OXYGEN DIFFUSION THROUGH SOIL COVERS ON SULPHIDIC MINE TAILINGS

Engineered soil covers are being evaluated under Canada's Mine Environ ment Neutral Drainage (MEND) program for their effectiveness in preven ting and controlling acid generation in sulphidic mill tailings. A cri tical parameter for predicting the performance of these covers is the diffusion coefficient of gaseous oxygen in the cover materials. Labora tory experiments conducted to determine the effective diffusion coeffi cient of a candidate cover material, a glacial till from an active min e site, are described. The diffusion coefficient is determined by fitt ing a semianalytic solution of the one-dimensional, transient diffusio n equation to experimental gaseous oxygen concentration versus time gr aphs. Effective diffusion coefficients determined at high water satura tions (85%-95%) were of the order of 8 x 10(-8) m2/s. The diffusion co efficients decreased with increase in water saturation as a result of the low diffusivity of gaseous oxygen in water relative to that in air and the low solubility of oxygen in water. Placement of soil covers i n high saturation conditions would ensure that the flux of oxygen into tailings underneath such covers is low, resulting in low acid flux. T his is confirmed by combined laboratory, field, and modeling studies.