Leading practice approaches to select post-mining land uses for residual mine voids
The Environmental Protection Act 1994 (Qld) (EP Act) requires that approval holders for mining activities (both new and existing) describe a plan to progressively rehabilitate land disturbed during mining to a ‘stable condition’. This means that the land is to be made safe and structurally stable, so that it does not cause environmental harm and that the land can sustain a post-mining land use (PMLU). Historically, open cut mining practices have left open pits in place at the end of mining.
In some instances, mine voids could be backfilled to create an area of land suitable for a PMLU such as grazing or natural ecosystems. When this is not possible, voids are left in place and typically become filled with water. Water filled voids can potentially be used for a range of post mining uses such as providing water for irrigation of crops or for stock drinking water, supporting the production of pumped storage hydropower, aquaculture or recreational areas. In practice, mine voids are usually left open and hold water with limited use without treatment.
There are some examples across Australia and internationally where mine voids have been repurposed, in many instances mine voids have limited practical uses and many are likely to remain perpetuity. Where a mine void cannot be rehabilitated to a stable condition, they may be legally allowed to remain in place without a use and be managed as a non-use management area (NUMA) under some circumstances. The vast majority of residual voids for existing mines are likely to be managed as NUMAs.
However, for both new and some existing mines, where a NUMA has not previously been approved, it is important to properly determine whether a suitable PMLU can be achieved for a residual void or not. This technical paper describes leading practice approaches to identify, assess and compare the feasibility of implementing options for PMLUs of voids. The methodology described here is to assess the viability and suitability of PMLUs for voids through the development and testing of strategic mine planning scenarios.
The information is then combined into a multi-criteria analysis that allows mine planning scenarios, as well as the consideration of associated risks and potential benefits, to be weighed up through the lens of multiple stakeholder perspectives. There can be several PMLU options for a void and they must be assessed and compared in a consistent and reliable way. Such a process requires the analysis of multiple mine planning scenarios, as well as the consideration of associated risks and potential benefits to stakeholders. Evaluating and comparing opportunities and constraints for post-mining uses for voids is linked to the mine planning process as this can influence where a residual void will be left and what form it will take.
Early planning to achieve a PMLU is likely to increase the number of possible post-mining uses available. Undertaking assessment and planning in an iterative way is likely to ensure that opportunities for rehabilitation of mine voids are realised where possible and that rehabilitation planning decisions made regarding void management are robust. Plans can also be used to assess the feasibility of backfilling or leaving a void open.
This technical paper proposes an integrated planning platform to assess the viability of filling the void with material or water, by analysing three specific strategic mine planning scenarios:
Maximise operational value, without specifying a preferred final land use for the void.
Deliver a material-filled void, which can support a range of land uses.
Deliver a water-filled void, which is non-polluting and can support a range of water uses.
The technical paper describes the architecture and elements of the integrated planning platform. One of its core objectives is to provide a wider range of financial results that can support a nuanced analysis of risks, benefits and values. As such, it considers the impact of variations in corporate assumptions, such as discount rate or commodity price outlooks, captured in scenario parameters. It also proposes a decision-making process that includes a qualitative valuation method for PMLU options and a multi-criteria analysis to compile the outputs from scenario analysis and PMLU valuation and present them in terms of several stakeholders’ perspective.
The approach can be applied to any stage of mining: pre-approval, operational, and prior to closure. The number of options and availability of data and information may vary along this timeframe and may lead to a re-consideration of PMLUs. In general, the earlier the PMLU options are included into the planning processes, the greater the potential for a positive overall outcome, and the lower the cost and risk of implementation. The approach will be iterative, but the number of iterations will depend on the scenario and on the number of PMLUs that are identified.
The scenario that considers a water-filled void is the most complex and will require detailed understanding of all connections between void design and water assessments, with inputs from multiple teams. For both new and existing mines, where a NUMA has not previously been approved, leading practice approaches support the identification and comparison of a range of PMLU options for voids. Such an assessment should determine:
• whether it is feasible to backfill the void or not, or
• if the void is not backfilled, whether it will hold a permanent water body.
Where voids are not backfilled, there is a need to assess the value of the PMLU and interaction with surface and groundwater. Water levels and water quality need to be quantified to assess whether the water quality can support a beneficial use into the future and whether there are risks of export of contaminants to the receiving environment (surface water and groundwater).
Void PMLU options are rarely incorporated into strategic mine planning. We propose that leading practice strategic mine planning includes consideration of final void PMLU options to explore the costs and benefits associated with PMLU scenarios. Costs should be assessed using a robust assessment that goes beyond Net Present Value to incorporate future risks, benefits and values.
Citation:
Côte, C.M., Holloway, E., Dunlop, J. and Chrystal, R. 2023. Leading practice approaches to select post-mining land uses for residual mine voids: Technical paper. Brisbane: Office of the Queensland Mine Rehabilitation Commissioner, Queensland Government.