Four tonnes of cyanide waste from a gold mine would accumulate long term in a tailings dam above a South Coast water catchment, and it was not a matter of if the tailings dam would leak but when, says an environmental scientist.
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Unity Mining's proposal to use cyanide to process ore at Majors Creek poses short and long-term health risks, says Dr Peter Beck, principal scientist for professional services consultancy GHD. The treatment of large volumes of waste is contrary to NSW Environmental Protection Authority policy, Dr Beck said.
Eurobodalla Shire Council commissioned GHD for an independent review of the gold miner's proposal to change its mining operation at Dargues Creek, near Majors Creek, within the upper catchment of the Deua River. The catchment provides drinking water for 40,000 permanent residents, and up to 100,000 people during the summer holidays.
Dr Beck said even though cyanide was cheap, effective and biodegradable, it was highly toxic, which has led to new methods elsewhere for extracting gold that are less toxic.
"Accidental releases of cyanide have occurred at all stages of its use at a number of sites around the world, even over recent years," the report says.
A large-scale cyanide release could potentially lead to large-scale extermination of aquatic fauna and potentially pose a health risk to residents, their livestock and pets and other beneficial land uses of land and water in the catchment.
After previously assuring people in the Braidwood, Majors Creek and Araluen districts it would not use cyanide, Unity Mining now wants to modify its operation, which would result in 180,000 tonnes of concentrated waste tailings that contain toxic substances.
Dr Beck says the commitment not to use cyanide helped resolve a previous legal challenge in the Land and Environment Court.
Assessing the likelihood of the tailing dam failing, GHD's scientist says based on Unity's estimate of a 1 in 1000 probability, the risk falls into the category "could occur".
"The consequence of such a failure could be that several thousand to tens of thousands of waste tailings are released into the catchment," Dr Beck said.
Modifying the mining operation proposes co-disposal of the flotation and concentrated waste tailings leading to interlayering and mixing of the two waste streams, says Dr Beck. "The consequence is that the co-disposal leads to a waste volume of 1,222,000 tonnes instead of 180,000. This is contrary to NSW EPA policy that dilution is not an acceptable solution to pollution," he said.
"Also the tailings storage facility in [the operation's modification] would not meet the requirements of a landfill that would contain similar type waste."
Other contaminants in the tailings dam would an 800 per cent increase in arsenic, cadmium, chromium, lead, 300 kilograms of mercury, and almost a 23,000 per cent increase in sulphur mass.
Unity Mining's chief executive Andrew McIlwain says: "I'm lost for words. The report misrepresents the facts to such a degree that it is laughable. This isn't science – this is scaremongering.
"For instance [Dr Beck] estimates the amount of various heavy metals in the tailings in a way which sounds alarmist. Three hundred kilos of mercury sounds terrible. What he omits is that it's 300 kilos in over 1.2 billion kilos of rock material. And many of those metals, like mercury, are trapped in the structure of the rock minerals," Mr McIlwain said.
Unity Mining's environmental consultant Mitchell Bland says the GHD report's language is sensationalist.
People in the Araluen Valley, east of Canberra, have also voiced their opposition to the mine.
