The previous issue of STEP Matters 185 described the risks to Sydney’s water catchment in the Illawarra region from longwall mining. Research is now pointing to mining also being a culprit in the drying out of Thirlmere Lakes. Thirlmere Lakes National Park is part of the Blue Mountains World Heritage Area. The two largest of the five lakes are currently empty and the other three are depleted.

The report, published by the UNSW Centre for Ecosystem Science, found that the dropping of water levels coincided with the onset of longwall coal mining in the mid-1980s, with the lakes having continued to dry out since then. Longwall coal mining is thought to have fractured the aquicludes that constrain the groundwater aquifers, causing diversion of groundwater resources. Another factor could also be considerable pumping of groundwater.

The report's findings contradict a 2013 NSW Government Commission of Inquiry which stated that:

… changes in the water levels in Thirlmere Lakes over the past 40 years are due to climatic variations such as droughts and floods.

The recent heavy rain that has led to nearby Warragamba Dam almost overflowing has had very little impact on the water levels in the lakes.

Commenting of the report, UNSW wetland ecologist Prof Richard Kingsford said gum trees were encroaching on the dry lakes. ‘What we are seeing here in terms of the vegetation is the slow death of a wetland,’ he said.

The Thirlmere Lakes are the 15-million-year-old remnants of a river whose original flow direction was stalled and gently reversed by mild tectonic tilting, and according to Prof Kingsford, hold a rich geology. The implications are significant for the ecological character of the Thirlmere Lakes.

There are many affected obligate aquatic species or species reliant on wet habitats, including:

• five species of waterbirds (Australasian Bittern Botaurus poiciloptilus, Australian Painted Snipe Rostratula australis, Great Egret Ardea alba, Cattle Egret Ardea ibis and Latham’s or Japanese Snipe Gallinago hardwickii)
• one fish species (Macquarie perch Macquaria australasica)
• two frog species (giant burrowing frog Heleioporus australiacus, Littlejohn’s tree frog Litoria littlejohni)
• three plant species (smooth bush pea Pultenaea glabra, Kangaloon sun orchid Thelymitra Kangaloon and the aquatic water shield Brasenia schreberi) that are listed as threatened

Interestingly, Thirlmere also carries the freshwater sponge Radiospongilla sceptroides.

The report has serious implications for governments and their responsibilities for managing the values of Thirlmere Lakes National Park along with the Blue Mountains World Heritage Area. These major changes in flooding regimes to Thirlmere Lakes will continue to degrade the Thirlmere Lakes National Park and its ecological, cultural and recreational values. The lakes were once popular with swimmers and canoeists.

Mine owner, Glencore announced in June that operations will be reduced and the mine will be closed in 2019 as a result of low coal prices. The local Wollondilly Council and Friends of Thirlmere Lakes are calling for action to restore the Lakes. Council has suggested that artificial injection of fresh water should be considered. It is a theory supported by groundwater expert Dr Philip Pells, who said recovery of the water table may otherwise take decades.

‘It's not beyond the realm of possibility to pump water up from Warragamba Dam, cycle it back through here, and it goes back down to Warragamba Dam,’ Dr Pells said.