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Mining at the Mt Leyshon Gold Mine in semi-arid north Queensland stopped in 2002. Newmont Australia has recently initiated a thorough post-closure water management study of the site by revisiting the existing information and conducting new water-related investigations. The focus of this paper. which is the first publication on post-closure environmental management of the site. is an overview of the site water quality in view of the sources and spatial distribution of polluted mine water, and also the performance of cover systems in controlling water flux though mine wastes.

Since the mine closed the rehabilitation of waste rock dumps and tailings dams have significantly improved water quality in the surface receiving waters. However. the Scats Stockpile, the Heap Leach and Old Plant areas. which can potentially produce acidic waters, enriched with heavy metals and arsenic. are the top management priority. Modelling of the potential transport of contaminants in groundwater needs further investigations. A detailed examination of the waters in seepage sumps showed high concentrations of manganese, cobalt, nickel and selenium, which were previously overlooked. The study also showed distinctive chemical signatures for waters at different locations: the waters draining front scats, supergene materials and waste rocks have distinctly lower pH and higher concentrations of dissolved iron, sulfate, heavy metals, Mg, Mn, U. Se and As. While, the tailings sumps are more alkaline, and are enriched in Na and Cl. The southern tailing sump is characteristically As rich. The waters draining from the supergene materials have higher Cu concentrations.

Assessment of the cover monitoring systems showed that although covers pass the design criteria, automated recording of data may ignore any failure of the monitoring system. Recalibration of the moisture sensors not only showed realistic moisture values compared to rainfall data but also showed that the permanently installed system records values that are higher than those of the mobile system. Event-driven recording of data overlooks infiltration events. Besides, with a higher time resolution (daily measurements) the breakthroughs could be clearly detected. Modelling of the water balance in covers is underway and an example is given here. Newmont is currently reviewing water management options and is willing to share the lessons learned.