Risk Assessment and Remedial Measures of Open-Cut Coal Mines
Mine Waste Management and Landform Design
The following report outlines the results of an application of a risk assessment modelling tool to the rehabilitation of a mine in the Bowen Basin. The study gives in depth mathematical results for a possible post mining land use, being forestry in this particular case. The financial viability of the rehabilitation investigated in considerable depth.
The mine is located in a semi-arid climate, with an annual rainfall in the range from 500 to 1,500 mm falling over the summer quarter. During this time maximum daily temperatures ranging from 35 to 20 degrees Celsius for the year, and experiences a mean pan evaporation of 3,000 mm
In addition with the above mine conditions, the mine domain is also developing and processing a low grade, sulphidic gold ore body with a halo of copper. Heap leaching and cyanide treatment are two methods used to extract the gold and copper respectively.
The mine has operated for 10 years, but more importantly, when considering the relevance to diagnosing remedial measures and ultimately rehabilitation measures, it has 3 years and 5 years left of mining and processing respectively, by most recent forecasting.
The general approach taken to assess and reduce the potential risk associated with each of the domain will be in the form of a qualitative risk chart/table approach The analysis that is being undertaken will include the following steps:
1. Firstly identify the hazards and targets associated as well as the pathways between these hazards and targets.
2. Compile an ultimate question, issue or statement to be addressed which stated what we are trying to achieve.
3. Consider the ultimate potential failure scenarios when the ultimate question may produce an unfavourable result.
4. Recommend operational and rehabilitation methods to effectively minimise environmental impacts whist keeping in mind the cost of the project.
The ACARP model is a mathematical analysis tool which attempts to evaluate the success of possible post mining land-use options for a particular domain. The model takes into consideration a number of factors including:
§ Prior Land Use
§ Agreed post mining land use
§ Selected land use
§ Stage of Rehabilitation
As a result of this the ACARP model is an extremely flexible tool, it can be used for a wide range of uses including selecting a more viable rehabilitation method prior to the completion of mining, carrying out costing estimates on a range of rehabilitation strategies and even evaluating the effectiveness of a strategy following the completion of the rehabilitation works. Due to the large number of variables accounted for the ACARP model provides an accurate assessment of the approach and allows its user to provide a defensible argument when recommending optimum strategies.
Tools such as this one allow mine operators and key stakeholders an opportunity to consider the requirements of all major parties, thus helping to select the best possible solution by minimizing the overall environmental impact while also keeping the financial and social cost as low as possible.
ACARP RISK ASSESSMENT TOOL
The risk assessment is carried out by completing the background information, including a description of the mine site, the assessment parameters, a description of the Domain being considered, assigning Likelihood ratings to the contributors to land-use failure and assigning Consequence ratings to these contributors. The Likelihood and Consequence ratings involve five point logarithmic scales from 1 (lowest rating) to 5 (highest rating), with the following meanings:
1 = Rare = May occur in exceptional circumstances (say 1 in 1,000 years).
2 = Unlikely = Could occur at some time.
3 = Moderate likelihood of occurrence (say 1 in 100 years).
4 = Likely = Will probably occur.
5 = Almost certain = Expected to occur during the life of the mine (say 1 in 10 years).
Note that the Likelihood rating can be assigned reasonably objectively, with potentially little difference in the Likelihood ratings assigned by different Stakeholders. The Risk Ranking is not highly sensitive to the Likelihood rating assigned to a particular "Fault". Hence, the likelihood values of most events pertinent to the mine site were kept relatively constant for all stakeholders.
1 = Negligible = Dealt with routinely.
2 = Low = Threatens efficiency or effectiveness.
3 = Medium = Requires significant review of or change to the operation.
4 = High = Threatens the community and/or the environment and the survival of the operation.
5 = Extreme = Permanently harms the community and/or the environment and threatens the survival of the organisation.
Note that the assignment of the Consequence rating is a reasonably subjective exercise, with different Stakeholders potentially assigning quite different Consequence ratings. Since the Consequence rating is applied to all "Faults" for the particular land-use failure mode, variations in its value can have a significant effect on the relative risk rating of the land-use failure modes and on the overall Risk Ranking. The risk ratings given to each failure scenario work on a relative scale of 0 to 5, with 5 presenting significant impact and a score of 0 indicating negligible risk, or impact.
The following representatives were invited to provide a risk assessment for the mine spoil domain:
For this particular model we have adopted to weighted approach for stakeholder input. The main reason for applying this approach is the considerable difference between the stakeholders with regard to –
§ Influence the stakeholder has in the running of the mine
§ Level of Responsibility in the decision making process
§ Potential impact in the event of mishap or failure
§ Cost incurred in the event of mishap or failure through repair or potential litigation etc
The risk rankings utilize a scoring system based on the level of exposure and responsibility taking into account the factors above. Ten stakeholders in total were considered and ranked as a part of this particular model; their assigned scores are shown overleaf:
Resident of Nearest Town
The mine owner carries the greatest level of responsibility and is affected in a large number of areas in the event that something goes wrong during the rehabilitation process, as a result of this the mine owner carries the greatest risk rating, this is contributed to by a number of factors:
§ The mine Owner is generally forced to take responsibility for any mistakes or problems which may occur in the rehabilitation process; this may result in large financial loss and poor public image.
§ In this particular case, contamination of water sources has the potential to cause the greatest concern for the mine owner, any contamination problems may place the health of himself, his workers and community at risk as well as being extremely costly for the company.
The Shareholder was assigned a moderate/low-weighted risk factor for the following reason/s:
§ Company Shareholders, even though most of them value environmental issues, are predominantly unaware of how the company address and resolve these issues. Shareholders, more often than not, invest in mining companies that provide high financial returns, not mining companies that have outstanding environmental practices.
§ Company Shareholders have an influence over how the company is managed. Hence, they potentially have a significant influence over the operation of the mine and ultimately the environmental responsibilities taken on by the company. This is particularly evident when the company’s public perception towards the environment is portrayed as being negative.
The Mine Manager is accountable for the efficient operation of the mine site and for ensuring that all the guidelines proposed by his company are satisfied. He/she does not directly set out these procedures but has to make sure that the employees carry out their assigned tasks proficiently. If the Environmental rehabilitation or the mining operation is not carried out correctly and complaints or concerns arise from mine workers or the local public, they are directed to the Mine Manager. It is then his/her responsibility to take appropriate action to remedy the situation or if needed to forward it onto the Mine Owner. The Mine Managers is job is at risk when operations at the mine site are not carried out smoothly and problems continue to arise.
Depending on the particular region, the mine workers may or may not be locals from a neighbouring town. For the purpose of this exercise, the latter is assumed. Non-local mine workers would have little care for the rehabilitation of the mine site as once the mining operation is finished, they will be gone. The mine worker is actively involved in rehabilitation measures use to promote the post-mining land use agreement and would therefore be satisfied that these adequate measures were taken. However, the mine worker would not necessarily consider the long-term impacts which are a significant aspect of mine site rehabilitation. Although a worker would know more than the average person about mine site rehabilitation, their level of knowledge about particular specifics would not be as high as their manager or the owner. Mine sites can be a significant place of employment and hence one would expect there to be a large number of mine workers. Due to their probable departure from the area upon mine closure, mine workers are not particularly vulnerable to any ill-effects caused by improper rehabilitation. Their responsibility is also considered low because they are merely taking orders from their superiors and rarely have any impact in the decision making process.
The adjacent grazier has a moderate risk score as the success of the mine rehabilitation has the potential to have a considerable impact on his / her livelihood.
§ We must assume that the grazier is a local community member and as such is directly impacted by the socio-economic prosperity of the town.
§ The grazier would be most concerned about possible vegetative failure and the effects on both the surface and ground water. Erosion and contaminants to either of the water sources may have severe detrimental effects on his ability to graze cattle, therefore affecting his livelihood.
Resident of Nearest Town
The Resident of Nearest Town was assigned a low weighted risk factor for the following reason/s:
§ The environment impacts resulting from the mine directly affect the town dweller. Hence, the mine can influence their quality of life (i.e. land contamination, dust clouds, pest production).
§ The advantages and disadvantages of the post-mine land use will directly affect the local community.
§ The mine site provides a significant boost to the local community’s business during the life of the project. Hence, the local community relies on the mine remaining operational.
The local council’s main interest is in the health and happiness of the local populace. Damage to the surrounding environment such as stability of the land is of concern to the local government especially after mine closure and rehabilitation. Any economical change (positive or negative) caused by the arrival or departure of the mining company is also of importance.
The weighting of the local council’s risk assessment is lower than average. Although in possession of knowledgeable individuals, they would most likely not have the specific technical expertise in dealing with mine site rehabilitation. There would typically be a handful of individuals assigned to deal with the case, hence the low size allocation. Due to their close proximity to the mine site, they would be somewhat vulnerable but not as much as local residents. They would also be accounted for minimal responsibility due to larger matters being handled by the State Government.
The State Government is considered a moderate risk factor for the following reasons
The State Government is concerned with the long term outcomes of the mine and has a key input regarding the mines future
The State Government may only be affected by the mine if a catastrophic event takes place, causing citizens to question their commitment to the people’s needs.
The Regulator’s primary objective is to guarantee that the mine reshaping and rehabilitation strategies are successfully implemented and that they adhere to the state and local government defined standards. Regulators must ensure that developed mine restriction guidelines are prepared for the Mine Manager to adhere to so that the environment can be successfully rehabilitated after the completion of the mine. Concerns needing to be addressed by the regulator include degradation of waterways from runoff, impacts to local area and communities and inefficient treatment and cover of spoil heaps. Mining Managers must seek approval from the regulator before major decisions are met, hence it is expected that the regulator be an expert in his field, being well practiced with relevant laws and a diverse range of rehabilitation strategies. Since the regulator is very influential with mining decisions, his/her position is particularly vulnerable when the rehabilitation plan is not implemented successfully and damaging impacts are not minimized on the surrounding community. The regulator is not concerned with cost efficiency as their main responsibility is to ensure laws and regulations are met, whatever the cost.
The City Resident was assigned the lowest weighted risk factor for the following reason/s:
§ A City Resident will not be directly affected by the environmental impact resulting from the mine.
§ If the mining company closes then the social affects it has on the city would not be felt anywhere near much as they would be in the local town.
Table 1.1 Summary of risk rankings
Unfactored Risk Ranking
Factored Risk Ranking
Resident of nearest town
Overall Risk Ranking
The ACARP tool was used to assess the cost-effectiveness of different types of land-uses post mining. It aims to evaluate the costs of alternative rehabilitation and remedial work options. For this analysis it should be noted that the reshaping is usually best compared when varied. However, a value was stated for this site so it was chosen not to vary this number. From the data it can be seen that the chosen agreed land-use of grazing is the least cost effective option to implement. Though cropping also produces a similar high result. However, these are the options likely to have the most economical gain. The ‘do nothing’ approach is not recommended for the negative perceptions associated with this approach. The most cost-effective approaches using the variables chosen were the land use options of forestry and native habitat. These are also the options that are likely to have the least economical gain. Also, it is of note that if the runoff controls are varied in any way the cost-effectiveness is decreased dramatically and in some cases can be more costly than grazing or cropping.
Upon mining completion, the open-pit will be at a depth of 350m, covering 30ha. The pit walls are expected to contain potentially acid forming sulphides. The hazards of this excavation are as follows:
The process of dewatering causes the pit to become a sink. This occurs when the rate of evaporation is larger than the rate of rainfall, which will cause water to start flowing towards the pit. However, contaminants may concentrate. The pit may depress the surrounding groundwater, however a rock layer will decrease this effect and eliminate contamination of the groundwater.
Pit Wall Failures
· Quick break down of exposed weathered rock
· Softening sped up by wet climate
· Initial instability evident from minor failures within pit benches
· Real possibility of significant failure
· Pit slope stability and erosion gullying of crest will continue to be a safety hazard after mining completion
Sedimentation and Concentration
Regardless of rehabilitation, the pit will eventually fill with sediment and dissolved minerals from water runoff. It is a natural environmental process, however if there remains some potential uses of the pit this process should be limited or controlled.
A practical approach to achieving the target of a water storage structure would be to increase the catchment area by diverting nearby creeks or streams into the pit. An environmental assessment would first need to be performed to analyse the impacts of this action. Other methods include diverting runoff water from haul roads and waste dumps.
The acid forming potential of the pit walls could be a problem, it is ideal that the pit be flooded quickly to limit the amount of chemical reaction. Though it is more expensive, the capping of exposed benches with inert material and vegetation will control the amount of acid produced.
Animal paths and pit wall failures are another issue. Erecting an electric fence will control the movements of larger animals around the pit to designated drink locations.
The target of the project is to maintain a post mining land use of a native habitat reserve. This is to be achieved using a cost effective approach that also limits further environmental impacts elsewhere.
A strategy to implement this would be to convert the pit into a water storage structure. This would function as a water source for native animals and the growth of vegetation. It would also provide a possible economic opportunity and positive social image for the operation as water features are attractive.
How can the project establish a cost effective, self-sustainable native rehabilitation plan?
Potential Failure Scenarios
Some potential failure scenarios are possible, including:
1. Repeated failure of the pit walls maintains a layer of sulphidic rock that prevents the establishment of plant life.
2. The pit does not fill and becomes a sink, causing concentrated acidic water and depressing local groundwater.
3. Water directed to the pit is contaminated and eventually fills the pit.
4. Cost of establishing capping and vegetation on exposed benches is unfeasible.
To reach a cost effective native rehabilitation plan, the local catchment area for the pit must be sufficient to ensure it is flooded, otherwise the pit will turn into a sink. In this state, the pit will not be suitable for native vegetation and will not be of any environmental or economic value.
There are numerous hazards which arise when a waste rock dump is present. Environmental impacts includes pollution of waterways, which in turn would destabilise the ecosystem, effecting vegetation and wildlife. This is because waste rock dumps are comprised of PAF waste rocks which have a retarding effect on the development on plants and animals. It is important to minimise erosion to combat this. If the waste rock dump is located near infrastructure or an urban development then the risk of hazard will be increased.
In order to rehabilitate the land after mining, it is important to perform a stability analysis to ensure that there is sufficient slope. If there is not sufficient slope, then the land cannot return to its native habitat. It is also important to consider monitoring the site during the mining process.
The contamination of the downstream waterway could potentially be treated by the introduction of lime into the rocks. Introducing lime to the PAF rocks would reduce the acidity and therefore neutralise the negative effects it has on the environment. Like with the mining process, constant monitoring of the waterway may be required after mine closure.
To achieve rehabilitation, restoring the post-mining land use to that of native habitat. This will most likely be bushland which can support native animals and is free of contamination so vegetation and wildlife can be sustained.
To achieve restoration to native habitat is the ultimate goal. Vegetation and wildlife must be sustained naturally in the end.
Rehabilitation Options and Recommendations
There are two primary potential scenarios for failure. These are when the ground deteriorates in composition and form which does not allow plant growth, and when deep gullying or erosion occurs.
Stabilisation of the slope must be achieved in order for the land to be restored to native habitat. This will encourage topsoiling and revegetation. The landform must also be aesthetically pleasing by conforming with the surrounding area while hydraulically being able to channel water effectively.
The slope should be shallow as this would not erode as easily and would allow vegetation to sprout easily. This would be done in accordance with the legislative demands and the contract. The shallow slope would also allow any runoff created by storm events to flow easily downhill without a large velocity which would cause corrosion. The time of concentration would therefore be very low. This is important as the mine experiences between 500-1500 mm of rainfall each year due to its semi-arid climate.
It is important to cover the waste rock dump with topsoil as to allow vegetation to grow. Soil amendment must be carried out on the waste rock to balance pH for the Potentially Acid Forming (PAF) waste rock. Limestone, dolomite and agricultural lime all perform the task.
This results in leaving the surface in a cloddy state. Because sandy soil particles are not usually strong enough to resist abrasion, this can be difficult to achieve. The soil surface must be kept damp by using sprays and water tankers. This will increase the aggregation of particles, resulting in an increased resistance to wind erosion. Trucks should be used over scrapers in replacing topsoil, as this would create dimples.
In the case of some steep slope sections, moonscaping may be considered to stabilise the ground and allow vegetation. It is very important that all craters and moulds have interlocking sections to avoid channel runoff. If the slopes become too long, bench or contour banks should be introduced.
By using larger particles of rock, over 150 mm in diameter, it is possible to provide a surface effective in resisting erosion. This blocky overburden material is very course and rests on the top layer. It is resistant to many forms of corrosion, even trapping smaller particles in crevices when there are high wind speeds. Hydro seeding may also be used to accelerate the rate of vegetation.
Rock Cladding is most likely the best rehabilitation technique when compared to dimping and moonscaping. This is due to the simplicity of remediation of the present dumps. These have already been divided into PAD, inert fresh rock and PAF mineralised rock.
The inert fresh rock later should be spread among the other two rock types, freeing up storage space, which would further limit damage to the environment. Rock Cladding is also an excellent option financially, as the cost is less than that of dimping and moonscaping.
Heap leaching for this mine site extracts gold and copper using a leach solution of cyanide and/or sulphuric acid. Covering 50 ha, the heap leach pads provide a platform from which the majority of the ore has been removed for further crushing, grinding and cyanide recovery of gold. Ore is placed on an impermeable liner and the leach solution is added to the top of the ore. The leach solution passes through the solution to the liner extracting the metals, before the solution is then drained so the metals can be removed.
Hazards and Pathways
There are various hazards regarding heap leach pads. The most important hazards that need to be addressed are; erosion of the ore, failure of the pad cover, stability of the pad and contamination of water. Erosion of the ore heap is the act of removal of the material from the heap leach pad via either wind or water sources. Due to the significant amount of rainfall that the mine site can encounter, the rainfall runoff can lead to erosion of the heap leach pads. Due to the mine being situated in semi-arid climate, wind erosion also needs to be considered. Wind erosion can affect a large area of land and has the potential to have an impact on the outside community as well by the transportation of the hazardous materials via dusting. Erosion reduces the capacity of the pad and can result in overtopping of the containment structure. The lining of the pad must be constructed with appropriate consideration as failure can have an extremely detrimental effect to the surrounding environment. The lining of the heap leach pads can fail due to sliding of debris, in turn, tearing the lining. It can also fail due to a miscalculation resulting in excess load placed on the pad.
Slope stability needs to be considered as every time some of the ore is removed, the shape of the pad is altered, potentially leading to pad failure. The stability of the heap leach pads needs to be properly maintained to prevent the collapse of the crushed ore. There is the possibility that the ore could travel down the pad and may possibly disturb the lower lying areas. Infiltration of the soil with wastes through the lining of the pad can be a possible pathway when the lining of the pad is compromised, ultimately leading to groundwater and soil contamination. Surface water can also be considered a hazard as it can be contaminated by rainwater run-off transporting small particles of heavy metals, sulfates or cyanides from the pad to the river systems, contaminating downstream water systems. Due to the majority of the heap leach pads being used for cyanide recovery of gold, there is an extreme danger of poisoning to the local environment as even the smallest amounts of cyanide concentration in the water can have damaging effects to both humans drinking water and the homes of aquatic life. Hence the need to keep contamination to water sources is at a minimum is evident.
§ To achieve the post-mining land use of forestry with the maximum economic efficiency and minimum environmental impact.
§ Minimise contaminants present both on the site of the heap leach pads, as well as the potential for contaminants to leave the site through either soil or water.
§ Remove the risk of contamination of both the ground and surface water supplies by means of an adequate drainage and treatment facility.
What is the most economically viable method of bringing the heap leach pads to the agreed post mining land use of forestry while limiting the environmental impacts?
Potential Failure Scenarios
The potential failure scenario involves the breakdown of the lining and the heap leach pads to contain hazardous materials. Water contamination, erosion or slope instability can lead to the failure scenario releasing hazardous materials to the environment outside of the heap leach pad. Considering the above facts it seems appropriate to assign the following risk assessment values:
Likelihood: Moderate (3)
Consequence: Negligible (1)
Risk is considered Significant.
In order to maximise cost effectiveness and minimise environment impacts, the initial design of the heap leach pads and ongoing maintenance is crucial. The leach process has the potential to contaminate ground water through seepage of the liner. In order to prevent seepage and maintain the integrity of the liner, care must be taken in the initial construction as well as adequate and regular inspections must be undertaken.
Revegetation is one of the most important aspects of the leach pad rehabilitation. Due to the harsh climate both the timing and vegetation used during this phase must be carefully considered:
§ Timing: Reshaping and Revegetation should commence as soon as possible after the end of the wet season. By commencing at this time erosion and wash off during high rainfall events will be reduced. The increased soil moisture content should create an ideal environment for vegetation.
§ Vegetation Type : The primary aim of revegetation is stabilise slopes and prevent the occurrence of erosion. With this in mind the best vegetation is local grasses and shrubs. By using plants with a high tolerance to dry conditions and substantial root systems the plants will hold together topsoil and reduce erosion, will having greater access to water supplies deeper beneath the surface.
Surface water runoff can disperse the leach solution off the heap leach pad and consequently contaminate ground water. However preventative measures can be taken to avoid this problem These measures include design of the heap leach pad to facilitate the maximum possible rainfall. In addition, leachate can be minimised by removing spent ore or by adding water to the ore heap to reduce the hazardous chemical levels within the leach solutions.
Passive abandonment or natural rehabilitation is the process whereby mine operators effectively let nature run its course:
§ Passive abandonment is a viable option due to the fact that cyanide degradation is a naturally occurring process over time.
§ The harsh semi arid climate will aid the degradation process however this solution is generally the slowest of the 3 options available.
§ Often the cheapest possible solution
§ Mine owners have little control of the detoxification process and as such it is difficult to estimate timeframes.
It is recommended that hard, durable rocks should be incorporated to the slope of the heap to assist in the prevention of erosion by increasing the stability. Previsions should be incorporated to limit and contain the leachate and water damage of the heap. Additionally, topsoil should be introduced in order to promote vegetative growth that will help reduce erosion.
Chemical treatment involves the use of chemicals to induce the degradation of cyanide and other chemicals into less dangerous – non toxic chemical compounds:
§ This method gives the mine operator a great deal of control over the detoxification process
§ Considerably faster and more effective than the previous to methods
§ Uses dangerous chemicals particularly Hydrogen peroxide and calcium hypochlorite
§ High cost of chemicals and transport for these products
§ Procedures for handling and protocol in case of a spill must be put in place in order to minimize both the likelihood of a spill and damage caused if a spill was to occur.
§ To minimise the geochemical fallouts, careful pH control and close monitoring of the system is required.
§ Instability can cause the tailings storage facility to fail resulting in the potential contamination of surrounding soils, groundwater and downstream water supply.
§ Acid seepage forming mine drainage resulting in the potential contamination of surrounding soils, groundwater and the downstream water supply.
§ Saline seepage resulting in the potential contamination of surrounding soils, groundwater and the downstream water supply.
§ Tailings can produce pollutant gaseous emissions which can affect nearby townspeople and workers.
§ Acidity of the tailings affecting soil quality resulting in the potential impeding of revegetation, which will in turn cause problems in trying to achieve the post-mining land use.
§ Onsite accidents including accidental drowning in the tailings dam and physical injuries resulting in loading of crusted tailings with an inadequate bearing capacity.
§ Proper placement of the tailings storage facilities.
§ Capping the tailings prior to revegetation will minimise toxic leachate generation.
§ Revegetation of the tailings storage facilities to minimise erosion and increase the stability.
§ Continual monitoring of the tailings storage facilities for significant changes in the environment.
§ Provide adequate danger warning signs and/or barricades around the tailings storage facilities to warn workers of potential hazards.
§ Ensuring that during inductions for the site potential hazards are made well aware to the workers.
§ Suitable work wear and protective equipment to be enforced when working around the tailings storage facilities.
§ To have a safe and accident-free workplace.
§ Minimise impacts on the surrounding environment, workers and nearby towns.
There are a number of possible techniques to achieve safe closure of tailings storage facilities. The risk issues with the closure of a TSF are embankment stability, surface water erosion, flora and fauna impacts, dust, seepage and radiation. To manage these issues two principles in the closure of the TSF should be incorporated, these principles are:
§ Water shedding – rainfall in transported to the cell perimeters and shed into the surrounding natural environment without entrainment of the cover or significant erosion.
§ Store and release – this is where infiltrated water is temporarily retained within the cover and is evaporated or is consumed in evapotranspiration where there is available topsoil materials.
Using a cover system that incorporates these two principles is the ideal way to ensure the safe closure of a tailings storage facility.
There are two primary types of Haul Roads used at mining locations. These include in-pit roads and surface roads. The in-pit roads pose little threat to the environment as any water runoff will be directed downhill into the mining pit where it can be treated accordingly. Surface roads, however, present numerous challenges such as compaction and contamination of both the land surface and water runoff.
Compaction in caused by loading that has been applied over an extended period of time. The weight placed upon the surface is typically over 300 tonnes and will be applied as trucks drive over every few minutes. Contamination will occur when passing vehicles leak or loose materials and substances they are transporting or debris from the actual vehicle such as petroleum.
These roads pose a threat to water ways, as they remain in operation during heavy rain. Because of this, the water is redirected off the road and potentially into waterways and natural channels. It is therefore important for a sufficient drainage system to be applied.
In order to achieve the nominated target as specified in the contract, all water runoff from the 6km surface haul road will need to be redirected into the pit. It can be assumed that 4km of haul road is in-pit at a grade of 1 is to 10.
The road should be covered with topsoil to assist in re-vegetation as well as for containing the contaminants. The alternate would be to rip up the road surface to combat the problem of compaction. This option is likely to be more costly. Finally, proper drainage should be installed to ensure effective water runoff at all times.
The target of the haul road usage post mining is to facilitate a native habitat by overcoming its potential hazards using an economic and environmentally sensitive approach. This approach will direct contaminated runoff water from the haul roads into the pit.
The haul roads only make up a small area of the site and are highly compacted, with contaminants in the road expecting to remain for a long period. For these reasons, complete re-vegetation may not be realistic or even required if they could be utilised post mining. It would be more efficient to use them as a path instead for navigation through the post mining reserve site for monitoring or scenic purposes.
Potential Failure Scenarios
There are several potential failure scenarios, which occur when water runoff cannot be channeled back into the pit. Contamination of offsite waterways is the most likely scenario, others include:
Due to the semi-arid environment, high-quality topsoils are required for successful re-vegetation, which would involve an expensive initial cost. The raised profile of the road also makes increases the difficulty for vegetation to assimilate with the existing environment. The cost to cover the entire length of road would be significant, and care must be taken to ensure vegetation is quickly planted so that topsoil is not lost to erosion.
Covering the entire road would limit access for other possible post mining land uses. Another solution could be to only cover a certain percentage of the road so that the remainder is suitable for park reserve access while still maintaining the planned native habitat.
Ripping up the haul road would be an effective method of solving the compaction problem. The road profile would largely remain the same and be more encouraging for vegetation growth. This option offers the best results but would be more expensive, as disposal of the contaminated surface will also be required before re-vegetation proceeds. Once this is disposed of, topsoiling may still be required to encourage growth.
Loss of access is also considered in this method with a similar solution of only treating a percentage of the road.
Redirecting Contaminated Runoff
Along with one of the rehabilitation methods above, drainage will be necessary to control contaminated runoff. Nearby natural waterways must be avoided, thus the contaminated runoff from the haul road should be redirected to be contained in the pit.
The preferred option to the target for complete re-vegetation of native habitat would be the rip-up method to undo compaction and dispose of the contaminated road surface, which would facilitate and encourage the best vegetation growth at a higher cost. A drainage system should be provided to manage the contaminated runoff to the containing pit.
Sediment collections dams are used to slow the rate of runoff from the mine which helps to allow settlement in the runoff to settle. It also helps to control the rate of runoff from the mine into the surrounding environment and can act as a barrier against AMD if the prior preventative measures fail to work. Though in normal cases it is assumed that sediment collection dams do not possess any AMD issues.
The target of the sediment collection dams is that they are to be used for the remainder of the mines life. Then to reformed as to comply with the post mining use of the land as sparse cattle grazing. As the land is to be used for cattle grazing a suitable use would be as a watering hole or wetlands for flora and fauna. This is to be accomplished in a manner that is beneficial to the environment but also cost effective.
The ultimate question is how to best reform the sediment collections dams in a way that will result in the best environmental outcome and remain cost efficient.
Hazards concerning sediment collection dams include.
Drying of the dam: This possesses many hazards such as the increase of salinity due to the reduced water levels. If the dam is to be reformed into a wetland it would result in the loss of fish and other flora and fauna. It is relevant during the operation of the mine and afterwards during rehabilitation.
Quality of Runoff from Dam: The water quality of the runoff from the dam must be monitored throughout the operation of the mine and during its rehabilitation. This will help to lead to an easier more cost efficient rehabilitation as any potential issues such as acidity salinity etc, Can be addressed as soon as they occur.
Erosion of the Dam and contained tailings: Erosion of the dam may cause a failure which will lead to an uncontrolled release of runoff into the environment. This may cause problems such as ADM increased salinity of unwanted sediment build up further downstream. During rehabilitation it may be controlled by the planting of grass trees etc.
Forming into wetlands: Reforming the sediment collection dams into wetlands would be beneficial as it would provide a suitable area for various flora and fauna. It may also provide a spot for activities such as fishing/ camping etc. Erosion and water quality could be improved by the addition of various plants in and around the dam. This would provide a more aesthetically pleasing reformation of the dam.
Using the surrounding drainage system: If the surrounding drainage system is used the dam would provide a suitable watering hole for the grazing cattle which will be present after the land is reformed. It would also help to control any runoff and help mitigate flooding etc if there is a problem present. The dam may also slowly develop into a wetland over time due to natural causes.
After analysing the proposed methods of rehabilitation of the dams it was decided that they will be reformed into wetlands. This will be in conjunction with the reformation of the drainage systems as they will help to provide water to the wetlands. Revegetation would be required as well as research into whether the wetlands will be able to sustain themselves in the arid climate. This option would help to improve the water quality and provide a habitat for fauna as well as a watering hole for the grazing cattle.
§ Seepage of chemicals used in the processing plant. This may result in the contamination of the local soils, groundwater and the downstream water supply
§ The seeping of chemicals from the processing plant causing salinity or acidity in the surrounding soils
§ Equipment removed from site which is not properly decontaminated, leaking chemicals at the next site that they are relocated to
§ Regular checks of practises and safeguards in the processing plant to prevent the leaking of chemicals, especially sulphur and cyanide, out of the designated waste dumps
§ Monitoring of the soil around the processing plant looking for changes in the chemical composition
§ Monitoring of the groundwater and downstream water supply for changes in chemical composition
§ Decontaminating and Registering of equipment to be removed from site
§ Be able to remove Processing Plant and Office Complex from site completely, leaving little or no permanent effects on the environment
§ To revegetate the soil to be able to be used for the agree post-mining land use of spare cattle grazing
After the final 5 years of processing, all equipment used in the Processing Plant and Office Complex need to be dismantled and removed from the area so that the 10ha that was taken up by the infrastructure can be revegetated and used for the agreed post-mining land-use of sparse cattle grazing.
Potential Failure Scenarios
The most likely potential failure scenario is contamination of the groundwater supply, surrounding soils or downstream water supplies that can be caused by:
Leaking of concentrated sulphur or cyanide waste water into surrounding soil
Equipment in the Processing Plant or Office Complex that leaves site after use not properly decontaminated and contaminating surrounding soil or water supply at the next site it is used at
Operational and Rehabilitation Processes
§ Essential that sources of clean and dirty water be kept separate, both during operation and after closure
§ Incorporating a “prevention is better than cure” ideology, with regular inspections of the processing plant to maintain safe operating levels
§ Prior to revegetation, testing of the soil after dismantling the processing plant will need to be carried out to determine the effects of the plant. Also, further testing of suitable species of plant is required
§ All equipment that is to be removed from site first needs to be decontaminated. After decontamination, all equipment needs to be certified and registered for radiation safety before being removed by site.
Hazards and Pathways
There are two main hazards when dealing with bore fields; the drying and also the contamination of the groundwater. The drying of the bore field occurs when the water is used at a greater rate than the natural replenishing process. This can lead to undesirable conditions such as high salinity and high concentrations of dissolved solids. This can cause the water to become unusable for mining purposes the natural environment, or for agricultural purposes. The other hazard is contamination of the water supply from the mine operations. The most common contaminants on the mine site are potentially acid forming sulphides and potentially acid forming mineralised rock. A pathway for this is the improper decommissioning of unused bore fields.
The targets of the bore fields are that they be used in such a way that they sustainable for the lifetime of the mine that is, for the remaining three years of mining operation and the five years of processing. Also the decommissioning of those bore fields not needed will need to be implemented to comply with the post-mining land use agreement of sparse cattle grazing.
To prevent the over use of the bore fields and thus the prevention of a potential hazard, the implementation of a monitoring system could be considered. The Snapper Mineral Sand Mine has implemented such a system wherein trigger levels are established. This is a proactive approach to prevention and it exceeded the legislative requirements at the time. This method observes the water quality and the groundwater level by setting levels for both total dissolved solids and water level (static depth below measuring point). The water levels were established by modelling and consultation with the land owner where applicable which can give such information as the historic level. If the trigger levels were exceeded an investigation would be conducted to assess if the impact was the result of the mine operation or another variant such as seasonal weather. A system like this one could be implemented in the mine site referred to in this report. Such measure, however, would need to be further investigated to account for cost-effectiveness.
The remedial actions agreed to be undertaken should the mine adversely affect the groundwater users in the case of the Snapper Mineral Sand Mine can give an indication of remedial actions that could be undertaken in the present case. These actions include, providing an alternate water supply of similar standard to the water before the bore was affected, until the effected supply is restored. Also, the use of appropriate measures to restore the supply, for example, bore reconditioning or lowering would be implemented if necessary. The remedial actions will also have to meet the statutory natural resource management responsibilities.
It should be noted that these remedial actions are not cost effective as they are not necessary to the operation of the bore fields. It may be seen that it is more cost effective to prevent the negative impacts before the stage is reached where remedial actions need to be taken. The cost of a monitoring system will need to be compared to the cost of remedial actions and the likelihood of the need for these remedial actions.
The most cost effective post mining land-use is classified as ‘do nothing’ however, seeing as the land is agreed to be used for grazing. Decommissioning measured should be undertaken. There are five steps to undertake to properly decommission a bore hole and well. That is to define the objectives for the specific case, remove the headworks and casing, backfilling the hole, sealing the top of the bore hole and recording the details. However, it is recommended that as each bore hole is different, they be considered on a case by case basis.
The objectives are defined in terms of safety and hazards. The method implement should meet the following requirements; eliminate the safety risk associated with an open hole, prevent bore hole acting as a conduit for contamination, prevent the mixing of contaminated and uncontaminated water, prevent the flow of water from geological horizon to geological horizon and, prevent wastage of water. Secondly, the removal of the headwork and casing ensures that the bore hole is free from obstruction that may hinder the capping process.
Next, the backfilling of the hole should result in a state that is of similar standard to the pre-drilled state. Backfilling must be clean, inert and non-polluting. Such minerals include pea gravel, concrete, bentonite, sand, shingle, cement grout and uncontaminated rock. There are two main methods of backfilling. The first is the use of low permeability materials for filling of the entire hole. Alternatively permeable aggregates can be placed adjacent to aquifer horizons and low permeability materials are placed adjacent to low permeability horizons. As there are limited materials on site that fulfil these criteria it would need to be established which method is the most cost effective. Consideration also needs to be given to the geochemical environment to ensure acceptable material behaviour.
Sealing of the bore hole is used to prevent contamination products entering the borehole backfill. Completion of the bore hole backfilling required the use of an impermeable seal or cap. It is recommended that this seal be at least two meters in depth and have a diameter at least one meter greater that the bore hole. Suitable materials include, cement, concrete or bentonite grout.
Also, accurate and complete records should be kept for future reference. Especially in regards to water depth, backfilling and sealing materials and levels, any changes i.e. casing removal and problems encountered.
In any post land use rehabilitation attempt, there are always going to be potential failure scenarios that need to be encountered for, acted upon, and minimized so as to attain successful rehabilitation.
Cover deterioration can rapidly occur in this particular environment the site is situated at, as data shows rainfalls can be expected of up to 1500mm during the 3-month period predominantly over summer. Topsoils can potentially erode away above the tailings storage facility, leaving tailings exposed to the natural elements such as wind and rain. Whilst the tailings themselves are understood to have limited potential acidic properties due to the nature of the counteracting alkaline tailings water they are submersed in, a tailings storage facility of this size storing approx. 60 million cubic meters of tailings and traversing an area of 300 Ha should always be encountered for in mitigating possible failure scenarios.
The existing topsoil itself is also considered to be poor in quality and highly erodible, so adequate care should be taken to ensure that it is placed and mounded correctly and distributed in proper quantities, so as to minimize the effect of damaging runoff, and also seepage of rainwater into the covering device, whilst promoting adequate transpiration needed.
The topsoil should also be enriched with fertilizers so as to promote the growth of vegetation needed for a post mine use of sparse cattle grazing. If no topsoil is present at all due to potential erosion, then the intended post mine land use agreement is null and void, as without sufficient vegetation for cattle to graze upon the mine site is effectively poorly rehabilitated and fails in its intended post land use agreement.
Contaminants within soil can lead to the death of vegetation upon rehabilitation of the site. Highly acidic or alternatively highly alkaline soils can pose problems to the native vegetation that already pre-exists at the site, as well as the intended additional vegetation to be put in place upon closure of the site.
A large proportion of the waste rock stored on site is potentially acid forming. This waste rock, coupled with heavy rainfall and erosion can instigate contamination within the soil on site, so therefore precautions must be made to minimize the effects of the PAF waste rock in order to achieve successful rehabilitation.
As well as this the walls of the open cut pit contain potentially acid forming sulphides whose effects again need to be mitigated. If adequate preventative measures to suppress soil contamination both within the pit and also within the surrounding land are not put in place, rehabilitative success will not be attained. A pit with sulphidic rock making up the walls of the enclosure is in essence a giant storage device for sulphidic runoff, which would then seep into the underlying and surrounding soil, and eventually make its way into the ground table water system, thus greatly spreading the area of potentially contaminated soil.
The last scenario is that acidic waste rock and or tailings will react with the surrounding atmosphere and coupled with water present within the rock will produce acidic transpiration. This could under the worst case circumstances produce acid rain which would not only re-contaminate the soil present on site but would greatly increase the potential for mass erosion of the pit walls, the heap leach pads and also the tailings storage facility.
The effects of potentially contaminated soil can have lead on effects as well. If vegetation is also contaminated then the livestock intended to be stationed on the post-land-use site will also be affected. Contaminated vegetation will lead to contaminated livestock, which in the case of cattle as a consumable resource, means that the product will not be viable on the market as it will not pass health and safety regulations within the food industry. Ultimately, contaminated soil must be managed and mitigated wherever possible so as to attain successful rehabilitation of the site and consequently usable land.
Rehabilitation of a mine site is possibly the most important undertaking of the whole mining operation. The possibility of inadequate rehabilitation can have disastrous effects for the environment, the surrounding population, the company’s coffers, and lastly the company’s reputation. Therefore, it is fundamental that successful rehabilitation be a No.1 priority in the operational plans of a mining firm.
With the ACARP assessment, coupled with the Risk Assessment factors, cost effectiveness and stakeholder requirements can all be accounted for, providing the most cost effective and satisfactory outcomes for all parties involved. If the rehabilitation is ultimately successful and all entities involved are satisfied with the results, then this is the most beneficial outcome for the company, the stakeholders, the population, and also the environment. With the recognition of hazards, the implementation of target outcomes, and the instigation of techniques used to mitigate hazards, then agreeable post-mine-use land is achievable. In this situation regarding the Bowen Basin mine site, all the factors have been weighed up and analyzed and ultimately this report has entailed the best possible rehabilitation scenario.