Subaqueous Disposal in a Waste Management Facility
Clayton Larkins, Geological Survey of Finland GTK, P.O.Box 1237, FI-70211, Kuopio, FINLAND, email: clayton.larkins(at)gtk.fi
Long term management of acid mine drainage (AMD) and heavy metals emissions from mine sites after closure poses a persistent challenge to sustainable mining practices. Subaqueous disposal of mine waste is recognized as the most effective strategy for preventing sulphide oxidation and subsequent AMD and heavy metals emissions from mine waste (Tremblay & Hogan 2001, EC 2009, Kauppila et al. 2013). Mine waste has been disposed in subaqueous settings that include constructed impoundments, flooded open pits, natural lakes, and marine environments (EC 2009). This page focuses on subaqueous mine waste disposal in constructed impoundments.
The use of a constructed impoundment as a waste management facility (WMF) for subaqueous mine waste disposal is considered the least environmentally and politically controversial subaqueous disposal option (EC 2009). As with other subaqueous disposal methods, this method utilizes water as a barrier to inhibit the oxidation of sulphide minerals in the mine waste. Sulphide oxidation can be effectively limited under a water cover because the concentration of molecular oxygen in water is significantly less than in air, and the diffusion rate of oxygen in water is approximately 4 orders of magnitude lower than in air (MEND 1998, Lottermoser 2010, Yanful et al. 2000). The processes acting to limit sulphide reactivity under a water barrier as described by Robertson et al. (1997) are summarized below:
- The relatively low molecular oxygen concentration and diffusion rate in water promote the development of anoxic conditions, especially in shallow sediments, which limit sulphide oxidation
- Iron hydroxide precipitation in anoxic conditions immobilizes heavy metals through absorption, and limits ferric iron availability as a sulphide oxidant
- Precipitation of Fe and Mn oxides in the oxygenated water of the sediment-water interface immobilizes metals through adsorption (oxide scavenging)
- Natural sedimentation on top of tailings acts to stabilize contaminants by further limiting oxygen availability in tailings and inhibiting metal diffusion from mine waste into overlying water
Technology Concept and Methods
For this method a man-made water retaining impoundment is utilized as a WMF, tailings are deposited under water or submerged after initial deposition, and the water level in the pond is controlled to ensure the tailings remain submerged indefinitely. The pond provides a water barrier that prevents the generation of contaminants by limiting oxidation of sulphide minerals in the tailings. The water barrier also acts to prevent contaminant transport by wind ablation, and can facilitate the development of a sediment layer on top of the tailings to further limit the potential for contaminant diffusion from pore water in the mine waste (Robertson et al. 1997, MEND 1998). Subsequently, it is expected that if the WMF design is incorporated into the watershed to receive sediment inputs from upstream, it will more effectively immobilize tailings (MEND 1998, Heikkinen et al. 2008).
For subaqueous tailings emplacement, mine waste in the form of tailings slurry is piped into the pond using floating or underwater piping systems. The piping system must discharge tailings evenly across the bottom of the pond to prevent the formation of a tailings beach or the occurrence of subaqueous slides (MEND 1998, Dillon et al. 2003). The use of piping is preferred to conveyor belts because it minimizes atmospheric exposure prior to subaqueous deposition. Mobile piping systems capable of spreading tailings beneath the water surface and throughout the WMF are utilized to achieve even tailings emplacement. For example, ropes used to pull floating piping in wide arcs across the WMF surface allow for even tailings emplacement (MEND 1998, Dillon et al. 2003). Achieving even tailings emplacement is more difficult in shallow impoundments with large surface areas and may require the use of additional technologies such as a motorized barge or specialized anchoring system (MEND 1998).
Once tailings are submerged, they must remain isolated from atmospheric conditions and oxygenated water to remain inert. To ensure the tailings remain inert, the impoundment is designed to meet the following conditions:
- Dams and impoundments are designed for long term structural stability and water retention
- Water is sufficiently available to maintain a satisfactory water barrier, even during extended dry periods
- Outflows are designed to maintain a water balance and prevent flooding or overtopping, even during extreme weather events such as large storms or the development of thick ice
- The water barrier is sufficiently deep, even during extended dry periods, to ensure that tailings remain physically and chemically stable
The MEND Report 2.18.1 summarizes seven case studies on the subaqueous disposal of sulphate bearing mine tailings, six of which utilize constructed impoundments (Yanful & Simms 1997). This report also summarizes various methods for predicting required depth of water in impoundments. These case studies demonstrate the application of different subaqueous disposal scenarios, as summarized below.
- Subaqueous tailings disposal in WMF: Equity Silver, British Columbia, Canada; Quirke Lake Tailings Test Site, Elliot Lake, Ontario Canada ; Hjerkinn Tailings Pond, Norway ; Lokken, Norway; Stekenjokk, Sweden; Solbec-Cubra, Quebec, Canada
- Subaqueous tailings disposal in WMF with material additions to increase chemical and physical stability: Cell 14, Quirke Lake Tailings Test Site, Elliot Lake, Ontario Canada and Solbec-Cubra, Quebec, Canada (lime); Stekenjokk, Sweden (sand layer)
- Use of water breaks in WMF to increase tailings stability: Stekenjokk, Sweden
- Subaqueous disposal of previously oxidized tailings in WMF: Solbec-Cubra, Quebec, Canada
The Stekenjokk mine site case study provided in this website summarises the application of subaqueous disposal at this site. A summary of maintenance on dam outlets and lessons learned at the Stekenjokk site is also provided in Section 126.96.36.199 of the European Commission’s Reference Document on Best Available Techniques for Management of Tailings and Waste-Rock in Mining Activities (EC 2009).
The work done at the Benambra mine site in Australia provides another example of subaqueous disposal in an impoundment with the addition of alkaline material to enhance tailings stability. This work is summarized as a case study in the GARD Guide (INAP 2009).
Dillon et al. (2003) report on the design methods and construction issues encountered at Europe’s largest subaqueous WMF, in Lisheen, Ireland (Link)
To date subaqueous disposal within a WMF has performed successfully at the Samatosum mine site.
Subaqueous disposal of mine waste is appropriate for potentially acid or dust generating tailings or mineral precipitate sludge (Kauppila et al. 2013). The suitability of this method must be determined using risk assessment and consideration of the site-specific variables collected during the design phase of the mine-life cycle, as described by The Reference Document on Best Available Techniques for Management of Tailings and Waste-Rock in Mining Activities (EC 2009). Primary physical design considerations are the long term stability of impoundment dams, and long term water balance management. Careful consideration of tailings and impoundment water chemistry are also necessary to predict the effectiveness of subaqueous disposal. The Design Guide for the Subaqueous Disposal of Reactive Tailings in Constructed Impoundments (MEND 1998) provides a detailed and systematic approach for the design and application of this method.
Subaqueous disposal is only appropriate if hydrologic conditions constitute sufficient water to maintain an adequate water barrier. Therefore the disposal site requires a local positive water balance and the WMF must be designed for long term water storage. For potentially acid generating waste, the WMF requires an impermeable basal structure, which ensures water retention and prevents exchange between WMF water and potentially oxygenated groundwater (Kauppila et al. 2013). The WMF requires sufficiently robust dam structures, with design specifications dependant on site-specific conditions including local seismicity, the nature and distribution of the waste material, and local weather extremes (EC 2009). Design considerations must prevent dam overtopping through implementation of adequate spillways, prevent subaerial waste exposure or ice entrainment with sufficiently deep water cover, and inhibit waste disturbance and oxidation by wave action. The effects of wave action can be minimized with sufficient water depth and the implementation of wave break structures (MEND 1998).
If tailings have undergone oxidation prior to subaqueous emplacement, soluble contaminants may be mobilized by submergence and result in unacceptable emissions (Tremblay & Hogan 2001, Lottermoser 2010). Tailings oxidation can occur during subaerial storage or through certain ore processing techniques, such as pressure oxidation for the extraction of gold and silver concentrates (MEND 1998). However, case studies have shown that emissions from submerged oxidized waste may be maintained within acceptable levels by introducing organics or neutralizing agents such as lime to the subaqueous environment (Tremblay & Hogan 2001). Therefore, the effectiveness of this method for the disposal of oxidized tailings should be evaluated on a case-specific basis (INAP 2009).
Advantages of this mine waste management technology include:
- It is the most effective method for minimizing ARD generation and related emissions from mine waste (Tremblay &Hogan 2001, EC 2009, INAP 2009)
- The constructed impoundment, as a controlled waste disposal environment designed to minimize emissions, is the most environmentally and politically accepted subaqueous disposal option (EC 2009)
- Relative to dry barriers, water barriers reduce the need for borrow material and the subsequent creation of borrow pits (EC 2009)
- Long term decommissioning costs have been found to be lower than those of dry cover decommissioning (Holmström & Öhlander 1999)
- Effective application of a wet barrier reduces the need for water treatment before outflow is discharged to surface water (EC 2009)
Disadvantages for this closure technology include:
- Application of the waste management strategy is limited by the following site-specific physical and chemical variables
- Insufficient long-term water availability to maintain adequate water barrier
- Site specific risks such as seismicity and severe storm events (Tremblay & Hogan 2001)
- Space and material restrictions for construction of permanent water retaining dams (MEND 1998, Kauppila et al. 2013)
- Soluble contaminants in tailings as the result of ore mineralogy, mill processing techniques, or prolonged atmospheric exposure prior to disposal (MEND 1998)
- Initial design and construction cost for permanent water impoundment facilities are relatively high (Kauppila et al. 2013)
Under suitable circumstances, the subaqueous disposal of mine waste into a constructed facility provides effective abatement of AMD generation from mine waste (Tremblay & Hogan 2001, EC 2009).
The capacity of impoundments is limited by geography, land availability, and construction material resources. Impoundment design is dictated by site geography and hydrology for long term water balance management (MEND 1998). For example, impoundments can be designed to be cross-valley, off-valley, or on flatland, to either maximize the availability of water for impoundment flooding, or reduce the need for water diversion (EC 2009). Subsequently, both site geography and land availability influence impoundment capacity. Capacity may be further limited by the relatively large footprint and abundant construction material needs of a dam designed to retain large volumes of water (Kauppila et al. 2013).
Maintenance needs may be identified throughout operation or during post closure monitoring. Maintenance to improve dam stability, address seepage, or other potential emission pathways may be required. Measures to improve dam stability include slope angle reduction, terracing, shoulder reinforcement, reducing pore water pressure by pumping, or erosion control (Heikkinen et al. 2008). Maintenance to address seepage may include construction of perimeter drains and return systems. Controlling seepage to groundwater may entail seal repairs, construction of grout curtains, slurry barriers, or a permeable reactive barrier (Heikkinen et al. 2008, EC 2009).
Maintenance at the Stekenjokk site was conducted to prevent overtopping of the dam (EC 2009). The impoundment outlet was reconstructed to prevent clogging under conditions of extreme ice formation, and an emergency outlet was constructed as a safety feature.
Environmental and Monetary Cost Aspects
The environmental cost of this application is dependent on site specific variables defined through environmental impact assessment and considered as part of the risk assessment process, as described in the Mine Closure Handbook (Heikkinen et al. 2008). Environmental costs of WMF construction include the need for construction materials, future land use restrictions and impact to the aesthetic value of the reclaimed landscape. Depending on the WMF setting and design, managing the water balance may require diverting clean surface water into the WMF. Therefore freshwater input requirements may also be considered an environmental cost of this waste management method.
The environmental costs of potential failure range from minor to catastrophic. The development of turbidity, or chemical instability in the WMF coupled with insufficient downstream water treatment could result in contaminant releases to surface water. Additionally, WMF dam or basal seepage could result in acid and metals emissions to surface and groundwater.
The largest potential environmental impact is catastrophic structural failure of the WMF. Such failures can result in the uncontrolled release of contaminated waste, destruction of property and loss of life. Work by Rico et al. (2007) reviews historical tailings dam failures world-wide and provides insight on both the cause and impacts of such failures.
The cost efficiency of a waste management method is dependent on site-specific conditions. In general subaqueous waste disposal during operations is slightly more costly than subaerial waste disposal due to special requirements and consideration for oxidation prevention during waste emplacement. However, mine site decommissioning costs for subaqueous disposal methods are generally significantly lower than subarial methods (UC 2009). The use of a water cover in a WMF was demonstrated to be a highly cost efficient mine closure strategy at the Stekenjokk site in Sweden (Broman & Göransson 1994). At this site a water cover was estimated to cost six to eight times less than utilization of a dry cover. In 1990 the construction of a water cover at Stekenjokk required an investment of approximately two U.S. Dollars (USD) per m2, relative to 12 USD per m2 estimated for a dry cover at the same site (EC 2009).
Site Specific Data Needs
The data needs for evaluating the application of subaqueous disposal using a WMF are collected during the design phase of the mine life-cycle (EC 2009). Site specific physical, chemical, and socioeconomic data are integrated into a risk assessment to evaluate the appropriate technology for waste management. While all of these design phase data are utilized to determine the appropriate waste management technology, fundamental site specific variables that dictate the applicability of this waste management method include tailings chemistry, water availability, and long term impoundment stability. Chemical characterization of tailings informs the need for ARD prevention measures, and is required to evaluate the effectiveness of a water barrier as a prevention strategy. Detailed assessment of site hydrology and hydrogeology is required to evaluate the feasibility of water balance management for a long term water barrier. Data on foundation geology, topography, construction material availability, and special environmental considerations such as storm events or seismicity are required for dam design considerations (MEND 1998, EC 2009).
For tailings to remain inert, the WMF is designed to prevent tailings resuspension by entrainment forces. Therefore site specific entrainment forces are evaluated for consideration in the WMF design. The primary entrainment forces acting on man-made impoundments, as discussed in MEND (1998), are ice entrainment and wave action. Kauppila et al. (2013) indicate seasonal water column blending as an additional important entrainment force in the Finnish climate. Site specific entrainment forces are predicted using site meteorology and topography data (MEND 1998, Kachhwal et al. 2010).
Data on chemical and physical inputs, including ore mineralogy, milling and metallurgical process inputs, water treatment inputs, and drainage inputs to the impoundment must be collected throughout the mine life-cycle (MEND 1998). Ongoing evaluation of inputs enables prediction of pond chemistry and facilitates effective treatment and management of pond outflow.
Requirements for Materials and Appliances
Primary material and appliance considerations include:
- dam construction material
- seepage management system appliances
- tailings deposition system appliances
WMF dams intended for subaqueous tailings disposal are relatively resource intensive, requiring large amounts of construction material and land area (Kauppila et al. 2013). Such dams are designed to permanently retain large volumes of water without the structural support of a tailings beach area (MEND 1998). The conventional dam and the downstream staged dam designs are best suited to meet the structural stability requirements for subaqueous disposal (MEND 1998, Kauppila et al. 2013, EC 2009). If overburden and waste rock are not suitable for use as construction material, borrow pits may be required to fulfil construction material needs (EC 2009).
Additionally, dams used for storing potentially AMD generating material require impermeable barriers to prevent contaminant release by leaching (Kauppila et al. 2013). Impermeable barriers can be constructed for zoned and rockfill dams, both of which require large amounts of material for construction. Zoned dams also require a range of material size for construction, and may subsequently require on-site crushing, or sourcing construction material from off-site.
Impermeable barriers are constructed using plastic (e.g. HDPE) or bituminous geotextile with welded seams to prevent seepage. The impermeable plastic or bituminous liner is installed on a sufficiently even surface to prevent stress failure. It is recommended the impermeable liner be underlain with a bentonite liner to further protect against failure. Further details on construction considerations for dams retaining AMD generating material are given by Kauppila et al. (2013).
Seepage control systems may also include pumps for returning seepage to the WMF or a separate treatment facility, or construction of barriers that require additional materials.
Tailings deposition systems specialized for subaqueous disposal will likely include appliances not required for other disposal systems. Piping that transports the tailings from the processing to disposal area may be more costly than conveyor or trucking methods (EC 2009). For example, systems for spreading tailings subaqueously may involve the use of track mounted vehicles, a specialized discharge vessel, or a barge (Dillon et al. 2003, EC 2009).
Minimization / Treatment of Discharge
Potential sources of emissions that require design consideration and monitoring are water outflow and seepage. Treatment of outflow is a common requirement, especially during the production phase of mining. The need for outflow treatment is initially assessed during the design phase, but is re-evaluated based on ongoing monitoring throughout the mine life-cycle (EC 2009). Case studies such as those summarized in the MEND report 2.18.1 (Yanful & Simms 1997) have shown that the need for water treatment can be reduced through the addition of organic material, sand, or neutralizing agents to the impoundment system.
Although impermeable barriers are required in impoundments that contain potentially AMD generating material, seepage from a flooded impoundment is considered inevitable (EC 2009). The most effective seepage control measure is selecting an impoundment site on naturally impermeable ground (EC 2009, Kauppila et al. 2013). Additionally, measures to control seepage include construction of perimeter drains from which seepage is either returned to the pond or is treated. Groundwater seepage controls include cut-off trenches, slurry walls, grout curtains, collection systems or the construction of treatment systems, such as a permeable reactive barrier (EC 2009, Kauppila et al. 2013, Heikkinen et al. 2008).
Monitoring / Controls
Post closure monitoring must be conducted to ensure to the ongoing effectiveness of the closure strategy. The details of the closure monitoring program are based on environmental permits and site specific environmental concerns. A monitoring plan should be established in conjunction with the mine closure plan (Heikkinen et al. 2008). Further information regarding the development of a closure monitoring plan can be found in the Mine Closure Handbook (Heikkinen et al. 2008). Primary monitoring considerations specific to WMF subaqueous disposal sites include the long term structural stability of impoundment dams and the chemical stability of the tailings within the facility (MEND 1998, Heikkinen et al. 2008).
The Design Guide for the Subaqueous Disposal of Reactive Tailings in Constructed Impoundments (MEND 1998) suggests that the monitoring plan for geotechnical structures be developed by a specialist, for use the operation’s technical staff in conducting ongoing monitoring. The guide also suggests that site specific performance parameters for structures can be established during operations to streamline the post-closure monitoring program (MEND 1998). Elements in a typical closure monitoring program for geotechnical structural stability include visual inspection of impoundment structures, assessment of pore water and surface water levels, monitoring for seepage, and measuring surface deformation, mass movement or failure (Heikkinen et al. 2008).
Water balance management influences both structural and chemical stability with the WMF. Work conducted at the Stekenjokk site in Sweden illustrates the use of ongoing water level monitoring to guide preventative action. At this site, monitoring observations led to the decision to reconstruct the primary outflow channel and construct an additional emergency outflow to prevent dam overtopping (EC 2009).
Post-closure monitoring of surface water and groundwater is conducted to evaluate the ongoing chemical stability of the tailings within the WMF, can signal the need for further mitigation efforts, and can inform ongoing water treatment measures (Heikkinen et al. 2008, MEND 1998). Existing groundwater monitoring networks can be used for post-closure monitoring, but may need to be supplemented to address specific environmental concerns at the time of closure. Surface water monitoring should be conducted upstream, downstream, and from the supernatant of the WMF. Monitoring upstream of the WMF provides ongoing baseline data, while monitoring downstream is conducted to detect emissions. Periodic monitoring of WMF supernatant can provide information on the chemistry within the pond and can improve understanding of changes in water quality conditions over time (MEND 1998).
The use of man-made impoundments for subaqueous waste disposal has been studied extensively as a mine waste management strategy. The design of a safe and effective WMF for subaqueous disposal requires the thorough evaluation of site-specific chemical and physical variables. Data is collected and integrated from many areas of expertise, including hydrology, chemistry, and geotechnical engineering. A detailed, iterative process for the design of subaqueous mine waste impoundments was produced by the MEND project (MEND 1998), and the methods used to inform design decisions have been refined through ongoing studies (Samad & Yanful 2005).
Extensive studies on the application of this method have resulted in continuing advances in its effective implementation (Yanful & Simms 1997, Samad & Yanful 2005). Examples of advances in understanding include the work of Holmström et al. (2001), which identified method limitations through the investigation of previously oxidized tailings that were flooded and subsequently produced metals emission. Other lessons can be taken from the Stekenjokk site, which highlights the importance in climate considerations with regard to water balance management after reconstruction measures were required to prevent ice damming (EC 2009). Additional lessons learned can be found through the review of other sites and case studies, such as those listed in the Technology Concept and Methods section of this page. Because the successful application of subaqueous disposal in waste facilities is dependent on site specific variables, there remains ongoing need for detailed case studies.
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Rico, M., Benito, G., Salgueiro, A.R., Diez-Herrero, A., & Pereira, H.G. 2007. Reported Tailings Dam Failures: A Review of the European incidents in the worldwide context. Journal of Hazardous Materials 152, 846-852.
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Samad, M.A. & Yanful, E.K. 2005. A Design Approach for the Selecting the Optimum Water Cover Depth for Subaqueous Disposal of Sulfide Mine Tailings. Canadian Geotechnical Journal 42, 207-228.