Preferential unsaturated flow and transport in heterogeneous mine waste rock dumps

English Abstract:

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 (such as compacted lift surfaces) is affected by the dumping process. Previous modeling studies have shown that zones of increased saturation and preferential flow exist within dumps as a result of textural contrasts which can in hydraulic and capillary breaks or variable flow paths depending on water flow rates.

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 could 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 created 2D domains of patterns of two texturally different waste rocks with a multiple-point geostatistical algorithm (FILTERSIM) using photos taken during 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 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 snowmelt infiltration front reached them within a few days. Hydraulic and capillary barrier effects occurred at various positions in the profiles. However, soil heterogeneity did not always result in preferential leaching at the bottom of the lifts.