The design of a proposed tailings dam for a gold mine at Majors Creek east of Canberra, which has applied to use cyanide processing, is seriously flawed, a former chief research scientist with the CSIRO says.
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Dr Emmett O'Loughlin said Unity Mining's data, in an environmental assessment to support its application to modify the mine, underestimated rainfall and overestimated evaporation. Consequently, the tailings dam would have a higher probability of discharges of contaminated liquids into nearby creeks and ultimately the Deua River, the major water supply for 100,000 downstream users.
"The predicted behaviour of the tailings dam, based on that modelling, has no credibility," Dr O'Loughlin said.
Rainfall data used to calculate monthly inputs to the tailings dam, claimed by Unity Mining to be from Braidwood Weather Station, were incorrect and significantly different from rainfall statistics published by the Australian Bureau of Meteorology, Dr O'Loughlin said.
"The bias is further increased by ignoring the quality controlled rainfall data from the Majors Creek weather station, which shows that rainfall at the mine site is 30 per cent higher than Braidwood rainfall," he said.
"Results of the water balance calculated for the proponent on a month-by month basis over the life of the mine, using incorrect and biased data, suggest that spillage of contaminated water will not occur. This is incorrect and misleading."
Dr O'Loughlin's submission to the NSW Department of Planning and Environment follows another scientist's report that said using cyanide to process ore posed long- and short-term health risks.
Dr O'Loughlin said the miner's hydrologic analysis of the tailings dam failed to include an examination of how the tailings dam would have behaved in the past, when subjected to weather data recorded over the past century by the meteorology bureau. This was best practice and was not done, he said.
Because wrong data was used, Unity Mining's environmental assessment documents, to support its case for modifying the gold mine, gave incorrect projections of contaminated water entering Spring Creek, he said.
Consultant Knight Piesold's analysis of water balance based on historical rainfall and evaporation data for Braidwood were wrong. Bias in evaporation figures were grossly overstated in every month, Dr O'Loughlin said. The use of incorrect data rendered the consultant's report "total nonsense".
Dr O'Loughlin said Majors Creek data should have been used, instead of Braidwood's, because the former was only three kilometres from the dam site, whereas Braidwood was 12 kilometres away. Majors Creek rainfall was 945 millimetres, compared with 735 millimetres at Braidwood, a point also made in submissions by Roger Hosking, a volunteer observer for the bureau since 1986, and Majors Creek Catchment Guardians president Matt Darwon.
Asked why Majors Creek data was not used, a Knight Piesold spokeswoman said a key consideration on the design volume of the trailings dam was the 1000-year-magnitude storm event.
"This was determined using the Bureau of Meteorology's own process and its own data specific to the Dargues project area."
The spokeswoman said the tailings dam was designed to much more stringent conditions than required, by a factor of 10.
"It would normally classify as a High C facility, requiring a design storm capacity equivalent to a one-in-100-year annual exceedance probability 72-hour-magnitude event. However, Unity Mining asked that the TSF [dam] be designed as a High B facility with a one-in-1000-year annual exceedance probability 72-hour-magnitude event. As such, we don't believe that the design is total nonsense."
Unity Mining chief executive Andrew McIlwain said: "Knight Piesold is a world leader in the design of tailings storage facilities. They know what they're doing."
