In closing a surface waste rock dump it is not simply a matter of constructing a cover retrospectively

urface waste rock dumps may not appear to generate acid and metalliferous drainage from the start of their construction. However, the scene is set for potentially contaminated seepage as soon as the first reactive waste rock is placed. Once reactive waste rock is exposed to oxygen, oxidation reactions commence. The placement of this material in a surface waste rock dump also exposes it to rainfall infiltration from the surface and to wetting up from the base by runoff along buried surface channels beneath the dump. Rainfall infiltration into an uncovered dump is likely to be of the order of 50% of cumulative rainfall, depending on the compaction of the top surface, the climate and the age of the dump. Initially, most of the rainfall infiltration and runoff will go into storage within the dumped waste rock, with little or none seeping into the foundation or emerging as toe seepage at topographic low points around the dump. The dump will gradually wet up over time, as a function of the climatic conditions, the rate of construction of the dump and the height it achieves, and the nature of the waste rock dump and how it breaks down with time. The wetter the climate, the more rapidly will the dump wet up. The faster the dump is constructed and the greater its' final height, the longer it will take for seepage to break through. The finer-grained the waste rock and/or the quicker it breaks down, the more water it will store and the more saturated it will need to become before base seepage will emerge. Typically, a waste rock dump is left open to oxygen ingress and rainfall infiltration during its operational life. This may be of the order of 20 years. During this time, it is most likely that some base seepage will emerge, associated with high rainfall events. The older the dump, the more likely is base seepage to emerge, the lower the trigger rainfall required to initiate seepage, and the shorter the lag between rainfall and seepage. If the dump is left open to oxygen ingress and rainfall infiltration for say 20 years, and even if an impermeable cover could be placed at that time, the water stored in the dump will drain down over a period of a further 20 years or more. Just as wetting up will increase exponentially with time to a degree of saturation at which drain down will occur, drain down will decrease exponentially with time to a rate that matches the infiltration rate. In reality, an impermeable cover will not be possible, and the drain down will eventually reach the net percolation rate through any cover. The paper provides guidelines on the selective placement and encapsulation of reactive waste rock, with a benign or alkaline base layer, wide side encapsulation by benign or alkaline waste rock, the exclusion of reactive waste rock from beneath side slopes, and a benign or alkaline low net percolation top cover. Drawing on the approach of progressive covering of landfills as they are built, the paper recommends means of progressively covering each lift of the dump as it is constructed, to limit oxygen ingress and wetting up, and hence limit potential acid and metalliferous drainage