The mention of acid mine drainage usually sets the environmental alarm bells ringing. But drainage from a mine in the Adelaide Hills may be offsetting another environmental problem - effluent from a nearby sewage treatment plant.
Scientists at Adelaide University and the Australian Nuclear Science and Technology Organisation have been investigating how well Dawesley Creek is holding up to acid drainage from Brukunga mine, 50 kilometres north-east of Adelaide.
They have also been studying the effect of effluent from the Bird in Hand sewage treatment works.
"Ironically, the acid mine drainage is effectively providing tertiary treatment for the sewage by stripping out the phosphorus nutrient," said Dr John Ferris, a radioecologist in ANSTOs Managing Mine Wastes Project.
But the team has recommended action to reduce or eliminate both mine and sewage pollution simultaneously after turning up evidence that the acid is harming aquatic life. They will announce their results on Wednesday, November 24, at the Third Brukunga Workshop, at the Australian Water Quality Centre at Bolivar, near Adelaide.
Adelaide Universitys Dr Peter Gell and Adam Sincock, and ANSTOs Dr Ferris have been using microscopic plants called diatoms as indicators of water quality. They found that drainage from tailings and waste rock at the iron pyrite mine was reducing the numbers of diatom species in the creek.
"Loss of diatom species diversity is a sign that other aquatic life will also be affected," said Dr Ferris. But if the mine drainage were stemmed without action to reduce the nutrient load from the sewage treatment plant, other problems would emerge. Diatom numbers would be likely to skyrocket, and other algae, including blue-green and green algae would probably thrive, producing floating green mats and brown slimes.
"Unless both problems are addressed together, we will be replacing one form of pollution with another."
Diatoms are single-celled algae up to 200 millionths of a metre long. These plants, which emerged in the age of the dinosaurs, have exquisitely-shaped glass shells that rank them among the most beautiful inhabitants of the microcosmos.
They are photosynthetic food producers, and this gives them an important role in freshwater and marine ecology. It also makes them sensitive indicators of the health of aquatic systems.
The team found only a few species of diatom surviving just downstream of the Brukunga mine. "We found more than 70 species in the wider area," Dr Ferris said.
"The area is quite diverse, with diatoms adapted to various environments - nutrient pollution, acid and metal pollution and, further downstream, pollution from dryland salinity.
"The only ones missing are the ones typical of unpolluted waters", he said. Interestingly, the scientists discovered what appeared to be two unknown diatom species in the most polluted section of Dawesley Creek.
"This is Darwinian natural selection in action," Dr Ferris said. "Only diatoms that have evolved to handle acid conditions survive here. Its a little ironic that fixing up Dawesley Creek could affect the local biodiversity by reducing the habitat available to these acid-tolerant algae," he said.
Meanwhile, the scientists have moved to allay fears that the mine and sewage pollution is lowering the water quality of other waterways in the Dawesley Creek system.
They said there was no evidence that the water quality of Mount Barker Creek, the Bremer River and winegrowing area, were affected at the time of sampling. But they did say more effort was needed to analyse the sediments of those waterways for heavy metal contaminants.