Final voids, the large holes left in the ground once open cut mining ceases, is now a significant issue to be faced by our community.

In the US they passed a federal law in 1977 called the Surface Mining Control Reclamation Act (SMCRA), which in simple terms says surface mines (their version of open cut mines) must return the land to its pre-mining landform.

In a word, no final voids.

While we have been worrying about the problems associated with open cut mining operations most notably air and noise pollution we have almost forgotten about what happens to the land when mining ceases.

But the stark reality of final voids was brought to our attention thanks to the recent Planning and Assessment Commission (PAC) report on the Warkworth and Mt Thorley (MTW) mine expansions.

In their report published in March, it states the commission does not accept that a mining legacy of large voids across the Hunter Valley is acceptable and considers that as a matter of priority a study should be undertaken by the Department (Planning and Environment) to review the cumulative impact of voids in the Hunter valley.

The Department of Planning told PAC there were approximately 30 voids currently approved with the one planned for the expanded WTW project covering 950 hectares and 300m in depth one of the largest. 

PAC estimated that this particular void would be equivalent to one-sixth of the size of Sydney Harbour.

In its response to the PAC report, the department says the NSW Government is considering a range of matters to strengthen the regulation of mining operations in NSW, including the nature and scale of final voids associated with open cut mines in the Hunter Valley.

One could argue this action is long overdue given overseas experiences and the fact that mine closures will become more of an issue as pits reach their end of life or pits are closed because they become unprofitable.

The US SMCRA legislation stated clearly more than 30 years ago that appropriate and necessary means to minimise so far as practicable the adverse social, economic and environmental effects of such (surface) mining operations.

Part of the act was to encourage underground methods but where surface methods were used the environment must be protected.

In their Environmental Protection Performance requirements, it states mining companies must restore the land affected to a condition of supporting uses of which it was capable of supporting prior to any mining or higher or better uses.

“With respect to all surface coal mining operations backfill, compact and grade in order to restore the approximate original contour of the land with all highwalls, spoil piles and depressions eliminated.”

For the US, the final void must be as small as possible.

The act also has very strict conditions on reclamation control for prime farm lands.

Searching for similar rules and regulations in NSW drew a blank and research on the subject for our valley was to say the least scant if non-existent.

Finding anyone willing to comment on the issue in the Hunter Valley also proved extremely difficult.

During the recent Hunter Valley Coal Festival a discussion panel was held in Singleton where some suggestions were made about what one could do with a final void. Some of suggestions included theme parks, rubbish dumps and aquaculture.

But given the depth and size of the voids (see table) it would appear that any future use is thwart with headaches.

Mine Max depth and size (hectare)

Warkworth Continuation 2014 310m                950

Mount Pleasant Project 160m                          510

Mt Arthur Mine 180m                                      1,420

Bengalla Continuation Project 240m                 324

Bulga Coal Mine 350m                                      500

Hail Mine Creek (QLD) 210m                          1,540

Wandoan Project (QLD) NA                             1,450

Caval Ridge Mine (QLD) 220m                        2,000

(Source: Warkworth Continuation 2014: Response to Planning Assessment Commission review report Appendix B.)

John Richards, managing director, Bloomfield Group (operators of two open cut mines) speaking at the Tom Farrell Institute Mine Rehabilitation Conference also held in Singleton during the festival, talked about the Hunter’s void problems due to their high salinity.

The Permian layers from which the coal is extracted are highly saline and therefore so will be the voids and the water they contain.

The proposed MTW void, which like all others is completely legal, is estimated to take 1000 years to fill. 

Or it will cost $2 billion, according to the company and the department, to backfill.

In its report approving the MWT expansion plans, the department says it accepts the relatively modest agricultural value of the land does not warrant the costs and impacts associated with filling the void and that options to fill the final void are not reasonable.

Then the question must be asked: what does the department and government propose will do with this void?

Finding innovative ways to utilise final voids is big business overseas with one project in Germany expected to cost $22billion to rehabilitate the site of former open cut coal mines.

In Canada it is estimated the liabilities for abandoned mine sites at over $C555million and in Australia it is estimated that there are 50,000 abandoned mines. Some of these are small shafts but others are large pollution mines. *

With the rapid expansion of open cut mining in Australia in recent years laws governing the post-mining environment were almost put on the ‘back burner’ as governments and regulators concentrated their efforts on issues relating to the impacts of the actual mine’s operations.

Now we are starting to think about what happens when large open cut mines close and can we learn anything from overseas.

Discussion on the use of final voids range from rubbish dumps to tourist attractions like the Eden Project in Cornwall UK, developing aquaculture industries and turning them into water storages. 

Rehabilitation work overseas has shown if it is undertaken with innovation and skill the void can ultimately be turned into an asset.

However in most of the overseas success stories the  voids are unlike the one we are now creating  were not as big and nor as deep and we have far less water available.

One of the pinup rehabilitation projects is to be found in the former East Germany in the Saxony and Brandenburg region about 150km south of Berlin.

The area in the late 1980s was considered the world’s biggest brown coal producer with an output of 300million tonnes/annum.

But mining ceased in 1990s leaving ‘a lunar landscape’ of abandoned mines. 

Rehabilitation work was begun most notably involving the LMBV company and included diverting rivers and flooding old mines creating 24 artificial lakes covering more than 14,000ha some of which are linked by canals.

The work has been described in newspaper reports as a long and difficult process and not without hiccups including the River Spree turning rust-orange a result of the increased iron hydroxide dislodged from soils by years of mining.

As far as the Hunter Valley is concerned all this work in Germany according to the literature took place in mines with voids at depths of only 60-70m.

The proposed void at Mount Thorley Warkworth mine will be 300m which poses an altogether different set of challenges and we can hardly divert our waterways to fill these voids.

In Germany the large scale long term rehabilitation by the state is unique in Europe.

And the vast sums of money involved achieving a reasonable outcome – these are the questions we must be putting to the Department of Planning and Environment and probably our federal government.

*'A jurisdictional maturity model for risk, management, accountability and continual improvement of abandoned mine remediation programs', Resources Policy June, 27, 2014.