Coalmining under part of Sydney's water catchment has resulted in an important feeder pool being completely drained and has also triggered extensive iron contamination, water specialists said after visiting a river that flows into the Woronora Reservoir.
"Pool N" on the Waratah Rivulet, which supplies about half of the reservoir, was found to be completely drained during the visit on February 13 with water running into big cracks in the pool's 150 metre-long floor.
"To see all of that water was gone … was a big surprise," said Peter Turner, the National Parks Association-nominated member on the Metropolitan Colliery's community consultative committee. He blamed longwall mining in the area for the subsidence and fractures of the pool's floor.
Longwall mining involves giant machines that shear coal from the seam which can then cause the rock above to collapse behind the machine as it moves forward.
A spokeswoman for the Sydney Catchment Authority said it was aware of the impact to Pool N but that "there is no evidence to date to suggest that mining activity has affected storage levels in Woronora Reservoir in a substantial way".
A spokeswoman for Planning and Infrastructure said "strict conditions around water management were part of the consent conditions for this project", and Pool N was "upstream of where the specific environmental protections detailed in the consent begin".
Dr Turner said the location of the pool was overridden by the requirements that the mine's longwall expansion had only a "negligible reduction to the quality or quantity" of the water reaching Woronora. There was concern that authorities had not been able to identify where the water was going, nor if the impact was negligible, he said.
Metropolitan Coal's owner Peabody said it monitors water levels in pools on the Waratah Rivulet either daily or using a continuous sensor.
Pool N had stopped overflowing in early September 2012, and the company reported the reduction to the Sydney Catchment Authority on the 6th of that month as required as part of the Longwall 20-22 Water Management Plan, a spokeswoman said.
"Most recently, water ceased flowing over the Pool N rock bar in early January 2014 after a protracted period of less-than-average rainfall," she said.
The company is undertaking "an innovative process of stream remediation rock bar grouting as approved by the Department of Planning and Infrastructure," the spokeswoman said in a statement, without indicating if grouting work will be done to Pool N.
"As detailed in our approved 2011 and 2012 annual reviews there is negligible reduction in the quantity of water reaching the Woronora Reservoir," she said.
Conservationists have called for the water data to be made more accessible to the public.
The water experts also found that iron contamination had spread to at least two kilometres of the Waratah Rivulet, and was affecting the area's ecosystem.
The mine's 2012 annual report found that iron concentrations in the rivulet had doubled since the expansion of the mine, to levels which exceed Australian Drinking Water Guidelines by 30 er cent.
Without an independent study of the mine's impact, and where the water from Pool N was ending up, the operation was "going ahead in ignorance," Dr Turner said. At stake was the health of part of the water supply to Sydney's 4.6 million consumers.