The slow drying up of the five Thirlmere Lakes in the World Heritage Blue Mts National Park, possibly because their bedrock has been fractured by longwall mines nearby, has already been written up in the media, also filmed on ABC Channel 2 recently. To recap briefly, the lakes:
- are a repository of Aboriginal culture
- contain endangered or rare birds and flora
- are an estimated 15 million years old
- have been a much-loved tourist and recreational facility for generations
But though the drought here broke in 2008:
- the lakes have been drying up for some years and are now sadly depleted
- underground coal mines have been allowed within 700m – 900m from the lakes, 1998 – 2001
- Wollondilly Council and the Rivers SOS Alliance have called on Water Minister Phil Costa to investigate
- His NOW Department is investigating, although an independent inquiry is preferable
- meanwhile boating, kayaking, canoeing, fishing and swimming is no longer possible
- warning signs have been erected around the previous lake shores because of the deep mud now evident
While awaiting the results of the Departmental investigation, some experts are analysing the issue. At a Lithgow meeting of the Rivers SOS Alliance last weekend, Dr Brian Marshall, retired professor of structural and engineering geology, and a past president and current committee member of the Blue Mountains Conservation Society, discussed the drying up of the lakes.
The lakes are sourced from surface water and register the local water table. They should have recharged by now, as was the case after previous severe droughts, said Dr Marshall, but instead the water levels continue to drop.
He said that something other than drought is causing the loss: mining has possibly caused fracturing of the local aquiclude (rock barrier) beneath the lakes, facilitating hydraulic connectivity between the lakes and the deeper aquifers affected by mine impacts. Water may now be migrating through a fracture network (permeability paths) extending both vertically and laterally (vertilaterally) down to the level of the mine workings.
Over time the rate of flow along the fractures increases along with the vertilateral connectivity, while mine workings form a low pressure “sump.” The role of the low pressure “sump” is increased by the pumping out of water from the mine workings at a rate of 4 megalitres per day from Xstrata’s Tahmoor Colliery into the nearby Bargo River. This ongoing dewatering of the mines could be a significant factor in the water loss from the Thirlmere Lakes.
Dr Marshall concluded that any theory of loss would be hard to prove. Isotopic tracers could be used to gauge where the water is going, but given the slow rate of flow through the permeability network this could take several years to provide a result.
Also, remediation could be extremely difficult if mining-induced enhanced permeabiltiy is the cause of loss. A full or partial grout curtain to block flow could be considered, but this would be very expensive and perhaps not effective in the long run. The use of epoxy filler if specific cracks could be located is also very expensive and environmentally damaging.
Yet another possibility, said Dr Marshall, could be to pump the water, once suitably treated, to the lakes until mining is finished, the workings fill up with water, and a more stable hydrologic regime is established.
Keith Muir, director of the Colong Foundation for Wilderness, added that the Australian Committee of the International Union for the Conservation of Nature should be called on to undertake an examination of the lakes and then provide a commentary. This was agreed on by the meeting.
Brian Marshall: Phone: 47841148 email: briannamar@bigpond.com
Caroline Graham: Rivers SOS Vice President: phone: 46309421 email: rivers@bigppond.net.au
