Oxygen transport through multi-layer caps over mine waste

Uncontrolled piles of mine waste containing sulphidic rock and tailings generate acid rock drainage (ARD) when the rock and tailings oxidize. ARD discharge laden with heavy metals can contaminate surface water and ground water, which results in serious impacts on wildlife. As a cost-effective strategy for ARD control, multi-layer caps consisting of geosynthetic layers and earthen materials for minimizing oxidation of sulphidic mine waste are used. However, very little is known about their capability to restrict oxygen transport. Quantifying oxygen transport in multi-layer caps was the primary objective of this study.; A finite element model was developed to simulate advective-diffusive oxygen transport through a multi-layer cap containing seven layers (a surface layer, a rooting layer, a drainage layer, a geomembrane layer, a GCL layer, a base layer, and a grading layer). The geomembrane contained a hole. An analytical solution was also developed for limiting cases to verify the finite element model. The finite element model was also compared to field data reported by others.; Results obtained with the finite element model show that the dominant mechanism for oxygen transport is gas-phase diffusion and the contributions of gas-phase advection, liquid-phase advection, and liquid-phase diffusion to oxygen transport through multi-layer composite cap are small. The numerical results also show that presence of the geomembrane is the most important factor, whereas breathing of the drainage layer is least important. Climate has modest importance, but its importance increases significantly when a geomembrane is not present. Thickness of the barrier layer is less important than the geomembrane or the climate, but more important than breathing of the drainage layer for oxygen transport.