A DEBATE has begun over the coal industry's reliance on "final voids" - large holes in the ground, often filled with saline water - that are left after open-cut mining is finished.
While final voids have been largely taken for granted over the years, Whitehaven Coal's plan for the Maules Creek mine in the environmentally sensitive Leard State Forest has drawn attention to the issue.
Reports on Maules Creek show Whitehaven expects to leave a 170 hectare lake on the site that will take 350 years to fill to a depth of about 100 metres and 1000 years to reach its full salinity at about a quarter the strength of ocean sea water.
An independent Planning Assessment Commission panel wanted Maules Creek to backfill its final open-cut rather than leave a void, but the NSW Department of Planning has rejected this, and the mine has been sent to another panel to be assessed again.
The department says backfilling the hole would waste 84 million tonnes of coal and cost between $388 million and $813 million
A 2005 report by the department on coal in the Upper Hunter noted that final voids were likely to cover about 1270 hectares of the Hunter Valley, and that all would become "saline water sinks" unless they were "configured for an alternative post-mining land use"; in other words, filled in.
The report raised "a number of general concerns" about final voids and groundwater, including the chance for "super saline void waters" to enter the environment.
The coal industry defends its practices, but environmentalists are increasingly concerned about the potential impacts.
Monash University engineering researcher Dr Gavin Mudd said various elements including arsenic and selenium could be leached from coal tailings and overburden into final void dams, creating serious concerns over water quality.
Environmentally, it was inappropriate for a salt-water lake to be created in a freshwater environment like the Namoi.
Hunter Valley Coal Report editor Colin Randall said surface mining had traditionally left final voids unless the land was so valuable as to make "restoration, rather than rehabilitation", worthwhile. Mr Randall said there were no big final void lakes in the valley that he knew of because the open-cuts had not finished their operations.
NSW Minerals Council chief executive Stephen Galilee said "the strict rehabilitation practices required by government form an important part of the responsible growth of mining in the region".
"Final voids, if deemed necessary, can vary from mine site to mine site and it is industry best practice that these voids are left in a safe, stable and non-polluting state," Mr Galilee said. "The nature and size of final voids are assessed on a case-by-case basis taking local geographical and other factors into account."
The final void debate began after the Planning Assessment Commission said it was satisfied with Maules Creek, but wanted a mine closure plan that did not "generate a pit lake". As well as rejecting it on a costs basis, the department said Maules Creek would probably extend beyond 21 years, meaning the location of any void would change.