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Several waste rock (WR) acid and metalliferous drainage (AMD) management options have been tested at Brukunga Mine site for their long-term ability to suppress or minimise acidity loads emanating from the WR. A 12–24 month field demonstration assessed alkaline caps (enhanced caustic magnesia, and natural limestone), and limestone blends in their long-term capacity to control acidity loads. The first 12 months of the trial suggest alkalinity producing covers may have significant potential as an effective and sustainable AMD minimisation strategy. Acidity load produced by the test pile (TP) capped with enhanced caustic magnesia was considerably decreased when compared with baseline. These results suggest alkaline covers allow inert neutralisation precipitates to coat preferential flow pathways, minimising the interface between water and acid salts. Blended limestone did not sustain long-term minimisation/suppression of acidity loads emanating from the WR, regardless of the ratio of limestone to WR or grain size. Even though limestone blended test piles showed the highest quality leachate measured throughout the trials, all test piles showed signs of impending leachate acidification during the latter half of the trial. In particular, the quality of leachate from all limestone blended test piles showed increasing acidity and sulphate concentrations, and a decreasing alkalinity and alkalinity to acidity ratio. Little residual alkalinity remained in the leachate from the limestone blended test piles after 12 months of the trial. The trend clearly shows, that in the long term, blended test piles are likely to produce leachate of quality comparable to untreated test piles. This trial illustrates the necessity for mine closure managers to be cautious in their application of short-term findings when planning for the longer term, in relation to management of contaminants such as AMD. These trial results signify that whilst testing of approaches is necessary, the longer the time frame the better the likely prediction of long-term outcomes. A hasty approach might well result in a failure of remediation, an inability to complete mine closure, and a hefty cost with little positive outcome.

Approaches to acid and metalliferous drainage management — implications of short-term trials for long-term remediation