Mine Closure Criteria and Indicators

by: Dr. A. Robertson and S. Shaw

In order for regulators, mining companies and society to evaluate the success and reliability of closure measures and the relative and cumulative impacts of a mine post closure, criteria are typically applied to 'test' the performance of those measures. The assessment criteria, as to what constitutes 'a reasonable level of post closure social, environmental and aesthetic impact, land use, active and/or passive care, costs and environmental risk' will differ for the various stakeholders with interests in the mining operations and the surrounding impacted region. Definition of appropriate indicators and assessments of the appropriate criteria, for each of these indicators, must be made during closure planning in order to form a basis for decision-making. The rapid changes that are being experienced world-wide in the increase in the number of indicators (for air and water quality, aesthetics, land use, re-vegetation, ecosystem restoration, social impacts etc. and the application of ever more stringent environmental standards for mining projects results in considerable uncertainty as to the acceptability of many criteria, particularly those involving on-going active care and risk of environmental impacts.

For most decision-making processes, there are a number of decision 'drivers', i.e. issues that are so important they tend to determine the conclusions of the decision process. These are often related to:

  • Surface and groundwater quality and impacts on the receiving environment,
  • Long term stability and erosion of structures that will remain on the site,
  • Land use and post closure aesthetics,
  • Social and economic impacts related to a potential reduction in economic potential of an area and the potential long term burden placed on future generations related to post mining maintenance,
  • Economic consequences to both the mining company and financial stakeholders of closure costs.

At a minimum, indicators, with associated criteria, are required to describe and test these driving issues. The use of indicators was described in the previous section on multiple accounts analysis.

Selection of inadequately 'low' criteria may result in the rejection of a Closure Plan or the imposition of additional and stringent closure requirements in the premits. Selection of 'high' criteria may result in closure costs that are not economically achievable or result in incremental costs which are out of proportion relative to the gain or improvement in performance. Unjustifiable closure cost expenditures waste fiscal resources that may otherwise be available for other local or national needs. The selection of criteria is therefore a balance between costs and benefits of reducing requirements for future active care and of future risk to the environment. Minimum criteria are those that just meet regulatory 'standards' and are protective of the environment, health and safety.

During mine planning and operations, it is often difficult, if not impossible, to predict, with the degree of confidence required, the precise impact on the environment and the resulting ability to meet closure objectives or standards. Regulatory criteria are likely to change over time ('moving goalposts'), and criteria selected for the purposes of Closure Planning early in operations, may not be applicable at the time of closure. Therefore, it is not necessarily appropriate during mine planning, development or early operations to select quantitative closure criteria which may or may not be realistic or valid at the time of mine closure. Early in the mine's life, it may be more appropriate to discuss closure objectives, or indicators for such issues as surface water quality, groundwater quality, stability and erosion, land use, revegetation etc. in more general terms. Thus the 'tests' by which the anticipated effects of various closure measures are judged typically range from quantitative values (believed to be most realistic for long term criteria values), to semi-quantitative or qualitative (descriptive) indicators of impacts or benefits.

Typical indicators for some of these areas are provided here.

Surface and Groundwater Indicators:

Secondary Copper Minerals Precipitation (Furry Creek Drainage, Britannia, B.C.)
Price et al., 1995)
Dark greenish-black drainage from a mine adit indicative of ferrous iron (Fe2+) (Gilt Edge, South Dakota)
(Photo taken by S. Shaw, 2000).
Rusty red water from underground indicative of ferric iron (Fe3+)
(Photo taken by A. Robertson).
Milky water in a stream often indicative of aluminum (Red River, Questa, New Mexico)
(Photo taken by S. Shaw, 2001).

Stability and Erosion Indicators:
Indicators and criteria are also typically applied to ensure the integrity and long term stability of structures. Typically civil engineering safety standards (such as a factor of safety of 1.5 for slope stability) applies during the operating life of the mine. Durability of structures, post closure, is dependent on the durability of the materials the structure relies upon for stability. Wooden support in underground openings, bridges, dams and buildings will fail over time resulting in failure of the supported structure. Not so obivious is the long term deterioration of other materials such as geomembrane liners or covers, reinforced concrete and corrosion protected steel structures. Yet less obvious is the long term weathering of rock, including tunnels, pit walls and waste rock, resulting in strength deterioration and failures. Rip-rap protection on a dam face or lining a ditch will last only for the period over shich the rock from which it was quarried is durable.

Structures are under the continual attack of both perpetual 'forces' such as weathering and corrosion, erosion by wind and water, sedimentation, biotic action by roots and burrowing animals and frost action.

Structures are also subject to extreme events, which, because of the long period of post closure interest have a much greater probability of occurrence than during the operating period of the mine. Further, the consequences (economic, environmental and socio-economic) of structural failures of, for instance mine rock piles, tailings dams etc., are also oftentimes large. Therefore, it is typical for long term closure planning, that measures will consider larger events where the consequences of failure would be catastrophic, i.e. 1:10,000 year or maximum credible earthquake (MCE).

The selection and computation of design floods is another important part of many mine closure plans for features associated with water management (e.g. spillways, ditches, tailings dams, diversions, ARD capture and transport facilities, water treatment plant storage ponds etc.). Non-critical structures are typically designed to accomodate the 1 in 100 year flood event while structures that would cause large, but not catastrophic impacts, or are critical to the operations of specific facilities would require design to accommodate the 1 in 1000 year event. Those structures for which failure could result in casualties or cause catastrophic environmental impacts should be designed for the probable maximum flood (PMF). The provision for long term post closure operation of water management structures is often one of the most challenging aspects of closure measures design.

Land Use and Socio-economic Indicators:
The post mining land use and socio-economic impacts are extremely important issues for consideration in closure planning. Mine site development is often responsible for significant changes in the local social and economic conditions in an area. In industrialized and highly developed countries, the priority for rehabilitation of mine sites located remote from population centers is usually to return the site to the conditions that existed prior to mining, or an agreed equivalent. In industrialized countries, large mines have often been the catalyst around which a larger industrial and urbanized community has developed (i.e. the Witwatersrand of South Africa; Bingham Canyon/Salt Lake City, Utah; Berkley Pit/ Butte, Montana). A return to pre-mining socio-economic conditions is sometimes not contemplatable, in particular in regions with very little previous industry development (e.g. areas of Peru, Indonesia, Papua New Guinea etc.). To the extent reasonably achievable, mine closure must address the facilities and conditions that should be maintained post mining, to sustain the social and economic benefits generated during mining. While it is not possible to select closure criteria for all these issues, comparative assessments using indicators such as maintenance of access and power, on-going protection of health and safety, on-going job opportunities, sustainability of tax revenue etc. should be completed.

The cumulative assessment of the various benefits and impacts of all the closure measres can be completed on this basis using the MAA methodology (also see Robertson and Shaw, 2004). The resulting evaluation of all stakeholder issues, including water quality, stability and erosion, land use, socio-economics and economics, provides a comparative 'test' of success of a Closure Plan without necessarily applying or committing to closure criteria too early in the closure planning process. This type of a comparative 'test' was utilized for the reclamation planning of the Zortman and Landusky mines in north central Montana and was used in writing the EIS document.

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