Mine Closure

MINE CLOSURE - INTRODUCTION
 

As we progress into the twenty-first century, there is increasing awareness of the need to provide for 'sustainability' of ecological and social settings in which mines are developed, operated and closed. The 'six tenets for sustainability of mining' provide the foundation for sustainability planning at a mine site. This gives rise to the need to do more than 'Design for Closure', requiring that we also prepare 'Post Mining Sustainable Use Plans' for the mine site and affected area. This concept is described by Robertson et al., 1998 and Robertson and Shaw, 1999. It also requires that all stakeholders, including the succeeding custodian, be consulted in the preparation of mine development, operations, closure and post closure sustainable use plans.
 

An example of innovate post mining land use development is the redevelopment of an abandoned mine in Cornwall called the Eden Project. In planning for closure, there are four key objectives that must be considered:
1. protect public health and safety;
2. alleviate or eliminate environmental damage;
3. achieve a productive use of the land, or a return to its original condition or an acceptable alternative; and,
4. to the extent achievable, provide for sustainability of social and economic benefits resulting from mine development and operations.
 

Impacts that change conditions affecting these objectives are often broadly discussed as the 'impacts' or the environmental impacts of a site or a closure plan. It is convenient to consider potential impacts in four groupings:
 

1. Physical stability - buildings, structures, workings, pit slopes, underground openings etc. must be stable and not move so as to eliminate any hazard to the public health and safety or material erosion to the terrestrial or aquatic receiving environment at concentrations that are harmful. Engineered structures must not deteriorate and fail.
 

2. Geochemical stability - minerals, metals and 'other' contaminants must be stable, that is, must not leach and/or migrate into the receiving environment at concentrations that are harmful. Weathering oxidation and leaching processes must not transport contaminants, in excessive concentrations, into the environment. Surface waters and groundwater must be protected against adverse environmental impacts resulting from mining and processing activities.
 

3. Land use - the closed mine site should be rehabilitated to pre-mining conditions or conditions that are compatible with the surrounding lands or achieves an agreed alternative productive land use. Generally the former requires the land to be aesthetically similar to the surroundings and capable of supporting a self-sustaining ecosystem typical of the area.
 

4. Sustainable development - elements of mine development that contribute to (impact) the sustainability of social and economic benefit, post mining, should be maintained and transferred to succeeding custodians.
 

Clearly the assessment of these types of impacts and closure requirements must address components of the site as well as the region and must select measures and allocate resources to address the major issues of impact. In order to minimize the various impacts, risks and liabilities, it is necessary to anticipate, as early in the process as possible, potential future liabilities and risks, and to plan for their elimination or minimization. In many areas, much of the liability or risk is associated with the uncertainty of the requirements for closure and rehabilitation from the succeeding custodian (be it a government agency, community organization or corporate entity). Early identification of the succeeding custodian, and their involvement in the development of the closure plan enables the closure requirements to be established and agreed and considered in the closure plan development. This allows the mining company to determine, and provide for, the requirements of the succeeding custodians, gain their support for the closure plan and minimize the risks and liabilities that may derive from succeeding custodian rejection or objection to the closure measures at the time of mine closure.

MINE CLOSURE - STEPS IN CLOSURE PLAN DEVELOPMENT

The typical steps for closure planning are shown in Figure 1. These steps also provide a logical order in which to develop and present the various sections of a Closure Plan Report. They provide the reader with a progressive description of the material required to understand the need for, nature of, effectiveness of, and cost the Closure Plan.
 

Any closure plan must consider the long-term physical, chemical, biological and social/land-use effects on the surrounding natural systems (aquatic, groundwater, surface water etc.). Therefore there must be an understanding of the pre-mining environment (step 1) and the effects of past and future mine development (step 2) on the pre-mining environment. Operational control measures must be selected (step 3) for implementation during mining in order to minimize the impact on the surrounding ecosystems. Impact assessments (step 4) must be done prior to measures selection as well as periodically during operations in order to determine the success of the measures implemented. Alternative mine closure measures are developed (step 5) and assessed (step 6) during mine design to ensure that there are suitable closure measures available to remediate the impact of the selected mine development.
 

If suitable remediation or closure measures cannot be identified or achieved, then it may be appropriate to revise the type of mine development proposed (return to step 2). Once a technically acceptable mine development and closure plan has been developed it is necessary to prepare a monitoring and maintenance plan (step 7) that will monitor the system performance during operations and post closure and provide for the maintenance necessary to ensure the long term functionality of the system components. Throughout this process, costing and scheduling evaluations (step 8) are completed, if the costs are too onerous, or if fatal flaws in the design are identified, the process returns to the design phase (step 2) and alternative measures are evaluated
 

Once an acceptable plan is completed, an acceptable form of financial assurance is developed and provided (step 9) in order to cover the costs of plan implementation, long term operations, monitoring and maintenance of the site post closure. The final stages of the closure plan process involve the application for (step 10) and approval by (step 11) the regulatory agencies of the Closure Plan, and implementation (step 12) at the end of mine life.

Figure 1. Typical Steps in the Closure Plan Development Process.

MINE CLOSURE - CYCLIC EVALUATION OF PLANS AT VARIOUS STAGES OF A MINE'S LIFE.

Each development, operating and closure plan comprises a design with drawings; specifications that define what will be constructed, and an operating plan which describes how the constructed facilities or machines will be operated. The design is completed to satisfy a number of design criteria and the operating plan specifies a number of operating constraints. Sometimes the permit conditions specified by the regulatory authorities include certain design criteria and operating constraints.
 

During the development of the design, the design engineer is continually doing informal risk assessments (or failure mode and effects analyses - FMEAs) to check that his/her design will meet operating requirements. If the current design has unacceptable risks of not meeting the design objectives, either the design or the operating procedures are modified until adequate performance characteristics are achieved (this evaluation process is represented by the upper half of the top circle on Figure 2). It is becoming more common for large mine developments to appoint Boards of Review to provide an independent check (audit or review) of the designs and operating manuals to ensure the appropriate 'International" standards of safety and environmental impact (risk or liability) are achieved. In effect these Boards perform FMEA's within the scope of their audits or review.
 

The FMEA primarily addresses the risk of designs and operating procedures not achieving the design intent. There are a number of other assessments that are important in deciding if a particular mine development or closure option is appropriate and represents a reasonably optimized plan. These assessments include impacts on the environment, the local and distant communities, costs etc (represented by the bottom half of the top circle on Figure 2). All significant stakeholders may need to participate in all or part of these evaluations and accounts must be taken of their values and concerns. A methodology, termed the multiple accounts analysis (MAA) has been developed as one of the tools (together with EA's, EIS's or ERA's) to perform such assessments (see Robertson and Shaw, 1998; Robertson and Shaw, 1999; and Shaw et al., 2001 for more details). The end result is the selection of a preferred Closure Plan.
 

Closure plans should be re-evaluated as the mine site development progresses since the initial plans are based on projected conditions which are expected to change in response to additional ore discoveries, changing conditions of product and mining economics, advances in technology and new regulatory requirements. Once the initial plan has been developed and is accepted, periodic, iterative re-assessments and revisions should be completed to ensure that the plan remains current, relevant and optimized. This results in a cyclical development of the plan and mine design over various stages of a mine's life as illustrated in Figure 2.

Figure 2. Cyclic Development of Plans and Designs at Various Stages of a Mine's Life.

Figure 2 provides an illustration of the successive activities at various stages of a mine project's life. The mining process, and the mine closure planning process, involve stages that evolve from conceptual to feasibility to permitting to operating to closure and finally to a post closure stage. These stages are shown on the figure down the left hand side. The circle at the top of the figure illustrates the decision-making activities as discussed above that are typically involved at each stage.
 

During the period of design, operating plan development and FMEA and MAA evaluations of a Closure Plan, there will be iterative modifications to the designs and operating procedures until a revised plan is agreed. The entire process is typically repeated periodically at intervals of about 5 years to ensure that the plans always remain relevant and current.
 

The FMEA is intended to minimize risk (financial as well as environmental) associated with complex, long duration engineered systems as represented by the closure measures. The MAA provides a basis for the evaluation of impacts and tradeoffs where large, high economic value projects also have high, and potentially long term, social and environmental impacts. The MAA provides the mechanism for communication of stakeholder values, as well as the accounting system by which they can be taken into consideration in the development of control and closure plans that address concerns from all stakeholders.

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