Evaluation of acid drainage risk from spent (leached) ore in arid climatic zones consists of two components: (1) whether residual and secondary minerals will release acid and metals when exposed to rainfall, and (2)whether ambient rainfall is likely to infltrate, and eventually impact surface or ground water resoures. The risk of acid drainage from spent ore at the El Abra on-off copper leach pad operation was evaluated using conventional gechemical methods and unsaturated flow models. Ahtough geochemical testing indicated poor quality water would result from any seepage, unsaturated flow modeling demonstrated that infiltration and seepage were unlikely given the arid climatic conditions. Modeling futurer demonstrated that the residual moisture content of the spent ore when it is removed from the leach pad controls the risk of seepage. To further reduce the risk of seepage, operational, controls were developed and implemented at the El Abra site.
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