freon-12) and a reactive tracer (hexafluorobenzene) were used to evaluate the relationship between the ratio of the gas diffusion coeff icient in soil (D-g(s)) to that in free air (D-g(a)) and the volumetri c air content (a) with a two-chamber diffusion system. The measured D- g(s)/D-g(a) values in our experiments (i.e., the relative gas diffusio n coefficients) showed a similar relationship to air content as data o btained from various studies reported in the Literature. Our data and values taken from other studies were used to test the validity of vari ous models. When compared with data obtained from experiments with dis turbed soils, the Penman model D-g(s)/D-g(a) = 0.66 a was found to ove restimate the measured relative diffusion coefficient, whereas the Mil lington-Quirk model D-g(s)/D-g(a) = a(10/3)/phi(2), where phi is the s oil porosity, underestimated it. Another Millington-Quirk relationship D-g(s)/D-g(a) = a(2)/phi(2/3), which has been largely overlooked in t he literature, was found to provide significantly better agreement wit h measured relative diffusion coefficients in various disturbed soils of different texture, On the contrary, no universal relationship was f ound when data from both disturbed and undisturbed soil experiments we re evaluated. The Troeh model D-g(s)/D-g(a) = [(a - u)/(1 - u)](v) had the flexibility to fit all of the experimental data when both of the model parameters were varied simultaneously; however, no obvious corre lation was found between soil properties and the parameters, The limit ations of any universal form for gas tortuosity model in natural soils were analyzed with percolation theory.