4.4 Tailings storage facilities and heap leach piles

4.4.1 Hydrology of surface tailings storage facilities

Surface tailings storage facilities (TSFs) generate high rates of base and/or embankment seepage and groundwater mounding during operation because of the large volumes of water discharged with the tailings. In dry climates, the volume of tailings water discharged to the TSF will be many times the volume of rainfall. Leading practice shows that thickened or paste tailings yield less water to the TSF and thereby mitigate many water-related problems.

Rates of base seepage generated during operations depend on:

  • whether or not the base of the TSF is lined, and the thickness and effective permeability of the liner and natural foundation
  • whether the TSF embankment is designed and constructed to seep with an interception system or to be water-holding
  • the particle size distribution and particle shape of the tailings and their potential to consolidate and to cement on desiccation, affecting their permeability
  • the rate of production and removal of supernatant water
  • superimposed incident rainfall and rainfall run-off (clean run-off should be diverted so it does not enter the TSF) (Williams & Williams 2007).

By understanding the key factors influencing the hydrology of TSFs, it is possible to plan appropriate monitoring programs.

Monitoring of the physical consolidation (including draining and drying) of the tailings during the life of the TSF enables the prediction of the final strength of the tailings surface to identify the types of equipment the tailings surface can safely support during rehabilitation, and the extent of ultimate post-closure consolidation so that the amount and distribution of required cover surcharge can be calculated.

Surface seepage can quickly attract wildlife, particularly in dry environments. Surface seepages and soaks also often produce lush vegetation that attracts more wildlife. The water quality of seepages and soaks needs to be assessed to identify risks to wildlife, for example from elevated cyanide or arsenic concentrations. The absence of carcasses around a TSF is not necessarily indicative of lack of impact or risks to wildlife, because the continuous deposition of tailings slurry often buries them and nocturnal scavenging animals remove them. Monitoring impacts on wildlife is difficult, but monitoring the water quality of seeps is much easier and should form the basis of a frequent and simple monitoring regime. If water quality suggests a possible risk to wildlife, further investigations into the extent of the impacts are warranted.

Vegetation that is growing on the surface of TSFs can also be a potential source of metals to animals that graze on it, so consideration should be given to periodic monitoring of metal uptake by vegetation such as grass.

4.4.2 Geochemistry of surface tailings storage facilities

Monitoring of tailings geochemistry during the life of a TSF, as well as water quality of intercepted seepage and monitoring bores, enables the potential future performance of the TSF to be evaluated and the most appropriate rehabilitation strategy selected.

Prevention is preferred over mitigation or treatment. For example, where tailings contain sulphides, the sulphides can potentially be largely removed by in-plant flotation. Comprehensive geochemical characterisation and monitoring of the hydrological and water quality performance of a TSF during operations will indicate whether the tailings can be directly revegetated or whether a cover system will be needed to meet rehabilitation objectives. Post-mining land uses having been defined during the planning stage of a mine (see Mine closure (DIIS 2016e) and Mine rehabilitation (DIIS 2016g)) and agreed in consultation with stakeholders, those uses must then guide the planning of the final rehabilitation of tailings.

A proactive leading practice approach requires the investigation of the potential for plants to take up metals by conducting pre-closure research trials to inform the final design plan. If covers are required, cover designs for TSFs should integrate what has been learned from monitoring and modelling of the hydrology and geochemistry of the tailings to achieve post-mining land-use objectives. Contaminant uptake by plants growing on the rehabilitated tailings may also need to be monitored to define potential impacts on grazing animals or humans eating bush food; for example, extensive grazing trials on tailings at Kidston showed insignificant uptake of metals and arsenic by cattle eating grass, although there was some uptake from ingesting dirt attached to the grass and roots (Bruce et al. 2002, 2003). Conversely, in some instances, metallophytes may be useful for reducing soil concentrations of particular metals (see Section 4.3.1). A proactive leading practice approach would be to investigate the potential for plants to take up metals by conducting a pre-closure research trial to inform the final design plan.

‘Store and release’ covers are not necessarily applicable in all situations. Other options include the use of wet covers and rock mulch for dust control in dry climates. In all cases, monitoring to assess their effectiveness and whether objectives are being met (or likely to be met) is critical.

4.4.3 Stability and water monitoring of surface tailings storage facilities

The slope stability of a TSF is most at risk during its operation. Hence, during its operation, monitoring of the stability of the TSF embankments is of key importance.

Monitoring to ensure geotechnical stability should include the use of piezometers within the embankments and tailings deposited against them to record the phreatic surface. Settlement plates are used to record embankment deformations. Visual inspections are an important TSF monitoring tool and should focus on critical embankment sections. They should identify seepage points, particularly those that are elevated on downstream embankment faces; obvious signs of embankment deformation or erosion; ponded water against sections of the embankment; and the condition of emergency discharge spillways.

Seepage monitoring will rely on data from an automated weather station (ideally installed on the TSF embankment). The volumes of tailings water input to and returned from the TSF should also be monitored to provide the data needed to calculate overall water balance. The volume and quality of base seepage should be monitored, particularly from low points around the TSF toe, since this will report directly to the surrounding surface catchments and into the foundation, where groundwater resources may be affected. A TSF may have a seepage collection trench installed along the outer foot of the embankment and wells installed with pumps to return the seepage to the TSF. Some TSFs have underdrainage systems that report to a seepage sump. Monitoring of the quality and volume of this seepage is essential. Borehole sampling should be employed to monitor groundwater quality upstream and downstream of the TSF. This data is also important for closure planning (DIIS 2016e; ANCOLD 2012).

Comprehensive geochemical characterisation and monitoring of the hydrological and water-quality performance of a TSF during operations indicates whether the tailings can be directly revegetated or whether a cover system will be needed to meet rehabilitation objectives. Once tailings deposition ceases and rehabilitation is undertaken, it may be necessary to also monitor erosion loss due to rainfall run-off, dust generation by wind, the stability of drainage and spillway works, the time-series evolution of seepage water quality, and vegetation establishment and sustainability, to verify when the works meet closure objectives (see Appendix 2). Further details are in the leading practice handbooks Tailings management (DIIS 2016h), Mine closure (DIIS 2016e) and Mine rehabilitation (DIIS 2016g).

Many wildlife species, such as microchiropteran bats and waterbirds, use TSFs as wetlands where they seek food, water and resting sites. The solution and slurry quality of operating TSFs can be poor, and the exposure of wildlife to such solutions can be, but is not necessarily, detrimental. Wildlife monitoring regimes for TSFs should be established, and can be simple and inexpensive to implement. Further details are in the Cyanide management handbook (DRET 2008b).

4.4.4 Monitoring of in-pit tailings storage facilities

In-pit tailings disposal can be designed so that there is no additional impact on the surface. However, the geochemical nature of the tailings and composition of process solutions need to be monitored through time to provide input to groundwater models that are used to predict the extent of interaction of the pore water in the deposited tailings with surrounding groundwater flows. The other critical parameters that need to be measured relate to the consolidation of the tailings, since this will determine the final settled level of the tailings and hence the depth of any ultimate pit lake, the depth of the final tailings surface below ground level if no surface water is intended, or the volume of backfill required in the event that the pit is to be backfilled to the original ground surface. Further details are in the leading practice handbooks Tailings management (DIIS 2016h), Mine closure (DIIS 2016e) and Mine rehabilitation (DIIS 2016g).

4.4.5 Monitoring of heap leach piles

Monitoring required for heap leach piles is somewhat similar to that required for TSFs and WRDs. Physical stability and leachate containment should be the focus of operational monitoring. However, gold heap leach operations use cyanide at concentrations toxic to wildlife, and copper heap leach operations use acidic leachate solutions that are also toxic to wildlife. Operational monitoring of such facilities must therefore also include recording the presence of ponded leachate solutions on the surface and seepage at the base. Wildlife is particularly attracted to surface ponds and toe seeps. Also, scavenger species have easier access to carcasses on heap leach pads compared to TSFs. Even small pools can attract large numbers of animals, so the position (in relation to native vegetation, and height compared to the surrounding terrain), and environmental conditions (drought), are more critical than the size of the pool. The irrigation of heap leach pads is dynamic, and surface pools can form and disappear rapidly during the course of operations. Heap leach pads are well suited to automated wildlife monitoring techniques, such as motion-triggered and infrared cameras (see Section 4.14.3).

Monitoring of disused heap leach pads will be needed to determine whether they are able to be rehabilitated in place once mining and processing cease, or whether the materials will need to be returned to a mine void or encapsulated in a dedicated waste storage facility (such as a WRD) to minimise environmental risk.

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