Background research of the tailings area of a Ni-Cu mine for the determination of an optimal method of revegetation
Elena I. Khozhina* and Barbara L. Sherriff
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, R3T 2N2, Canada.
ehozhina@inbox.ru, BL_ sherriff@umanitoba.ca
* Corresponding author
Abstract
This research determined the agricultural properties of pyrrhotite-rich exposed tailings of a Ni- Cu Mine, at Thompson, Manitoba, the state of plants colonizing them, and did greenhouse tests of standard revegetation techniques. Top- and sub-soils of the tailings were compared with glaciated clay soils in a Control Area. The most important characteristics of the Exposed Tailings are low Net Neutralization Potential (–1.27 to –243.67 kg CaCO3eq./tonne) and high salinity (2.58 to 19.0 dS/m) in the root zone. The tailings and the plants growing on them contain high Ni. In the green- house experiment, plant growth was suppressed even though tailings were neutralized and treated with organic matter and fertilizer. Therefore, the standard revegetation methods can not be applied. We recommend a protective layer of waste rock fines to limit root growth into the tailings. This layer should be covered by organic layer for plant growth. The protective layer would also decrease the production of windborne dust and the flow of oxygen into the tailings.
Keywords: sulphide tailings, Ni-Cu Mine, background research, salinity, revegetation
1 Introduction
It has been known since Roman times that vegetation reflects the nature of its substrate. The type and function of plant populations developing on a particular type of soil depend on its physical (texture, compaction, structure, water content, temperature), chemical (pH, salinity, organic carbon, chemical composition), and biological (micro- and mezobiota) characteris- tics (BROOKS1983; KABATA-PENDIASand PENDIAS2001). The relationship between vege- tation and substrate has been used for prospecting for minerals in geobotany by visual examination of the vegetation cover and biogeochemistry by chemical analysis of vegetation (BROOKS1983).
The ecology of a plant community is greatly influenced by the pH of the soil, as this affects the availability of elements (BROOKS1983; COREY1990; KABATA PENDIAS and PENDIAS2001). Redox potential (Eh) of the soil also affects elemental availability by con- trolling the oxidation state of an element. For example, halophytes form a characteristic plant association growing on saline soils with sodium chloride, carbonate, or sulphate. They usually have a high osmotic pressure in the cells and can accumulate high levels of salts (BROOKS 1983). Calciphilous floras, forming over limestone and dolomite, include both calcicolous plants, which absorb Ca, and calciphilous plants, which require the specific growth conditions of calcarious rocks (good aeration and conductance of water and heat) (BROOKS1983). Selenium flora, which either have a specific requirement for or can tolerate large concentrations of Se, indicate high concentrations of this element in the soil. Some species of selenium flora are able to absorb up to 1 % dry wt. Se and emit a garlic-like odor (BROOKS1983). Soils rich in Cr, Co, Fe, Mg, and Ni, and deficient in nutrients, Ca, Mo, N, P,
and K support serpentine floras, which are characterized by short species and individuals. In areas of sulphide mineralization, Galmei flora grow on soils high in Cu, Pb, or Zn (BROOKS
1983).
The most important aspect in the revegetation of mine waste sites is, therefore, the improvement of growth conditions in the plant habitat (reclamation), as this determines the composition of plant species and the physiological state and chemical content of individual plants. To do this, adequate background research is required. The properties of waste as a growth medium, its physical, chemical, and biological characteristics, should be tested to reveal key differences between the waste and natural soils, which are responsible for poor soil conditions. The type of revegetation technique should be selected to reduce these differences. The success of plant growth has been shown to be directly proportional to the amount of substrate research (PECKand SOLTANPOUR1990).
Tailings of base metal mining are characterized by poor physical properties, such as fine texture with little aggregation, high hydraulic conductivity, low water holding capacity, and low organic carbon. The high content of sulphides leads to acidity and high metal concen - tration in ground water (ZHANGet al. 1996; DUDKAand ADRIANO1997; YEet al. 2000; HAO
et al. 2004; VEGAet al. 2004). These characteristics create a complex stress for the living organisms and can restrict their growth and development.
Revegetation of mine tailings usually includes: (1) the addition of neutralizing materials (carbonates, oxides, hydroxides, and silicates), (2) the addition of inorganic fertilizers or/and organic materials, such as soil, synthetic or natural organic wastes (municipal sewage sludge, animal manure, paper mill sludge or sawdust), and (3) seeding with standard seed mixtures or seeds of native plants, that should be endemic to a region and found in mining areas (ROBERTSet al. 1988; SEAKER1991; FEAGLEYet al. 1994; PICHTELet al. 1994; MARIONand LUCON1998; DUNKERand BARNHISEL2000; HAERINGet al. 2000; NORLAND2000). Each mine waste site is individual and has its own specific characteristics including; the topography of tailing discharge area, the type of the discharge and length of time for storage, the mineralogy of the tailings, the potential for acid production and metal release, and the climate. Thus, background research must be done for each specific site.
The objectives of this study of the tailings area at the Ni-Cu Mine of INCO Ltd., Thompson, were (1) to examine agricultural properties of the tailings and the state of the plants naturally colonizing the tailings surface, (2) to test the standard method of revegetation of minesoil in greenhouse conditions, and (3) to choose the most appropriate strategy for revegetation of the tailings on the basis of the background studies.
2 Methodology
2.1 Area of studyThe mine is located about 5 km south of the town of Thompson, approximately 645 km north of Winnipeg, Manitoba. Glaciation of the Precambrian rocks produced a gentle relief with elevations varying from 180 to 240 m. Thompson has a continental climate, typical of central Canada, with about 85 frost-free days per year. The average annual precipitation is 550 mm with 67 % as rain and 33 % as snow. Annual lake evaporation is about 400 mm and losses due to evapotranspiration about 300 mm (KLOHNLEONOFF1992). The mine is in the Thompson Belt, which is the largest nickel deposit associated with ultramafic rocks in the Canadian Shield. The mine has been operated for the extraction of Ni and Cu by INCO Ltd.
from 1960 to the present (KLOHNLEONOFF 1992). Processing of the ore includes milling,
selective flotation, smelting of flotation concentrates, and electrochemical separation of Ni and Cu. About 40 million tonnes of sulphide tailings have already been produced by the mine and about 15 million tonnes will be deposited before the closure (SEACOR 1996) into the Tailings Management Area formed in Thompson Lake.
The tailings basin is in a zone of discontinuous permafrost (KLOHNLEONOFF1992). The sulphides present are pyrrhotite, pentlandite and chalcopyrite with minor pyrite, violarite and mackinawite (KLOHN LEONOFF 1992; SIDENKO et al. 2007). Pyrrhotite is the most reactive sulphide with an oxidation rate up to 100 times higher than that of pyrite at 22 °C in atmospheric oxygen (NICHOLSONand SCHARER1994). The rest of the tailings consists of quartz, alumino-silicates, calcite, and minor dolomite (KLOHNLEONOFF1992). Most of the tailings are covered with water (Submerged Tailings), however, about 3.0 km2of the tailings area are elevated above the water level and exposed to air and atmospheric precipitation (Exposed Tailings, Fig. 1). Currently tailings are discharged into Area 4 of the Submerged Tailings. The Exposed Tailings in Area 3, Area 2, Tailings Beach, Abundant Tailings, and Emergency Tailings were chosen for this investigation (Fig. 1). Area 3, which is partly covered by water, contains the youngest tailings with discharge ceasing in 2005. The Abundant Tailings are the oldest, being about 40 years old. The Emergency Tailings Area was used for discharges in cases of pipeline breakdown until 2003. Currently, Area 1 is used for emergency discharge.
Waste rocks, at the Thompson Operations, consist of quartzite, schist, mineralized schist, skarn, and amphibolite. They are classified as having marginal acid generation potential with net neutralizing potential (Net NP) of –9.1 kg CaCO3eq./tonne KLOHNLEONOFF1992).
Mineralized schist has the most negative Net NP (–352.48 kg CaCO3/tonne) and the highest Ni content (1.39 wt. %) compared to other rocks (quartzite 0.19, schist 0.09, skarn 0.04, amphibolite 0.04 wt. %) (KLOHNLEONOFF1992).
Thompson
Winnipeg
0 1000 m
Mill
Exposed Tailings Submerged Tailings Surface sampling 1–8 Profile sampling N
–55°
–56°
Control Area Tailings
Beach Abundant
Tailings
Emergency Tailings
Area 1 Area 2
Area 3 Area 4
1
2 3 4 5
7 6
8
Dam B
Fig. 1. The location and the schematic of the mine site, Thompson, Manitoba, Canada.
An uncontaminated Control Area was chosen for comparison with the tailings in a forested area near a small pond (about 400 m2), 3.9 km west of the Dam B (Fig. 1). The mine area is in the boreal forest, which consists of stunted trees and low bush. The soils in this region are glacial-lacustrine grey clay, covered by brown weathered clay and finally with humus. There is a layer of water bearing sand overlying the bedrock below the clay.
2.2 Sampling
In order to evaluate the lateral properties of tailings, twenty two surface samples (up to 15 cm depth) were collected from all of the exposed areas, except the Abundant Tailings, using a stratified random sampling method. Eight of the twenty two sampling locations were selected for profile sampling, up to a depth of 70 cm, to study the vertical properties of tailings. These are areas of plant growth, which are usually close to ponds or natural forest.
The length of the profiles was equal to the depth of root zone or the depth to the water table if this was less. Profiles were stratified into layers according to colour that varied from dark grey for the reduced tailings to reddish orange for the most oxidized ones and each layer sampled randomly to give forty one samples. Additional samples were collected from profile ATP10, where the roots of fowl bluegrass (Poa palustrisL.) were surrounded by 1 cm of oxidized tailings in the reduced grey tailings. A composite sample of oxidized tailings con- sisted of ten sub-samples from different spots of the rhizosphere. After being mixed, each profile sample, including the rhizosphere sample, was divided into three sub-samples, which were analyzed separately.
A sample of waste rock fines (20 L), provided by INCO Ltd., was thoroughly mixed in the laboratory, with three sub-samples taken for analysis.
In the Control Area, four composite surface samples were taken to represent the layer of highest organic carbon content (0–15 cm). An area of one hectare was divided into four equal squares. A composite sample, consisting of three sub-samples collected equidistantly along a diagonal, was obtained from each square. Since the lacustrine clay soils do not vary with depth (up to 1 m), profiles were not collected. The dominant plant species within the Tailings Area water sedge (Carex aquatilis Wahl.) was sampled in early July at each of the seven tailing surface sampling locations where it was found. At each location, 3 to 5 sampling points were chosen randomly within a cluster to obtain a composite sample of 15 to 20 plants. These were divided into live and dead parts. Dead shoots were from the previous season but were still standing and attached to the rhizomes. The plants were washed thoroughly in distilled water at the laboratory. Three composite samples of live and dead shoots of water sedge were collected from the Control Area.
At the end of September, seeds of water sedge were collected for germination tests by random sampling from each of the plant sampling locations of the Tailings and Control Areas.
2.3 Analysis
The particle size of tailings and waste rock fines was measured on 100 to 200 g of air-dried and well mixed samples, which were mechanically passed through 2.0, 1.0, 0.5, 0.25, 0.125, 0.063 mm sieves over a period of 5 to 10 minutes (JELHNEK1974; DONAHUEet al. 2000). The finest fraction (0.063 mm) was separated into silt and clay using a hydrometer (SHELDRICK
and WANG 1993). The particle size of natural clay soil was also determined using the hydrometer. The texture of the samples was classified using U.S. Department of Agriculture system of soil classification (PCA 1973; JURYet al. 1991). Soil water content was determined
gravimetrically and calculated as a percentage of moist soil mass (wet mass % H2O) (TAN
1996). Available water content, the difference between water content at the field capacity (0.3 bar) and wilting point (15 bar), was determined using a pressure plate method (TAN
1996). Hydraulic conductivity of waste rock fines and core samples of natural soils were measured by a falling-head method (JURYet al. 1991). Organic matter and pH were analysed on samples, which passed through the 2-mm sieve. Organic matter was analyzed by the Walkley and Black method, which consisted of oxidation of organic carbon in a solution of acid dichromate followed by back titration of the remaining dichromate with ferrous sulphate solution (ALLISON1965; TIESSENand MOIR1993). The pH was measured on 1:2 soil-water mixtures (LIEROP1990) using an AP62 Accumet pH/Eh Meter with an Accumet 13-620-AP50 Ag/AgCl electrode. Electrical conductivity was determined by a saturated paste method at 25 °C using an AP65 Accumet Conductivity Meter with 13-620-168P (1.0 cm–1cell constant) and 13-620-171 (10.0 cm–1 cell constant) Accumet electrodes (JANZEN
1993; SPAC 2000). No adjustment for soil texture is required for this method (SPAC 2000).
For elemental analyses, samples of natural soils, tailings, and waste rocks were air-dried and ground to pass a 63-µm screen. A 0.5 g portion was dissolved using HF/HNO3/HClO4
digestion. Concentrations of B, Na, Mg, K, Ca, Cr, Mn, Fe, Co, Ni, Cu, Zn, P, and S (JEFFERY
1981) were measured, using a Varian Liberty 200, inductively coupled plasma, optical emission spectrometer (ICP-OES) referenced to ICP standard solutions (SCP Science®) at the Department of Geological Sciences, University of Manitoba. A 3.0 g portion was analyzed for CaCO3content by a gravimetric method based on measuring the loss in sample weight from CO2released after reaction with acid (GOHet al. 1993).
A 1.0 g portion of tailings, waste rocks, and soil samples was mixed with 1 M NH2OH·HCl solution in 25 vol. % acetic acid at 96 °C (HALL et al. 1996). This extracts water-soluble sulphate absorbed onto the surface of Fe-oxyhydroxides, both amorphous and poorly crystalline Fe-phases, as well as crystalline phases (goethite and jarosite). The total sulphur in the extractant was determined by ICP-OES and the sulphate content calculated. The Net NP of the tailings and soils were calculated as the difference between neutralization potential (NP) and acid generating potential (AP) using the mNP mineralogical ABA computer program (PAKTUNC1999, 2003). NP is calculated from the content of carbonate as calcite and AP values, which depend on sulphide content taking into account that pyrrhotite (Fe1-xS) is the major sulphide present. Sulphide content was calculated as the difference between total sulphur and sulphate S.
All plant species found within the Tailings and the Control Areas were collected and identified using the University of Manitoba Herbarium (WIN).
Seed germination tests were done according to BRADBEER (1988) and KOVAL and SHAMANIN(1999). A sample of 100 seeds was placed in a 90 mm diameter sterile plastic disposable Petri dish, with a filter paper at the bottom (BRADBEER1988). The seeds were then stratified for four days (KOVALand SHAMANIN1999). The Petri dishes were placed in a growth chamber at 25 °C for 24 hours and then in a cold room at 2 °C for another 24 hours.
This cycle was repeated once more. Stratified seeds were germinated in a growth chamber at 23 °C with 16 hours of light per day and at 18 °C during dark periods. During the stratification and germination of seeds, the filter paper in Petri dishes was kept wet. Three replicates were tested for each sample.
Samples of live and dead shoots of water sedge were air-dried and macerated in a coffee- grinder. Portions (2.5 g) of the crushed plant material were digested with HF/HNO3(BOCK
1984) and analyzed for Ni, Cu, and Zn using ICP-OES. The precision of the analyses is 10 % of the total value.
One-tail t test with αequal to 0.05 was used to determine significance of the difference between two mean values (KEPPEL1982).
2.4 The greenhouse experiment
A greenhouse experiment was established to test the standard revegetation method with organic materials and fertilizers being incorporated into neutralized tailings to improve the quality of the tailings as a soil (YEet al.2000). Three types of growth medium were tested:
(1) a mixture of tailings and limestone (T-medium), (2) a mixture of tailings neutralized with limestone and potting soil (TS-medium), and (3) a control mixture of sand and potting soil (SS-medium). Knopp solution (205 mg/L N, 57 mg/L P, 231 mg/L K, 234 mg/L Ca, 49 mg/L Mg, 66 mg/L S) diluted five times was added as a fertilizer daily in dry periods and every second day in wet periods (KOVALand SHAMANIN 1999). In contrast to solid fertilizers, nutrients from Knopp solution are available immediately and distributed evenly in the soil.
Plants were grown using a Na light (400 W) under daily conditions of 16 hours of light at 26 °C, and 8 hours of dark at 19 °C.
Limestone fines (carbonates, 10 mesh) were applied at the rate of 55.6 g of limestone per kg of tailings according the mean value of Net NP for the oxidized tailings (Table 1). Half of the limestone was applied more than six months before other amendments (PETERS1995).
Potting soil was applied at the rate of 33.7 g per kg of tailings and 47.5 g per kg of sand. This provided an organic content of the mixtures equal to that of the natural soils from Control Area (3.8 %, Table 1). The ratio was calculated from the organic matter content of the potting soil (82.9 wt. %), tailings (1.2 wt. %, Table 1), and sand (0.0 wt %). Seeds of water sedge, fowl bluegrass (Poa palustris L.), and foxtail barley (Hordeum jubatum L.) were collected from the Control Area and than stratified and germinated in the growth chamber in the same way as for the seed germination test. Species were planted separately with four replicates of ten seedlings into each medium.
Since water sedge was the only species growing on the oxidized tailings, the TS-medium for the water sedge was prepared using oxidized tailings, whereas reduced tailings were used for fowl bluegrass and foxtail barley with appropriate rates of limestone (24.2 kg per tonne, Table 1). Fowl bluegrass and foxtail barley were not planted into the T-medium.
3 Results
3.1 Tailing surfaceSurface samples of the tailings were divided into reduced and oxidized tailings according to their color and pH values. The color of reduced tailings varied from dark bluish grey (GLEY2 4/1) to bluish black (GLEY2 2.5/1), and of oxidized tailings from olive-yellow (2.5Y 6/6) to red (5YR 5/4). Reduced tailings have neutral or slightly alkaline values of pH, according to the classification of soil reactions (TAN1982; BRADY1990). Oxidized tailings are acidic (pH 3.5 ± 0.6) due to sulphides being partly oxidized with production of sulphate and sulphuric acid (Table 1).
Both reduced and oxidized tailings are classified as loamy sand (Table 1). In contrast, natural soils have a clay texture with 54.85 ± 2.33 % clay, 30.6 ± 2.69 % silt, and 14.50 + 0.42 % sand. The difference between mean water content in reduced (27.4 ± 6.8 wet %) and oxidized (23.2 ± 8.4 wet %) tailings is not significant (t[20] = 0.73, p= 0.247) (Table 1 and 2). However, there is a significant difference between water content in natural soils (48.4 ± 12.5 wet %) and reduced as well as oxidized tailings (Table 1 and 2). This was attributed to the higher con- tent of organic matter (3.84 ± 1.13 %) in natural soils, which is significantly different from the means of organic matter content in both types of tailings (Table 1 and 2). There is no signifi- cant difference between organic matter content in reduced and oxidized tailings (Table 2).
Table 1. Physical and chemical characteristics of the surface samples of the tailings and natural soils.
1particle size classification according to ISSS (International Society of Soil Sciences) (JURYet al. 1991);
2NNP is negative; 3NNP is positive and higher than 20 kg CaCO3eq./tonne Tailings Area
Reduced Oxidized Control Area
Mean SD Mean SD Mean SD
Coarse sand, %1 23.78 11.70 31.51 19.02 4.50 0.42
Fine sand, % 55.64 8.29 55.95 12.38 10.00 0.50
Silt, % 10.00 5.03 7.25 3.30 30.6 2.69
Clay, % 10.58 7.23 4.45 1.30 54.9 2.33
Wet mass % H2O 27.4 6.8 23.2 8.4 48.4 12.5
Organic matter, % 1.17 0.62 1.19 0.72 3.84 1.13
pH 7.3 0.4 3.5 0.6 6.7 0.8
EC, dS/m 13.1 4.7 8.3 0.3 0.41 0.02
Total S, % 2.02 1.87 2.39 1.47 0.04 0.02
Sulphate, % 0.26 0.18 0.24 0.12 0.006 0.003
Sulfides, wt% 5.63 5.10 6.48 3.65 0.17 0.08
CaCO3eq., wt % 2.27 0.96 1.69 1.16 3.00 1.38
NNP, kg CaCO3eq./tonne –24.2 21.3 –56.5 31.7 26.9 15.7
Potential for ARD Likely2 Likely Non3
B, ppm 170 78 240 110 46 3.7
Na, % 1.0 0.17 0.87 0.21 0.76 0.055
Mg, % 1.0 0.33 0.97 0.25 1.2 0.13
P, ppm 180 42 130 34 450 77
K, % 1.9 0.57 1.5 0.32 2.3 0.25
Ca, % 1.2 0.13 1.0 0.43 0.97 0.23
Cr, ppm 310 170 240 110 75 4.1
Mn, ppm 770 180 730 190 580 260
Fe, % 8.9 5.2 11 4.3 3.3 0.25
Co, ppm 25 6.7 41 28 19 3.3
Ni, ppm 2000 700 2200 1300 81 29
Cu, ppm 120 39 150 70 34 9.8
Zn, ppm 95 19 80 16 78 6.2
Conductivity measurements revealed the high salinity of both the reduced (13.1 ± 4.7 dS/m) and oxidized (8.3 ± 0.3 dS/m) tailings. These can both be classified as strongly saline soils having conductivity values between 8.1 and 16.0 dS/m (Table l) (SPAC, 2000) but do not belong to sodic or saline-sodic soil categories because they have pH values lower than 8.5 (BRADY and WEIL2000). Only tolerant crops such as barley (Hordeum vulgare L.) can survive in such. strongly saline soils since threshold soil salinity for barley is 8.0 dS/m (MAAS
1985). Natural soils at Thompson were classified as nonsaline soils having conductivity between 0.0 and 2.0 dS/m. The difference between conductivity of natural soils (0.41 dS/m) and both types of tailings is significant (Table 2).
The mean content of sulphides in reduced (4.16 %) and oxidized (6.48 %) tailings varied greatly between 2.0 and 8.3 % for reduced tailings and 1.7 and 11.3 % for oxidized tailings (Table 1). There is no significant difference between the two means (t[20] = –0.98, p= 0.182) (Table 2). Sulphide content in soils was between 0.12 % and 0.3 % with a standard deviation
of 0.08 %. The differences between mean sulphide content in reduced tailings and soils, and oxidized tailings and soils are significant (Table 2). The average values of sulphide content in the tailings are an order of magnitude higher than for natural soils (Table 1).
The calcium carbonate equivalent was low for both reduced (2.27 ± 0.96 %) and oxidized (1.69 ± 1.16 %) tailings as well as for natural soils (3.00 ± 1.38 %) (Table 1). There are no significant differences among the values (Table 2). Net NP is between –8.6 and –66.6 kg CaCO3eq. per tonne for reduced tailings and between 12.5 and –97.8 kg CaCO3eq. per tonne for oxidized tailings indicating that the potential for ARD is likely (Table 1). In contrast, there is no potential for ARD in natural soils with Net NP from 9.1 to 39.6 kg CaCO3eq. per tonne and the mean value of 26.9 (Table I).
There is no significant difference between the metal concentration in reduced and oxidized tailings (Table 2). However, concentrations of P differ significantly with a higher mean content of P in reduced (180 ppm) tailings (Table 1 and 2). Significant differences exist between reduced tailings and natural soils for B, Na, P, Cr, Ni, and Cu. Average metal concentrations in reduced tailings are higher than in soils by factors of 3.6 for B, 4.2 for Cr, 3.4 for Cu, 1.3 for Na, and 25 for Ni (Table 1). The large difference for Ni makes this metal the most problematic. The concentration of P in the reduced tailings is 2.5 times less than in natural soils.
Significant differences were revealed between the metal content in oxidized tailings and natural soils for B, P, K, Cr, Fe, Ni, and Cu (Table 2). Mean concentrations of metals in oxidized tailings are higher than in natural soils by factors of 5.4 for B, 3.2 for Cr, 4.5 for Cu, 3.4 for Fe, and 27 for Ni (Table I). The concentration of P in oxidized tailings is one quarter that in natural soils.
Table 2. One-tail t tests between the mean values of reduced and oxidized tailings (Reduced-Oxidized), reduced tailings and natural soils (Reduced-Soils), and oxidized tailings and natural soils (Oxidized- Soils).
Reduced-Oxidized Reduced-Soils Oxidized-Soils
t(20) p t(14) p t(12) p
Wet mass % H2O 0.73 0.247 –2.56 0.031 –3.47 0.007
Organic matter, % –0.040 0.485 –3.59 0.012 –4.13 0.003
EC, dS/m 2.41 0.026 4.68 0.005 44.06 4. 6E-9
Sulfides, wt% –0.98 0.182 3.14 0.017 2.90 0.014
CaCO3eq., wt % 0.72 0.248 –0.75 0.247 –1.45 0.099
B –0.95 0.188 2.75 0.026 2.96 0.013
Na 0.90 0.201 2.33 0.040 0.86 0.210
Mg 0.15 0.444 –0.98 0.192 –1.45 0.099
P 1.86 0.056 –5.33 0.003 –8.36 0.00008
K 1.30 0.120 –1.11 0.164 –3.67 0.005
Ca 0.76 0.237 1.51 0.103 0.11 0.458
Cr 0.72 0.249 2.39 0.037 2.51 0.023
Mn 0.29 0.390 1.04 0.178 0.95 0.189
Fe –0.62 0.278 1.86 0.068 3.00 0.012
Co –0.94 0.191 1.62 0.090 1.37 0.109
Ni –0.24 0.409 4.74 0.0045 2.73 0.017
Cu –0.67 0.264 3.70 0.010 2.77 0.016
Zn 1.20 0.137 1.47 0.107 0.20 0.423
3.2 Tailing profiles
Within the tailing profiles, pH values vary from very strongly acid (2.94 ± 0.017) to slightly alkaline (7.94 ± 0.06). Acidic layers were found beneath tailings with higher pH values. In profile 1, a strongly acidic layer (pH 4.31 ± 0.02) was found at a depth of 22 cm beneath moderately acid layer (pH 5.69 ± 0.27) (Fig. 2A). A second strongly acid layer (pH = 4.45 ± 0.22) at 72 cm is located under a neutral one (pH = 7.04 ± 0.19) in the same profile. In profiles 2, 3, and 5, acidic layers of tailings with mean pH of 4.71 (20 cm depth), 2.94 (30 cm depth), and 3.27 (20 cm depth) were found beneath layers with higher mean values of pH of 7.63 (10 cm depth), 4.70 (20 cm depth), and 6.94 (10 cm depth) (Fig. 2A, 2B, and 2C). In pro- file 3, a strongly acidic layer with a pH of 4.70 (20 cm depth) was located under a moderately acidic one (pH = 5.88 ± 0.23). The colour of the acidic layers varies from olive-yellow to red, reflecting the degree of pyrrhotite oxidation and replacement by the secondary Fe phases, goethite, lepidocrocite, and jarosite (SIDENKOet al. 2007; KAVALENCH2004).
In the less oxidized profiles (6, 7, and 8) the pH varied from slightly acid (6.28 ± 0.25) to slightly alkaline (7.94 ± 0.06) (Fig. 2C and 2D). However, acidic tailings (4.13 ± 0.15) were found within the rhizosphere of the fowl bluegrass (Poa palustrisL.) (Fig. 2D). Roots, found within the layer of bluish grey tailings (GLEY2 6/1) at a depth of 7 cm, were surrounded by a 1cm thick layer of red tailings.
Within the tailing profiles, electrical conductivity values vary from 2.58 ± 0.12 to 19.0 ± 0.42 dS/m (Fig 2A–D). Except for profile 2, the tailings are classified as moderately, strongly or very strongly saline (Fig. 2A–D). In profile 2, tailings deeper than 10 cm are slightly saline.
In general, the highest salinity values are in the top layers of the profiles. In profiles 2, 3, 6, 7, and 8, the mean conductivity values of the top layers are 7.6, 19.0, 16.4, 10.9 and 15.1 dS/m, respectively (Fig. 2A-D). However, in profile 1, 4, and 5, the highest conductivity was deter- mined for the deeper tailings. In profile 1, the highest value is 16.6 ± 1.6 dS/m at 72 cm depth (Fig. 2A–C). In profiles 4 and 5, these are 9.9 ± 0.29 dS/m at 30 cm and 17.85 ± 0.07 dS/m at 40 cm depth.
The lowest values of NNP were determined for profile 1 (from –1.27 ± 4.56 to –6.71 ± 6.5 kg CaCO3eq./tonne) and the highest values for profile 7 (from –180.88 ± 4.94 to –243.67 ± 27.58 kg CaCO3eq./tonne).
0 10 20 30 40 50 60 70 80
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h, cm
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0 2 4 6 8 10 12
0 2 4 6 8 10 12 14 16 18 20
EC, dS/m EC, dS/m
0 2 4 6 8 10 12 14 16 18
0 2 4 6 8 10 12
EC, dS/m
0 2 4 6 8 10 12 14 16 18
EC, dS/m A
Profile 1 Profile 2
B
Profile 3 Profile 4
C
Profile 5 Profile 6
D
Profile 7 Profile 8
Fig. 2. The pH ( ), electrical conductivity (EC, ), and Net Neutralisation Potential (NNP) within the root zone of Exposed Tailings.
3.3 Plants
In the Tailings Area, plants mainly grow along the perimeter close to the natural forest, where there is less than one meter of tailings above natural soil. Small plant communities grow in other locations of the Tailings Area where organic matter, such as animal excrement and dead plants, accumulates on the surface. In these regions, there was no evidence of organic layers or soil up to 1.25 m beneath the tailings.
Eleven plant species, including three species of trees, were found within the Tailings Area (Table 3). Water sedge is the dominant species, with toad rush and needle spike rush being widely spread. White poplar dominates among the trees.
In the seed germination tests, the average viability of water sedge seeds from the Control Area (54.0 ± 8.9 %) was three times higher than for those from Tailings Area (14.0 + 2.1 %) This difference is significant (t[8] = 7.58, p= 0.0008). The germination period for the control seeds was longer (15 ± 5 days) than for seeds from Tailing Area (7 ± 2 days).
The difference between the mean elemental content in the live shoots of sedge from the Tailings Area and from the Control Area is significant for Na, S, Ca, Mn, Ni, and Cu (Table 5). The mean contents of Na, S, and Ni, in live shoots from the Tailings Area were higher than those for the Control Area by factors of 6.3, 3.5, and 22, respectively (Table 4). The mean contents of nutrients such as Ca, Mn, and Cu were about half those for Control Area.
Cr and Co were not detected in live shoots of sedge for either the Tailings or Control Areas (Table 4).
Significant differences were found for Na, P, S, K, Ca, Fe, Ni, and Cu between live and dead shoots of water sedge from the Tailings Area (Table 5). Na, S, Ca, Fe, Ni, and Cu con- tents in dead shoots were higher than in live shoots by factors of 1.5, 2.5, 5.2, 4.3, 2.7, and 6.6, respectively (Table 4). P and K contents were decreased by 3.3 and 6.9 times, indicating that these nutrients were washed out from the dead shoots. Cr was not detected, but there was 6.1 ± 1.4 ppm Co in dead shoots from the Tailings Area (Table 4).
In the Control Area, significant differences between element content in live and dead shoots were found for B, P, K, Ca, Mn, and Ni (Table 5). In contrast to the Tailings Area, the mean content of most elements in dead sedge shoots from Control Area was lower than in the live shoots or equal, whereas Ca and Mn concentrations were slightly higher (Table 4).
Therefore, in the Control Area, composition of shoots after the death remains about the same. Cr and Co were not detected in live shoots from the Control Areas (Table 4).
Table 3. Plant species found within the Tailings Area.
Scientific name Common name
Calamagrostis inexpansaGrey Northern Reed Grass
Carex aquatilis Wahl. Water Sedge
Eleocharis acicularis L. Needle Spike-Rush
Hordeum jubatum L. Foxtail Barley
Juncus brevicaudatus Engelm. Rush Juncus bufonius L. Toad-Rush
Juncus dudley Wieg. Slender Rush
Poa palustris L. Fowl Bluegrass
Picea glauca Moench White Spruce
Populus balsamifera L. Balsam Poplar
Populus tremuloides Michx. Trembling Aspen
There are significant differences for all elements, except Zn, between dead shoots from the Tailings and Control Area (Table 5) with concentrations of B, Na, Mg, S, Ca, Fe, Ni, and Cu being 2.8, 11, 2.3, 10, 2.1, 5.9, 85, and 3.0 times higher from the Tailings Area (Table 4).
Concentrations of nutrients P, K, and Mn in these shoots were about half those from the Control Area. Thus, elevated contents of many metals would be expected in plant litter that would form within the Tailings Area with Ni being the most concentrated.
Table 4. Concentration of elements in water sedge shoots (ppm). SD – standard deviation.
Tailings Area Control Area
Live shoots Dead shoots Live shoots Dead shoots
Mean SD Mean SD Mean SD Mean SD
B 48 27 42 14 21 0.64 15 0.64
Na 1000 410 1500 200 160 35 140 35
Mg 2900 1700 3600 300 1700 74 1600 100
P 1100 410 330 74 1300 98 800 100
S 5300 2300 13000 1900 1500 210 1300 210
K 11000 2900 1600 310 9100 190 4200 190
Ca 2100 730 11000 810 3600 180 5200 190
Cr < 5.0 < 5.0 < 5.0 < 5.0 < 5.0 < 5.0 < 5.0 < 5.0
Mn 250 98 290 71 600 53 710 11
Fe 1500 950 6500 1500 1500 370 1100 340
Co < 5.0 < 5.0 6.1 1.4 < 5.0 < 5.0 < 5.0 < 5.0
Ni 200 79 530 70 9.3 1.8 6.2 1.8
Cu 10 2.9 66 15 19 6.6 22 5.1
Zn 19 2.9 19 6.1 16 2.9 13 2.9
Table 5. One-tail t tests between the mean values of metal content in live shoots of sedge from Tailings Area (TA) and Control Area (CA), in live shoots from TA and dead shoots from TA, in live shoots from CA and dead shoots from CA, and in dead shoots from TA and CA.
Live shoots TA/ Live shoots/Dead shoots Dead shoots TA/
Live shoots CA TA CA Dead shoots CA
t(8) p t(9) p t(4) p t(5) p
B 1.67 0.066 0.41 0.347 11.48 1.6E-4 3.26 0.011
Na 3.42 0.005 –2.25 0.025 0.70 0.261 11.38 4.6E-5
Mg 1.18 0.136 –0.80 0.223 1.39 0.118 10.87 5.7E-5
P –0.81 0.221 3.64 0.003 6.18 0.002 –7.21 0.0004
S 2.76 0.012 –5.65 0.0002 1.17 0.154 10.37 7.2E-5
K 1.10 0.153 6.32 6.9E-5 31.59 3.0E-6 –12.68 2.7E-5
Ca –3.40 0.005 –18.74 8.0E-9 –10.59 2.3E-4 11.89 3.7E-5
Mn –5.70 0.0002 –0.71 0.249 –3.52 0.012 –9.92 8.9E-5
Fe 0.00 0.500 –6.86 3.7E-5 1.38 0.120 5.98 0.0009
Ni 4.04 0.002 –6.92 3.5E-5 2.11 0.051 12.65 2.7E-5
Cu –3.14 0.007 –9.95 1.9E-6 –0.62 0.284 4.77 0.002
Zn 1.50 0.086 0.00 0.500 1.27 0.137 1.55 0.090
3.4 Greenhouse experiment
All of the water sedge seedlings in T-medium (tailings) died on the 5thday after experiment started. In contrast, there was 100% survival of seedlings in TS-medium (tailings with potting soil). On the 12thday, each plant from the control pots with SS-medium contained 2–3 root branches compared to 0–1 for those in TS-medium. On the 57thday, the height of water sedge in TS-medium (18.7 ± 5.9 cm) was two thirds of the one in SS-medium (29.5 ± 7.7 cm) (Table 6). On the 152ndday, it was one quarter of the height. No seeds were formed in either medium.
There was no significant difference between the height of fowl bluegrass in TS- and SS- medium (t[78] = 0.72, p= 0.237) on the 57thday (Table 6). Plants in both types of medium had formed flowers (Table 6). However, on the l52ndday, the bluegrass height in TS-medium (58.2 ± 12.4 cm) was half that in SS-medium (104.9 + 5.6 cm). Plants in both media had formed seeds by the 152ndday.
All of the foxtail barley seedlings in TS-medium died on the 28thday. The height of the plants was 8.9 + 2.4 cm (Table 6). The height of the control plants in SS-medium was 28.3 ± 4.6 cm on the 57thday and 59.9 ± 6.6 cm on the 152ndday (Table 6). They had all formed seeds.
Table 6. The height and seed bearing ability of plants grown in two different medium in the greenhouse conditions. TS-medium – tailings plus potting soil; SS-medium – sand plus potting soil; SD – standard deviation; * the height measurements were done on 28thday; NA – not available since no live plants left.
Water sedge Fowl bluegrass Foxtail Barley TS-medium SS-medium TS-medium SS-medium TS-medium SS-medium 57thday
h, cm mean 18.7 29.5 26.1 24.9 8.9* 28.3
SD 5.9 7.7 8.3 4.8 2.4 4.6
seeds/flowers no no flowers flowers no seeds
152ndday
h, cm mean 34 128.6 58.2 104.9 NA 59.9
SD 5.4 6.1 12.4 5.6 NA 6.6
seeds/flowers no no seeds seeds NA seeds
3.5 Waste rocks
The texture of the waste rocks consists of 4.38 ± 1.60 % gravel, 45.36 ± 5.00 % fine gravel, 48.99 ± 2.30 % coarse sand, and 1.05 ± 0.30 % fine sand and can be classified as gravely sand (Table 7). The bulk density of the waste rocks (1.58 ± 0.02 g/cm3) was double of that for natural clays (0.70 ± 0.05 g/cm3). The mean permeability (hydraulic conductivity) of the waste rocks (4.37 · 10–7m/s) was less than permeability of the natural clay soils (5.37 · 10–5m/s). The mean value of available water (water holding capacity between 1/3 and 15 bar pressure) for the waste rocks was half that for the natural clays. (t[4] = 4.66, p= 0.005) (Table 7). The waste rocks were slightly alkaline (pH 7.42 ± 0.03), and sulphide content (1.43 ± 0.55 wt. %) was
less than in tailings but 8.4 times higher than in natural soils (Table 7). The difference between the last two values was significant (t[4] = 3.88, p= 0.009). The potential for ARD was likely because NNP was negative (–13.1 ± 7.6 kg CaCO3eq./tonne). Organic matter con- tent in waste rocks (1.17 ± 0.17 %) was one third that of soils. The difference between Ni content in waste rock fines and reduced tailings (t[13] = 0.76, p= 0.245) and between waste rock fines and oxidized tailings (t[11] = 0.35, p= 0.370) is not significant. The mean Ni con- tent in waste rock fines (2500 ± 900 ppm) is 30.9 times higher than for soil (81 ± 25 ppm) (Table 7).
Table 7. Physical-chemical characteristics of the waste rock fines (WRF) compared to the natural soils.
NA – not available since mean values are equal to zero.
Natural soils (Clay) WRF
Mean SD Mean SD
Gravel, % 0.00 NA 4.38 1.60
Fine gravel, % 0.00 NA 45.36 5.00
Coarse sand, % 4.50 0.42 48.99 2.30
Fine sand, % 10.00 0.50 1.05 0.30
Silt, % 30.60 2.69 2.00 0.60
Clay, % 54.85 2.33 0.00 NA
Bulk Density, g/cm3 0.70 0.05 1.58 0.02
Permeability, 10–5m/s 5.37 0.876 0.0437 0.00178
Available water, wt% 14.4 0.4 9.7 1.7
pH 6.7 0.8 7.42 0.03
Sulfides, wt % 0.17 0.12 1.43 0.55
NNP, kg CaCO3eq/t 26.93 15.69 –13.1 7.6
Organic matter, wt % 3.85 1.13 1.17 0.17
Ni, ppm 81 25 2500 900
4 Discussion
A soil profile is formed as a result of the weathering of parental material through complex interactions of the lithosphere, biosphere, atmosphere, and hydrosphere. As the result, natural soils consist of mineral and chemical components that are relatively stable and equilibrated in the particular soil environment (KABATA-PENDIASand PENDIAS2001). In contrast, mine tailings (minesoils) can be considered as very young soils developed on un - stable materials, characterized by instability and lacking cohesion (NORMAN1998; VEGAet al.
2004). The tailings of the Ni-Cu mine in Thompson, Manitoba, contain sulphides, which are predominantly highly reactive pyrrhotite. Both reduced and oxidized tailings have a fine sand texture, low water and organic matter content, and high salinity compared to the natural soils. They are potentially acid generating with higher amount of B, Cr, Ni, and Cu and lower P than in natural soils. As the tailings are oxidized, pyrrhotite is replaced by secondary Fe phases, such as goethite, lepidocrocite, and jarosite (SIDENKOet al. 2007; KAVALENCH2004).
Oxidized tailings have a reddish colour due to goethite and lepidocrocite or olive-yellow due to jarosite (KRAUS1959). The oxidized tailings are acidic (pH 3.5) with slightly less P than in the reduced tailings (Table 1) indicating that it may be leaching from the tailings.
The pH values of the tailings varied with depth. Within the root zone (up to 70 cm depth), the pH ranged from very strongly acid (2.94) for oxidized tailing layers to slightly alkaline (7.94) for reduced. The profiles of older tailing (Delta Tailings and Emergency Tailings) con- sisted of alternating layers of reduced and oxidized tailings. According to KLOHN-LEONOFF
(1992), the carbonate buffering capacity of the surface of the tailings would be exhausted by four years of exposure. Thus, discontinuous deposition of tailings could cause oxidized tailings to be covered by reduced ones forming the alternating layers. To raise the low pH values within the root zone, all layers of oxidized tailings should be neutralized. However, the incorporation of neutralizing agents to deep layers (up to 70 cm deep) is technically difficult. If limestone is applied only to the surface, it would take many years for the limestone to dissolve and reach the deeper oxidized layers, although the effectiveness would depend on the type of neutralizing agent (carbonates, oxides, or hydroxides) and the particle size. This slow rate of neutralization would delay the establishment of a vegetation cover.
Neutralization of tailings is a more challenging issue than neutralization of the natural soils because of the acid caused by the oxidation of sulphides (BLOWESand PTACEK1994;
PETERS1995; NORMAN1998; YEet al. 2000). Within these Ni-Cu tailings the average content of sulphides (as pyrrhotite) did not differ significantly between reduced (5.63 %) and oxidized (6.48 %) tailings for the upper layer or with depth. However, there was a large vari- ation in sulphide content of individual samples giving Net NP values from –1.27 to –243.67 kg CaCO3eq. per tonne. Thus, areas with low and high NNP should be mapped to estimate the optimal amount of neutralizing agent to add. If too much was added alkaline areas could form, and if too little the acidic conditions would regenerate rapidly (GUNNet al. 2001).
Another problem associated with oxidation of sulphides is the high salinity of the tailings.
Salt increases the energy that must be expended by a plant to extract water from the soil and to make the biochemical adjustments necessary to grow under stress (SUTCLIFFE 1962;
MAAS1984, 1985). In dry periods, the surface of tailings was covered by a thin evaporitic layer of Ni, Cu, and Fe sulphates including morenosite, and melanterite (SIDENKO et al.
2007). The upper 15 cm of both reduced and oxidized tailings were classified as strongly saline soils (8.1 to 16.0 dS/m) (SPAC 2000). Deeper layers of tailings (15–70 cm) were moderately (4.1 to 8.0 dS/m), strongly, or very strongly saline (> 16.1 dS/m). Standard methods of reclamation of salt-affected soils such as leaching (by water), and bioremediation can not be applied to sulphide tailings because they have complex of interrelated problems (HOFFMAN1986). Leaching of the tailings would increase acid mine drainage which might impact surrounding lakes. Limestone added to the tailings to increase the pH can also increase salinity because concentration of base cations usually increases after limestone treatment (GUNN et al. 2001). It would be difficult to find appropriate plant species for phytoremediation of salts because few species are able to survive on the tailings.
The species composition of plants naturally colonizing the Tailings Area, their chemical composition and health reflect the conditions of the habitat. There were few species compared to the natural forest and these plants were mostly growing along the perimeter of the Tailings Area close to the forest, where the depth of tailings is less than a meter above natural soil. Water sedge, the dominant species within the Tailings Area, formed seeds at the end of growth season but they had a viability one third of those from the Control Area.
The concentration of Ni was 22 times higher in live shoots of sedge from the Tailings Area than in the Control Area. This might cause an increased concentration of Ni in animals and insects feeding on plants (RAVENet al. 1999). The concentration of the major nutrients in sedge live shoots from the tailings was the same as those for the Control Areas, except for lower levels of Ca, Mn, and Cu.
The concentrations of elements apparently change after the death of the shoots.
Concentrations of Na, S, Ca, Fe, Ni, and Cu have increased and concentration of P and K
decreased in sedge from the Tailings Area. This may be due to dead shoots passively taking up elements as the result of chemical reaction between their organic compounds and the bleeding sap. Since the dead shoots were still attached to the rhizome, bleeding sap could be lifted into the conductive tissues of the shoots (RAVEN et al. 1999). Another possible explanation is the formation of evaporate salts in the conductive tissues of the dead shoots as the result of evaporation of the bleeding sap through their open stomata (RAVENet al.
1999). Phosphorous and K may be released from dead tissues by nutrient resorption during the senescence of plant tissues (KILLINGBECK2004). In contrast, concentrations of most of the elements in the dead shoots of sedge from the Control Area remained the same or became slightly lower than in live shoots. This may be due to lower concentrations in the soils compared to the tailings.
There are higher concentrations of B, Na, Mg, S, Ca, Fe, Ni and Cu and lower P, K, and Mn in dead shoots from the Tailings Area than for the Control Area. The highest difference (85 times) was found for Ni. The litter, forming from these shoots would have very high Ni and lower P, K and Mn. The composition of the plant litter would influence the mature zone of a soil, since it is one of the main components of soil organic matter (VAUGHANand MALCOLM
1985).
The reddish colour around the roots of the fowl bluegrass showed a relationship between the oxidation state of the tailings habitat and the plants. Plants are able to release oxygen from roots and oxidize metal sulphides in tailings, hence increase the local acidity and salinity.
Oxygen is released from the roots to oxidize toxic rhizosphere compounds and protect young root tissues (RAVENet al.1999).
The growth of water sedge, fowl bluegrass, and foxtail barley in tailings in the greenhouse was suppressed even when amended by limestone, organic matter and fertilizer. The height of water sedge and fowl bluegrass was lower than the control samples and all of the foxtail barley plants died. Only fowl bluegrass formed seeds.
Based on the results of the tailings and plant analyses and of the greenhouse experiment, it was concluded that a standard revegetation method would not be effective and could not be recommended for the Ni-Cu mine tailings at Thompson. A protective layer to prevent contact of the plant roots with the tailings should be placed on the surface. A soil layer should be created on top of the protective one to provide substrate and a nutrient supply for plants. The use of freely available waste organic materials such as sewage sludge, paper sludge, or coal production waste for the soil layer would significantly reduce the revegetation expenses.
There is an upper limit of soil bulk density of about 1.7 g/cm3where resistance to root penetration is high enough to limit or stop root growth (CANNELL1977; BENGOUGHand MULLINS1990; KUZNETSOVA 1990). This limiting bulk density is strongly influenced by soil texture, which affects the pore size and mechanical resistance of compacted soil, with coarse textured soils having a higher limit than fine-textured soils (VEIHMEYER and HENDRICKSON 1948; SCHUURMAN 1965). A protective layer could be constructed from compacted clay soils, fly ash or waste rocks.
NEUSCHUTZet al.(2003) found that plants were unable to grow in a layer consisting of 80 % fly ash and 20 % water. However, they did not reveal whether the chief cause of the inhibited root growth was increased pH, the compaction or toxicity of the fly ash, or lack of nitrogen. The application of clay is usually expensive and complicated. If clay is contaminated by silt or sand particles, it becomes more permeable for plant roots as well as atmospheric oxygen and precipitation (DOMENICOand SCHWARTZ1998). For the Tailings Area of the Ni-Cu Mine at Thompson, waste rocks were found to be the most effective material for the protective layer. The bulk density of waste rock fines (1.58 g/cm3) is close to the upper limit of soil bulk density (1.7 g/cm3). An inactive road made from waste rock fines on the surface
of tailings was almost impermeable for plant roots, which penetrated less than 10 cm. The hydraulic conductivity of the waste rock fines (4.4 · 10–7m/s) was close to that of clay (10–7to 10–12m/s) (DOMENICOand SCHWARTZ1998). Waste rocks located within the mine area are freely available with low transportation costs. A layer of waste rocks would also strengthen the tailings surface allowing vehicles to carry out revegetation work in summer when the surface of tailings is soft. One negative characteristic of waste rocks is that they are poten- tially acid producing with 1.43 wt. % of sulphides and a negative Net NP. However, as the sulphide content is lower than in the tailings and the Net NP higher the waste rock layer could be neutralized with less limestone than for the tailings, hence causing less of an increase in salinity. The Ni content in waste rock fines was as high as in the tailings. This creates a potential source of Ni into plants but it would be limited by the low permeability of the waste rocks for plant roots. Moreover, a soil layer placed on top of the waste rocks would provide an additional barrier between them and the roots and limit Ni uptake by the plants.
The protective layer would decrease the formation of windborne dust and flow of oxygen to the tailings. Its effectiveness as a barrier for oxygen would increase with thickness.
5 Conclusions
Wastes within the Tailings Area of the Ni-Cu Mine at Thompson are acid producing with a high rate of oxidation. Repeat discharges of tailings have resulted in alternating layers of acidic oxidized tailings and slightly alkaline reduced tailings within the root zone. Extremely high salinity of the tailings on the surface and in the root zone creates the major problem for the tailings reclamation since the standard methods, such as leaching or phytoremediation used for salt-affected soils can not be applied.
The few species of plants that naturally colonize tailings reflect the conditions in the tailings habitat. These plants have low seed viability, and a high content of Ni. This creates a risk of translocation of Ni into animals and insects feeding on plants and, thus, into food chains. Litter forming within the Tailings Area is expected to accumulate Ni, Cu, B, Na, Mg, S, Ca, and Fe and release P, affecting the chemical composition of the mature zone of the minesoil.
Results of analyses of the tailings and plants, and the greenhouse trials showed that standard techniques would not be effective for the revegetation of these tailings. A protective layer made from a compacted material, which is resistant to the penetration of plant roots, should be placed on top of the tailings and covered by soil layer prior to the establishment of a vegetation cover.
Acknowledgements
The work was jointly funded by INCO Ltd., Thompson Operations and an NSERC, Collaborative Research and Development Grant. We thank Elizabeth Punter for the help in identification of the plant species, Cathy Stewart and the staff of the Environmental Health and Safety Office of INCO Ltd. and many undergraduate summer students for field assistance. We also thank Dr Gregory Vandeberg and anonymous reviewers for helpful comments on this manuscript.
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Revised version accepted December 12, 2006
