Numerical Modelling of Flow and Capillary Barrier Effects in Unsaturated Waste Rock Piles

Abstract

Flow systems in unsaturated waste rock piles were simulated using a two-dimensional numerical model (HYDRUS). The conceptual models are based on homogeneous (unstructured) waste piles, and on structured piles that include either horizontal or inclined fine-grained layers within a coarser host material, forming a capillary barrier system. The approach considers fully transient conditions and uses observed climatic data from a mine site in northern Quebec, Canada. All physical properties of the porous media, including the water retention curves, were obtained from measured data. Different geometric configurations were tested to determine their effect on moisture distribution and water flow, which ultimately control the potential for acid rock drainage (ARD). The simulations begin with a relatively dry initial condition under hydrostatic equilibrium. After an initial transient period, the simulated internal moisture distribution became periodic with a regular pattern of seasonal fluctuations. The simulations suggest that flow can be controlled in such systems using inclined fine-grained layers that retain and divert moisture due to capillary barrier effects. With horizontal layers, the local flow regimes become unstable, causing vertical preferential flow zones to develop below the barriers wherever the local water pressure first exceeds the entry pressure of the underlying coarser material. In this scenario, ARD production can remain high since a large fraction of the internal pile is being flushed. A shallow downward slope in the layers forces drainage toward the outer boundary and maintains lower saturation in the centre of the pile, thus potentially reducing the amount and mobility of ARD.