Sustainable mine-water management strategies for open-pit mines

Some people in the mining-engineering field mistakenly believe that mine-water management primarily revolves around the treatment of mine waters that fail to meet applicable water-quality standards. While water-quality control is undoubtedly important, it represents only a fraction of what constitutes comprehensive site-wide, mine-water management. A more holistic perspective recognizes that it also involves mine dewatering to maintain safe and workable conditions, securing sufficient water supply for operations, strategically handling surface water within and surrounding the openpit to prevent issues like flooding, developing robust post-closure plans to mitigate long-term environmental effects, and proactively minimizing any adverse impacts on local communities and ecosystems. This article provides a brief overview of topics that fall within the concepts of mine-water management. The focus is on openpit mines, as they are generally larger than underground mines. The management of mine water is highly dependent on local and regional climate such as tropical, desert or temperate climate. Each climatic setting has its own unique challenges, but there are also commonalities. Mine-water management topics Overall water management. A key to managing water is to know the sources and volumes of water into and out of the mine. The sources are direct precipitation on the mine and surrounding areas, surface-water flows to and from the mine, and groundwater flows into and out of the mine. Understanding these flow components requires an understanding of a water balance for the mine. The water balance must include an understanding of flow volumes and spatial and temporal distributions of flows. This understanding requires the collection and analysis of data prior to the initiation of mining so that preliminary water conveyance systems can be designed, and the designs and costs can be integrated into mine plans. One commercially available software that can be used in understanding water balance is GoldSim. Data-collection systems also need to be designed and installed that provide information on volumes, rates and water quality. Thought needs to be given not only to the locations of data-gathering systems but also how the data will be collected and analyzed. The construction of data-management systems also needs to be considered early in the pre-mining process. Water serves several important functions within and outside the mine. These include its use in ore processing, heap-leach operations (where implemented), dust suppression, providing potable water for the mine camp, and supplying water to support surrounding communities and maintain sensitive ecosystems. The latter items will be important if the mine operations are likely to impact the quantity and quality for communities and ecosystems. Surface-water management. Surface-water inflows into the pit can have detrimental impacts such as mine flooding and slope instability. Efforts need to be made to divert as much surface water away from the pit as possible. Diversion canals or ditches need to be designed to carry the necessary volume flows of water for a specific precipitation/flow event that has a specific recurrence interval. For example, this could be the 100-year flood event or some other flow depending on the risk that mine management is willing to accept. The authors highly advise that diversion canals/ditches be lined to prevent seepage from the canals/ditches. This seepage could percolate from the structure to groundwater and potentially cause slope instability. The collection of water within the pit is also of high importance. Due to the dynamic nature of the pit/mining operations, the control of surface water within the pit is generally limited to the haul roads. The slope of the roads can be constructed such that water is diverted away from the slope face and into ditches. If the road will be in existence for a significant amount of time, then consideration of lining ditches should be made. Again, the lining will minimize surface water from infiltrating into the slopes. Water within the ditches should be directed to lined sumps where it can be pumped to the land surface and used (such as for dust suppression). Figure 1: Example of hybridized dewatering/depressurization systems. Groundwater management. Within the pit, the management of groundwater is critical to maintain dry working conditions as well as prevent slope instability due to high pore pressures within the slope. Dewatering wells are a primary means to dewater/depressurize a pit. Dewatering wells are generally located just outside the pit shell to minimize the possibility that they will be destroyed due to mine operations. However, as the pit expands in width and depth, dewatering wells surrounding the pit become less effective in dewatering the center of the pit or slopes due to limited cones of drawdown. The extent of drawdown is dependent on the permeability of the rock. Alternative methods of dewatering/depressurization need to be considered as the pit expands and deepens. In-pit dewatering wells can be used to achieve this, but their longevity may be limited due to mine operations. The in-pit dewatering wells may be mined out as the mining progresses. Locations within the pit should be considered where the wells will be effective in dewatering and where there will be a significant amount of time when they will not be disturbed. In cases where the pit slopes consist of relatively low-permeability rock, subhorizontal drains can be installed to depressurize the slopes. Subhorizontal drains are relatively inexpensive to install and maintain. One challenge is to collect the water from the drains and direct it to lined sumps where it can be eventually pumped to the land surface. One misconception that most mine operators have is that if the subhorizontal drain is not flowing it is not working. As mentioned previously, subhorizontal drains are generally installed in lower-permeability rock. The water that initially flows from the drain is derived from storage within the rock. Once depleted, water is then derived from the rock due to drawdown in the rock slope. If the rock is of lower permeability, then the flow rate into the drain can be very low. The volume of water extracted is not as important as the fact that the drain is still depressurizing the slope. The authors’ advice is not to abandon these subhorizontal drains if they stop flowing, given that they are still serving their intended purpose of depressurization. Water from dewatering/depressurization systems is available for use in mine operations. Examples of various dewatering/depressurization systems are shown in Fig. 1. The depressurization drainhole shown in Fig. 1 is constructed downward. This is because the geologic unit is over pressurized. In most cases the drains are installed horizontally or inclined upward at a low angle (for example, three degrees). Figure 2 shows the installation of a subhorizontal drainhole. Figure 3 shows a typical dewatering well that is used at mines in northeastern Nevada. Figure 4 shows the wellhead assembly of a typical dewatering well. Figure 2: Example of installation of a subhorizontal drainhole. An important component in understanding the distribution of water levels and pore pressures within the mine is to have strategically placed piezometers and monitoring wells. These monitoring systems should be installed early in the mining process, ideally before mining commences, and then expanded as critical geotechnical domains are identified for pore-pressure monitoring and potential depressurization. It is good practice to use real-time monitoring systems (for example, SCADA) to continuously track water levels and adjust dewatering processes accordingly. It is also good practice to have a telemetry system in place so that the data collection can occur in a timely manner and in one location. Figure 3: Typical components of a dewatering well or water-supply well. Water-use minimization management. The minimization of water use within the mine is critical especially for mines located in arid environments. Water supplies for the mine may be obtained from water sources that are located some distance from the mine, necessitating the installation of pipelines and other infrastructure. The extraction of water from external locations to the mine may be subject to stringent permitting requirements. This is necessary to ensure the protection of water resources for surrounding communities and sensitive ecosystems, preventing any adverse effects on their water supplies. Thus, it is imperative that the mine optimizes the use of water at the mine. The consumption of water at the mine can be reduced through water reuse and this involves water-treatment systems, storage of precipitation and surface water, storage and use of water from dewatering operations, and implementation of efficient dust suppression. Utilizing water-treatment systems to treat and reuse water can minimize the need for water from outside the mine’s boundaries. Collecting and using precipitation and surface water for mining operations can reduce reliance on other water sources. Further, implementing efficient dust suppression systems that utilize chemicals, such as magnesium chloride, instead of water will minimize consumption of water. If the use of chemicals is not cost effective for a particular mine, then an alternative solution is to apply water to roads and rock piles within the pit during times of low evaporation (for example, nighttime) to help in reducing overall water consumption. Water-treatment management. Treatment of water may be required where the quality of the water exceeds regulatory requirements or specifications for ore processing. This situation is especially applicable to locations where ores are associated with sulfidic minerals, and where these minerals are exposed to oxygen, resulting in acidic conditions. Understanding the locations of sulfidic minerals within the mine can assist in handling of waste rock or rock exposures that contain such minerals. Depending on the volume of waste rock, lining of waste-rock or tailings storage facilities may be necessary to collect and treat leachate from the piles before it can be used in the mine or discharged to downstream surface-water features. If lining is impracticable, then minimization of infiltration into such facilities can reduce water-treatment demands. Establishing closed-loop systems where water is recycled and reused within the mining process can significantly reduce the need for water that meets water-quality goals for ore processing and other mine uses. Water treatment may include active treatment such as flocculation/settlement, filtration, neutralization or other active processes. Once treated, on-site use or off-site discharge can occur. Passive methodologies, such as the use of artificial wetlands, may be an alternative option. Such an option may be appropriate where land area, climatic conditions and chemical loading rates exist. Other options include permeable reactive barriers for the treatment of groundwater. Permeable reactive barriers are designed to enhance bacterial sulfate reduction and metal sulfide precipitation. They have the potential to prevent acid mine drainage and the associated release of dissolved metals. The barrier is usually constructed with an excavator and the associated trench is backfilled with the reactive materials. This methodology is most applicable where shallow groundwater is impacted by acid-rock drainage. Treating and reusing water from mining operations for agricultural and community use can ensure water availability for local communities and ecosystems. Adopting practices that balance operational demands with environmental responsibility, such as minimizing water extraction and preventing adverse water-quality impacts are examples of sustainable water-management practices. It is important, however, to collaborate with local communities and all stakeholders when developing water-management plans to effectively address both mining needs and community water requirements. Figure 4: In-pit dewatering well with piping to collection system. Post-closure water management. The closure of a mine should be part of the overall planning process, with consideration of post-closure scenarios initiated even before mining begins. The time and expense of post closure can be significant depending on the size of the mine and regulatory requirements. Studies for post closure can take years to complete to arrive at a cost-effective closure. Post-closure considerations include minimizing infiltration of precipitation into waste-rock storage and tailings, control of surface water to minimize erosion of waste-rock and tailings storage facilities, control of surface water to minimize inflows to the former pit depending on how the pit is closed (backfilled, partial backfill, open), capture and treatment of surface and groundwater that is impacted above regulatory standards, and length of time for treatment of water. It should be noted that each of the closure options have tradeoffs, especially the closure of pits. The management of water during closure and post closure is critical. Waste-rock and tailings storage facilities The components of water management that need to be addressed are varied. One issue is to minimize infiltration of precipitation into waste-rock storage and tailings facilities especially if these facilities can generate acidic conditions. The minimization of infiltration can involve the use of impermeable caps utilizing geomembranes or evaporative covers that use a low-permeability cover (such as clay or fine-grained geologic materials) below a layer of soil that can support vegetation. The vegetation will then remove water from the facility by evapotranspiration. The design of an evaporative cover requires studies on the appropriate thicknesses of the layers to minimize water inflows as well as vegetative studies to determine the species of plants that have a high likelihood of survival in the future. Geomembrane caps covered by soil are generally used where the footprint of the facility is not large. The cost and effectiveness of capping a facility with a geomembrane will govern the size of the facility to be covered. Surface-water flow control on waste-rock and tailings facilities is also necessary. Assessments of the potential for erosion need to be made. These analyses may result in specific contouring of the sides of the facilities. Lined drainage ditches may be installed on the slopes to convey runoff to collection sumps and then the water is discharged from the facilities. This water may not need to be treated as it should not have contacted chemically reactive waste materials. Designing surface-water contours to direct precipitation away from waste rock and tailings storage areas can help to minimize infiltration and erosion. Surface-water and groundwater capture and treatment Again, the identification of potential post-closure water-quality impacts needs to be made early in the mining process so that appropriate management scenarios can be synthesized and cost estimates prepared. In cases where rock piles and pit slopes consist of reactive minerals that generate acidic conditions the capture and treatment of surface and groundwater may be required. Developing long-term water treatment plans that consider the projected duration and cost of treatment is essential. This includes planning for the maintenance and operation of treatment facilities over extended periods. Setting aside financial provisions or bonds to cover the cost of long-term water treatment and management is crucial for sustainable post-closure water management. Summary In conclusion, mine-water management is not a one-time effort with a sole focus on water-quality treatment. Mine-water management encompasses broad applications, and is an ongoing process that requires continuous monitoring, maintaining and adapting to changing conditions that include mine plans, closure plans and climate throughout the mine’s lifecycle. (References available upon request.)

Sustainable mine-water management strategies for open-pit mines

Authored by: Rennie B. Kaunda and Robert J. Sterrett

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