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The principal tool used for modelling the effect of contaminant releases during the various phases of the Project is IMPACTTM - the Integrated Model for the Probabilistic Assessment of Contaminant Transport. IMPACTTM is a probabilistic model that has been developed to calculate effects of contaminant emissions on environmental media (e.g., water, sediment, air) and for identifying risks to biological receptors including wildlife and humans. The model uses established mathematical models and principals to predict concentrations in environmental media and ecological risk to biota and humans. For the this EIS, IMPACTTM has been used to predict:
The prediction of concentrations in environmental media such as water is the first step in the prediction of metal exposure and uptake with aquatic and terrestrial food chains.
IMPACTTM has been evaluated as part of BIOMOVS II ( Biosphere Model Validation Study -Phase II), an independent, international, comparative study of models used to predict transport, fate and biological uptake of contaminants in the environment. BIOMOVS II involved modelling of environmental effects using a standard hypothetical data set and included atmospheric, groundwater and surface water pathways. Results of BIOMOVS II evaluation demonstrated that IMPACT TM produces valid and credible predictions of contaminant transport and fate (SRPI 1996).
The model operates on the basic impact assessment approach of source, pathway receptor and as such to use the model to predict water quality concentrations, mine sources, transports pathway (air and water) and receiving environment need to be defined.
Source Characterization
In order to predict downstream water quality, the IMPACTTM modelling of the Project incorporated information on contaminant sources and receiving environment characteristics. The model considered both atmospheric sources (i.e., deposition of atmospheric releases on surface water) and direct and indirect (seepage) aquatic releases from four major sources of contaminants:
1. North Tailings Basin (aquatic releases operation and post-decommissioning);
2. Headwater Pond (aquatic releases operation and post-decommissioning) ;
3. open pit (dust during operation, aquatic releases post-decommissioning); and
4. mine mill (aquatic and atmospheric releases during operation
only).
These sources were characterized in terms of the contaminants associated with their discharge, concentrations of these contaminants and discharge rates. The contaminants considered in the model were metals (copper, nickel, cobalt, lead, zinc, arsenic, aluminum and cadmium). These metals were selected based on review of expected minerialized mine rock and tailings chemistry and their biological sensitivity (CCREM 1987). In order to predict transport and deposition to the sediment, each metal was characterized in terms of its propensity to allocate itself in either dissolved or particulate phase.
The IMPACTTM model scenario did not consider iron, pH or ammonia which are expected to be associated with the North Tailings Basin and Headwater Pond. The concentrations and potential effects of these parameters were modelled separately using source term chemistry and receiving environment information. Modelling was conducted to assess the potential influence of iron oxidation on pH and the implications of iron precipitation on fish habitat and aesthetics. Since the oxidation of iron generates acidity, all iron from seepage and direct decant discharge was assumed to oxidize and pH levels were predicted downstream as a function of dilution and limited alkalinity (worst case). The concentrations of total ammonia is expected to be elevated in the North Tailings Basin and Headwater Pond due to the use of ammonia based explosives during mining operations. Since ammonia concentrations are a result of active operations and will not persist post decommissioning, the assessment of ammonia concentration downstream of the North Tailings Basin and Headwater Pond was limited assessing operational conditions. During operations, seepage will be the only source of ammonia to surface water and therefore only areas downstream of seepage sources were assessed.
Information of the chemistry of the aqueous releases (North Tailing Basin, Headwater Pond and the mine mill) were provided. Ranges of concentrations were provided over time to correspond with the various phases of operation and decommissioning. Concentrations were provided for both direct discharge (surface water releases) and indirect discharges (seepages). Concentrations were based on design information and mineralized mine rock and tailings chemical characterizations. In addition, information from similar facilities was used to validate predicted concentrations. For modelling purposes the high end of all concentration ranges were used to provide for conservative predictions.
Direct decant discharge and seepage rates through perimeter dams were provided. A water management plan for the Project was prepared through all stages of the mine operation. This provided information of direct decant discharge rates, as well as the direction and rate of flows during the various stages of operation. This information was incorporated into the model for the appropriate years.
Air quality information input based on air modelling of dust associated with construction and operation of the open pit, atmospheric releases from the mine/mill and releases from the concentrate handling facility at the port site. Information was provided on air plume characteristics during the operating stages of the mine. This information was incorporated directly into the model for the spatial extent of the air plume.
Receiving Environment
Following the characterization of the sources, the receiving environment was defined in order to model metal concentrations and transport downstream. To make predictions spatially, the surface water system was divided into distinct physical units which represented areas with unique attributes such as a pond or a reach of a river. These units in the model are called aquatic polygons and some 200+ polygons have been established in the model for the Project. Concentrations of water and sediment quality may be predicted in each polygon, however, in order to make reporting and interpretation of results more manageable results were only reported for 15 representative polygons. Each polygon was characterized in terms of attributes which would affect processes such as dilution, transport, deposition and dispersion. Information for the freshwater polygons was based on data obtained through field surveys. This data included information describing streams and ponds in terms of habitat, depth, bathymetry, volume and sediments.
The rates of water flow through each of the defined surface water polygons were calculated based on unit area runoff coefficients and the catchment area for each defined polygon. This flow information was input based on the hydrological data at the site. The inflow data was provided in terms of maximum, minimum and mean annual flow. For ponds, the model used inflow and outflow estimates together with volume and mean depth to incorporate hydraulic retention times for each pond. The mean annual flow rates were incorporated into the IMPACTTM model to reflect changes in surface water flows associated with the various phases of operation and decommissioning. This includes draw down from specified ponds for mill process water and all changes in flow due to engineered diversions, such as those to be constructed at the North Tailings Basin. Because seasonal flow variation at the site can be substantial, water quality was also modelled in IMPACTTM on a short terms basis to account for seasonal variability. Mean monthly flow rates provided by Acres International were incorporated only for water polygons receiving metal loadings via seepage. These are the only polygons where contaminant loading would remain constant, due to relatively constant seepage rates, while inflow or dilution water would vary on a seasonal basis. The loading rates to water polygons receiving contaminants via surface water pathways are expected to increase and decrease in parallel to seasonal changes in rates of inflow dilution water. Seasonal variation in water quality was predicted for operation and post-decommissioning stages of the mine for the selected polygons.
Once information on source terms were entered into the model and each polygon was characterized in terms of its physical attributes, the polygons were linked to allow for transport of metals downstream and prediction of concentrations in those downstream polygons. The flow pattern for the surface water courses reflected the baseline flow patterns on the site except where the Project design will alter these.
Model Predictions
The model predicted water quality concentrations in selected polygons over a 140-year period, where this time frame represents equilibrium concentrations post-closure. Concentrations of each of the metals modelled were provided for six time periods, each representative of different stages:
Year 5: Represents 2.5 years of construction and 2.5 years of operation of the open pit and mine/mill. Airborne sources, direct discharge to Edward's Cove and seepage from Headwater Pond are included.
Year 25: Represents underground operations, airborne sources, direct discharge to Edward's Cove and Kangaklualuk Bay, seepages from both Headwater Pond and the North Tailings Basin.
Year 30: Represents the period following the end of operations, but where water is still retained, treated and discharged from both Headwater Pond and the North Tailings Basin. Airborne sources have been discontinued at the end of operation.
Year 50: Represents the post-decommissioning period when flow diversions are removed and water is no longer treated but flows through Headwater Pond and the North Tailings Basin. Seepage from these facilities in included during the decommissioning phase. Headwater Pond flows east toward Throat Bay during closure.
Year 75: Represents a mid point in terms of the modelled post-closure period.
Year 140: Represent long term equilibrium in concentrations post-closure.
Concentrations of the selected metals were predicated in these time steps in 15 freshwater polygons representing areas downstream of mine sources. Predicted concentrations were compared to chronic toxicity thresholds established for fish and invertebrates which are present on the site. Locations which exceeded thresholds were identified for the various time frames reported.
Plots of seasonal concentrations were presented for each metal modelled for operation and post-decommissioning stages. Ranges in concentrations were identified and peak concentrations noted. Since peak seasonal concentrations are expected to be short in duration in response to short-term flow periods, these concentrations have been compared to acute toxicity thresholds.
Predicted concentrations of iron and ammonia as well as levels of pH
were reported for operational and post-decommissioning downstream of the
North Tailings Basin and Headwater Pond. These concentrations were compared
to chronic toxicological thresholds.
CCREM (Canadian Council of Resource and Environment Ministers) 1987. Canadian Water Quiality Guidelines. Ottawa, ON.
Swedish Radiation Protection Institute (SRPI). 1996. BIOMOVSII Technical
Report No. 11.
(i) metals;
(ii) thiosalts generated during the milling process;
(iii) ammonia;
(iv) total suspended solids;
(v) total airborne particulates; and
(vi) pH.
The following discussion has been extracted from two principal source documents:
(i) CCME (1995) Canadian Water Quality Guidelines
(ii) AQUAMIN (1996) Assessment of the Aquatic Effects of Mining in
Canada: AQUAMIN, Final Report.
Other references are quoted, where appropriate, throughout the text.
(i)Metals
Metals regulated under the Metal Mining Liquid Effluent Regulations (MMLER) include arsenic, copper, lead, nickel, zinc, pH, total suspended matter, and radium 226. Radium 226 is normally associated with uranium ores; it has not been reported in the ore for the Project and is excluded from the discussion. Aluminum, cadmium and iron, however, were identified as being a potential issue and are addressed below. Three metals (lead, arsenic and cadmium) are recognized as being potentially "toxic" under the Canadian Environmental Protection Act (CEPA).
Metals and metalloids generally occur in surface waters at concentrations of less than 1 mg/L. At these trace amounts, they are often essential for biological/physiological systems; at higher concentrations many become toxic.
The biological activity of many metals is determined both by the total
concentration and the availability, as determined by the speciation or
form of the metal or its ions. The form present in solution is determined
by many factors, including:
Nickel
Nickel is regulated under both MMLER and Newfoundland Department of Environment and Labour (NDOEL) regulations. CCME provides guidelines for the protection of aquatic life is 0.025 mg/L at a hardness 0-60 mg/L (CCME 1995).
Nickel naturally occurs mainly in combination with sulphur, arsenic and antimony. Nickel enters surface waters through the weathering of minerals and rocks within the watershed. Nickel concentrations as high as 0.10 mg/L have been found in natural surface waters, although it generally ranges from 2.9-7.2 mg/L in drinking waters; it is absent in most groundwater. Nickel occurs in most metal mining effluent in solid or dissolved form. Nickel species are soluble at a range of pHs.
Copper
Copper is regulated under MMLER and NDOEL Guidelines are provided by CCME. Copper is commonly present in metal mine effluent as particulates, dissolved copper., or copper-cyanide or copper-iron-cyanide complexes (AQUAMIN 1996).
Copper is necessary for animal health. Toxic effects to humans have never been reported as a result of high concentrations in drinking water; however, they may result when natural sources and dietary supplements contain an excess of copper. Copper is toxic to aquatic plants, invertebrates and freshwater fish. It is readily accumulated by plants and animals; however, it is not thought to be biomagnified to any significant extent. Copper is not acutely toxic to humans (Moore and Ramamoorthy 1984).
Cobalt
Cobalt is not regulated under MMLER, NDOEL or in federal guidelines for the protection of aquatic life. It is, however, considered to be an air quality issue and a component of airborne particulate matter that may enter aquatic systems. The amount of cobalt in aerobic surface waters is very small, generally below 20 mg/L. Concentrations normally increase during fall circulation and winter, primarily in organic fractions.
Lead
Lead is regulated under MMLER and NDOEL Guidelines are provided in CCME for the protection of freshwater life. Lead is present in metal mine effluents in the form of galena particulates or particulates containing sphalerite and/or chalcophyrite. It may also infrequently occur in solution as lead-iron.
Chemical speciation of lead compounds in water is complex, and depends upon several factors including the solubility of lead compounds, pH, dissolved oxygen and the presence of coexisting inorganic and organic compounds. In the absence of soluble complexing species, lead is almost totally sorbed as precipitated species at pH greater than 6.
Many environmentally important lead compounds such as halides, sulphates, phosphates, and hydroxides are insoluble and thus are of relatively low toxicity in aquatic systems (Moore and Ramamoorthy 1984).
Zinc
Zinc is regulated by both MMLER and the NDOEL. In addition, CCME (1995) provides guidelines for drinking water and for the protection of freshwater life. Zinc sulphide is a common zinc mineral. Zinc sulphate, an oxidation product, is soluble in water and therefore usually found in most metal mining effluent. According to theoretical calculations, the greatest dissolved zinc concentrations in fresh waters are possible at low pH, low alkalinity and high ionic strength (Hem 1972).
Iron
Iron is regulated under NDOEL, but not MMLER. Guidelines are provided by CCME for drinking water, and for the protection of freshwater life. AQUAMIN (1996) has recommended that iron not be regulated under MMLER, but that it should be included in the list of parameters to be measured in periodic effluent characterization.
Iron is naturally released into the environment from weathering of sulphide ores such as pyrite and pyyrhotite. Concentrations in aerated surface waters are usually less than 0.5 mg/L. Total iron found in oxygenated surface waters of pH 5-8 typically ranges from 0.050-0.2000 mg/L, almost none of which occurs in ionic form (Wetzel 1983).
The chemical behaviour of iron in water is determined by oxidation-reduction reactions, pH and the presence of coexisting inorganic and organic complexing agents. In the presence of oxygen, which in natural systems may occur during the autumn circulation, ferrous iron is oxidised and precipitates as ferric iron. As a result, iron is usually found in the aquatic environment as colloidal suspensions of ferric hydroxide particles. Under anoxic conditions and in the presence of sulphide, ferrous sulphide (FeS) is precipitated (Wetzel 1983).
Iron may also be acted upon by microorganisms. Aerobic bacteria may catalyse the oxidation of ferrous iron, resulting in the precipitation of ferric hydroxide. The growth cycles of freshwater algae and aquatic microorganisms can influence the concentration of iron in surface waters.
Aluminum
Aluminosilicate minerals are abundant in all soil/rock types. Aluminum is mobilized from soils and sediments by natural weathering and accelerated acidification processes, resulting in detectable levels in surface waters (Harvey et al. 1981, in CCME 1995). Most natural surface waters contain less than 1 mg/L, although acid waters may contain higher concentrations. Aluminum is not regulated under MMLER, but CCME guidelines are available for the protection of freshwater life.
Generally, the chemistry of aluminum in water is essentially that of aluminum hydroxide. The forms of aluminum are determined primarily by pH and the nature of coexisting inorganic and organic ligands. In general, aluminum is relatively soluble in water, especially under acidic conditions. Aluminum has a minimum solubility at pH 5.5-6.0; concentrations of total dissolved aluminum increase in water at higher and lower pH values (CCME 1995).
Soluble aluminum concentrations in water have been found to increase greatly in lakes having surface water pH values below pH 5-6. Numerous organic materials, such as humic acid, fulvic acid, reducing sugars and organic acids, are capable of mobilizing aluminum in the aquatic environment. Levels of soluble aluminum in lake and stream waters have been correlated with pH and total organic carbon (TOC); aluminum concentrations increased as pH decreased and organic carbon increased (Driscoll et al 1980).
Cadmium
Although cadmium is not regulated under MMLER, limits are set by the NDOEL and guidelines provided under CCME. It is recommended as a parameter for monitoring during periodic metal mine effluent characterization (AQUAMIN 1996). As water hardness affects cadmium toxicity, AQUAMIN (1996) recommends a sliding scale of regulated levels.
Cadmium and its compounds are recognized as potentially "toxic" under the CEPA. Cadmium occurs in some metal mine effluents, particularly effluent from ores containing zinc because cadmium is commonly associated with zinc and is expected to respond to treatment for zinc.
Cadmium occurs in trace concentrations in freshwater, generally less than 0.001 mg/L. The form and fate of cadmium in water are complicated. They depend upon its chemical speciation, which is determined by water pH and hardness, as well as the presence of ligands and coexisting metal cations (Moore and Ramamoorthy 1984).
(ii) Thiosalts
During milling (grinding, aeration and flotation) of sulphide ores, part of the sulphide content is oxidized in the processing streams. In addition to sulphates, partially-oxidized sulphur oxyanions such as S2O3-2, S3O6-2, and S4O6-2 are formed. These species are collectively known as "thiosalts". In many mills, thiosalts are eventually completely oxidized to sulphate before the effluent leaves the mine property. However, at some mining operations, thiosalts persist in the mill effluent and ponds, and they pass through conventional effluent treatment operations largely unaffected. Upon entering receiving waters, they oxidise, in the presence of bacteria, to form sulphuric acid. However, effluent discharges to the marine environment will reduce the effects of thiosalts oxidation(AQUAMIN 1996).
Research studies have found that the amount of thiosalts produced is influenced by the grinding and media, pH, aeration, SO2 additions, pulp densities, sulphur content of the ore, residence time during flotation and temperature. Major types of thiosalts which appear to be a problem include thiosulphate, trithionate, and tetrathionate at concentrations ranging from 100-1000 mg/L.
(iii) Ammonia
CCME (1995) provides guidelines for total ammonia for the protection of freshwater life, based upon pH and temperature. For waters at pH 6.5 to 7.0, the guideline for total ammonia ranges from 2.5 mg/L at 0°C to 1.5 mg/L at 20°C.
(iv) Total Suspended Solids
Total suspended solids (TSS) is a direct measure of the concentration of particulate matter, organic and/or inorganic within the water column. Sediment can be defined as inorganic, undissolved matter ranging in size from fine colloidal clays (particles less than 0.004 mm in diameter) (McCubbin et al. 1990) to larger particles most commonly referred to as fine pebbles (particles 2-4 mm in diameter). While not necessarily acutely toxic (except at high concentrations), solids can transport other contaminants, including oxygen demanding substances, nutrients, metals, xenobiotic organic compounds and pathogenic microorganisms.
(v) Total Airborne Particulates
The deposition of airborne particulate matter (dust) into the aquatic environment is influenced by dust levels, distribution patterns, composition and the length of the ice/snow-free seasons. When dust particles are deposited on terrestrial or peatland habitats, particulates are effectively attenuated by the soil (mineral and organic). When deposited directly into watercourse or waterbodies, dust produced by the crushing of ore may increase levels of TSS. The nature of the sulphide ores (pentlandite, chalcopyrrite and pyrrhotite) which are insoluble in water, means that the bio-availability of the constituent metals is megligible.
(vi) pH
Hydrogen ion (H+) and hydroxide ion (OH-) are considered to be controlling variables in aqueous systems because they influence both physical-chemical and biological processes in the aquatic environment. The level of pH, the measure of hydrogen ion, is regulated under MMLER, the Newfoundland Water and Sewer Regulations. Acceptable levels vary with potential use (e.g., agricultural use vs. drinking water), but are generally between 5.5-9.0.
In aquatic systems, pH is usually a result of the geology and geochemistry
of the rocks and soils in the watershed (CCME 1995). Fluctuations in pH
occur naturally on a seasonal basis, usually influenced by runoff, nutrient
cycling, atmospheric input and ice cover.
CCME (Canadian Council of Ministers of the Environment). 1995. Canadian Water Quality Guidelines. Prepared by the Task Force on Water Quality Guidelines of the Council of Resource and Environment Ministers. Inland Waters Directorate, Environment Canada.
Driscoll, C.T., J.P. Baker, J.J. Bisogni and C.L Schofield. 1980. Effect of aluminum speciation on fish in dilute acidified waters. Nature (London) 284: 161-164.
Harvey, H.H., R.C. Pierce, P.J. Dillon, J.R. Kramer and D.M. Whelpdale. 1981. Acidification in the Canadian Aquatic Environment: Scientific Criteria for Assessing the Effects of Acidic Deposition on Aquatic Ecosystems. Associate Committee on Scientific Criteria for Environmental Quality, National Research Council of Canada, Ottawa. NRCC No. 18475. 369p.
Hicks, B.J., J.D. Hall, P.A. Bison, and J.R. Sedell. 1991. Responses of salmonids to habitat changes. Am. Fish. Soc. Spec. Publ. 19: 483-518.
Hem, J.D. 1972. Chemistry and occurrence of cadmium and zinc in surface waters and groundwater. Water Resour. Res. 8: 661-678.
McCubbin, R.N., A.B. Case, D.A. Rowe, and D.A. Scrutton. 1990. Resource road construction: Fish habitat protection guidelines. Fisheries and Oceans and Canadian Forestry Service, Ottawa, ON. 78p.
Moore, J.W. amd S. Ramamoorthy. 1984. Heavy Metals in Natural Waters. Springer-Verlag, New York, NY.
Wetzel, R.G. 1983. Limnology. W.B. Saunders Co., Philadelphia, Pennsylvania.
743p.
Acid generation occurs as a result of oxidation of sulphide minerals (enhanced by biologic activity) under aerobic conditions. It is generally accepted that, in most settings, gaseous diffusion of oxygen to the mineral surfaces is the process that controls the rate of acid generation. The rate of oxygen diffusion through water (or water filled voids in soil) is several orders of magnitude lower than that through air filled voids; consequently the rate of oxygen diffusion into tailings can be greatly reduced by either saturation or flooding.
Underwater placement of tailings will result in organic matter and natural lake sediments will accumulate on top of the tailings. Once buried under 0.05-0.10 m of normal lake sediment, diffusion conditions will largely control the geochemical behavior of the tailings (Rawson 1992). As the organic matter in the deposited sediments is degraded, reducing conditions will occur. This may result in precipitation and co-precipitation of metal sulphides, which reduces the leaching of mobile metals.
The following discussion examines the existing research and case history data available with respect to underwater tailings deposition. The advantages of underwater tailings deposition are compared to above-ground methods. The principles discussed also apply to mineralized mine rock disposal.
Collaborative federal government and mining industry sponsored research into the efficiency of subaqueous tailings disposal as a means of controlling oxidation of sulphide minerals was initiated in 1988. The first study was a review of underwater disposal of mineralized mine rock and tailings in the marine environment.
In early 1989, the Mine Effluent Neutral Drainage (MEND) group was formed. MEND is a collaborative group of industry and government that was organized to focus research on the methods to control acid rock drainage associated with the mining of sulfide ore bodies. A secretariat was established under the auspices of the Canadian Center for Mineral and Energy Technology (CANMET) in Ottawa. Under the direction of this group, research programs were developed to study the various methods for controlling acid rock drainage. One of the research programs was to investigate the suitability of water covers to control oxidation of sulfide minerals. In order for the study to have a broader applicability across the country, the underwater cover studies were focussed on tailings and mineralized mine rock placed in the freshwater environment. The initial study in 1989 was a detailed literature review of subaqueous disposal of reactive mine waste in freshwater environments. From this initial study, four lakes that had reactive tailings historically or actively discharged were selected for in depth study. These lakes covered two geo-climatic zones, the coastal rain forest and the boreal forest in the Canadian Shield. Two additional case studies were later added to the study, a control lake with no mine waste influence and a closed tailings impoundment with a water cover.
The following paragraphs summarize the MEND case studies carried out at Anderson Lake, MB and Buttle Lake, BC. These sites have been monitored over a period 3-8 years and while none of these tailings locations mimic conditions which will be encountered at the Voisey's Bay property they do demonstrate the feasibility of subaqueous disposal of tailings. The following discussion has been paraphrased from Pedersen et al. (1997).
Anderson Lake is a horseshoe-shaped, small (6 km long) mesotrophic to, eutrophic lake located in central Manitoba on the Precambrian Shield. The lake contains about 9 Mt of tailings from the processing of Cu-Pb-Zn massive sulphide ores, which were discharged via a floating, pipeline. Tailings were deposited in the western basin of the lake and a column of standing water up to 8 m in depth overlies the tailings. The tailings are sulphide-rich and acid generating. There is extraneous contamination (a road made of mineralized mine rock along one edge of the lake) which is continuously adding trace metals and sulphate to the tailings pond.
Results of the ongoing study at Anderson Lake indicate that there is no oxidation occurring in the upper 0.10 m of the submerged tailings. Reducing conditions which occur as a result of degradation of organic matter are actually resulting in diffusion of metals out of the surface water column and co-precipitating them as metal sulphides in the upper few centimetres of lakebed sediments.
Buttle Lake is a large (35 km long by 1 km wide by maximum 87 m deep) lake in which high sulphur tailings were discharged in deep water (below the thermocline). Biological affects of tailings deposition were not addressed, however it was determined that as of 1993 oxidation of the tailings has not occurred (deposition ended in 1981).
In addition, a significant amount of data compilation and investigation into subaqueous tailings disposal was conducted through Environment Canada, CANMET, under the auspices of the MEND program (MEND 2.11.1a, 1990; MEND 2.11.1b, 1990: MEND 2.11.2a, 1993: MEND, 2.11.4a, 1995: MEND 2.11.3abc, 1996; MEND 2.11.5ab, 1996; MEND 2.11.5c, 1996; Robertson et al. 1997; Pedersen et al. 1997).
This work has increasingly supported the position that in many cases subaqueous tailings deposition is the most environmentally-sound solution to acid mine drainage and metal release to the environment. The work has culminated in the development of guidelines for the subaqueous deposition of tailings which were presented in draft form at the Fourth International Conference on Acid Rock Drainage held in Vancouver, British Columbia, May 31 to June 6, 1997. These guidelines are currently undergoing final review and will be released in final form by CANMET (MEND) in December 1997.
Research and case studies which investigate various tailings disposal scenarios, including underwater deposition include work by the Waterloo Centre for Groundwater Research (David 1993; Nicholson et al. 1995; Nicholson et al. 1994; Bernard et al. 1995; and, Payant et al. 1995).
Case Studies
The following case studies summarize successful operation and/or post-closure of three Canadian mining operations using submerged disposal of sulfide bearing tailings.
Louvicourt Mine, Val d'Or, Quebec, Canada
Detailed discussions on the subaqueous disposal of tailings at the Louvicourt Mine are presented in Filion et al. (1994), Li (1997), and Li et al. (1997). An overview of the results is summarized below. The mine operator, AUR Louvicourt Inc, also provided additional information.
The Louvicourt mine is a base metal (copper/zinc) underground mine located approximately 20 km east of Val d'Or, in northern Quebec. The ore reserves exceed 24 million tonnes, averaging 4.3 percent copper, 2.1 percent zinc, 27.4 g/tonne silver, and 1.06 g/tonne gold. Mine production commenced in mid-1994 and is expected to continue for 18 years. The current mine production rate is 1200 tonne/day and it is predicted that 8 million tonnes of tailings will be generated throughout the life of the mine. The tailings contain between 30-50 percent sulphides, mainly as pyrite, 5-24 percent carbonates and 0.6 percent sphalerite, with the remainder consisting of quartz and other silicate minerals. Recent acid base accounting testing on the tailings indicated that the tailings and mineralized mine rock are net acid producing. By comparison, the tailings generated at Voisey's Bay will be dominated by pyrohotite and pyrite sulfide minerals and will also be net acid producing.
The mine site is located in a typical subarctic climatic zone in northwestern Quebec. These climiatic conditions are similar to the Voisey's Bay area. The average temperatures in the coldest months, December and January, are -13.2 and -16.8 degrees Celsius, respectively. The average temperature for the seven ice-free months (April through October) is 10 degrees Celsius. The average annual precipitation is 954 mm, with a net annual precipitation, subtracting evaporation and evapotranspiration, of 413- 465 mm.
Low topographic relief, generally poor drainage, with frequent swamps and scattered, small, relatively shallow lakes, characterizes the area surrounding the mine site. The tailings basin is located approximately 9 km from the mine site. The basin has a total capacity of approximately 12 million tonnes, covering 164 hectares. The typical depth of the basin is 6 m, however, maximum depths may reach 18 m. The initial average water cover depth was 3 m, which is gradually decreasing as the basin is filled. Current closure plans specify a minimum water cover depth of 1 m.
The tailings basin design for the Louvicourt facility is very similar to that proposed for Voisey's Bay. The design of the tailings basin included creek diversion and the construction of low permeability dam cores and key trenches. Bedrock grouting and the construction of low permeability blankets were also completed to reduce seepage losses. During ice-free periods, the tailings are deposited using a floating pipeline. Initially, tailings were deposited in deeper portions of the basin, however, tailings are currently spread by continuously moving the pipeline to ensure even deposition throughout the basin. For deposition during the winter, when the pond surface is frozen, one to two locations, capable of accommodating all the tailings produced during that period, are selected. The insulated pipeline is run directly over the ice and a hole is cut through the ice to permit deposition in the dedicated location. The pipeline is flushed with water to prevent freezing when not in use. Bathymetry surveys are completed twice a year to ensure that the tailings are being evenly distributed in the basin.
In accordance with provincial and federal regulations, monitoring of the discharge from the tailings basin is completed on a regular basis. The presence of thiosalts in the mill process water resulted in the requirement for lime addition to the effluent to ensure discharge from the tailings impoundment met all regulatory guidelines. In addition, as a precautionary approach, a polishing pond was constructed to reduce the suspended sediments loadings in the discharge water prior to release to the surrounding environment.
Falconbridge Limited, Sudbury Ontario:
Falconbridge Limited investigated options for closure of existing above
ground acid generating tailings impoundments in the early 1990s. Nicholson
et al. (1994; 1995) used the WATAIL geochemical model to predict the relative
performance of the following options for an acid generating tailings with
approximately 12-15 percent pyrrhotite and 0.3 to 4 wt. percent carbonate:
The tailings had been oxidizing in place for about five years at the time of the study. Even so, Nicholson et al. (1995) indicates that the lowest total release of sulphate and iron would occur by flooding the impoundment and relocating all of the tailings to below the final water level.
Oxidation rates were estimated by David (1993) for the same site. Field measurements of oxygen and solute profiles were recorded at various locations to simulate the three closure options listed above. The oxygen flux rate and sulphate release rate predicted for flooding was the lowest.
Heath Steel Mines, Newcastle, New Brunswick:
Payant et al. (1995) and Bernard et al. (1995) evaluated of the best available technologies for reducing acid generation potential in a waste rock from the Stratmat site at Heath Steel Mines, NB. The waste rock contained about 19 percent pyrite minerals and minimal (less than 0.2 wt. percent) carbonate as CO3.
Based on column tests running over a period of three years less than,
Payant et al. (1995) reported that providing a water cover was the most
effective of several techniques tested for the attenuation of acid production.
Relative to exposed tailings, a water cover reduced oxygen flux by approximately
99.7 percent. The next best option was a multilayer oxygen barrier type
of soil cover, which had an initial effectiveness of 98.3 percent in the
laboratory. Importantly, however, the results from parallel outdoor tests
indicated the effectiveness of the same soil cover exposed to weathering
was reduced to 46.5 percent (due to freeze-thaw effects and desaturation
effects).
Conclusions
Robertson et al.(1997) summarizes the results of the MEND research with the following statement:
The work conducted to date has demonstrated that reactive tailings can have a very stable geochemistry when placed under a water cover. On this basis, the study sponsors are confident that with proper and thorough engineered conditions, subaqueous tailing disposal can be considered to be one of the best available preventive technologies for disposing of reactive tailing material.
Of the technologies available for tailings disposal, subaqueous deposition
is the best method to minimize the effects associated with acid mine drainage
at Voisey's Bay. As organic matter and natural sediments are deposited
over of the tailings and mineralized mine rock the biologic and aquatic
ecosystem will gradually re-establish itself.
Filion, M.P., F.W. Firlotte, M.R. Julien, and P.F. Lacombe. 1994. "Regulatory Controlled Design - Louvicourt Project - A case Study" Pp. 22-31. In Proceedings of the Third International Land Reclamation and Mine Drainage Conference and the Third International Conference on the Abatement of Acidic Drainage, Pittsburgh, PA, April 24-29, Vol. 2.
Li, M., A. Bernard, and L. St-Arnaud. 1997. "Considerations in the Use of Shallow Water Covers for Decommissioning Reactive Tailings". Pp 117-130 In Proceedings of the Fourth International Conference on Acid Rock Drainage, May 31 - June 6, Vancouver, BC.
MEND 2.11.3abc. 1996. "Geochemical Assessment of Subaqueous Tailings Disposal in Anderson Lake, 1993 - 1995 Study Program". Prepared by: Rescan Environmental Services Ltd.
MEND 2.11.5ab. 1996. "Shallow Water Covers - Equity Silver Base Information on Physical Variables". Prepared by: Hay & Company Consultants Inc.
MEND 2.11.2a. 1993. "Literature Review Report: Possible Means of Evaluating the Biological Effects of Sub-Aqueous Disposal of Mine Tailings". Prepared by: INRS-EAU.
MEND 2.11.5c. 1996. "Geochemical Assessment of the Equity Silver Tailings Pond". Prepared by: Rescan Environmental Services Ltd.
MEND 2.11.4a. 1995. "Geochemical Assessment of Subaqueous Tailings Disposal in Buttle Lake, B.C., 1993 Study Program". Prepared by: Rescan Environmental Services Ltd.
MEND 2.11.1a. 1990. "Geochemical Assessment of Subaqueous Tailings Disposaal in Buttle Lake, British Columbia." Prepared by: Rescan Environmental Services Ltd.
MEND 2.11.1b. 1990. "Geochemical Assessment of Subaqueous Tailings Disposaal in Anderson Lake, Snow Lake Area, Manitoba." Prepared by: Rescan Environmental Services Ltd.
Nicholson, R.V., J.M. Scharer, E. Kwang, and G. Williams, 1994, "Additional Modellingand Verification Sampling at the New Tailings Facility: A Study of the Effects of Implementing Management Options After Nine Years of Being Inactive". Report submitted to Falconbridge Limited.
Nicholson, R.V., J.M. Scharer, E. Kwang, W. Annable, and G. Williams. 1995. "An Application of the WATAIL Model to Develop a Management Strategy for a Pyrrhotite Tailings Impoundment". Pp. 1043-1052. In Sudbury '95, Conference of Mining and the Environment, Sudbury, Ontario, May 28 - June 1, 1995. Conference Proceedings, Vol. 3.
Payant, S.; L.C. St-Arnauld, and E. Yanful. 1995. Evaluation of Techniques for Preventing Acidic Rock Drainage. Pp. 485-494. In Sudbury '95, Conference of Mining and the Environment, Sudbury, Ontario, May 28 - June 1, 1995. Conference Proceedings, Vol. 2.
Pedersen, T.F., J.J. McNee, D. Flather, B. Mueller, A. Sahami, and C.A. Pelletier. 1997. Geochemistry of submerged tailings in Buttle Lake and the Equity Silver Tailings Pond, British Columbia, and Anderson Lake, Manitoba: What have we learned. Pp. 991-1005. In Proceedings; Fourth International Conference on Acid Rock Drainage, Vancouver, BC, May 31 - June 6, 1997. Vol. 3.
Rawson (The Rawson Academy of Aquatic Science). 1992. "A critical review
of MEND studies conducted to 1991 on subaqueous disposal of tailings".
SSC File No. 03SQ.23440-1-9131.
Robertson, J.D., G.A. Tremblay, and W.W. Fraser. 1997. Subaqueous tailing
disposal: A sound solution for reactive tailing. Pp. 1029-1040. In Proceedings;
Fourth International Conference on Acid Rock Drainage, Vancouver, BC, May
31 - June 6, 1997. Vol. 3.
