Closure guidance has increasingly recognised the risks and opportunities surrounding pit lakes, and these should be considered in closure planning
Pit lakes have long been a legacy of large-scale open cut mining. However, pit lake closure plans have historically been developed (if at all) very late in the life of the mine. At this late stage, remediation and reuse options are more limited. This lack of early and thorough consideration in closure planning has resulted in thousands of abandoned and unrehabilitated pit lakes globally, either filling or already filled.
As typical of most closure planning, risks related to pit lakes are primarily considered in the context of long-term post-mining use. Long-term vision is needed when considering the complexity of the issue and the changing relevance of aspects of pit lakes over time. Nevertheless, there can also be a number of short-term risks.
Pit lake risks typically include general compliance definitions for ‘safe’ and ‘non-polluting’, as well as other definitions of how pit lakes as final landforms must meet general mine closure objectives. However, an increasing number of jurisdictions now include specific compliance requirements for pit lakes (Figure 1).
As with any large final mining landform, unrehabilitated (or poorly rehabilitated) pit lakes can present a number of hazards as follows (McCullough and Lund, 2006).
Safety may be a concern to both human and animal end- users around the pit lake. Safety is typically addressed through rehabilitation or access to void walls, details of which are often omitted in closure planning. Steep pit sides above and below the water present fall and drowning risks and these risks should be addressed to achieve successful pit lake relinquishment. Commitments to fit perimeter fencing to prevent human access may be unreasonable for pit lakes with large perimeters. Equally, the maintenance of such a fence may impose an unreasonable or unworkable burden on the following land owner.
Stability of final pit lake walls may be considered using geotechnical techniques, but also must consider the influence of rebounding groundwater pressures (including seasonal or other fluctuations) and adjacent mining landforms – in particular, waste rock dumps.
Erosion is a concern for pit lakes that have constant active surface water dynamics. Shoreline erosion is an increasingly recognised risk, especially when roads or other community infrastructure is near the pit edge. Excessive erosion can also lead to further problems regarding safety, pollution and sustainability closure objectives. A starting place for considering shoreline erosion can be through hydrodynamic sediment transport modelling of proposed shoreline batters.
Pollution and water quality
In terms of mine closure, ‘non-polluting’ refers to the risk of pit lake water causing environmental harm. Typically, low pH and elevated salinity and metal concentrations are the most relevant concerns regarding water quality. Long-term concentrations can be generally predicted through modelling pit lake water balances and water quality. However, applying generic water quality criteria against contaminant concentrations to determine closure success may be inappropriate for an artificial pit lake environment, and may equally not be relevant to the lake’s environmental context. Furthermore, water column stratification and other complex lake dynamics such as surface water inflows may be needed to extend modelled understandings of long-term water quality evolution. Sustainability is key to whatever end use is proposed for the pit lakes so that the post-closure objectives will continue to be met now and into the future.
Sustainability and end uses
In terms of sustainability as intergenerational equity, pit lakes are dynamic landscape features. Water quality of pit lakes may change over time, evolving without an equilibrium. This continual change must be factored into closure planning, whereby changes result in different risk and opportunity profiles compared to long-term conditions. For instance, short-term water quality may be attractive to wildlife as either planned or unplanned habitats. This end use may be problematic, however, if water quality still presents risks either directly to wildlife or indirectly through their food chain, eg via sublethal concentrations of biologically accumulating elements. Conversely, short-term water quality opportunities may eventually be lost if water quality declines.
In this latter case, the pit lake (although initially meeting closure criteria) may yet not be on a sustainable trajectory, and may fail to meet these same criteria when assessed at a later time. Although initially meeting closure criteria is important, so is being able to continue to meet criteria into the future (McCullough, 2016).
Any of the above hazards presenting at closure may prevent relinquishment from succeeding. Failure to relinquish the pit lake landform may entail ongoing costs in administration and maintenance. Future expectations in regulatory and social licence may also require increasingly rigorous criteria to then be attained.
Loss of social licence to operate
Even if relinquishment were achieved, failure to avoid these risks afterwards may still lead to loss of social licence to operate, with the pit lake considered an ongoing liability to communities, regulators and the broader environment.
How these risks are best addressed depends upon establishing a clear understanding of what hazards the pit lake presents, determining the likelihood and magnitudes of these hazards as risks, and qualifying knowledge gaps and uncertainty.
Begin planning early
Like all good closure planning, final landform design begins early. Pit lake closure strategies and designs should be developed, and these should build upon next land use statements that define closure objectives. Pit lakes should have clearly defined objectives and closure criteria (McCullough et al, 2018). These statements should exist throughout the life-of-mine from before operations begin to closure. To determine what objectives are required, an explicit next land use is required to be determined in conjunction with stakeholders first.
The tendency to let planning for above-ground landforms overshadow the planning needed for pit lakes should be resisted – even if above-ground landforms are what a particular regulatory body tends to emphasise. There should be a closure vision for the pit lake too, or the opportunity to include designs that would facilitate a better closure may be missed.
Backfill isn’t necessarily your only option
Stakeholders may expect backfill despite it being unpracticable (due to the low waste-to-ore ratios of many coal and iron operations) or economically no longer viable given historic waste placements. Many voids may also reduce risk relative to backfill.
Targeted trans-disciplinary studies are essential to successful pit lake planning. Planning should involve coordinated cross-disciplinary collaboration. An effective plan will come from involving the multitude of disciplines present with pit lakes: surface and groundwater experts, stakeholder engagement and permitting specialists, geotechnical engineers, ground engineers, construction and design experts, ecologists, ecotoxicologists and others. A holistic, trans-disciplinary approach will lead to a visible difference at closure.
Ensure a focus on water quality
Much of the risk, and much of any proposed opportunity will depend on water quality. Closure strategies can often improve water quality or at least mitigate closure risks presenting from poor water quality. If all else fails, there are often treatment systems that can sometimes improve water quality to meet regulatory requirements.
Think beyond risk, see the opportunities
Consider both long-term planned and unplanned opportunities for the pit lake in the region (McCullough et al, 2018). While the large size, large volume and often poor water quality and connection to regional water sources can present significant potential risk and liability, there are a host of proven end uses that can produce valuable benefits to the community, the mining company and the environment.
Communities will often express special interest in next use opportunities and, if proposed, should be engaged early and regularly to discuss these. These
- swimming, diving and boating
- activities such as sightseeing, walking and bike riding
- wildlife habitats and fisheries
- treatment of other mine waters on the reclaimed or active mine site
- water resources for flood attenuation and primary industries.
Closure guidance has increasingly recognised these risks and has recommended approaches to considering pit lakes in closure planning. Recent Australian (DIIS, 2016) and other global (APEC, 2018) guidelines explicitly advise approaches for pit lakes in closure planning. A number of industry and academic publications provide context to leading international practice – for example, see ‘Opportunities for sustainable mining pit lakes in Australia’ (McCullough and Lund, 2006); ‘Ecological restoration of novel lake districts: new approaches for new landscapes’ (McCullough and Van Etten, 2011); and ‘Key issues in mine closure planning for pit lakes’ (Vandenberg and McCullough, 2017). Recent books also include information on pit lakes globally (Mine Pit Lakes: Characteristics, Predictive Modeling, and Sustainability; Castendyk and Eary, 2009), the science of acidic pit lakes specifically (Acidic Pit Lakes – Legacies of Surface Mining on Coal and Metal Ores; Geller et al, 2013) and closure planning for pit lakes (Mine Pit Lakes: Closure and Management; McCullough, 2011).
APEC (2018). Mine Closure Checklist for Governments. Asia Pacific Economic Consortium (APEC), Canada. 70p.
Castendyk, D. N. & Eary, L. T. (2009). Mine Pit Lakes: Characteristics, Predictive Modeling, and Sustainability Society for Mining, Metallurgy and Exploration (SME), Colorado, USA. 312p.
DIIS (2016). Leading Practice Sustainable Development Program for the Mining Industry – Preventing Acid and Metalliferous Drainage Handbook Department of Industry, Innovation and Science (DIIS), Canberra, Australia. 221p.
Geller, W.; Schultze, M.; Kleinmann, R. L. P. & Wolkersdorfer, C. (2013). Acidic Pit Lakes – Legacies of surface mining on coal and metal ores. Springer, Berlin, Germany.
McCullough, C. D. (2011). Mine Pit Lakes: Closure and Management. Australian Centre for Geomechanics (ACG), Perth, Australia. 183p.
McCullough, C. D. (2016). Key mine closure lessons still to be learned. Proceedings of the International Mine Closure 2016 Congress. Perth, Australia. Fourie, A. B. & Tibbett, M. (eds.), Infomine, 319-332pp.
McCullough, C. D.; Harvey, B.; Unger, C. J.; Winchester, S. & Coetzee, J. (2018). From start to finish – a perspective on improving sustainable development aspects on life-of-mine practices. In, From Start to finish: Life of Mine Perspective, AusIMM, Brisbane, Australia, 395-400pp.
McCullough, C. D. & Lund, M. A. (2006). Opportunities for sustainable mining pit lakes in Australia. Mine Water and the Environment 25: 220-226.
McCullough, C. D.; Schultze, M. & Vandenberg, J. (2018). Realising beneficial end uses for pit lakes. Proceedings of the International Mine Closure 2015 Congress. Leipzig, Germany. ACG, 497-504pp.
McCullough, C. D. & Van Etten, E. J. B. (2011). Ecological restoration of novel lake districts: new approaches for new landscapes. Mine Water and the Environment 30: 312-319.
Vandenberg, J. & McCullough, C. (2017). Key issues in mine closure planning for pit lakes. In, Spoil to Soil: Mine site rehabilitation and revegetation, Chap. 10. Nanthi Bolan, N.; Ok, Y. & Kirkham, M. CRC Press, 175-188pp.