Case Studies Chapter 6
- 1. Pamour Tailings – Encapsulation
- 2. Lupin Tailings – Elevated Water Table Covers for Reactive Tailings
- 3. Strathcona Tailings – De-Sulphurized Tailings Cover
- 4. Poirier Tailings – HDPE Synthetic Cover
- 5. Normetal Tailings – HDPE Membrane Cover
- 6. Denison Tailings – Reclamation by Flooding Post Closure
- 7. East Sullivan mine site – Organic Covers
- 8. Benambra Mine Site - Alkaline Covers
1. Pamour Tailings – Encapsulation
A historic gold tailings impoundment in the Timmins area, Ontario, Canada is sulphide bearing with moderate to high carbonate contents. Expansion of an open pit required relocation of a tailings stack (T1). During milling, the tailings were separated into a sulphide concentrate and a carbonate-rich flotation tailings and were placed in different locations within the T1 stack. The sulphide concentrate tailings were on surface and exposed to the atmosphere for decades and therefore had developed acidic pore water within the near-surface tailings. Relocation planning included investigations to mitigate existing acidic pore water as well as ongoing acid generation. The selected option included the relocation and deposition of the acidic sulphide concentrate tailings onto a nearby high-carbonate tailings stack (T2) followed by a cover composed of the remaining high-carbonate tailings from the relocated T1 stack. This encapsulation approach was investigated extensively to minimize the risk of acidic leachate. An investigation was also completed to evaluate the quality of pore water that would evolve when acidic waters migrated down through the high-carbonate tailings and the results were presented in MEND Report 2.46.1 (2010).
The tailings in both the T1 and T2 stacks were characterized in the field during the options investigation. Samples of the sulphide concentrate and carbonate-rich T1 tailings were collected from multiple depths and at several locations. The solids and soluble (pore water) products were analyzed in all samples. Acid base accounting (ABA) was complemented by total sulphur and sulphide-sulphur analyses as well as carbonate contents to illustrate AP/NP values for the carbonate-rich tailings and to quantify available NP in the stack below the sulphide concentrate layer.
The acidity in the existing pore water represented only a small portion of the AP. The AP/NP balance clearly showed that there was adequate NP to consume all acidity in low-pH pore water as well as all AP that could be generated in the sulphide concentrate layer if all of the remaining sulphide reacted to form sulphuric acid.
The investigation of mitigated water quality was completed in two phases. The initial phase included batch studies that added acidic tailings to neutral carbonate-rich tailings, after which pore water was extracted and analyzed. The second phase included column studies involving acidic tailings overlying neutral tailings, which were completed over a one-year period with field-like infiltration rates. Samples of pore water were collected within the neutral tailings and column drainage was collected at the base of the column and analyzed. The results showed that concentrations of most soluble constituents in the neutralized pore water were attenuated or mitigated to low values as waters migrated through the neutral tailings. Field monitoring was conducted to follow the performance of the encapsulated tailings.
- Mine Environment Neutral Drainage Program (MEND), 2010. Evaluation of the Water Quality Benefits from Encapsulation of Acid-Generating Tailings by Acid-Consuming Tailings – December.
2. Lupin Tailings – Elevated Water Table Covers for Reactive Tailings
The Lupin former gold mine in Nunavut, about 400 km NE of Yellowknife, NT, Canada operated from 1982 and closed in the mid-2000s. The tailings were potentially acid generating. Tailings were deposited over a 600-ha area in multiple shallow cells that were originally small lakes and were contained by multiple small dams. The closure concept for the tailings included construction of “dry covers” between 0.6 to 1.6 m thick with the added design feature that the final water table in each tailings cell would occur within the cover layer as described in MEND (2004). With the water table above the tailings surface, the raised water table provides an “effective” water cover to prevent sulphide oxidation. The cover design therefore was a hybrid system that represented a dry cover with no standing water within the tailings cells while maintaining submerged conditions for the sulphide tailings. Although this concept was implemented at a mine site in permafrost terrain, permafrost was not a necessary aspect of the design and adaptation is possible at other sites with an appropriate water balance and topography.
Deposition of tailings in cells provided an opportunity for progressive reclamation that started in 1995. The cover material consisted of esker sands. There was no need for low-permeability materials to prevent infiltration of water or ingress of oxygen. The elevated water table above the tailings represented the design feature that prevented oxidation and generation of ARD. The cover system, however, included a second feature to complement and maintain the raised water table. The surface layer was composed of coarse granular material that reduces evaporation and promotes infiltration to maintain an elevated water table.
While it is commonly accepted that a water cover in a pond, for example, should have a thickness of 1 to 3 m in order avoid physical resuspension of tailings, the depth below the water table can be minimal because any water cover thickness is effective at limiting oxygen access and there is no potential for mixing within a porous medium. Therefore, the design criteria for a raised water table above the tailings surface can include a minimum depth below the water table and the thickness of the cover in constrained only by the preference to avoid free water over the cover layer surface.
- Mine Environment Neutral Drainage Program (MEND), 2004. Covers for Reactive Tailings Located in Permafrost Regions Review. Report 1.61.4, Natural Resources Canada
3. Strathcona Tailings – De-Sulphurized Tailings Cover
The Strathcona tailings facility, near Sudbury, Ontario, Canada has an active area of 100 ha and has been operating since 1968. For the first two decades, the nickel tailings with a sulphur content of 15% S, mainly as pyrrhotite, were discharged as an unsegregated slurry in cells within an impoundment. When tailings were exposed to the atmosphere with no further addition of fresh tailings slurry, the surface material oxidized rapidly, producing acidity, soluble metals and a visible crust, or hard pan, of iron hydroxide. Tailings management alternatives were considered in the early 1990s and the preferred option was to use the scavenger tailings with a sulphur content of less than 1% to cover the high-sulphur tailings. Lime kiln dust or reject material from lime production was also added to the low-sulphur tailings to increase the carbonate NP as well as the NP/AP ratio. The high-sulphur tailings fraction with 30% S was deposited underwater in the Oxidation Pond that is also an integral part of the water treatment system for the mining complex in the Onaping area. The low-sulphur cover placement was initiated in 1995.
The low-sulphur tailings cover is produced as the cyclone overflow from the scavenger flotation units that generate a sandy material for mine backfill. The overflow contains the fine-grained fraction referred to as slimes and therefore has the value-added property of moisture retention capacity. The high-moisture retention acts to reduce oxygen ingress due to low values for the gas diffusion coefficient.
The low-sulphur tailings slimes cover was evaluated through modelling to determine the appropriate thickness required to protect the underlying high-sulphur tailings from oxidation. A minimum cover thickness of 1.5 m was selected as being adequate as an oxygen barrier based on the degree of moisture retention or saturation anticipated in the lower zone of the cover layer. The desulphurized tailings cover was almost completed by the summer of 2011.
Interim assessment of the cover layer has shown that the sulphur content is consistently less than 1% S and that the NP/AP ratios ranged between 1.4 and 12. The lime demand for water treatment in the adjacent oxidation pond has decreased substantially since the cover construction was initiated. The lime demand in the pond, however, represents other possible sources of acidity and therefore does not directly reflect the performance of the desulphurized tailings cover.
4. Poirier Tailings – HDPE Synthetic Cover
The Poirier Mine was an underground copper-zinc operation that opened in 1965 and closed in 1975, producing about 5 million tonnes of tailings containing between 6 and 20%S. A reclamation options assessment was initiated in 1996 for the surface oxidized and acid generating tailings. The preferred option that was accepted by the regulators was the construction of a HDPE membrane over the 46-ha tailings impoundment with a protective earth materials cover over the membrane. The objective of the cover was to reduce long-term loadings of acidity and metals from the tailings to the adjacent environment by limiting infiltration into the tailings that contained substantial loads of soluble oxidation products as a result of more than 20 years of exposure since the mine closed.
A follow-up study with four years of performance data showed that the loadings of acidity and metals from the tailings impoundment were reduced to small percentages of pre-reclamation values (Maurice and Wiber, 2004). The water table in the tailings had declined to elevations near the base of the tailings as a result of limited infiltration to recharge the subsurface water. The outflow from the tailings had also declined in response to the decrease in the water table.
- Mauric, A. and Lottermoser, B.G. Phosphate amendment of metalliferous waste rocks, Century Pb-Zn mine, Australia: Laboratory and field trials. Applied Geochemistry, 26 (2011), 45-46. P. 45-56.
5. Normetal Tailings – HDPE Membrane Cover
The underground Normetal Mine operated from 1937 to 1975, producing more than 10 million tonnes of sulphide-bearing tailings that were deposited over a 60-ha area. An HDPE membrane was placed over 56 ha of the tailings in 2005-06. The objective of the membrane cover system was to reduce oxygen and water infiltration into the tailings, thereby reducing acidity and metal loadings to the environment.
The cover system included 0.3 m of clay till on membrane slopes and 0.5 m of clay on flatter membrane surfaces. Rip-rap was also placed on slopes as added protection and for stabilization. Toe drains were installed at the base of tailings slopes below the membrane to collect and divert tailings seepage to desired locations. Surface drainage networks were constructed to collect and divert non-contact waters. The exposed clay cover layer was re-vegetated. Construction was completed in September 2006.
In the first two years after construction, the water table decreased an average of about 6 cm per month resulting in a reduction of tailings seepage flow from 150 to 103 L/min. The downstream conditions improved over that time, showing increases in pH and decreases in concentrations of acidity and metals. The pH increased from 3.0 before reclamation to 6.8 in 2009. Concentrations of iron decreased from a high of 300 mg/L to values near 10 mg/L and zinc declined from a high near 7 mg/L to values near 0.2 mg/L over the same period. The overall cover performance was considered to be successful to 2009 with plans to complete detailed monitoring to 2011. One technical issue noted was partial blockage of seepage drain pipes by iron precipitates and organic “slime” thought to have resulted from iron-oxidizing bacteria. Mitigation of the precipitate formation was being considered.
6. Denison Tailings – Reclamation by Flooding Post Closure
The Elliot Lake Uranium Mining District in Ontario was one of several early sources of uranium in Canada. Many of the Elliot Lake mines ceased operation in the early 1960s. Some mines, however, operated until the early 1990s producing almost 200 million tonnes of tailings before closure. The Denison Mine operated from 1957 to 1992, producing about 60 million tonnes of tailings that were stored in two tailings management areas (TMA).
Description of Denison Mine TMA
Elliot Lake is 30 km north of Lake Huron and 130 km west of Sudbury on the Canadian Shield, a region of exposed Precambrian rock that mantles a large area across Canada. The Denison property is located about 11 km north of Elliot Lake and is within the Serpent River watershed that drains to Lake Huron (Figure 1). The underground mine began operation in 1957 and continued production until 1992. The mill was located on site and the original production rate of 5,000 tonnes per day was increased to 6,350 tonnes per day in 1977 and 13,600 tonnes per day in 1982. More than 60 million tonnes of ore were processed over the life of mine and the tailings were stored in two tailings management areas (TMAs), TMA1 and TMA2 (Figure 2). The combined area of the TMAs is 280 ha.
The TMAs were originally lakes that had dams constructed at lower sections of the perimeters in order to increase capacity. Tailings deposition was typical with low density slurry delivery to the impoundments. Spigotting resulted in the development of long beach areas near the spigot and ponds where fines settled near the dams.
The Denison tailings were typical of those from the Elliot Lake ore body. The ore was a Precambrian conglomerate similar to the gold deposits in South Africa. The uranium grade was on the order of 0.2% U. Pyrite was an accessory mineral with typical sulphide contents in the range of 5 to 10 % S. The ore was milled with a sulphuric acid leach process that removed any potential carbonate or other neutralizing minerals. The only neutralization potential (NP) in the tailings originated from residual traces of carbonate minerals in the lime that was used to neutralize the tailings prior to deposition in the TMAs. Studies of other tailings deposits in the region suggested that the NP of Elliot Lake tailings was less than 0.1% CaCO3 or 1 kg CaCO3/t (Dubrovsky et al., 1985).
Acidic water in the Denison tailings was recognized in the 1980s and water treatment plants were constructed to manage the effluents from each of the TMAs separately. The treatment systems were designed for pH control as well as for the removal of radium-226, a daughter isotope of uranium that has a prescribed effluent release limit of 0.37 Bq/L (10ρCi/L).
The Decommissioning Concept
The proposed decommissioning plan for the TMAs was based on the concept of mitigation to lessen the need for active management in the long term, in accordance with the principles developed by the federal nuclear regulatory agency. While the decommissioning plan considered all mine facilities and infrastructure, the tailings represented the main focus for water quality effects, and therefore, is the focus of this discussion.
Pyrite oxidation and ongoing acid generation in exposed on-land tailings was identified as the key concern with the need to develop chemical stability for the protection of water quality in the long term. Therefore, flooding and permanent underwater storage of the existing tailings was selected as the preferred closure option at Denison, as well as several other TMAs at Elliot Lake. Although dams were in place to contain tailings in TMA1 and 2, they were generally not suited to raise water levels sufficiently to flood the beached tailings. Flooding was achieved through a combination of modest dam raises and upgrades to hold water at appropriate elevations, as well as the relocation of some beached tailings to lower elevations. Some beached tailings in TMA2 were relocated to TMA1 and some were sent underground. Some drainage redirection in the watershed was also required to have a positive water balance and to maintain a water cover on TMA1.
A cornerstone of the decommissioning plan for Denison, and for other TMAs at Elliot Lake, was the follow-up monitoring plan that consisted of three tiers of surveillance. These included a range of physical scales from the footprint of the tailings facility to the greater watershed that receives water from most of the TMAs.
Water Quality Before And After
Acidic conditions had developed in the beached tailings in TMA1 and TMA2. The acid conditions were observed in the effluent from the Denison tailings. More detailed investigations at other Elliot Lake TMAs showed that the shallow tailings pore waters were commonly characterized by pH values less than 3 with elevated sulphate and acidity, iron and other metals (Dubrovsky et al., 1985). In fine-grained tailings, the acidic zones were generally limited to the top few metres of tailings while in coarser and well-drained tailings acid zones had developed to depths of 10m. At Denison, about 1.7Mm3 of the most acidic tailings in TMA2 were relocated during decommissioning with about 50% sent to underground workings and 50% to TMA1. During relocation, lime was added to the acidic tailings to raise the pH.
The acidity loads generated in the TMAs can be estimated from the quantities of lime and/or sodium hydroxide used in the treatment plants that receive runoff and seepage from the tailings. Prior to flooding TMA1, typical lime consumption was on the order of 4,000 tonnes of CaO per year (Figure 3a). Lime consumption declined dramatically during the decommissioning process by three orders of magnitude (99.9%). Similarly in TMA2, sodium hydroxide consumption prior to decommissioning was on the order to 1.5 t/a, which declined to zero post decommissioning (Figure 3b).
Water quality of the influent to the water treatment plant reflects the effects of flooding to some extent as shown for sulphate, acidity and iron concentrations in Figure 4. All parameters that were elevated prior to flooding exhibit decreases after flooding. The differences do not appear as dramatic as those for lime use, primarily because the chemistry of the tailings basin water was altered by in-situ lime addition prior to reporting to the treatment plant. Part of the overall trend in pyrite oxidation at TMA1 is exhibited by the concentrations of sulphate in the treated water, as shown in Figure 5, that were typically in the 1,500 to 2,000 mg/L range prior to decommissioning and declined to the 20 to 100 mg/L range after flooding. The lower concentrations of sulphate are expected to persist for an extended period because of the presence of gypsum (CaSO4 • 2H2O) in the tailings that dissolves and releases sulphate to pore water. Subsequently, sulphate in the pore water is released to the overlying water as a result of diffusion at the water-tailings interface. In the tailings at the Denison site, gypsum originated from two sources: it formed when the sulphuric acid (H2SO4) in the leach solution was neutralized with lime (CaO) and in the oxidized tailings when sulphuric acid was produced by pyrite oxidation and reacted with available calcium.
The effects of decommissioning are also observed in the Serpent River, immediately downstream of the Denison effluent discharge as shown in Figure 6. While the pH values have become more consistently in the range of 6.5 to 7.5, the sulphate, acidity and iron concentrations have all declined to low levels. For example, prior to 1994, sulphate concentrations as high as 1,000 mg/L were commonly measured in the river while the average value from 2010 to 2011 is less than 20 mg/L.
Quirke Lake is the largest water body immediately downstream of the Denison TMAs in the Serpent River chain of lakes with a volume of 803.0Mm3 and an annual average outflow of 183Mm3/a. Quirke Lake also receives treated effluent from the other TMAs that were also decommissioned and flooded at about the same time as the Denison TMAs. In the 1960s, Quirke Lake had pH values near 4 as a result of early milling practices that did not include complete neutralization of mill process water prior to tailings discharge. Neutralization of tailings was implemented in the 1970s and the pH in Quirke Lake increased. The water quality at the outlet of Quirke Lake from the late 1980s to 2011 is summarized in Figure 7. The pH was typically in the range of 5 to 6 in the early 1990s and has since increased to values consistently near 7. The concentrations of sulphate have declined from about 200 mg/L in 1991 to less than 50 mg/L in 2011. Similarly, concentrations of acidity and iron have declined since decommissioning of the Denison and other upstream TMAs.
The uranium mines in Elliot Lake were developed and operated over decades at a time when environmental effects from sulphide oxidation and acid generation on water quality were not clearly understood. Planning for decommissioning and mitigation of acid generation was therefore only considered in the late stages of operation, prior to mine closure. During closure planning, sulphide oxidation and tailings acidification were recognized as the most important influences on future water quality and ecological health of the watershed. The implementation of effective mitigation strategies for potentially acid generating tailings produced excellent results for water quality improvement at the individual TMAs and in the watershed, overall. The case study of the Denison TMA and results for other flooded TMAs in the Serpent River Watershed represent excellent examples of the reversibility of environmental effects from mining operations that were not initially designed to mitigate acid generation. The commitment by Denison Mines Inc. and other former uranium mine operators at Elliot Lake has clearly demonstrated that historic mines need not represent a negative environmental legacy for future generations.
- Blair, R.D (1981). Hydrogeochemistry of an Inactive Pyritic Uranium Tailings Basin, Nordic Mine, Elliot Lake, Ontario M.Sc. Thesis, Department of Earth Sciences, University of Waterloo, Waterloo, ON, Canada.
- Dubrovsky, N.M., J.A. Cherry, and E. J Reardon (1984). Geochemical Evolution of Inactive Pyritic Tailings in the Elliot Lake Uranium District. Department of Earth Sciences, University of Waterloo, Waterloo, ON Canada.
- Ludgate, L., Morrell, R., Knapp, R.A. and Kam, S.N. (1997). Decommissioning of Denison Mines Tailings Management Areas. Proceedings of the Fourth International Conference on Acid Rock Drainage, Volume II, pp. 917-933, May 13-June 6, 1997, Vancouver, British Columbia.
- Moffatt, D. and M. Tellier (1978). Radiological Investigations of an Abandoned Uranium Tailings Area. J. Envir. Qual., 7(3).
7. East Sullivan mine site – Organic Covers
A mixture of organic materials (sawdust and sewage sludge) was emplaced into a mine spoil backfill to stimulate microbial growth and generate an anoxic environment through sulphate reduction (Rose et al., 1996). The results of the organic matter injection process caused no change in water pH, about a 20% decrease in acidity and a similar decrease in Fe, Mn, and Al. The results indicate that the process works, but improvements in organic material injection and the establishment of a reliable saturated zone in the backfill are needed for maximum development.
The nature of the organic material used to prevent the oxidation of sulfidic mine tailings is varied. Forestry wastes have been laid down since 1984 at the East Sullivan mine site in north-western Québec (Germain et al., 2003).
The wood waste cover on the East Sullivan mine tailings serves several functions. The cover is considered to act as a prevention method for potential ARD production. The organic matter consumes oxygen and therefore acts as an oxygen barrier. The wood waste also promotes higher infiltration rates and therefore maintains a higher water table in the tailings than would occur without the cover. The higher water table is considered to be protective of the reactive tailings if oxygen does migrate through the cover. In addition, the cover has had a role in water treatment. Water collected in perimeter ponds is pumped onto the wood waste cover with the intent of removing soluble sulphide oxidation products, such as iron and other metals, and to neutralize the water with alkalinity produced by decomposition of the organic matter (Germain, et al., 2010).
A field-scale experiment was conducted at the Greens Creek Mine on Admiralty Island, Juneau, Alaska, USA to evaluate various organic carbon sources (peat, spent-brewing grain, municipal biosolids) as amendments for passive treatment of tailings in pore water (Lindsay et al., 2011). The field experiment proved that the technique can effectively limit the transport of sulphide mineral oxidation products in mine tailings impoundments. The experiment reported that the amendments containing a large proportion of labile organic carbon compounds should be used sparingly, as rapid development of reducing conditions can lead to enhanced mobilization of iron and associated trace elements.
- Germain, D. Normand Tassé, and Johanne Cyr 2003. Treatment of Acid Mine Effluents Using a Wood-Waste Cover.Proceedings of the 6th International Conference on Acid Rock Drainage (ICARD), Cairns, Australia, 12-18 July 2003, Australian Institute of Mining and Metallurgy, Melbourne.
- Germain, D., Normand Tassé, and Johanne Cyr 2010. The East Sullivan Mine Site: Merging Prevention and Treatment of Acid Mine Drainage. Presentation in Proceedings of the 16th Annual BC-MEND ML/ARD workshop, Vancouver, BC., December 2-3, 2009, W.A. Price and K. Bellefontaine (Eds.).
- Lindsay, M.B.J., Blowes, D.W., Condon, P.D., Ptacek, C.J. Organic carbon amendments for passive in situ treatment of mine drainage: Field experiments. Applied Geochemistry, 26 (2011), p. 1169-1183.
- Rose, A.W., J. Stalker, and L. Michaud. 1996. Remediation of acid mine drainage within strip mine spoil by sulfate reduction using waste organic matter. p. 321-335. In:Proceedings, Thirteenth American Society for Surface Mining and Reclamation Conference,May 18-23, 1996, Knoxville, TN.
8. Benambra Mine Site - Alkaline Covers
Rehabilitation and Post-Closure Monitoring and Performance at the Benambra Base Metal Mine Site in Australia
A Closure and Rehabilitation case study prepared by Earth Systems Pty. Ltd.
The Benambra Mine Site in south-eastern Australia was operated as an underground base metal mine from 1992 to 1996. During operations, 927,000 tonnes of ore was processed on site to produce copper and zinc concentrate, and nearly 700,000 tonnes of sulfidic tailings from the process plant was delivered by pipeline to a nearby tailings dam. The tailings are dominated by pyrite (65 wt.%), with lesser amounts of quartz (13 wt.%) sphalerite (1-8 wt.%), dolomite (4-5 wt.%) and chalcopyrite (1-2 wt.%).
The mining company was liquidated in 1996 and the site is now managed by the state government of Victoria, Australia. A detailed rehabilitation strategy was developed for the tailings dam, process plant site, geological core storage facility and road network. The rehabilitation strategy aimed to restore the environment to as near pre-mining conditions as possible, in accordance with industry best practice and stakeholder, community and legislative requirements. The key environmental risk was the potential for ARD generation from the tailings dam (Earth Systems, 2003) which is therefore the focus of this case study.
The tailings dam was engineered as a competent water-retaining structure. Prior to rehabilitation, the dam contained about 160 ML of supernatant water, with near-neutral pH and elevated concentrations of dissolved metals (zinc, arsenic, copper, lead, and manganese) and sulfate. Due to the irregular bathymetry of the tailings surface, the supernatant water depth varied from 0 to 8 m. Prior to rehabilitation, the supernatant water was treated with hydrated lime (calcium hydroxide; Ca(OH)2) on several occasions as required to achieve suitable water quality for off site discharge. The residual metal hydroxide precipitates arising from these treatment activities remained in the tailings dam but progressively began to dissolve, with acidic catchment inputs releasing metals back into the water column, over subsequent months after treatment. Seepage from the dam wall, flowing at around 1 L/s, was also affected by ARD. This water was characterised by slightly acidic pH, elevated zinc, manganese, copper, lead and arsenic, and high sulfate concentrations. The source of this ARD was attributed to the presence of sulfidic rocks in the dam wall rather than the tailings. This is based on the water chemistry and low flow rates observed, and effectively saturated condition of the tailings.
The primary objective of site rehabilitation was to manage ARD in the tailings dam by creating a permanent water cover over the tailings, with a minimum 2 m depth, and utilising passive treatment systems for long-term water quality control, as follows:
- Diversion channels around the tailings dam were removed and original creek alignments were reinstated in the upstream catchment, to direct water back into the tailings dam to maintain the permanent water cover.
- A spillway was constructed to allow controlled overflow and ensure long term stability of the dam wall. Long term climate and water balance modelling was conducted to determine the required spillway elevation to maintain the permanent water cover of at least 2 m.
- The tailings were redistributed to remove irregularities in the bathymetry and ensure a consistent water cover across the tailings surface.
- The levelled tailings were then covered with a layer of limestone sand to minimise wave-based re-suspension of tailings in the water column and therefore minimise the potential for oxidation of sulfidic material near the water surface.
- Organic matter layers (jute matting and sawdust) were installed above the limestone sand to promote sulfate reducing bacterial activity, in order to convert existing metal hydroxide precipitates into relatively stable metal sulfide complexes and lower sulfate concentrations in the supernatant water. Furthermore, the organic matter was intended to further minimise the potential for sulfide oxidation by inhibiting the migration of dissolved oxygen from the water column into the tailings.
- The tailings dam perimeter was revegetated to permit a constant supply of organic inputs to the tailings dam via leaf litter. This was aimed to promote ongoing sulfate reducing bacterial activity, to convert any residual metal hydroxides to sulfides and further reduce sulfate concentrations, in the long term.
- Passive alkalinity addition systems were installed in the upstream catchment to raise the naturally acidic pH of creek water to near-neutral levels prior to entering the tailings dam, to minimise acidity addition to the tailings dam water.
- The dam wall was strengthened by creating a 4:1 (H:V) downstream batter slope to maintain geotechnical stability in the event of a “maximum credible earthquake”.
- An anaerobic vertical upflow wetland was installed to passively treat seepage from the base of the dam wall. The wetland has a design life of 20-30 years, after which maintenance will be required if ARD-affected seepage continues to emanate from the dam wall.
The water monitoring program during rehabilitation involved the collection of in-situ water quality data and samples for laboratory analysis. At the completion of rehabilitation, a remote monitoring system was installed at the tailings dam to continuously monitor rainfall, surface water level and key water quality indicators (pH, temperature and electrical conductivity), in order to monitor the effectiveness of rehabilitation and provide early warning of potential water quality issues. Results from the monitoring system were compared with field results collected manually during the first 12 months post rehabilitation, to confirm the reliability of the system. Sensor calibrations are checked quarterly (rainfall gauge six monthly) or as required. Seepage water quality and flow rate has also been monitored (manually) on a regular basis to verify the performance of the constructed wetland system.
Key findings from the surface water monitoring before and after rehabilitation were:
- A minimum water cover of 2 m over the tailings material has been maintained.
- The pH was elevated at around 8.5-9.0 due to water treatment during rehabilitation, but has subsequently stabilised at near-neutral to slightly alkaline values.
- Dissolved metal concentrations have decreased by around 85-95% following rehabilitation. For example, copper concentrations decreased from 2.4 mg/L to 0.03 mg/L, lead decreased from 1.95 mg/L to 0.001 mg/L, manganese decreased from 1.3 to around 0.1-0.2 mg/L, zinc decreased from above 1 mg/L to around 0.1-0.2 mg/L, and arsenic decreased from 0.068 mg/L to 0.001 mg/L.
- Similarly, there has been an 80-90% reduction in sulfate concentrations, from 1600 mg/L to less than 200 mg/L following rehabilitation. Trends in salinity have been similar to those observed for sulfate.
- Detailed review of the sulfate data, taking into account the effects of dilution and evaporation in the dam, indicates no evidence of sulfate addition associated with ARD generation.
Key findings from the seepage water monitoring before and after rehabilitation were:
- There has been no apparent increase in seepage flow rates from the dam wall.
- The pH has increased from slightly acidic (5.6-6.0) values to near-neutral values (6.3-7.1) and salinity has a progressively decreased following wetland installation.
- A comparison of dissolved metal concentrations before and after wetland installation indicates a decrease of around 85-95% for copper, lead, zinc and arsenic (generally below detection limits). These metals are effectively converting to stable metal sulfide precipitates in the wetland.
- Unlike copper, lead, zinc and arsenic, the precipitation of manganese requires oxidising conditions. This is effectively occurring in the receiving creek line, within 1 km of the wetland overflow, where dissolved manganese drops from around 10 mg/L to less than 0.5 mg/L.
- There has been substantial improvement in surface water quality in the tailings dam post rehabilitation.
- The permanent minimum water cover of 2 m over the tailings is providing an effective long-term solution to water quality control in the tailings dam.
- Passive alkalinity addition systems, including a limestone sand layer over the levelled tailings, have maintained near-neutral to slightly alkaline pH in the dam.
- Addition of organic matter to the dam has promoted sulfate reducing bacterial activity and generated at least a 75% reduction in its soluble sulfate load since the completion of rehabilitation activities. This sulfate reduction is improving water quality by promoting metal sulfide precipitation, and concomitantly contributing alkalinity to the surface water.
- Decreasing soluble copper concentrations have gradually permitted algal growth which has in turn encouraged water birds to the dam. Faeces from water birds is introducing new organic matter and phosphorus to the system, which is further encouraging sulfate reducing bacterial activity and probably metal phosphate precipitation, ultimately improving water quality.
- Continued vegetation growth around the dam perimeter will promote a self-sustaining biological remediation system within the dam (via passive carbon addition) and ensure long-term passive water treatment through natural biological processes.
- The anaerobic wetland and downstream oxidation processes are combining to effectively treat ARD seepage from the dam wall.
Earth Systems (2003). Geochemical study of the Benambra Tailings Dam, Eastern Victoria. Prepared for Department of Primary Industries – Minerals and Petroleum Division. Earth Systems, Melbourne, Australia.