Mine tailings that are dumped Minto lakes and other bodies Mof water are sometimes viewed with suspicion. Many people worry that the tailings contaminate drinking water. But studies done by Prof Thomas Pedersen, in the Department of Oceanography at the University of British Columbia, indicate that this is not necessarily the case. Dissolved zinc, copper and cadmium concentrations in interstitial water (pore water), collected from a tailings deposit in Buttle Lake, B.C., indicate that the tailings are not releasing heavy metals into the overlying lake. Similarly, a tailings deposit in Rupert and Holberg Inlets, also in British Columbia, is not releasing significant amounts of copper and molybdenum into the seawater there. However, aquatic tailings deposits are not always free from severe environmental impact. A study by the Greenland Geological Survey, for example, concludes that tailings from the Black Angel mine (jointly owned by Sweden’s Boliden company and the Danish government) on the central west coast of Greenland contributed high concentrations of lead and zinc to the seawater in the fjord, which received the tailings discharge. Prof Pedersen concludes the deposition of mine tailings in water is a reasonable practice only under certain conditions.
The oceanography of the area and the mineralogy of the tailings are important factors in determining the environmental impact of tailings disposal in water. Mining operations often generate large quantities of mill tailings. Two storage options for these tailings are available: terrestrial and aquatic.
Terrestrial disposal of waste is expensive and may have long-term environmental consequences, including surface and groundwater contamination from acidic drainage enriched in leached heavy metals. Aquatic disposal, in contrast, is relatively inexpensive and has often been justified on the grounds that tailings have low chemical reactivity in fresh water and seawater. Prof Pedersen has investigated the release of metals into the environment as a result of aquatic tailings disposal. He has focused on the post-depositional (diagenetic) behavior of tailings in both fresh water and seawater. Determining the nature and extent of remobilization of dissolved heavy metals to a host body of water has been the primary objective.
In an attempt to understand the post-depositional reactivity of the tailings, Pedersen investigated the distribution of a broad range of metals in interstitial waters collected from the upper metre of tailings deposits and from proximal natural sediments. The degree of diagenetic reactivity of submerged mine tailings in fresh water at Buttle Lake, B.C., was assessed by investigating the distribution of zinc, copper, cadmium, manganese and iron in interstitial waters collected from several sediment cores. He also studies dissolved iron, manganese, molybdenum and copper distributions in interstitial waters collected from deposited mill tailings in Rupert and Holberg inlets, B.C. Buttle Lake is a large (30-km long by 1.5-km wide) body of water occupying a U-shaped valley in an area of high relief on central Vancouver Island. The lake drains northeastwards into Georgia Strait via Campbell Lakes and the Campbell River. The south basin of the lake reaches a maximum depth of 87 m, and it received tailings, by way of a slightly submerged outfall, for 17 years from 1967 to 1984. Land disposal is now being practiced. The tailings came from Westmin Resource’s zinc- copper-lead massive sulphide mine 6 km west of the south basin, adjacent to Myra Creek. The tailings consist largely of sand- and silt-sized silicate gangue minerals from the host metavolcanic rocks and iron, zinc, copper and lead sulphides. Concentrations of zinc, copper and lead in the tailings range widely but average about 7,000, 1,300 and 900 mg per kg respectively. Following the opening of the mine and prior to 1983, the concentration of dissolved zinc (and possibly copper and cadmium) in the lake increased significantly. It appeared that most of the increase was due to bacteriological leaching of sulphide minerals in the mine-site waste dump near Myra Creek. Collection and treatment of the leachate from the waste rock dump near the mine site began in 1983. As a result, dissolved metal levels in Myra Creek water and in Buttle Lake have been falling steadily and are now near historical background levels. At the time of high metal concentration in Buttle Lake prior to 1983, Prof Pedersen studied the south basin tailings deposit to determine whether it may have also contributed significant metal fluxes to the lake.
Four gravity cores ranging in length from 50 to 80 cm were collected. Three cores were collected from the tailings deposit near the discharge raft, and a fourth (B4) was raised from natural sediments 4 km to the north. The interstitial water samples were analysed for zinc, cadmium, copper, manganese and iron. Dissolved zinc concentrations in all interstitial water samples were significantly lower than in overlying lake water, and cadmium was undetectable (fewer than 0.5 parts per billion) in nearly all samples. Pore water copper levels in general were low (less than 7 ppb), but in a few samples it approached or marginally exceeded bottom water concentrations. There was no evidence in any of the cores of an increase in dissolved metal concentration with depth. In fact, the opposite was clearly evident in B4 and also appeared to be true in B3. Although the pore water metal concentrations ranged quite widely, Prof Pedersen concluded that the tailings deposit was not contributing a flux of dissolved zinc, copper or cadmium to Buttle Lake water as a consequence of diagenetic reactivity.
At Buttle Lake, the key factors which mitigated against release of metals were the high natural sedimentation rate, which placed a strong limit on the amount of oxygen that could diffuse into the tailings from bottom water to support oxidation of detrital sulphide minerals, and a relatively high organic matter content. The organic matter encouraged rapid consumption of oxygen by bacteria in the sediments. Pedersen’s research suggests oxidation of the tailings on the lake bottom was not occurring at the time of the study, presumably because the rate of tailings discharge was sufficient to bury the deposits continuously and quickly, thus minimizing the time of exposure to dissolved oxygen in the bottom water. Also, the absence of detectable dissolved zinc or copper in the natural sediments underlying the tailings in core B3 and the decreasing zinc gradient in core B3 indicated that these metals were diffusing into the natural sediment from the overlying metal-rich lake water rather than vice versa. (At the time of the study, Myra Creek was contributing a high metal content to Buttle Lake.)
Prof Pedersen has also investigated the remobilization of metal in a submerged tailings deposit in Rupert and Holberg Inlets, B.C. These inlets, 10 and 34 km long respectively, are relatively shallow fiords located in a glacially-scoured trough on northern Vancouver Island. The average depths are 110 m in Rupert and 80 m in Holberg Inlet. Since 1971, Utah Mine’s Island Copper mine, on the north side of Rupert Inlet, has been discharging copper- and molybdenum-bearing tailings into the inlet via a submerged outfall. Tailings now cover much of the floor of the recipient fiord and extend in a thin layer well up into adjoining Holberg Inlet. The Rupert Inlet tailings are typically silt-sized and are dominated mineralogically by quartz (50%-70%), biotite and chlorite (5%-10%) and feldspar (2%-20%). Magnetite (2%-4%), pyrite (2%-4%), calcite (2.5%) chalcopyrite (0.2%) and sphalerite (0.02%) occur in much smaller concentrations. Copper and molybdenum are expectedly enriched to mean concentrations of approximately 700 and 40 parts per million, respectively. The copper concentration increases markedly as a function of grain size: fine silt contains 200 to 400 ppm, coarse silt several hundred parts per million more. Sediment cores were collected from three facies: slowly and rapidly accumulating tailings and natural Holberg Inlet sediments.
The interstitial water samples were analysed for iron, manganese, molybdenum, copper, cadmium and lead. It was concluded that reactions in the tailings on the inlet floor are supporting fluxes of both dissolved copper and molybdenum to the overlying water. However, the copper flux from the tailings is similar to that in natural sediments in upper Holberg Inlet and many other locations in the world and, therefore, the extent of diagenetic remobilization of copper from the tailings is comparable to or less than the evasion of copper from uncontaminated near-shore sediments. Although there is a small flux of molybdenum to solution from the tailings on the inlet floor, this release will have no measurable effect on the molybdenum concentration of the overlying seawater.
A similar aquatic tailings disposal study is now in progress near the old Britannia mine, north of Vancouver. Graduate student Kary Drysdale is investigating the reactivity of the tailings lying below the seawater in adjacent Howe Sound. These ongoing research projects headed by Prof Pedersen are intended to determine the extent to which metal-rich submerged tailings deposits release dissolved metals into host waters. The research will assist in outlining future tailings discharge programs and may also prevent the contamination of bodies of water. The Buttle Lake project was funded by Westmin Resources at the request of the provincial government. The Department of Fisheries and Oceans and the Natural Sciences & Engineering Research Council funded the Rupert and Holberg inlets study and the Howe Sound project. REFERENCES Pedersen, T. F. (1983): Dissolved heavy metals in a lacustrine mine tailings deposit — Buttle Lake, British Columbia. Marine Pollution Bulletin, Vol. 14, No. 7, pp. 249-254. Pedersen, T. F. (1984): Interstitial water metabolite chemistry in marine mine tailings deposit, Rupert Inlet, B.C., Canadian Journal of Earth Sciences, Vol. 21, No. 1, pp. 1-9. Pedersen, T. F. (1985): Early diagenesis of copper and molybdenum in mine tailings and natural sediments in Rupert and Holberg inlets, British Columbia. Canadian Journal of Earth Sciences, Vol. 22, No. 10, pp. 1474-1484. Pedersen, T. F. and Losher, A. J. (1988): Diagenetic processes in aquatic mine tailings deposits in British Columbia, Management of mine tailings and dredged material (W. Salomons and U. Forstner, eds.). Springer-Verlag. pp. 238-257. (In press.)
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