Measurement and prediction of the oxygen diffusion coefficient in unsaturated media, with applications to soil covers

Molecular diffusion is an important mechanism for gas transport in various natural and man-made systems. This is particularly the case with soil covers installed on acid-generating mine tailings, where oxygen availability has to be controlled. One of the most important roles of such covers is to limit gas flux, which depends on the effective diffusion coefficient D-e of the cover materials. This paper presents an experimental procedure and results from oxygen diffusion tests performed on different types of materials, at various degrees of saturation. The determination of D-e in the laboratory from the test data is based on analytical and numerical solutions to Fick's laws. The ensuing values of D-e are compared to values calculated from available models that relate D-e to basic material properties, including porosity and degree of saturation. Statistical indicators are used to evaluate the accuracy of selected models, individually and on a comparative basis. It is shown that modified versions of the Millington-Quirk (M-Q) and Millington-Shearer (M-S) models provide D-e values close to the measured data. A semi-empirical expression, ensuing from these models and measurements, is proposed as a simple means of estimating D-e.