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The following updates the previous case study included in the last SoE Report (SDAC 1997).
The King River is arguably Australia's most polluted river. Mining started in the 1880s, with the Queen River, a major tributary of the King River, being used for waste water disposal from the Mt Lyell copper mine. Between 1922 and 1995 low grade ore was concentrated on site and the tailings (ore-washing residue) dumped in the river also.
About 1.5 million tonnes of sulfidic tailings entered the river system each year up to 1995, along with huge volumes of acidic, metal-rich water flowing from the workings. This 'acid mine drainage' is derived from water leaching through the exposed and oxidised sulfide rocks. When it was in operation, the fumes from the ore smelter produced acid rain which also leached minerals from the bare Queenstown hills.
In 1992 the King River was dammed above the confluence with the Queen River to generate hydroelectric power. This changed the flow regime in the King River, and affected the way tailings were transported through the river system. The tailings in the river greatly affect the water quality. Being alkaline they neutralise some of the acid water draining from the mines; they also provide surfaces to which the metals adhere, thus greatly reducing their effect on the biota.
About 100 million tonnes of tailings have been deposited on the banks and bed of the King River and in a delta at the mouth of the river where it enters Macquarie Harbour.
Since the closure of the mine in late 1995, and the construction of a tailings dam by the new operators, tailings no longer enter the river system. However, acid water continues to enter the river due to mine dewatering and run-off from the waste rock dumps. Without the buffering previously provided by the alkaline tailings, the acidity in the Queen and King rivers has increased, and dissolved metal concentrations have greatly increased-to levels highly toxic to aquatic life.
In 1993, a three-year study was begun under the guidance of Environment Tasmania to investigate water quality and sediment movement in the King River and Macquarie Harbour (Koehnken 1996). The aim was to document the environmental impact the mining operation has had on the King River and Macquarie Harbour, propose remediation strategies, and to investigate the relationship between power station operation and downstream water quality. The results of the hydrological studies indicate the dissolved metal concentrations fluctuate widely with power station operation. When the power station is not operating, the acid Queen River water dominates the chemistry of the lower King River, and dissolved copper concentrations of up to 10 mg/L have been detected (1000 times the Australian and New Zealand Environment and Conservation Council guideline). When the power station is operating, there is a much greater flow of water and dissolved copper concentrations are typically diluted to less than 1 mg/L.
The study found that each time the power station started up, a wave of sediment-rich water, with suspended matter concentrations up to 10 000 mg/L, moves down the river (Locher 1995). Associated with this sediment movement is a pulse of high concentration of dissolved metals. Except for this initial pulse, the concentrations remain fairly constant (0.3-0.6 mg/L) due to the buffering the tailings provide regardless of flow. The water quality monitoring and sediment movement investigations are being used to determine the long-term environmental impacts of ongoing pollution in the King River, and will form the basis for future management decisions in the region.
A toxicological evaluation of the acid mine drainage wastewater was conducted in 2000 to assist in determining treatment options at Mt Lyell. The primary aim is to partially or completely rehabilitate the ecosystems in the lower King River (down stream of the junction with the Queen River) and the upper Macquarie Harbour. The principal findings of the trials conducted indicated that treatment of acid drainage to pH 6.3 and/or 7.5 is likely to lead to limited or high levels of recovery of macroinvertebrate communities in the lower King River and the potential recovery of fish populations in the lower King River provided other factors-such as sediment toxicity and undesirable variation in flows from power station operation-don't impede recovery. Also such acid drainage treatment is likely to result in partial to complete recovery of native fish recruitment in tributaries of the lower King River and is highly unlikely to cause mortality or growth inhibition of rainbow trout in the sea cages in north-western Macquarie Harbour. Initial estimates of threshold concentrations for copper, aluminium and sulphate below which significant toxicological impacts in the lower King River and Macquarie Harbour are unlikely are 35mg/L, 350mg/L and 40 mg/L (Davies et al 2000). Further investigations are required of the toxicity of the sediments to evaluate the likelihood of macroinvertebrates and fish colonisation under improved water quality conditions. Treatment options are being further investigated in the light of current advances in technologies.
King River update provided by Greg Dowson, Environment Division, DPIWE.
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Last Modified: 14 Dec 2006
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