COMING THROUGH! SIMULATION OF PREFERENTIAL UNSATURATED FLOW AND TRANSPORT IN MINE WASTE ROCK DUMP

Surface mining operations often create large dumps of waste rock that may release chemicals associated with the rapid weathering of the waste rock. The rate of leaching of these dumps depends on the hydraulic and geochemical properties of the types of waste rock. In addition, the textural heterogeneity of a dump is partly controlled by spatial patterns created during the dumping process, e.g. compacted lift surfaces. Previous modeling studies have shown that zones of increased saturation and preferential flow exist within dumps as a result of textural contrasts which can lead to the creation of capillary breaks or variable flow paths depending on water flow rates. These studies have suggested that dump construction could potentially be used to control water flow and leaching pathways.

However, the variability of flow and transport within lifts of a dump, where material segregation is limited and heterogeneities have a more random character, are poorly understood. This is problematic, because these local heterogeneities could exert a significant control over leaching and even override some of the influence of larger spatial structures. In this modeling study, we explore how structure and heterogeneity within a lift profile affect breakthrough of water and solutes. We hypothesize that the spatial structure is the main driver of variation of water fluxes and concentrations of solutes at the bottom of a lift. We created 2D domains of patterns of two texturally different waste rocks with a multiple-point geostatistical algorithm (FILTERSIM) using photos taken during and after lift construction as a training image. The water flow and transport through these heterogeneous profiles were then simulated under climate conditions representative for western Canada.

We found that snowmelt during the spring freshet was the main driver of spatiotemporal variation of flow and transport. Preferential flow through coarse lenses occurred when these were located close to the top surface and the freshet infiltration front reached them early in the season. Capillary barrier effects occurred at various positions in the profile. Overall however, the majority of flow was vertical and preferential flow distances were relatively short, diminishing the effects of structure on potential fast leaching through the pile.