The derelict Lake George Mine is located at Captains Flat in NSW, a community of approximately 450, which lies some 43 km southeast of Queanbeyan. It straddles the Molonglo River, which flows to Queanbeyan and thence the nation's capital, Canberra. Catastrophic tailings dam failures in 1939 and again in 1943 resulted in the transport and dispersion of thousands of tonnes of sulfidic wastes downstream, with terminal settling in Lake Burley Griffin, in Canberra. The mine was closed in 1962, and major rehabilitation in 1996 - 1997 addressed the risk of further collapses, and reduced water flow into the underground mine._x000D_
Nevertheless, acid mine drainage from the mine, and seepages of contaminated water from the capped tailings dumps continue to pollute the Molonglo River for some 30 km downstream._x000D_
Environmentally, the highest contributor to the total contaminant load of the river is the Main Adit Spring. This was shown to be a constant groundwater-driven acidic flow, with contaminant generation resulting almost entirely from the ingress of gaseous oxygen into the underground workings. Long term remediation may be possible at source, by pumping reducing gases into the sealed mine, but the probability of this technology succeeding to the required degree is yet to be demonstrated._x000D_
For at least the medium term, treatment of contaminated water flow(s) will be required to achieve any of the water quality targets that may be applicable to the river. Three methodologies were short listed, but a number of strategic uncertainties remain, which the NSW Derelict Mines Program will address. Future risk arising from long-term contaminant release by the treatment wastes must be considered. Appropriate data, and realistic long-term performance parameters regarding the relative stabilities of water treatment sludges are scarce, and make proper evaluation of competing technologies problematic.