A former WaterNSW hydrogeologist has found rock beneath Woronora Reservoir south of Sydney is cracking in "unrecognised" ways because of continued coal mining.
In work commissioned by the Nature Conservation Council (NCC), Peter Dupen reviewed the environmental performance of Metropolitan Mine at Helensburgh against its planning approval.
His report describes an "unrecognised" subsidence impact in the catchment.
"What I'm now saying is the style of the impact is new," Mr Dupen said.
"It wasn't predicted at the time the coal mine was being approved and it hasn't been really properly reported.
"It may be a very major change in the ability of these reservoirs to supply high-quality drinking water to greater Sydney."
Owned by Peabody, Metropolitan Mine extracts coal from the Bulli seam using underground longwall mining in and around the restricted Woronora special area, including under Woronora Reservoir.
In 2017, surface water impacts around the eastern tributary above the mine were reviewed as part of the Independent Expert Panel for Mining in the Catchment report.
It recommended "additional monitoring be undertaken towards the Woronora Reservoir and the installation of additional inclinometers in the upper sections of the overburden for detecting any significant shear plane movement extending towards the reservoir".
The panel report found it did not have "any firm view on whether the security of water storage in Woronora Reservoir could be unacceptably impacted by basal shear planes", but it found "the likelihood of unacceptable leakage into the mine is very low".
Basal shear plane formation is a term for a fracture in rocks at the bottom of a valley where steep terrain has been undermined.
Mr Dupen said his analysis found evidence of the new fractures based on a kilometre-long dry stretch of the eastern tributary.
"It has become clear to me that there is widespread opening of basal shear planes and, once they are open, they are almost certainly never going to close again," he said.
Impact on water quality
Mr Dupen said water that would have otherwise run down the sides of the ridges and along the creek was now being diverted into the rock vertically and moving into the centre of the valley.
"It means the rainfall that falls into the catchment is no longer running along the surface or soils," he said.
This meant the the water would not be filtered as effectively because it was taking a different course.
"That is what the whole idea of these special areas has been the vegetation cleanses and filters the water," Mr Dupen said.
He said the findings did not suggest a substantial reduction in water entering the reservoir.
But he said water quality could impacted and was "quite alarmed by it".
The NCC recommended that mining beneath the reservoir be stopped immediately until the implications were reviewed.
Released just before election
A spokesperson for Peabody pointed out the research paper had been commissioned and paid for by the NSW NCC and released in the final days of the state election campaign.
They said Mr Dupen's claims of environmental impacts had "been considered by government authorities and independent experts many times across many years".
"His theories have been investigated by two previous independent scientific panels, the NSW government's own Independent Expert Panel into Mining in the Catchment, and the Woronora Reservoir Impact Study," the spokesperson said.
"The six eminent scientists on the NSW Independent Expert Panel concluded that the operations of the Metropolitan Colliery were having a negligible impact on the quality and quantity of water reaching the Woronora Reservoir, two of the most important issues for local residents."
A Department of Planning and Environment spokesperson said as a result of a 2017 investigation into impacts of mining on the eastern tributary, the mine operators were ordered to prepare and implement a remediation plan.
The work is due to be complete by 2024.
"We have not received any reports regarding any sub-horizontal plane movements, and we encourage the Nature Conservation Council to share its analysis with us," the spokesperson said.
"If anyone has concerns on potential breaches regarding any state significant development, we urge them to contact us."
