A VAST new coalmine planned for Sydney's south-western outskirts will damage the city's natural desalination plant - the ''hanging swamps'' that filter pure water down into the Georges River.
More than 50 swamps in the little-known Dharawal State Conservation Area, south-east of Campbelltown, will be undercut by longwall coalmines, which the mine owner, BHP Billiton, admits are likely to crack the bedrock and drain swamps. Aboriginal rock art above the mine site is also at risk.
The proposal, being considered by the NSW Government, calls for a huge expansion of existing coalmines near Appin, which would lock in mining there for 30 years.
Opposition to the plan is growing, and a coalition of local residents and environment groups and the National Parks Association are calling for mining to be excluded from the conservation area.
''It is literally underground and metaphorically under the public radar,'' said Sharyn Cullis of the Georges River Environmental Alliance. ''There should be widespread outrage or, at the very least, public debate about whether we really want the landscape desiccated … sacrificed for the sake of coal.''
The hanging swamps are shallow sandstone bowls, packed with matted sedge, native grasses and banksias that act like a sieve and a sponge, holding water in dry times and allowing it to seep out and feed some of the state's cleanest creeks.
''I would rather they mined under my own house than in the conservation area,'' said Julie Sheppard, of environment group Rivers SOS, whose home lies above another planned longwall panel near Appin.
O'Hares Creek, which flows through the conservation area and provides more than two-thirds of the water to the Georges River, is itself fed by the swamps. ''A total of 226 swamps have been identified within the entire Bulli seam project area, of which the Dharawal State Conservation Area is a part,'' a BHP Billiton spokeswoman said in a statement. ''There is some potential for impact but a monitoring and management plan has been developed.''
The company said its plans had been designed to minimise impact on larger rivers.
''Illawarra Coal has not mined directly beneath rivers since 2002, and consistent with this commitment, we have positioned longwalls away from major rivers and streams in the Bulli seam project.''
A detailed study by the staff at the NSW Department of Environment and Climate Change found the swamp network to be a ''priority fauna habitat'' for several endangered species, including the ground parrot and the giant burrowing frog.
The area also contains dozens of Aboriginal sites.
"Once you take the coal away, there's nothing to support the sandstone, and our artwork is cracking," said Alan Carriage, an elder of the Wadi Wadi people.
A June 2009 report produced by Biosis Research for BHP Billiton found that 11 Aboriginal rock art sites in the southern coalfields had already been damaged by subsidence from longwall mining. But the report concluded that "overall there is a low risk of significant impact to Aboriginal cultural heritage values''.
Before damaging a recognised Aboriginal site, a mining company must obtain a "permit to destroy" under the NSW heritage protection system. However Bev Manton, of the NSW Aboriginal Land Council, dismissed this process as a "regime to manage destruction".
She said five permits allowing damage to Aboriginal heritage sites are being issued a week by the State Government, and called for a new regime that gives more power to Aborigines to protect their significant sites.
Details about the BHP Billiton proposal will be aired at a public meeting on February 17.