Effective Oxygen Diffusion Coefficient and Field Oxygen Concentrations below a Geosynthetic Clay Liner (GCL) Covering Mine Tailings

In this paper, the theory of gas diffusion through porous media is briefly summarized and then experimental and analytical methods are presented to estimate the effective oxygen diffusion coefficient (D_e) based on a finite mass equation. Laboratory oxygen diffusion tests were performed on potentially acid generating (PAG) tailings at various water contents to determine D_e. These tailings originated from the Premier Gold Project (PGP) located near Stewart, British Columbia, Canada (~ 56°05^'N, 130°00^'W). Analytical results from these tests were in excellent agreement with estimates obtained with well-known software (Pollute V6) and a method based on the soil parameters of volumetric air and water contents and tortuosity. D_e for the PGP tailings was approximately 5.3 x 10^–8 m²/s at the soil saturation of 80% and 1.9 x 10^–6 m²/s at the soil saturation of 4 to 23%. Oxygen concentrations in two PGP field test plots are also presented. One test plot contained 0.75 m potentially acid generating (PAG) tailings overlain by 0.5 m coarse sand and gravel and 0.1 m top soil (Control Plot) and the other contained the layers (from bottom to top) of 0.75 m PAG tailings, 0.1 m fine sand, a Geosynthetic Clay Liner (GCL), 0.1 m fine sand, 0.3 m coarse sand and gravel, and 0.1 m top soil. The placement of the GCL significantly lowered the oxygen concentrations in the tailings below the GCL. Most oxygen concentrations below the GCL were lower than 1.0 % Vol whereas Control Plot oxygen concentrations ranged from 0.3 to 20% Vol. The estimated oxygen flux was 26.3 kg/m²/year for the Control Plot and 0.187 kg/m²/year for the GCL Plot.