Acid mine drainage results from the oxidation of sulphide-bearing rocks (e.g. rocks with the common sulphide minerals pyrite, and pyrrhotite). It is recognised as one of the major sources of heavy metal pollution in many waterways near metal-mining sites in Tasmania. The Disturbance of Acid Sulphate Soils is a related environmental issue in Tasmania and is discussed in a separate issue report.
There are over 4,000 recorded mineral activity sites in Tasmania and of these about 681 sites are classified as metal-related abandoned mines of various categories. Most of the large metaliferous mines in Tasmania are on the west coast, but there are many abandoned mine sites elsewhere in the State (e.g. north-east). The mine workings and ore treatment are both significant sources of contaminants. Mining operations can expose large quantities of sulphidic rocks, and produce mine waste rocks and tailings rich in sulphide minerals. The percolation of oxygenated water through the workings and waste rock dumps generates an acidic fluid, rich in metals leached from the minerals in the rocks. Both the low pH and metals in this liquid can cause severe ecological damage when it enters streams and groundwater.
Ore is usually treated on site to produce a concentrate. Processing also generates wastewater, polluted mainly with heavy metals and suspended solids. Most mines treat the wastewater to reduce the pollutant load by using tailings dams or reducing the acidity of the wastewater by treating it with lime.
This 'At a glance' section provides an overview of the issue of acid mine drainage in Tasmania. More detailed information and references are contained in the Tasmanian Acid Drainage Reconnaissance report on acid drainage from abandoned mines in Tasmania (Gurung 2001), which is available through the Mineral Resources Tasmania web site. A brief summary of the report's findings is provided here, as well as some of the management responses to this issue since the previous SoE Report. A recommendation is also provided on Acid Mine Drainage.
- A reconnaissance investigation was conducted by Mineral Resources Tasmania and DPIWE in 2000-01 (Gurung 2001). It provided a basis for 'an inventory of sources of acid drainage from historic mine workings', which is required under Section 37.5 of the Tasmanian State Policy on Water Quality Management (1997). The development of remedial programs to address priority problem areas is also required under this policy. The program is a component of the State water quality and quantity programs and has been largely funded by the National Heritage Trust.
- Various environmental management plans, environmental improvement plans and environmental decommissioning and rehabilitation plans are in place for many of the mines investigated in the reconaissance survey.
- The Rehabilitation of Mining Lands Trust Fund was established in 1997 following the proclamation of the Mineral Resources Development Act 1995. The target of this trust fund is the rehabilitation of abandoned mining lands for which no person or organisation can now be held responsible. A portion of mining royalties raised by the Act is allocated for this purpose. Through the trust fund various projects have been supported including the following.
- As part of the rehabilitation of abandoned tin mines around Gladstone and Rossarden in the north-east of the State, remedial works were conducted at the Endurance mine site to start to address the acid drainage problem into the Ringarooma River system and to stabilise gully erosion (MRT 1998). Drainage diversions at the Rossarden mine site were also carried out (MRT 2000).
- Remedial work has been carried out at the Storys Creek mine site to ameliorate the effects of acid mine drainage and metal contamination of Storys Creek and the South Esk River (MRT 2000 and 2001).
- Support was provided to the Acid Drainage Reconnaissance investigation (MRT 2000 and 2001).
- Major mining operations in Tasmania that are handling waste in line with best practice in treating potential acid drainage include ABM Savage River Mines, Renison, Pasminco Rosebery, and Mt Lyell.
- ABM, Savage River Mines is the largest waste handling operation. Alkaline rock flow-through structures are in place at the mine, neutralising water and lowering the copper levels. Acid generating rock is separated and sealed in cells within waste rock dumps to minimise the potential for acid drainage. Tailings handling and storage is under water cover again to minimise acid drainage formation.
- At Renison, tailings handling is acknowledged in the Best Practice Environmental Management in Mining series. Tailings are layered in the storage facility to cover acid generating phyrotite with non-acid forming material. Stockpiles of acid generating waste have been taken underground for use in stope fill.
- At Pasminco Rosebery, tailings are flooded to minimise acid generation. A prize-winning sewerage co-treatment facility uses the tailings dam to reduce metal emissions and organic discharges to the Stitt River. Waste rock is again stored in cells.
- Successful wetlands have been developed at Hellyer and Beaconsfield to minimise discharges of metals into downstream waterways.
- A report on the Feasibility of Remediating Mt Lyell Acid Mine Drainage was produced in 2001 detailing three main options and the costings, including full neutralisation, partial neutralisation, and copper recovery through solvent extraction. Public consultation was invited between December 2001 and February 2002, and the most supported option was for full neutralisation.
- The Clean up the King River Project was established, funded by the Natural Heritage Trust, with one of its key aims being to improve the ecological health of the lower King River and Macquarie Harbour through treatment of the acid drainage associated with the Mount Lyell mine site. The project released a report in 2001 concluding that it was not possible using current technologies to use a self-funding extraction plant for the removal of copper and for treatment of the acid mine drainage. The project committee is currently investigating a staged approach to addressing the acid mine drainage problem (Environment Australia 2003).
- The Mt Lyell Acid Drainage Reduction Act 2003 has been passed to facilitate a reduction of acid drainage pollution at Mt Lyell and for related purposes.
- The Mt Bischoff Rehabilitation Program is being developed by Mineral Resources Tasmania and funded by RiverWorks (a Natural Heritage Trust program) to conduct site improvement and ameliorate the effects of acid mine drainage on the Arthur and Waratah Rivers, which has emanated from the former Mt Bischoff tin mining operations at Waratah.
- The Savage River Rehabilitation Project (SRRP) has been established, including a Strategic Plan (2001) for long term remediation of environmental harm resulting from pre-1997 operations at the Savage River and Port Latta sites in the north-east of the State. The principal cause of the degradation is thought to be acid drainage emanating from approximately 200 million tonnes of waste rock deposited in dumps around the site.
- In an Australia-wide survey of acid drainage at operating mine sites in 1997, Harries (1997) identified seven mines in Tasmania (Beaconsfield, Hellyer, Henty, Mt Lyell, Renison, Rosebery and Savage River) as containing potentially acid generating wastes. The data related only to major deposits that contained significant amounts of pyrite and pyrrhotite and did not include abandoned mines with low sulphide but high acid generating potential.
- The Tasmanian reconnaissance investigation (Gurung 2001) found that about 215 of the 681 metal-related abandoned mines across the State have a history of base-metal mining and contain sulphidic rock materials that are either currently producing acid drainage or have the potential to do so if exposed to oxidising conditions. A majority of these sites also fall within current mining leaseholds. Consequently, several are indicating pollution problems from acid drainage and heavy metal contamination.
- Surface waters impacted by abandoned mine sites typically show high sulphate and metal distribution at a low pH range of 2.0 to 5.0. The impacted waters generally show metal distribution in the high acid/extreme metal to low acid/low metal range. Statewide distribution maps of surface waters with low pH-high sulphate and low pH-high metal in catchments impacted by abandoned mines are available through the Mineral Resources Tasmania web site.
- Surface waters impacted by acid producing abandoned mines generally contain metal pollutants about 10 times higher than the ANZECC (1992) values for aquatic environments.
- Geochemical results indicate that occurrences of high sulphate and high metal waters are closely associated with abandoned mine sites containing sulphidic rock materials with high net acid producing potential (NAPP). Many of the volcanic rocks hosting base-metal abandoned mines in the West Coast mineral fields generally have a high NAPP of >500 kg H2SO4/t and low to negligible acid neutralising capacity. Carbonate-hosted base-metal abandoned mine sites were found to be generally high acid producers, mainly because of the presence of high NAPP ore rocks and restricted availability of acid neutralisation from the host rock. A geochemical distribution map of potential acid forming rocks hosting abandoned mines in Tasmania is available through the Mineral Resources Tasmania website.
- Waste rocks and tailings materials at many of the abandoned mine sites also commonly contain unusually high concentrations of trace metals (arsenic, copper, lead, zinc, tin and tungsten). Similar high levels of trace metals reflected in surface waters and stream sediments near mine sites suggest that there is a significant release of these metals into the environment.
- The extent of the impact of acid drainage and heavy metal contamination on the environment is difficult to assess from the currently available information. There is lack of detailed hydrogeochemical characterisation, as well as baseline data on mass loadings and environmental parameters. The lack of sulphur analyses limits the geochemical characterisation of acid-forming rocks from abandoned mine sites. A hydrochemical assessment of acid drainage requires analysis of specific parameters not always measured in routine water quality analyses carried out for environmental monitoring of aquatic ecosystems.
- The inventory, established by the reconnaissance investigation (Gurung 2001), is not complete and its reliability is largely dependent on the accuracy of the historical data used for geochemical classifications of rock types at or close to abandoned mine sites.