A pit lake larger than Sydney Harbour: Hazelwood owners plan for coal mine site

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At 640 gigalitres, the Hazelwood coal mine’s void would hold more water than Sydney Harbour. The entire site, encompassing the decommissioned brown-coal-fuelled power station, in Victoria’s east, is bigger than Melbourne’s CBD.

Standing at edge of the streaked, coal-rich slopes that surround the vast pit, the enormity of the rehabilitation task that confronts Hazelwood’s owners, French energy giant Engie, is evident.

The owners of Hazelwood want to turn the former site into a giant pit lake. Credit: Justin McManus

Engie wants to turn the former open-cut mine into an artificial “pit lake” between 70 and 130 metres deep, which could take as long as two decades to fill and require almost double the amount of water Melbourne consumes in a year.

After environmental advocates called for greater scrutiny, the federal government in February confirmed it would scrutinise the proposal under the Environment Protection and Biodiversity Conservation Act, a process Engie says it welcomes.

The company argues a pit lake is the best way to provide geological stability to the mine void, saying it would end the need to constantly pump groundwater, minimise the risk of fire in the coal seam and provide an asset for the local community. (There is currently some groundwater in the pit, which is below the water table).

But environment groups have urged caution, saying it could put huge strain on the local Morwell river system, worsened by a warming climate and diminishing water resources.

Engie’s David Moran says a pit lake is the best way to ensure geological stability of the mine void. Credit: Justin McManus

David Moran, Engie Australia’s planning and approvals manager, says the oldest section of the mine closest to the town of Morwell has already been extensively rehabilitated, with the “batters”, or mine walls, graded and covered in grassed hills. As well as being more aesthetically pleasing, these batters maintain the mine’s geological stability and control erosion.

The next rehabilitative step is groundwater control. Brown coal, which is sedimentary rock formed from peat, is notoriously polluting because of its high moisture content, and all the Hazelwood deposit was below the water table.

This groundwater places destabilising pressure on the floor of the mine pit, which can only be mitigated by constant groundwater pumping.

“The best way to do this [rehabilitation] from a safe, stable and sustainable perspective is to apply [lake] water, which counteracts the pressure in the groundwater, keeps the batters stable, and covers the coal,” says Moran.

Environmentalists have warned that plans to flood the former Hazelwood coal mine to create a pit lake risk water security and possible contamination. Credit: Justin McManus

Globally, this approach has been widely used where brown coal deposits are below the water table, he says.

Engie has existing water contracts with Gippsland water it would use to fill the pit lake, as well as applying for additional surface waters, from the Morwell River. The company’s existing contracts have clauses that reduce and even limit water use in times of drought, says Moran.

“We want to tweak them so that they are representative of what we need, but also representative of climatic conditions,” he says.

Environmentalists are concerned about potential contaminants from a large coal-ash dump at one end of the pit. Engie proposes to cap this, using earth or concrete, and it would be inundated.

Internal advice recently provided to the federal Department of Climate Change, Energy, Environment and Water recently said there was a risk of creating a large pit lake with deteriorating water quality that would be unsuitable for swimming for coming decades.

If approved, the proposed pit lake would cover the former coal mine site. Credit: Justin McManus

Other risks include “serious or irreversible environmental damage” to the downstream Ramsar-listed Gippsland Lakes and insufficient flows to meet the huge volumes of water required to fill the lake, it found.

Environmental Justice Australia lawyer Chloe Badcock had said alternatives to diverting rivers – such as using recycled and desalinated water – should be considered.

“This half-baked rehabilitation plan will drain water resources, create a filthy, toxic pit lake, and choke the water supply to the Gippsland Lakes,” she said.

Moran disagrees that water contamination poses a substantial risk, saying independent auditors would be required to ensure the site, from the mine void to ash landfills, are legally compliant.

The proposal is currently being assessed for its potential environmental impacts.

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