Waste Rock

The majority of the rock extracted in an open pit operation does not contain an economic mineral resource and is placed in waste rock stockpiles (Figure 5). These piles commonly have a large footprint and can be tens to hundreds of meters in height. Waste rock piles at large, open-pit operations may contain up to several billion tons of material. Rain or snowmelt that infiltrates and percolates beyond the near-surface zone of the stockpile continues to move down in an unsaturated flow regime towards the base of the pile. If the foundation has low hydraulic conductivity, then a portion of this water will pond on the pre-mining ground surface and move laterally, emerging at the base of the pile as toe seeps around the perimeter of the stockpile. The movement of this lateral flow component is markedly influenced by the topography of the pre-mining ground surface, and it usually determines the locations of toe seeps. Any water that does not discharge at the pile perimeter acts as a source of recharge to the underlying groundwater system. At some mines a basal drainage network is installed beneath the stockpile to reduce the potential for widespread development of a saturated zone at the base of the stockpile, and to facilitate capture of the contact water. Waste rock that contains sulfide minerals such as pyrite (FeS₂) has the potential to release acidic drainage with elevated metal concentrations for many decades after mine closure. In some cases, the outflow may have a neutral pH but carry contaminants of potential concern (neutral mine drainage), or have high salinity due to leaching of soluble salts if present within the waste rock. An assessment of this potential, and a means for mitigation if required, are issues of detailed evaluation in modern mining practice.

Figure 5 – Example of a multi-lift waste rock stockpile.

Mill tailings

Large volumes of mill tailings can be generated over the operational life of a mine. Large mines produce hundreds of millions of tons of tailings that need to be safely contained during operations and following closure of the facility. A mill processing 100,000 tons of ore per day, over a twenty-year period, will produce in excess of 700 M tons of tailings requiring secure disposal. Tailings are commonly discharged into a storage facility as a slurry that segregates to form a settled tailings deposit and a supernatant water pool. A dam, or set of dams, is built to retain both the tailings and the slurry transport water. The elevation of the supernatant pond rises through time as the volume of tailings placed in the facility increases. Over time, the tailings deposit forms an exposed beach near the slurry discharge points (Figure 6). The height of the tailings dam is raised on a regular interval to ensure there is sufficient capacity to hold the volume of tailings to be produced according to the mine plan, in addition to maintaining freeboard requirements to contain the design flood event associated with large rainfall events.

Figure 6 – Photograph showing the interior of a tailings storage facility with an older beach deposit where tailings release is not active (dry sandy area), a current beach deposit forming beyond the slurry discharge points located to the right, and the supernatant pond to the left in this tailings storage facility.

A comprehensive review of tailings management technologies, including numerous case histories, is available in “Study of Tailings Management Strategies”, MEND Report 2.50.1, by Klohn Crippen Berger, October 2017. Figure 7 provides a schematic representation of the principal water transfers at a conventional slurry tailings storage facility. This schematic illustrates a tailings facility incorporating a downstream seepage recovery pond, from which water can be pumped back to the tailings facility for re-use. Water enters the tailings facility as direct precipitation, surface runoff from the upstream catchment, with the tailings slurry, and as seepage reclaim. Mine operators remove varying amounts of water from the tailings before it is piped to the tailings management facility, with thickened tailings (or paste) having lower water contents than slurry tailings. Water recovered at a thickener at the mill can be re-circulated to the mill directly, reducing the volume of water lost to evaporation in the tailings pond.

Water leaves the facility via evaporation from open water, evaporation from the tailings beach, seepage through the foundation, and as release of surplus water to the surrounding environment. Evaporation rates differ between an open pond, a wetted beach with active tailings deposition and a dry beach, with the dry beach having the lowest rate of evaporative losses. Water is also retained in the pore space of the settled tailings deposit. Not shown in the diagram is the seepage that reports to the internal drains in the dam that is returned to the tailings pond or released to the environment if not needed for other purposes (with treatment if required). In addition, as the deposited tailings consolidate over time, pore water is expelled into the pond. This diagram highlights the difference between pond seepage that is captured near the toe of the tailings dam at a seepage recovery pond and the unrecovered seepage that bypasses the collection system. Assessment of these seepage pathways is a central topic in environmental permit evaluations. Seepage recovery systems are discussed in Section 7.

Figure 7 – Schematic diagram of the water transfers at a conventional slurry tailings facility (modified from MEND Report 2.50.1)

An assessment of the rates of process water seepage from a tailings facility requires prediction of both the flow though the dam and its underlying foundation, and an evaluation of the potential for subsurface flow to adjacent valleys through the ridges that form the flanks of the basin in which the facility is located. In areas with little topographic relief, tailings dams are typically constructed as a ring dyke, leading to a radial seepage pattern outward from the facility.

If the ore processed in the mill contains sulfide minerals (pyrite being an example), those minerals will be present in the tailings that are produced unless sulfide minerals are segregated from the bulk of the tailings in a separate mill circuit. Tailings that contain sulfide minerals that are then exposed to atmospheric oxygen on a tailings beach are capable of producing acidic water with elevated dissolved metal concentrations. This process, known as acid mine drainage, requires appropriate controls be in place to minimize contaminant release. Tailings with sulfide minerals but contained under water-saturated conditions are not prone to acid generation because the oxidation of sulfide minerals requires oxygen at concentrations higher than those present in the pore water of saturated tailings. However, dissolution reactions controlled by the redox setting remain a potential concern for impairment of the quality of the water in the facility. Some tailings are geochemically benign with the principal seepage issue then being the chemical characteristics of the process water that is discharged into the pond with the tailings. This is the case at some gold mines where cyanide is used to recover gold from the ore. The concentration of cyanide in the water discharged in the tailings stream could be the principal solute of concern. Management of seepage pathways and mitigation of water quality impacts requires a sound understanding of the groundwater flow system in the watershed hosting the tailings facility.

In some underground mining operations, a proportion of the tailings produced are partially dewatered, amended with binders to increase their strength, and returned underground for disposal in the mine workings. Waste rock is sometimes also placed in abandoned mine workings. This practice introduces a hydrogeologic issue if dissolved metals and other contaminants of potential concern (e.g., nitrates from blast residues) are released from backfill materials and then migrate offsite in the groundwater flow system. These risks are commonly examined with the aid of a groundwater flow model and a coupled solute transport model, developed for the groundwater flow system envisioned to occur following mine closure. This approach allows for prediction of solute concentrations at downstream compliance points.

Spent Heap Leach

Contaminants can be released to groundwater from heap leach facilities should the leach solution migrate through defects in the underlying liner system and bypass any seepage collection system. Modern heap leach facilities often incorporate leak detection systems. Closure of a spent heap leach facility requires consideration of the drain-down time following cessation of the leaching cycle and completion of any fresh water rinsing cycles. In the long-term, meteoric waters that percolate through the spent heap leach can transport chemical contaminants from the heap leach residue to adjacent surface water and groundwater. This risk forms a key element in the evaluation of closure plans at heap leach facilities.

Filtered Tailings

Several failures of large tailings dams in recent years have resulted in the loss of life and the release of large volumes of tailings to the environment. As a result, there has been increasing interest in the option of placing tailings in stockpiles of filtered (partially dewatered) tailings that have a sufficiently low water content to ensure that the tailings will not liquefy and flow. While this approach has a number of geotechnical advantages in comparison to deposition as slurry in a tailings pond, a stockpile of filtered tailings, depending upon the mineralogy of the tailings, can be a potential source of groundwater contamination that requires an assessment of the hydrogeologic regime within and in the vicinity of the filtered tailings stack to assess possible impacts.