Tailings Rehabilitation in an Arid Climate

Mine Site

Introduction

This report critically analyses the tailings storages facilities presently utilised on a mine site in the Kalgoorlie Mining Region of Western Australia. The mine site is located on a relatively flat elevated plateau with occasional low hills and experiences an average annual rainfall of 260mm. Since the commencement of gold mining operations in 1958, the subsequent retrieval and processing methods implemented have resulted in environmental hazards, in particular by Tailings Storage Facilities (TSF’s).

The Scope of the report is focused on three main topics:

·      Potential hazards of the tailings storage facilities are first identified, including likely pathways and impact targets.

·      A range of options are then identified to address these hazards, including consideration of relocating the tailings from existing storage facilities to small open pits, and covering the tailings to reduce the risk of contamination.

·      Finally, cover systems are suggested for each of the tailings storage facilities and in-pit tailings to achieve a native habitat post-mining land use. This process required the selection of a suitable cover material and cover layer thickness.

In an open pit mine there are five different types of material that can be found. These include: overburden, ore, mine rock, mineral concentrate and tailings. The tailings are a mixture of finely grained rock and water that has been left over after the mineral concentrate has been removed. They are then permanently stored in a secure facility at the mine site and generally contain around 10-20% of the economic materials that couldn’t be recovered from the ore (Engles, 2005). The main function of this secure facility or tailings dam is to ensure safe, long-term storage of tailings and also to prevent any detrimental effects on the surrounding environment.

2.0 Identification of hazards posed by the tailings storage facilities

2.1 Instability of tailings storage facilities

Failure of the tailings storage facility due to instability can arise through foundation failure or rotational sliding. Gravity is the main causing force with the shear strength of the soil acting as the main resistance.  Foundation failure is caused when the stress from the applied load is greater than the shear strength of the soil in the weak layer. This causes a movement in the failure plane from the weak layer of soil or rock at shallow depth in the foundation.  Rotational sliding occurs when the shear stress on the failure surface is equal to or greater than the shear strength of the soil. The most common rotational failure occurs on a curved or convex cylindrical shape but can also appear as local failures (Craig, 2004).

2.2 Erosion

Erosion of the TSF’s poses an environmental risk which if not properly managed can lead to instability and spreading of the contaminated tailings to other areas. The Kalgoorlie mining region experiences reasonably well distributed rainfall throughout the year but mainly in intense short duration events. The mine site is also subjected to relatively high winds and therefore can be affected by wind erosion as well as water erosion. Two main failure modes are associated with erosion, these being internal and external erosion (Bõhm, J, 2004). Internal erosion is caused by water movement within the TSF and has no visible signs of evidence. The mine site currently experiences seepage which can be exacerbated by internal erosion. External erosion occurs from the wind and rainfall with dusting being a particular problem due to the relatively flat hills and low vegetation.

2.3 Overtopping

Overtopping is one of the most common forms of failure and can potentially have drastic impacts on mine operation and the environment. Overtopping occurs when a large volume of fluid comprised of tailings, storm water or other materials enters a storage pit. If this volume becomes too large for the pit to handle, the pit will break its banks causing a large volume of tailing slurry to be released over the edges.

The damaging effect overtopping has is not limited to the raw effects of the slurry. Erosion and cavitation will often also occur as the friction this slurry imposes on surrounding surfaces is great. If this occurs then entire sections could be eroded and will fail, resulting in even more catastrophic failures.

2.4 Cyanidation

Cyanidation occurs when tailings containing a cyanide concentration are discharged into a particular area where they are allowed to sit. This low intensity cyanide solution can be toxic, not only to humans, but the entire ecosystem. The cyanide compounds distort molecular properties and isolate the cyanide ions which are potent.

They result in lower than normal pH, and will often prevent vegetation from growing and in turn affecting the wildlife as well. Additionally, it has a direct impact on wildlife, compromising the efficiency of oxygen in the bloodstream and reducing the effectiveness of an animal’s repertory system.

It is important to consider cyanidation for the Kalgoorlie mine as it has been stated that grazing would become the primary use for the land after mining had ended. In 2002, there was a call for the contractors to revert the landscape back to its native habitat. Regardless, it is important that soil does not test positive for an unsafe cyanide concentration.

There is a wide range of cyanide compounds that can occur given the appropriate conditions (Fig 2).

Figure 2: Common cyanide compounds

 

2.5 Acid mine drainage

Acid Mine Drainage (AMD) occurs when Oxygen, Water and Pyrite react together. They react via chemical, electrochemical or bacterial oxidation methods and the resultant chemical composition can be considerably harmful to the environment.

The two primary areas where AMD is likely to occur are at the central thickened discharge TSF and TSF 5. This is due to the fact that both contain sulphide due to their production between 1980 and 1988. In this instance oxidation of the sulphide may occur which results in the AMD. The effects can last for decades, extending well beyond the expected closure date of 2014.

Surface runoff is likely to pose the greatest risk; however soil contamination from AMD bacteria also poses a threat to the environment, both on the ground and subsurface. If this occurs then vegetation will die upon exposure and the soil will be infertile and will not support any flora or fauna. Because the vegetation is altered, the habitat will also suffer as the food chain in broken. The mine will never be able to rehabilitate the land if this occurs and would violate both the existing and proposed contract.

2.6 Salinity

Salinity is another major hazard posed by the tailings storage on site. Salinity results from the evaporation of water and the depositing of salts due to flowing water through the mine wastes. High salinity and low pH conditions occur in direct correlation. (Williams, 2008). Previously used TSF’s for the given mine contained considerable amounts of pooled surface water carrying suspended loads (salts) that may have originated from waste rock dumps and other run-off sources (Williams, 2008). When this water evaporates, these salts that were contained within the water are left behind and deposited on the surface. The ability to re-vegetate the site will be severely compromised if action is not taken however if the areas affected by salinity are contained and controlled, problems may not occur.

The major environmental risk involving salinity occurs if this saline material escapes the dams and is deposited downstream. Salinity can devastate the local environment as it destroys most vegetation growth, and as a result, can significantly effect on the post-mine land use as well as agriculture downstream. Also, mining operations could be affected economically as mining equipment could be corroded due to the surrounding saline matter.

2.7 Liquefaction

Liquefaction occurs when (typically) silty saturated soil is subjected to rapid oscillating loading. The soil compacts which leads to an increase in pore water pressure. If the water pressure becomes significant this leads to the loss of contact friction between the soils particles resulting in a loss of shear capacity. The soil will appear to be liquefied and will flow easily due to the loss of shear capacity. Liquefaction of soil can cause a domino effect where previously stable portions of soil become liquefied due to the loss of stress.  There is a greater risk of liquefaction in soils that are silty (and poses similarly size particles) and saturated.  Occurrence in gravel or clay soils is rare.

Loading which may cause liquefaction includes:

·      Earthquakes.

·      Rapid placement of material on top of tailings.

·      Blasting/demolitions

·      Vibration from machinery (Plant equipment, Vehicles, etc.)

2.8 Location of Tailings Storage Facilities

Slope instability due to structural failure of embankments can have substantial consequences on open pits in close proximity to each other. In regards to the situation outlined in the report (See Appendix – Figure 1) TSF’s 1 and 2 poses a potential threat to pit 1. The consequence of potential structural failure of embankments could be quite significant and would almost certainly have a major economic and environmental impact. Also located in close proximity to pits 3, 4 and 5 is the Central Thickened Discharge TSF. Any flooding that was to occur or anything that could cause overflow would give runoff the potential to travel into pit 3 due to its depth and location. This would most likely present a major hazard and in the long term pose an economic and environment problem for the mine site.

2.9 Likely Contamination Pathways

Three main pathways for contamination of the Kalgoorlie mine site were identified. These include:

·      Surface and Groundwater

·      Drainage Corridors/Waterways

·      Air-Flow

All these pathways pose potentially devastating consequences for the environment and in the case of the waterways, acid drainage could contaminate the entire mine site. Although with the correct implementation of control techniques, safety plans and procedures, this contamination of the environment can be contained.

2.10 Impact Targets

Identification, control and remediation techniques are required to minimize the hazards that have been outlined and to return the site to its previous state, which is self-sustaining and secure from an environmental point of view. These techniques should aim to control and minimize the hazards and encourage future rehabilitation to the area. For this particular mine site the impact targets include:

·      Constant monitoring of salinity

·      Tailing dam stability

·      Erosion and Seepage rates

·      Contamination levels

General social, economic and environmental issues are the focus of these targets and due to the current global environmental situation they play a very significant role. These targets can also influence mining companies as sustainable development is a very sensitive topic and extremely important when considering the hazards and their outcomes (Williams, 2008).

3.0 Mitigation and Prevention Techniques for Identified Hazards

3.1 Instability of tailings storage facilities

High pore pressures and exceedance of the stress limit of the foundations are repeatedly associated with the instability of tailings storage facilities. A simple solution to address this hazard is by placing a straight line marker vertically down the slope so that early detection of problems is possible. Also, piezometers could be implemented to monitor the pore pressure within the soil as well as the use of pressure gauges and inclinometers to observe the behaviour of the tailings. Soil nailing could also be a possible technique to use to increase soil cohesion in the event of a problem.

3.2 Erosion 

Tailings affected by wind erosion in the form of dusting are best mitigated with the employment of a covering system. This is the most suitable option as other methods such as watering/submerging the tailings do not suit the arid climate.

For internal erosion due to rainfall, adequately designed drains should be implemented. The use of riprap could also be included to filter excessive tailings and sediment resulting from erosion.

3.3 Overtopping

The primary reason why overtopping occurs is due to heavy precipitation in a short period of time. These high intensity, short duration storm events can result in heavy inundation of mine TSFs and if unmonitored will overtop. As with many major problems, prevention is the best solution. Channelling the stormwater and diverting it away from the TSFs is the best and often most cost-effective option (Ricey, 2004). Structures which could assist in diverting and managing stormwater are spillways, penstocks and benches.

Other techniques for managing the situation are to increase the TSF wall height and increase drainage capacity for both the TSFs and the surrounding area. More drain inlets should be added and the drainage network must be extended in order to facilitate for extra inlets. In some cases it may also be necessary to apply geo-textile and geo-grid fittings to prevent spreading of harmful substances in tailings.

It is also important to note that in the Kalgoorlie mine, TSF1 and TSF2 are located very close to the large low grade open pit.  If these overtopped, the result could be similar to a dam break into the open pit and the economic impacts would be extensive. These storage facilities in particular must be protected from such incidents by implementing the above safety precautions.

3.4 Cyanidation

Like with overtopping, it is best to prevent cyanidation rather than treated. The main risk of cyanide leaking into the environment is if the TSFs are not properly sealed and cyanide escapes. This is because all TSFs have low cyanide concentrations due to processing of the gold ore.

Similar to overtopping, cyanidation can arise when there are intense storm events, inundating the TSFs and transporting cyanide. The solution for this problem is the same, diverting the stormwater is the best option. This can be achieved using a variety of structures such as spillways, benches and penstocks.

The final risk cyanidation poses is when the wildlife is exposed to it. Birds in particular may drink the contaminated water.  Wildlife such as birds may be scared away by an occasional gunshot, as is done in many Australian mines (Whitehead, 2001).

3.5 Acid Mine Drainage

The obvious method to prevent AMD from occurring is by eliminating one of the elements from the cycle, or isolate it so that it is not exposed to one of the other key elements. The most fundamental reactor in this process is water. It is idealistic to completely remove water exposure to the tailings however this is obviously not always achievable due to storm events. Regardless, the TSFs should be designed to avoid being inundated by stormwater as much as possible. Methods for achieving this are similar to those for avoiding overtopping and cyanidation. The most efficient is to provide water-routing structures which divert stormwater away from the TSFs.

The second method for reducing the possibility of AMD is by submerging potentially acidic rocks and minerals in water, hence denying oxygen contact. Without contact to oxygen, oxidation cannot occur and AMD is avoided. It is often seen as being more achievable then draining the tailings of water. It is just necessary to maintain the water level and not allow evaporation to lower the water levels.

It is also possible to naturalise the potentially acidic tailings with basic solutions. Alkaline substances are for instance, most frequently used to neutralise limestone, ammonia, soda and hydrated lime. The main drawback to this solution is that the base level must be accurate. If there is not enough of the basic solution then AMD will still occur, and inversely, if there is too much then the tailings will also become dangerous.

The final point to consider is that a cover or barrier must be applied to prevent the spreading of AMD. Oxidation of sulphide materials is a natural reaction, and while the extent of this reaction can be altered, the outcome cannot. It is therefore important to provide an adequate cover for the tailings. The cover material should consist of an inert soil or liner which makes an impervious cover or membrane. It is also possible to use plastic liner or a geo-fabric as this is the most efficient way to reduce spreading of acidic substances. The disadvantage of this material is its cost, both as an initial investment and for maintenance costs.

3.6 Salinity

Salinity and suspended load issues can be controlled in the following ways:

  • Minimising the surface area of water stored on site (the central thickener should help);
  • Discharging surface water as it accumulates;
  • Introducing vegetation to affected area as soon as practical to minimise the risk of saline runoff; and,
  • Minimising erosion from waste dumps, dam walls, and other run-off sources to prevent surface water from containing a suspended load (Williams, 2008).

3.7 Liquefaction

There are various techniques which are available to help prevent liquefaction including:

·      Improving drainage of the soil to prevent the trapping of groundwater.

·      Prevent an inflow of water into the soil. This can be achieved by applying a non-permeable membrane or surrounding and covering with a dense low permeation soil. This would help to reduce the risk of erosion from sudden periods of high intensity rain.

Both of the above methods are not particularly suitable for the site being examined. This is due to the lack of rainfall in the arid climate.

·      Compaction of the soil. This causes the shear strength of the soil to increase therefore increasing its resistance to inflow of water and susceptibility to loss of shear strength from increased water pressure. The compaction of the soil will also help to reduce any erosion of the soil mass. This is an economically viable option as the site features low rainfall.

3.8 Rehabilitation

The process of rehabilitating a mine needs to fall within regulations and also be within the requirements of the post-mining land use agreement. Rehabilitation of a mine works best when consideration is taken during all phases of the operation. Taking a more integrated approach to mine rehabilitation, and doing it progressively, can achieve effective mine rehabilitation. This can help to ensure that the post mine requirements are easier to achieve. It is also necessary if it is accommodating to changes (Department of Industry Tourism and Resources).

In reducing voids throughout the mine life and also after the mine is discontinued backfilling can be an option but has been found to be uneconomic in some situations. Any voids left when a mine is no longer in use require careful consideration and planning (Department of Industry Tourism and Resources).

The climate, size and soil or rock types of the site affect the options available to rehabilitating a mine to the set requirements (Department of Industry Tourism and Resources). The site of the mine in the Kalgoorlie region of Australia is considered to be arid with low rainfall amounts that are distributed throughout the year, however occur mainly in intense events of short duration. This will affect the slope that will be required to ensure some runoff but also reduce erosion. The site is located on a flat elevated plateau with occasional low hills of up to 50m elevation. The hills are capped with cemented sediments and gravels, while the flat areas have a sandy soil cover. The size and shape of a site will be a factor when considering issues where there is a strong edge effect (Department of Industry Tourism and Resources).

For the mine in consideration the topsoil that is available is limited and is low in nutrients. The site is in under arid conditions and along with the low nutrients, vegetation may struggle to grow (Department of Industry Tourism and Resources). Native vegetation will be cultivated firstly to help with this and continued maintenance will be needed to meet the post mine requirements.

3.9 Cover

There are a few options for cover of a mine site to convert it to a site that can be re-vegetated and not cause further damage to the environment. Covers that are designed to limit rainfall percolation into underlying mine wastes are used for dry climates such that are found in Australia. This consideration limits potentially contaminated seepage from the mine wastes. Incorporating a moist cover will also provide a barrier to oxygen ingress (Department of Industry Tourism and Resources).

The components of a cover include a growth medium, seal and capillary break. The growth medium is the topsoil of the cover and is where the vegetation will grow from. This layer has a high water storage capacity (Department of Industry Tourism and Resources). The seal layer has a low hydraulic conductivity and a high air-entry value (Department of Industry Tourism and Resources). The capillary break layer is required when there are potentially contaminating wastes and will be the first layer on top on the waste.

At the site in the consideration the materials available for the components of a cover are limited. Hard rock pit is available for the capillary break however it is potentially sulphuric; clayey oxide waste is available for the seal, and oxide waste, with a fertiliser addition, for the growth medium, although it is erodible.

As the growth medium that is available is erodible the slope that is required will be small so as to still allow runoff but decrease the erosion that may occur due to the rainfall. However when the vegetation on the site is sufficient it will help with stability of any slopes. Also the topsoil for the growth medium is limited and may not have an available thickness to ensure sufficient vegetation growth. Growth mediums as thin as 0.3m can support grasses however struggle with most terrestrial vegetation types.  Having the fertiliser added to the growth medium will help with this issue and initial seeding with a re-fertilising and seeding with grasses 12 months later also helps the support of vegetation (Department of Industry Tourism and Resources). 

The clayey oxide that is available for the seal is to be compacted to reduce oxygen entry and rainfall infiltration. The loose surface of hard rock above the compacted clay protects it from erosion and desiccation. Ensuring the clay layer is compacted will reduce the hydraulic conductivity as compared to if the clay was not compacted.

A store/release cover would possibly be best for this type of climate as the pan evaporation is significantly greater than the rainfall in this area. This type of cover is considered to be sustainable and relatively easy to construct when using mine site equipment. The problem of erosion is also addressed however they rely heavily on getting the vegetation right for the climate. The store/release cover allows rainfall to infiltrate the cover to a point and then excess rainfalls will runoff along a designated pathway Department of Industry Tourism and Resources).

3.10 Backfilling

Backfilling is the process of filling unused pits with waste rock disposal. This is not usually an economic option and can lead to failure due to long term settlement. However if there are many unused pits they can be sequentially filled as the mine site is used and therefore can reduce settlement issues that can occur with backfilling.

At the site in consideration there are five small scale open pits that are no longer in use and can therefore be used for disposal, and there is also a large low grade open pit that can be considered for use at the end of the mine life in 2014.

3.11 Relocation of tailings

Relocation of tailings is sometimes necessary if the pile poses a threat to inhabitants or the environment, for example, through being situated too close to populated areas, on top of aquifers or other sources of water, or in unstable areas such as flood plains or faults near earthquake zones (McCracken 2005).

Disturbance of tailings which accelerates oxidation tends to exacerbate metal and acid generation and this is unwanted, however if the tailings can be moved without accelerating oxidation then relocation and removal is an option (Fourie & Tibbett 2007). Under controlled circumstances where much care is taken to provide the latter, mobilising of the tailings will not increase the short term metal release (Fourie & Tibbett 2007). And removal of the tailings from one site and then placed in a site with similar pH’s may raise turbidity but increase in metal release are very unlikely (Fourie & Tibbett 2007).

4.0 Tailings Storage Facility Covering Systems

4.1 Tailings Storage Facilities 1 – 4

The first tailing storage facilities to be used on the Kalgoorlie site were tailings storage facilities one to four. These were established in 1958 and were used until 1988. The facilities were used to store sand-sized tailings produced by the milling of the oxide ore from the small open pits. The tailings were of low salinity and were non-acid forming and hence display limited crusting and are dust-prone. The process water used to discharge the tailings to the storage facilities was of a low cyanide concentration. Tailings storage facilities 1-4 are conventionally structured facilities. They include central decants, are raised by the upstream method and have the tailings on the upstream face and waste rock on the downstream face. The tailings beach slopes average 1%.

The agreed post-mining land use was established in 2002 and was that of native habitat. This report will work off this assumption but, it should be noted, that trials are underway to determine whether this is achievable and sustainable. The results of those trials may alter the recommended cover system.

The climatic setting and the environmental liabilities presented in the overview, that is; the arid climate, the pan evaporation being 2,400mm while the annual rainfall averages 260mm, and the Tailing storage facilities slopes showing evidence of erosion, indicate that the ‘store and release’ cover systems may be a suitable choice of cover system.  This is opposed to the ‘barrier’ system as this system is best used for wet climates and is likely to fail in semi-arid and arid climates. The store and release cover system aims to store rainfall and release it during the dry season through evapotranspiration and can be utilised to minimise desiccation, vegetation dieback and erosion. The advantages to the system are that the construction of the system is relatedly easy using mine site equipment and, that the moisture state is maintained by a rocky soil mulch protective layer over the sealing layer. However, the system relies on having the right vegetation for the climate so natural vegetation can be concluded to be a viable option. This system comprises of the growth medium, the seal, the capillary break and the waste rock or tailings.  For this system the use of material will be limited to the material available at the Kalgoorlie mine site to maintain cost effectiveness.

The first layer in this system is usually that of the capillary break or waste rock construction pad. This is best used over hyper saline material. In the case of tailings storage facilities 1 to 4 the salinity is low and the oxide ore is non-acid forming, therefore this layer will not be required.

The second layer is that of the sealant layer. For the tailings storage facility 1, 2, 3 and 4 this will be the first layer which overlays the waste rock material. The sealant layer aims to fulfil two of the three philosophies of a cover system; to function as a water infiltration barrier and as an oxygen ingress barrier. The material best suited for this layer is one which displays a low hydraulic conductivity (less than 10-8 m/s or 315 mm/year) and has a high air entry value to help maintain saturation. Typically this layer is 0.5m thick on a 3% grade surface. Choices for this layer are, compacted and self-healing silty, sandy clay, compacted clayey oxide waste rock, inert fine-grained tailings or compacted tailings/waste rock mixtures. The available material for this layer is clayey oxide waste which, with sufficient compaction, is therefore a suitable choice of sealant product.

 The next layer, which overlays the sealant layer, is the growth medium. The growth medium fulfils the third factor of the cover systems philosophy by providing a suitable layer for the growth of vegetation. It should also be able to absorb a sufficient amount of water and thus also acts as a medium to prevent erosion. The main properties of the growth medium are that of, having a high water storage capacity and having a sufficient depth for plant roots i.e. much greater than 0.5m. The choices for this layer are usually nutrient rich topsoil or oxide waste rock with the addition of fertiliser. As the natural topsoil thickness is limited as it was not stockpiled before 1988, and the nutrient content is low, and thus would not provide well for native habitat, it may be more suitable to choose oxide waste with fertiliser addition for this layer. This choice also has problems as it is erodible and it is important to prevent erosion as there is already considerable erosion sediment and salinity downstream of the tailings storage facilities.

To help prevent erosion a concave profile should be utilised. Also, the implementation of an erosion-resistant coarse-textured waste rock slope known as a riprap layer which uses the natural erosion process to utilise fine-grained material to retain moisture to support vegetation could be used. The riprap material available is limited to hard pit rock, though this is potentially sulphidic. It is recommended that the sulphidic nature of this pit rock be investigated before a riprap layer is implemented. If this is the case an alternate form of riprap may be implemented though the sourcing of this material will lead to greater cost.

The last issue regarding tailings storage facilities 1 to 4 is that of ponding. Ponding of rainfall runoff against the tailing embankment has the capability of overtopping the wall of the embankment and causing erosion on the downstream face. This issue can be addressed by the controlling of evaporation by the encouragement of the runoff to spread over a large area, as well as any runoff directed off the tailings storage facility to be transported in adequately lined drains.

Estimated volumes of medium needed for the store/release cover system:

Tailings storage Facility

Area (km2)

Sealant (m3)

Growth Medium (m3)

1

1.0

0.5x106

1.5x106

2

1.0

0.55x106

1.5x106

3

2.2

1.1x106

3.3 x106

4

0.7

0.35x106

1.05x106

Total

4.9

2.45x106

7.35x106

These figures were obtained by simple calculations taking into account the area of the Tailings Storage Facilities. It should be noted that this estimation relies on the assumption that the material is suitably compacted, if not the estimated volumes will be greater than those specified.

4.2 Tailings Storage Facility 5

Tailings storage facility 5 was established in 1980 and was used until 1988 to store the tailings from the underground operation which extracted a higher grade sulphide ore. The tailings initially produced were sand-sized and water of low cyanide concentration was used to discharge them to tailings storage facility 5. Tailing storage facility 5 is, as in 1 to 4, a conventional tailings storage facility.

The main difference between this tailings storage facility and the 4 previous facilities is in the type of tailings stored. The tailings in TSF 5 were produced from the underground operations for sulphidic ore and thus the tailings are known to be potentially acid forming. The ore is still of low salinity. However, there is evidence of acid seepage and thus this leads to the recommendation that a capillary layer be introduced to the cover system. By the introduction of a suitable capping system the protection the ecosystem from potential hazards can be assured.

A capping system’s main aim is to limit the diffusion of oxygen as wells as the infiltration of rainfall to the tailings. Even though, as a general rule, potentially acid forming tailings may tend to remain saturated and are less prone to oxidation and seepage but alternatively, may produce acidic runoff, seepage is present in TSF 5. Therefore an adequate capping system should be able to prevent further contamination from TSF 5 and decrease the risk of erosion by the promotion of vegetation growth, by the appropriate use of water storage.

In the previous sections, tailings storage facilities 1-4, the advantages of the use of a growth medium in the store and release cover system, have been evaluated in regards to the storage of water and minimisation of erosion. Even though a loose surface layer can be shown to increase infiltration, it helps to protect the underlying material from erosion or desiccation. If utilised as a growth medium the advantages of this layer are seen through the encouragement of transpiration from vegetation to prevent further contamination of the environment by helping to eliminate the risk of water reaching the tailings. As with tailings storage facilities 1-4, it is recommended that this layer be of suitable depth, that is, greater than 0.5m.

The addition of a capillary break to the store and release cover system is recommended when capping tailings that are potentially acid forming. The capillary break’s main aim is to prevent the upward mitigation of oxidation products.  The materials recommended for this layer are ones that possess a low air-entry value and have the ability to store water. The recommended depth of a capillary break is from 0.3 – 1m. The materials most commonly used for this layer are fresh waste rock with minimal fines or quarried rock with minimal fines. The material available at the Kalgoorlie mine site are hard pit rock though, these have the potential to be sulphidic. It is more ideal to use inert quarried rock for this purpose but to do so would incur additional expensive associated with attaining and transportation of the material and therefore be less cost effective. The sulphidic nature of the material should also be investigated, and if proves to be of an unacceptable nature then the quarried rock may be considered. At this time, the use of the available hard pit rock is recommended to be used if it contains minimal fines.

The other layers and features of the store and release cover system in tailings storage facilities 1-4 should also be used in tailings storage facility 5. The sealant layer should be the clayey oxides waste which of sufficient compaction and depth. Also, there should be use of adequately lined drains allowing for heavy rainfall.

Estimated volumes of medium needed for the store/release cover system:

Tailings storage Facility

Area (km2)

Capillary Break (m3)

Sealant (m3)

Growth Medium (m3)

5

2.2

1.1x106

1.1x106

3.3x106

4.3 Central thickened discharge tailings storage facility

The Central Thickened Discharge Tailings Storage Facility (CTD TSF) was operational during the decade spanning the years from 1988 to 1998. The upgrading of the processing mill in 1988 necessitated the opening of a brand new storage facility, resulting in an expansive 5km2 large TSF that would harbor silt-sized tailings processed with hyper saline (ground) water of low cyanide concentration. The tailings production was initially comprised from oxide and then later sulphidic ores, and the process water used in the production was also highly saline as opposed to previous methods that used freshwater attained from Perth via pipeline. Since 1998 the discharge has been stored in considerably smaller, old, open pits, and the CTD TSF is no longer in use.

When taking into consideration the possible covering systems for this particular TSF, a number of factors must be evaluated in order to determine the best possible covering system in order to achieve successful rehabilitation. Firstly, environmental liabilities will inevitably be present within an operation of this magnitude. Indeed, there is evidence suggesting that the Central Thickened Discharge TSF has acidic seepage, thus implying that the hyper saline tailings are potentially acid forming (PAF). This will affect the underlying ground table water resulting in highly acidic groundwater, which will have a disastrous follow on effect for both pre-existing and future surrounding vegetation.

As well as environmental liabilities, the proposed post-mining land usage agreement also needs to be upheld. In 1975, an agreement was settled upon, dictating that the mine-site land be used for sparse grazing upon termination of the mining operations. This has since been revised and updated in 2002, and at present the intended usage for the post-mining site is that of a nature habitat. Whilst trials to see if this proposal is viable are still in place and it is not definite yet, it is important to at least rehabilitate the land to a ‘sparse grazing’ level of return. However, it is obviously desirable to take the extra initiative and furthermore rehabilitate the land to a ‘nature habitat’ standard of rehabilitation.

This desire for environmentally-friendly rehabilitation then leads into the last major issue of concern: erosion, as the site is prone to bouts of intense rainfall sometimes reaching 100mm of downpour within a 24hr period. In rehabilitating the site and implementing cover systems, erosion will be an important factor to mitigate if an effective and safe cover system is to be put in place.

As the silt-sized tailings are hyper saline and are also potentially acid forming (PAF) a capillary break layer must first be applied to the top of the CTD TSF to essentially act as a barrier against the movement of the thickened tailings, and to also decrease infiltration of water and diffusion of oxygen into the tailings mound. This capillary break layer will be approximately 0.5 meters thick and will be constructed from the mining operations waste rock, which in this case will be hard pit rock extracted from the open pit mine. This hard pit rock can potentially be sulphidic however so necessary precautionary steps must be put in place to rectify this eventuality and prevent the possibility of acidic seepage, such as a sealant layer of material.

Therefore, on top of this capillary break layer will be a sealant layer comprised of clayey oxide waste, which will be compacted to further decrease hydraulic conductivity by several orders of magnitude as opposed to an uncompacted layer. This compacted layer of clayey oxide waste will be approximately 1 meter thick and will have extremely low hydraulic conductivity thus preventing the infiltration of rainfall into the tailings mound and the capillary break layer, consequently preventing the instigation of acidic seepage from the tailings into the surrounding ground water.

As the site is intended for sparse grazing use after termination of the mining operations, a growth medium approximately 1.5 meters thick must be added on top of the sealant layer to facilitate the growth of native shrubbery/grasses/trees etc which will complement the pre-existing native bushes and grasses. This final capping layer will also protect the vital clayey oxide layer and prevent erosion and desiccation of its surface. The growth medium will be a combination of oxide waste and fertilizer, as the topsoil presently stored at the site is deprived of nutrients and will not be suitable for rehabilitation purposes. The oxide waste/fertilizer mixture will be placed in a series of mounds over the sealant layer, thus exposing more surface area for maximum possible evaporation as well as maximizing wastewater runoff. It will however also possess the high water storage capacity needed to grow native flora in arid climates, such as this one. Lastly, it must be thick enough to enable plant roots to grow without disturbing the clayey oxide layer underneath.

Riprap should be added as the final layer to the cover to prevent erosion. As well as this, there should be considerable quantities of riprap placed in the small valleys and channels interspersed between the topsoil mounds covering the clayey oxide sealant. This will act as prevention against erosion caused by the erratic rainfall downpours experienced in the region. The Riprap will be comprised of waste rock which again in this scenario will be hard pit rock extracted from the open pit mine.

This method of covering is relatively simple and cost effective. All of the materials bar the fertilizer are already present at the mine site, therefore meaning that only handling costs are to be factored into the final cost. The fertilizer is essentially the only material that is not present on site, and this will consequently have to be bought and transported to the mine site upon termination of the operation.

Figure 2: A profile view of the required layering.

Tailings storage Facility

Area (km2)

Capillary Break (m3)

Sealant (m3)

Growth Medium (m3)

Central Thickened Discharge

5.0

2.5x106

5.0x106

7.5x106

Figure 3: Estimated quantities (maximum amount) of material required for cover.

4.4 In-Pit Tailings Storage Facility

The Small Open Pits 1-5 on the site were initially used for Gold Mining from 1958 to 1978 in which oxide ore was extracted. In 1980 the Gold mining moved to underground operations, extracting higher grade sulphide ore, the Small Open Pits 1-5 were then left unused. In 1998 is was decided to then utilise the Small Open Pits for Tailings Storage, where hyper saline, sulphidic, thickened silt-sized tailings have been stored ever since. The choice to use the previously unused pits for tailings storage had two advantages; filling in the open voids in the landscape, and providing somewhere nearby to store the tailings. If managed properly, the use of In-Pit Tailings Storage Facilities can be Environmentally Friendly, as well as efficient in both land use and cost.

The rehabilitation of the In-Pit Tailings Storage Facilities needs to work toward the overall Agreed Post-Mining Land Use Policy. The initial agreement in 1975 was for the land to be used for grazing, however an application was put forward in 2002 for the post-mining land use to be changed to native habitat. This means that the rehabilitation of the small pits needs to consider reshaping the landform, as well as the ability to re-grow vegetation and mitigating the effect of acid run-off downstream and into groundwater.

A covering method needs to be chosen to sit on top of the pits, to help contain the tailings (thus preventing water surface run-off downstream and into groundwater) as well as to help re-shape the landform back to suit a natural habitat in line with the post-mining land use policy.

From the earlier research into covering options (See section 3.9 Cover), the most suitable solution for these In-Pit Tailing Storage Facilities would be a Store and Release Cover System. This system is relatively easy to construct, with most of the materials already available on site. This method is well suited to these particular pits because of its reduction of oxidation of the tailings, and the promotion of vegetation at the surface to help with the post-mining land use. With a well-layered Store and Release Cover System and an established vegetation system, this cover method will be able to withstand erosion from the seasonal rainfall in the area, whilst still promoting seepage and retaining water to maintain vegetation in dry periods.

The Store and Release system will incorporate three layers.

First will be a capillary break directly on top of the tailings. This layer is made of hard pit rock approx 0.5m diameter with no fine material. This prevents capillary uptake of the tailings and evaporation through the cover system.

Second will be a sealant layer, made up of compacted clay oxide waste to reduce oxygen entry and rainfall infiltration. The compactness of the clay also reduces the hydraulic conductivity of the sealant layer.

Lastly will be the Growth medium on top, this will help with re-forming of the land shape, and provide a base for the vegetation to grow from. This layer is important in not only allowing growth of the vegetation, but the vegetation will also help maintain stability of the cover to protect against erosion.

Providing an effective cover system for the In-Pit storage facility will help with the rehabilitation of the site for post-mine land use, but will also help to provide another storage option for tailings. This method in the long run will be both cost and environmentally beneficial to the project, taking a negative output of land voids from earlier Gold mining and being able to use it efficiently for waste storage.


6.0 Conclusion

This report has focused largely on the storage and rehabilitation of mine tailings in arid climates such as the Kalgoorlie Mining Region in Western Australia. All of the topics outlined in the introduction have been addressed by identifying the hazards associated with on site tailings storage facilities, looking into the options available to address these problems and finally suggesting cover systems for each of the tailings storage facilities/in-pit tailings storages. These topics have been addressed in order to achieve a native habitat after the site has been mined.

The major hazards that have been identified in section 2 are:

·      Instability of tailings storage facilities

·      Erosion

·      Overtopping

·      Cyanidation

·      Acid mine drainage

·      Salinity

·      Liquefaction

·      Location of (TSF’s)

·      Likely contamination pathways

·      Impact targets

One other major conclusion that has been reached in this report is the recommended cover systems for each of the TSF’s and in-pit tailings. The summary for each of the storage facilities is as follows:

Potentially Acid Forming Locations:

·      1.5m of growth median

·      1m of compacted clay

·      0.5m of capillary growth material

Non-Acid Forming Locations:

·      < 0.5m of growth median

·      0.5m of waste rock

After this it was found that the only available material for the capillary breaks and rip raps were sulphuric in nature. This potential may not be a problem for capillary material due to the clay seals it will be for riprap as this is located directly on the surface. There is a good chance that this could lead to an acidic runoff problem so it is suggested that limestone treatment regime be used to neutralize and acid.

Rehabilitation and tailings management will always remain a significant problem for the mining industry to solve especially with the recent push for environmental stability amongst the community. From this case study it demonstrates the need for careful planning and review of mine waste and tailings management to avoid the entire issues highlight within the report.  In this day and age environmental management in mines is at the forefront of consideration for companies and it’s this sort of critical thinking that will ensure the best solution to potential problems both technical and economical.

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