For more than a century, copper and gold have been mined in the Mt Lyell area. Previous operators of the Mt Lyell mine discharged smelter slag and mill tailings into the river system until smelting stopped in 1970, and discharge of tailings to the river ceased in 1994. The current mine operator, Copper Mines of Tasmania (CMT), contain their mill tailings in a tailings dam in accordance with best practice.
Acid mine drainage is created by natural processes when the pyrite bedrock is exposed to oxygen and water. The last century of mining at Mt Lyell exposed massive areas of broken up pyrite-rich rock which will continue to react with oxygen and water for years to come, leaching acid mine drainage into the river system. The pyrite rock at Mt Lyell is different to other rock types (such as granite or sandstone) as it contains suplhides which react with water and oxygen to form sulphuric acid that leaches out the other metals. Drainage water from these historic mine practices contains high concentrations of copper, aluminium, iron, manganese and zinc. These reactions are very difficult to stop and will continue until either the oxygen, water or sulphides are exhausted.
Acid mine drainage due to historic mining practices continues to discharge into the river system, and previous studies have shown that this is by far the most significant source of ongoing environmental harm. Within the lease site, the North Lyell Tunnel has water derived in part from underground workings and has approximately 78% of the lease site copper loading. Discharge from other tunnels and waste rock dumps accounts for 21%. The remaining 1% is from smaller sources.
Copper Mines of Tasmania (CMT) is not responsible for the environmental damage caused by, or resulting from, previous operations, as outlined in the Copper Mines of Tasmania Act Pty Ltd (Agreement) Act 1999. However CMT has continued to be involved in the active management of acid drainage from the site, to the extent permitted by their current operations.
Unless remedial action is taken, acid mine drainage will continue to contaminate the river system and harbour for hundreds of years. It is important to realise that mine closure would not solve the problem because of the high rainfall and large volumes of sulphidic rock at the site. The only solution is to treat the acid drainage effluent from the Mt Lyell site by extracting metals or neutralising the acid.
Legacy waste rock dumps at Mt Lyell. When rainfall meets these rocks, acid drainage is created
Environmental impact of acid mine drainage on the King and Queen Rivers and Macquarie Harbour
The environmental impact of Mt Lyell’s acid drainage has been the subject of numerous investigations, theses, models and monitoring programs spanning decades. All investigations have consistently reported a biologically dead lower King River due to low pH and high metal (copper, aluminium) levels, and impacted ecosystems in Macquarie Harbour, due to high copper levels.
Further information is available from the Australian Government’s Department of the Environment website Final report: Mount Lyell Remediation Research and Demonstration Program.
Due to historic mining practices, metal rich acid mine drainage leaves the site and enters the Queen River
A large amount of work has been undertaken studying the problem and looking into a cost-effective remedy for the acid mine drainage.
The Mt Lyell Acid Drainage Remediation Act 2003 was passed to facilitate the treatment of the legacy acid mine drainage.
Water management works on the mine site to divert clean water from the acid producing areas and to eventually collect and store acid drainage for treatment have been completed.
Investigations completed in 2002 did not identify any cost-neutral options for treatment of acid drainage emitting from the Mt Lyell mining lease. Options investigated to treat the drainage ranged in cost from $10 million to $16 million in capital costs and from $1.6 million to $10 million in annual operating costs. Due to the high costs, full treatment was unlikely to proceed so a staged approach to the problem was adopted.
The Tasmanian Government signed a Memorandum of Understanding for the project with the Australian Government in December 2004, for the implementation of a copper extraction plant. However due to the technical complexities of developing the treatment system, the Memorandum of Understanding expired prior to the completion of the project.
In 2013, the Tasmanian Government advertised a request for proposals to develop pollution reduction operations at the site on a commercial footing. It was envisaged that the sale of extracted metals from mine effluent could form a viable, self-funding enterprise. A number of proposals were received, however ultimately none of them met the key criteria of being able to operate without government subsidies, within the constraints of the Mt Lyell Acid Drainage Remediation Act 2003. The request for proposals process concluded in 2014.