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Introduction: Mining and metals in the environment
ELSEVIER
Journal of Geochemical Exploration 58 (1997) 95-100
JOURNALOF GEOCHEMICAL EXPLORATION
Introduction: Mining and metals in the environment Rod Allan National Water Research Institute, Canada Centre fi)r Inland Waters, 867 Lakeshore Road, Burlington, Ontario L7R 4A6, Canada
In the planet earth, metals are ubiquitous from the co,re to the upper atmosphere. Terrestrial life evolved on earth, as it were, on the 'slag' or crust of a planetary blast furnace/smelter. Metals are a fundamental component of life on earth and part of all food chains. Thus, metals cannot be eliminated from the environment as can be attempted for toxic, persistent, bioaccumulating, completely anthropogenic organic chemicals such as polychlorinated biphenyls (PCBs). Metals are natural and, in large part, essential components of global ecosystems. Some metals such as copper (Cu) and zinc (Zn) are essential to life. In high concentration, however, the same metals can be toxic. Some metals such as lead (Pb) and mercury (Hg) are not known to perform any useful biochemical function. Human activities simply redistr'bute metals within and between ecosystems. Mining and metals in the environment is a sub-area of the broader field of study of metals in the environment. Not all environmental metals research is directly related to mining. Examples of this would be Hg releases from the mercury cells in chlorine-producing chlor-alkali plants or the bioavailability of cadmium (Cd) in various soils and related surficial Quaternary deposits. However, I selected these exaunples to show that environmental metals research o:' many types can be very relevant to metal mining environmental issues. Some smelters can be sources o ' Hg emissions but so can chlor-alkali plants and both must be considered and compared when assessirg the impacts of atmospheric releases of this metal. Likewise, some mine wastes may be sources of Cd to soils. When this occurs, the bioavailability of this
Cd, relative to the naturally present soil Cd, becomes of critical importance. Thus, the research area of metals in the environment encompasses an extensive range of investigations involving media from water to humans and often indirectly important in answering questions concerning mining and metals in the environment issues. Mining and metal beneficiation industries have developed into sophisticated operations, yet the basic causes of any metal pollution by them remain unchanged (UNESCO, 1988, 1989). Mining by its nature involves the removal, processing and disposal of vast volumes of rock and wastes. A typical metal mine uses more water by weight in production of the metal concentrate than the weight of the ore grade material itself. Water losses and water gains at a typical metal mine show that the main direct release of metals is from tailings and polishing ponds and emissions later in the beneficiation stage. The papers in this special volume are directly related to mining and metals in the environment. However, even this is a broad area of research. In this introduction, I intend to discuss briefly only a few key research areas, using primarily Canadian examples, which should perhaps be given more global attention in the future.
1. Metal bioavailability and effects assessments In terms of effluents and wastes, mining is largely a local issue but one of global importance. Alternatively, mining-related emissions, although again pri-
0575-6742/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. P'I S 0 3 7 5 - 6 7 4 2 ( 9 7 ) 0 0 0 0 4 - 6
96
R. Allan/Journal of Geochemical Exploration 58 (1997) 95 I00
marily of local or regional concern, can be considered a global concern when atmospherically transported emissions of some metals cross international boundaries. Adequate and scientifically sound assessments of the effects of effluents and emissions, both local and long-range, are likely to be a priority research area for some time to come. In this context, many of the papers which follow involve chemistrybased impact assessments. Such assessments need to be supported by more biology-based assessments. Comparisons are often drawn between persistent, bioaccumulating, toxic (PBTs) organic chemicals and metals. Because of their lipophilicity, PBTs biomagnify to high levels in top aquatic and terrestrial predators. The fate of PBTs in the environment is complex but major advances in predicting fate and effects have now been made for many of these substances. However, metals are quite different. They are present in the environment in the form of many species and they exhibit a wide range of oxidation states and coordination numbers. Some metals may bioconcentrate or bioaccumulate but, with the exception of metals which can be transformed in the environment to organometallic compounds, do not biomagnify to a significant extent (ICME, 1995). Metals, unlike synthetic chemicals, occur naturally and function as often essential micronutrients. Organisms have developed mechanisms to deal with excess exposure to metals. While some organisms may consume others with higher metal concentrations, physiological mechanisms can prevent bioaccumulation. The effects of metals are thus more difficult to predict because of the variety of biogeochemical conditions and processes in the environrnent as well as internal processes which control metal fate in organisms. Biological studies will be essential if environmental effects monitoring (EEM) techniques are developed for mines. A Canadian study of the importance of including biological as well as chemical techniques to assess metal impacts took place in lakes near two former gold mines. Jack of Clubs Lake in British Columbia and Larder Lake in Ontario were contaminated by mine tailings. These sites have been examined in detail over the last few years, including assessments of metal impacts on benthic biology (Azcue et al., 1994; Reynoldson et al., 1995). Differences in benthic community structure between Jack
of Clubs and Larder lakes were related to metal bioavailability and speciation rather than to total metal content in the lake bottom sediments. Total metal concentration is not a good indicator of biological effects, and knowledge of metal speciation is critical.
2. Remediation and rehabilitation of mine sites Steps in the remediation of a mining site are to: (1) assess the extent of the local pollution and quantify the sources of acid mine drainage, metals and other contaminants and pollutants; (2) control the major sources of pollutants (control effluents/emissions; reduce leachates; bury, lime, remove and treat contaminated soils, sediments and wastes, etc.); (3) assess regional pollution and identify 'hot spots' (remediate hot spots - - dredge rivers; lime lakes; treat in situ, i.e. create anoxic conditions; flood, bury, cover them, etc.); (4) let natural recovery take its course or accelerate it (increase erosion/deposition of clean sediments in bays/rivers; divert alkaline streams to acid streams; eutrophy or flush lakes/rivers, create wetlands, etc.) (Allan and Salomons, 1995). Terrestrial remediation techniques include: (1) covering tailings (with soil, vegetation, clay, till, polymer or cement-like materials) to reduce infiltration; and (2) creation of artificial hard pans (using limonite, goethite, iron hydroxide) or blending alkaline and acidic wastes. Polluted soils can be removed (treated or disposed of), given new soil covers (buried), or limed. Many such terrestrial remedial improvements have been made in the Sudbury area in Canada (Crawford, 1995), Metal-contaminated aquatic sediments can be dredged (dewatered, leached, treated, and disposed of in new, well-designed waste sites), buried, treated in situ (limed), or left to nature to restore. Other aquatic techniques include creation of water covers (anoxic conditions), or wetlands (to leach, filter contaminants and create anoxic conditions). Subaqueous disposal of metal mine tailings is perhaps feasible in abandoned and flooded, meromictic 'lakes' in abandoned open mine pits such as the Crown Pillar pit at the former Mattabi Mine, near Ignace, Ontario (Brassard and Mudroch, 1994). Passive techniques for
R. Alhm / Journal ~! Geochemical Lrploration 58 (1997) 95-100
remediation of metal-enriched, acidic groundwater using in situ, reactive walls is also a potentially important mitigation and remediation method being investigated in Canada (Blowes et al., 1995). Publications on mine site remediation and rehabilitation are seldom found in the scientific journal literature because they are often more site management oriented. Of course, a great deal of research is usually required to determine what site management techniques to apply. Published papers on remediation often revolve around before and after photographs of mining sites, for example the Sudbury area in Canada (Crawford, 1995), and in this context, the reader is referred to Mining Encironmental Management, an excellent publication with many reports on successfitl remediation and rehabilitation activities at mining sites around the world.
3. Local and long-range atmospheric deposition of metal emissions Recently, there is a renewed and increasing intere~t in both local and long-range atmospheric transport and deposition of metals in Europe and North America, particularly mercury, cadmium and lead (UNECE, 1994). Some of the main anthropogenic, atmospheric sources for certain metals are related either directly to mining, as with emissions from smelters, for example, or indirectly as with the burning of coal to produce electric power. For metals emitted primarily in particulate form, concentrations over background are usually found only within tens of kilometres of the source. Several of the papers in this special volume deal with local and regional geochemical impacts of smelter emissions, for example those concerned with the Russian smelter complexes in the Kola Peninsula. For metals emitted as very fine particulates or in gaseous form, atmospheric transport distances are considered to be far greater. In European countries, evidence for hmg-range atmospheric transport and deposition of metals from anthropogenic sources has been assembled and reduction of certain metal emissions proposed (UNECE, 1994). However, long-range atmospheric transport in Europe is often synonymous with transboundary movement of metals, i.e. transport from one country to another followed by deposi-
97
tion. This definition is also valid in North America but also implies atmospheric transport of metals over truly long distances (hundreds or even thousands of kilometres). Some of the more severe cases of transboundary anthropogenic atmospheric metal pollution in Europe are almost local in scale or at least sub-regional. In Fennoscandia, the definition of long-range transport is closer to that used in North America. In Canada, the South-East and the Arctic are the two regions where long-range transboundary atmospheric transport and deposition of metals are known to occur (Allan, 1996). Some of the metal sources to the former region have been traced to metal emissions in the eastern United States (Schroeder and Markes, 1994) and some of the sources to the latter region to similar emissions in central and eastern Europe (Sturges and Barrie, 1989). The vertical distribution of metals, especially mercury, in Canadian lake sediment cores has been interpreted as evidence of increasing, long-range, anthropogenic input from the atmosphere (Lockhart et al., 1995). The ratio of recent to geological Hg flux to lake sediments has been calculated for 51 lake sediment cores from lakes in five of the eight circumpolar countries (Landers et al., 1996). Recent increases range from zero to close to zero in Alaska and north-central Russia to values of two times and higher in southeastern Canada and southern Fennoscandia. Proper collection of freshwater lake sediment cores is not simple. A recent workshop of experts convened by the Electric Power Research Institute (EPRI), prepared a Protocol for the collection and analytical requirements needed to obtain lake sediment core data that can be adequately interpreted. A critical requirement is radionuclide age dating with 2J°pb and 137Cs. Such dating is essential to ensure that the surface layer of a core has been collected and that the sedimentation rate is known. This, in turn, allows accurate historical dates to be assigned to the sediment layers analysed (EPRI, 1996). In Canada, concentrations of Hg can exceed food consumption guidelines in specific native food items and in freshwater fish in many remote areas (Allan, 1996). Mercury concentrations in the blood of many aboriginal Canadians, especially the Inuit, sometimes exceed 20 ppb, the lower limit for increasing risk (less than 20 ppb is the normal acceptable range)
98
R. Allan/Journal of Geochemical Exploration 58 (1997) 95-100
(Wheatley and Paradis, 1995). An unquantified component of this Hg may come from long-range, anthropogenic atmospheric emissions such as those from coal-fired power stations and smelters. There is always far more non-organic Hg present, for example in lake sediments, than present in aquatic biota where virtually all of the Hg is in the methylated form. The processes and factors controlling Hg methylation in aquatic ecosystems in Canada have been researched for many years. Reduction or removal of point source inputs has resulted in clear reduction in Hg concentrations in downstream biota in cases of extreme Hg pollution of aquatic ecosystems (Allan et al., 1984). However, when contamination of aquatic ecosystems with anthropogenic Hg is less severe, for example from long-range atmospheric transport and deposition of Hg from anthropogenic sources, the results of this on Hg concentra':ions in aquatic biota are not so clear. The proportion of annual emissions of natural and anthropogenic Hg to the atmosphere is a crucial question. Early estimates of natural Hg emissions in lhe 1970's relied on analyses of deep glacier ice from Greenland (Weiss et al., 1971) and from sites around the world (Jaworowski et al., 1981). New clean techniques for sample collection and analyses of metals in water and ice were implemented in the late eighties. Although the earlier estimate of Weiss et al. (1971) continues to be cited in recent publications (IPCS, 1976, 1989), estimates since 1990 give the ratio of global annual natural/anthropogenic emissions as roughly 5 0 / 5 0 (OECD, 1994). The most recent and widely accepted estimate of natural ttg emissions arose from an expert panel assembled in 1993 by the EPRI in the United States. Their conclusions were that annual pre-industrial emissions of Hg to the atmosphere were in the order of 1600 tonnes and that anthropogenic emissions contributed 5:0 to 75% of the total annual natural plus anthropogenic emissions of some 5000 tonnes (EPRI, 1994). Anthropogenic Hg emissions in Canada in 1990 were 38.8 tonnes of which 30 tonnes came from base-metal smelters and associated plants, 4 tonnes from coal-fired power stations, 3.3 tonnes from municipal waste incinerators and 1.5 from other sources (Doiron et al., 1996). This 1990 Canadian total anthropogenic emission of some 39 tonnes of I-[g can be compared to the United States and Euro-
pean totals of 301 and 626 tonnes, respectively (Doiron et al., 1996). In Canada, a program for Voluntary Emission Reductions by mines and smelters has since resulted in dramatic reductions in metal emissions to the atmosphere. For example, an 83% reduction in Hg emissions (from 25.2 to 4.4 tonnes) occurred between 1988 and 1995 (Mining Association of Canada, 1996), with plans in place to result in further reductions of this and other metals such as Pb and Cd. When estimating anthropogenic emissions of Hg, consideration should also be given to sources such as former railings of abandoned gold mines, even centuries old ones. This volume contains several papers on Hg releases from present-day gold mining operations in the Amazon River basin of Brazil. Emissions from Hg-contaminated waterways, such as those downstream of former chlor-alkali plants may be an important source of anthropogenic Hg to the atmosphere. Emissions of anthropogenic and natural Hg from flooded soils, for example due to reservoir construction, also need to be better assessed.
4. Natural metal anomalies
The fact that metals are natural in origin and that background concentrations for metals vary across terrestrial, aquatic and atmospheric ecosystems, must always be taken into account in relation to all of the above issues concerning metal contamination or pollution by mining-related activities. It is therefore of great importance to continue to try to complete national and global geochemical surveys of natural metal concentrations in a variety of media. For smaller countries, metal surveys using sufficial media such as lake or stream sediments or soils have been completed (Darnley et al., 1995). For larger countries, only certain districts have been surveyed geochemically. In Canada over the last twenty-five years, federal and provincial agencies have extended geochemical mapping over vast areas using aquatic sediments (Painter et al., 1994). The maps reveal global-scale metal concentrations of several metals, extending over areas of tens to thousands of square kilometres in area. These natural concentrations of metals in aquatic sediments vary widely and can often exceed chemistry-based sediment quality
R. Allan/Journal of Geochemical Exploration 58 (1997) 95-100
guidelines associated with metal pollution. Even when aquatic systems have been contaminated or polluted with metals, it is still possible to derive natural historical metal concentrations, for example bv collecting deep lake sediments or by sampling buried layers of overbank sediment as found in some of the papers contained in this special volume. Determining background metal concentrations for soil surface horizons can be more difficult but could involve samples of the same soil types from very remote m-eas or soils that have been buried along with their natural metal content, for example by historical excavations. For natural inputs of metals to the atmosphere, we require more sampling of emissions from both terrestrial and sub-aqueous volcanoes, forest fires, windblown dust, and ocean spray as well as other sources.
References Allan, R.J., 1996. Long range atmospheric transport of heavy metals, particularly mercury, in Canada: sources, fate and effects. National Water Research Institute, Contribution 96-80, 81 pp. Allan, R.J. and Salomons, W.. 1995. Heavy metal aspects of mining pollution and remediation. J. Geochem. Explor., 52(1 and 2), 284 pp. Allan, R.J., Brydges, T., Dodge, D., Hamilton, R.D., Jeffs, D.G. and Shikaze, K., t984. Mercury Pollution in the WabigoonEnglish River System of Northwestern Ontario, and Possible Remedial Measures. Final Report of the Steering Committee, Minister of Supply and Services Canada, Cat. No. Em 3767/1984E, Vol. 1:18 pp.; Vol. 2:538 pp. ,azcue, J.M., Mudroch, A., Rosa, F. and Hall, G.E.M., 1994. Effects of abandoned gold mine railings on the arsenic concen:rations in water and sediments of Jack of Clubs Lake, B.C. Environ. Technol., 15: 669-678. Elowes, D.W., Ptacek, C.J., Balm J.G., Waybrant, K.R. and Robertson, W.D., 1995. Treatment of mine drainage water using in situ permeable reactive walls. In: T.P. Hynes and M.C. Blanchette (Editors), Sudbury "95 - - Mining and the Environment. CANMET, Ottawa. Catalogue No. M3966/1995E, 2: 979-987. E;rassard, P. and Mudroch, A., 1994. Potential use of Crown Pillar Pit in a demonstration project of the feasibility of subaqueous disposal of acid generating metal mine tailing. Environment Canada, National Water Research Institute Contribution No. 94-04, 34 pp. Crawford, G.A., 1995. Environmental improvements by the mining industry of the Sudbur3' Basin of Canada. J. Geochem. Explor., 52: 267-284. Darnley, A.G., BjiSrklund, A., B~lviken. B., Gustavsson, N.,
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Koval, P.V., Plant, J.A., Steenfelt, A., Tauchid, M. and Xuejing, Xie, with contributions by Garrett, R.G. and Hall, G.E.M., 1995. A Global Geochemical Database for Environmental and Resource Management: Recommendations for International Geochemical Mapping. Final Report of IGCP Project 259. UNESCO Publ. Earth Sci., 19, 122 pp. Doiron, C.C, Whalen, P.J. and Novaczek, I., 1996. Background Information Paper for a Heavy Metals Protocol under the United Nations Economic Commission for Europe Convention on Long Range Transboundary Air Pollution. Pub. EPS, Environment Canada, Ottawa, Vol. 1., 133 pp. EPRI (Electric Power Research Institute), 1994. Mercury Atmospheric Processes: A Synthesis Report. Publ. Electric Power Research Institute, Palo Alto, Calif., EPRI/TR- 104214, 31 pp. EPRI (Electric Power Research Institute), 1996. Protocol for Estimating Historic Atmospheric Mercury Deposition. Publ. Electric Power Research Institute, Palo Alto, Calif., EPRI/TR-106768 3297, 53 pp. 1CME (International Council on Metals and the Environment), 1995. Persistence, Bioaccumulation and Toxicity of Metals and Metal Compounds. Publ. ICME, Ottawa, Ont., 95 pp. IPCS (International Programme on Chemical Safety), 1976. Environmental Health Criteria for Mercury, 3.1. Natural Sources. World Health Organisation, Geneva, p. 42. IPCS (International Programme on Chemical Safety), 1989. Environmental Health Criteria (86) for Mercury - - Environmental Aspects. World Health Organisation, Geneva, p. 13. Jaworowski, Z., Bysiek, M. and Kownacka, L., 1981. Flow of metals into the global atmosphere. Geochim. Cosmochim. Acta, 45: 2185-2199. Landers, D,H., Gubala, C , Verta, M., Lucotte, M., Johansson, K. and Lockhart, W.L., 1996. Using lake sediment mercury flux ratios to evaluate the regional and continental dimensions of mercury deposition in arctic and boreal ecosystems. Atmos. Environ. (in press). Lockhart, W.L., Wilkinson, P., Billeck, B.N., Hunt, R.V., Wagemann, R. and Brunskill, G.J., 1995. Current and historical inputs of mercury to high-latitude lakes in Canada and to Hudson Bay. Water Air Soil Pollut., 80: 603-610. Mining Association of Canada, 1996. Voluntary Emissions Reduction (The Mining Industry in Canada and the ARET - Accelerated Reduction/Elimination of Toxics - - Program). Publ. Mining Association of Canada, Ottawa, 56 pp. OECD (Organisation for Economic Co-operation and Development), 1994. Mercury Risk Reduction. OECD, Monograph No. 4, pp. 37 and 38. Painter, S., Cameron, E.M., Allan, R.J. and Rouse, J., 1994. Reconnaissance geochemistry and its environmental relevance. J. Geochem. Explor., 51: 213-246. Reynoldson, T.B., Mudroch, A., Rosa, F. and Azcue, J.M., 1995. Studies of effects of metal mine discharges on benthic organisms. In: T.P. Hynes and M.C. Blanchette (Editors), Sudbury '95 - - Mining and the Environment. CANMET, Ottawa, Catalogue No. M39-66/1995E, 3: 703-714. Schroeder, W.H. and Markes, J., 1994. Measurements of atmospheric mercury concentrations in the Canadian environment near Lake Ontario. J. Great Lakes Res., 20(l): 240-259.
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Sturges, W.T. and Barrie, L.A., 1989. Stable lead isotope ratios in Arctic aerosols: evidence for the origin of Arctic air pollution. Atmos. Environ., 23:2513-2519. UNECE (United Nations Economic Commission for Europe), 1994. Task Force on Heavy Metals Emissions: State-of-the-Art Report. Publ. EGU Prague, PLC, CZ 190 11 Prague 9, 266 pp. UNESCO (United Nations Educational, Scientific and Cultural Organisation), 1988. International Hydrological Programme 1988, Workshop on Metals and Metalloids in the Hydrosphere; Impact Through Mining and Industry, and Prevention Technology. Unesco, Paris, 192 pp. UNESCO (United Nations Educational, Scientific and Cultural
Organisation), 1989. International Hydrological Programme, 1989. Metals and Metalloids in the Hydrosphere; Impact Through Mining and Industry, and Prevention Technology in Tropical Environments. Asian Institute of Technology, Bangkok, 191 pp. Weiss, H.V., Koide, M. and Goldberg, E.D., 1971. Mercury in the Greenland icesheet: evidence of recent input by man. Science, 175: 692-694. Wheatley, B. and Paradis, S., 1995. Exposure of Canadian aboriginal peoples to methylmercury. Water, Air Soil Pollut., 80: 3-11.
Journal of Geochemical Exploration 58 (1997) 95-100
JOURNALOF GEOCHEMICAL EXPLORATION
Introduction: Mining and metals in the environment Rod Allan National Water Research Institute, Canada Centre fi)r Inland Waters, 867 Lakeshore Road, Burlington, Ontario L7R 4A6, Canada
In the planet earth, metals are ubiquitous from the co,re to the upper atmosphere. Terrestrial life evolved on earth, as it were, on the 'slag' or crust of a planetary blast furnace/smelter. Metals are a fundamental component of life on earth and part of all food chains. Thus, metals cannot be eliminated from the environment as can be attempted for toxic, persistent, bioaccumulating, completely anthropogenic organic chemicals such as polychlorinated biphenyls (PCBs). Metals are natural and, in large part, essential components of global ecosystems. Some metals such as copper (Cu) and zinc (Zn) are essential to life. In high concentration, however, the same metals can be toxic. Some metals such as lead (Pb) and mercury (Hg) are not known to perform any useful biochemical function. Human activities simply redistr'bute metals within and between ecosystems. Mining and metals in the environment is a sub-area of the broader field of study of metals in the environment. Not all environmental metals research is directly related to mining. Examples of this would be Hg releases from the mercury cells in chlorine-producing chlor-alkali plants or the bioavailability of cadmium (Cd) in various soils and related surficial Quaternary deposits. However, I selected these exaunples to show that environmental metals research o:' many types can be very relevant to metal mining environmental issues. Some smelters can be sources o ' Hg emissions but so can chlor-alkali plants and both must be considered and compared when assessirg the impacts of atmospheric releases of this metal. Likewise, some mine wastes may be sources of Cd to soils. When this occurs, the bioavailability of this
Cd, relative to the naturally present soil Cd, becomes of critical importance. Thus, the research area of metals in the environment encompasses an extensive range of investigations involving media from water to humans and often indirectly important in answering questions concerning mining and metals in the environment issues. Mining and metal beneficiation industries have developed into sophisticated operations, yet the basic causes of any metal pollution by them remain unchanged (UNESCO, 1988, 1989). Mining by its nature involves the removal, processing and disposal of vast volumes of rock and wastes. A typical metal mine uses more water by weight in production of the metal concentrate than the weight of the ore grade material itself. Water losses and water gains at a typical metal mine show that the main direct release of metals is from tailings and polishing ponds and emissions later in the beneficiation stage. The papers in this special volume are directly related to mining and metals in the environment. However, even this is a broad area of research. In this introduction, I intend to discuss briefly only a few key research areas, using primarily Canadian examples, which should perhaps be given more global attention in the future.
1. Metal bioavailability and effects assessments In terms of effluents and wastes, mining is largely a local issue but one of global importance. Alternatively, mining-related emissions, although again pri-
0575-6742/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. P'I S 0 3 7 5 - 6 7 4 2 ( 9 7 ) 0 0 0 0 4 - 6
96
R. Allan/Journal of Geochemical Exploration 58 (1997) 95 I00
marily of local or regional concern, can be considered a global concern when atmospherically transported emissions of some metals cross international boundaries. Adequate and scientifically sound assessments of the effects of effluents and emissions, both local and long-range, are likely to be a priority research area for some time to come. In this context, many of the papers which follow involve chemistrybased impact assessments. Such assessments need to be supported by more biology-based assessments. Comparisons are often drawn between persistent, bioaccumulating, toxic (PBTs) organic chemicals and metals. Because of their lipophilicity, PBTs biomagnify to high levels in top aquatic and terrestrial predators. The fate of PBTs in the environment is complex but major advances in predicting fate and effects have now been made for many of these substances. However, metals are quite different. They are present in the environment in the form of many species and they exhibit a wide range of oxidation states and coordination numbers. Some metals may bioconcentrate or bioaccumulate but, with the exception of metals which can be transformed in the environment to organometallic compounds, do not biomagnify to a significant extent (ICME, 1995). Metals, unlike synthetic chemicals, occur naturally and function as often essential micronutrients. Organisms have developed mechanisms to deal with excess exposure to metals. While some organisms may consume others with higher metal concentrations, physiological mechanisms can prevent bioaccumulation. The effects of metals are thus more difficult to predict because of the variety of biogeochemical conditions and processes in the environrnent as well as internal processes which control metal fate in organisms. Biological studies will be essential if environmental effects monitoring (EEM) techniques are developed for mines. A Canadian study of the importance of including biological as well as chemical techniques to assess metal impacts took place in lakes near two former gold mines. Jack of Clubs Lake in British Columbia and Larder Lake in Ontario were contaminated by mine tailings. These sites have been examined in detail over the last few years, including assessments of metal impacts on benthic biology (Azcue et al., 1994; Reynoldson et al., 1995). Differences in benthic community structure between Jack
of Clubs and Larder lakes were related to metal bioavailability and speciation rather than to total metal content in the lake bottom sediments. Total metal concentration is not a good indicator of biological effects, and knowledge of metal speciation is critical.
2. Remediation and rehabilitation of mine sites Steps in the remediation of a mining site are to: (1) assess the extent of the local pollution and quantify the sources of acid mine drainage, metals and other contaminants and pollutants; (2) control the major sources of pollutants (control effluents/emissions; reduce leachates; bury, lime, remove and treat contaminated soils, sediments and wastes, etc.); (3) assess regional pollution and identify 'hot spots' (remediate hot spots - - dredge rivers; lime lakes; treat in situ, i.e. create anoxic conditions; flood, bury, cover them, etc.); (4) let natural recovery take its course or accelerate it (increase erosion/deposition of clean sediments in bays/rivers; divert alkaline streams to acid streams; eutrophy or flush lakes/rivers, create wetlands, etc.) (Allan and Salomons, 1995). Terrestrial remediation techniques include: (1) covering tailings (with soil, vegetation, clay, till, polymer or cement-like materials) to reduce infiltration; and (2) creation of artificial hard pans (using limonite, goethite, iron hydroxide) or blending alkaline and acidic wastes. Polluted soils can be removed (treated or disposed of), given new soil covers (buried), or limed. Many such terrestrial remedial improvements have been made in the Sudbury area in Canada (Crawford, 1995), Metal-contaminated aquatic sediments can be dredged (dewatered, leached, treated, and disposed of in new, well-designed waste sites), buried, treated in situ (limed), or left to nature to restore. Other aquatic techniques include creation of water covers (anoxic conditions), or wetlands (to leach, filter contaminants and create anoxic conditions). Subaqueous disposal of metal mine tailings is perhaps feasible in abandoned and flooded, meromictic 'lakes' in abandoned open mine pits such as the Crown Pillar pit at the former Mattabi Mine, near Ignace, Ontario (Brassard and Mudroch, 1994). Passive techniques for
R. Alhm / Journal ~! Geochemical Lrploration 58 (1997) 95-100
remediation of metal-enriched, acidic groundwater using in situ, reactive walls is also a potentially important mitigation and remediation method being investigated in Canada (Blowes et al., 1995). Publications on mine site remediation and rehabilitation are seldom found in the scientific journal literature because they are often more site management oriented. Of course, a great deal of research is usually required to determine what site management techniques to apply. Published papers on remediation often revolve around before and after photographs of mining sites, for example the Sudbury area in Canada (Crawford, 1995), and in this context, the reader is referred to Mining Encironmental Management, an excellent publication with many reports on successfitl remediation and rehabilitation activities at mining sites around the world.
3. Local and long-range atmospheric deposition of metal emissions Recently, there is a renewed and increasing intere~t in both local and long-range atmospheric transport and deposition of metals in Europe and North America, particularly mercury, cadmium and lead (UNECE, 1994). Some of the main anthropogenic, atmospheric sources for certain metals are related either directly to mining, as with emissions from smelters, for example, or indirectly as with the burning of coal to produce electric power. For metals emitted primarily in particulate form, concentrations over background are usually found only within tens of kilometres of the source. Several of the papers in this special volume deal with local and regional geochemical impacts of smelter emissions, for example those concerned with the Russian smelter complexes in the Kola Peninsula. For metals emitted as very fine particulates or in gaseous form, atmospheric transport distances are considered to be far greater. In European countries, evidence for hmg-range atmospheric transport and deposition of metals from anthropogenic sources has been assembled and reduction of certain metal emissions proposed (UNECE, 1994). However, long-range atmospheric transport in Europe is often synonymous with transboundary movement of metals, i.e. transport from one country to another followed by deposi-
97
tion. This definition is also valid in North America but also implies atmospheric transport of metals over truly long distances (hundreds or even thousands of kilometres). Some of the more severe cases of transboundary anthropogenic atmospheric metal pollution in Europe are almost local in scale or at least sub-regional. In Fennoscandia, the definition of long-range transport is closer to that used in North America. In Canada, the South-East and the Arctic are the two regions where long-range transboundary atmospheric transport and deposition of metals are known to occur (Allan, 1996). Some of the metal sources to the former region have been traced to metal emissions in the eastern United States (Schroeder and Markes, 1994) and some of the sources to the latter region to similar emissions in central and eastern Europe (Sturges and Barrie, 1989). The vertical distribution of metals, especially mercury, in Canadian lake sediment cores has been interpreted as evidence of increasing, long-range, anthropogenic input from the atmosphere (Lockhart et al., 1995). The ratio of recent to geological Hg flux to lake sediments has been calculated for 51 lake sediment cores from lakes in five of the eight circumpolar countries (Landers et al., 1996). Recent increases range from zero to close to zero in Alaska and north-central Russia to values of two times and higher in southeastern Canada and southern Fennoscandia. Proper collection of freshwater lake sediment cores is not simple. A recent workshop of experts convened by the Electric Power Research Institute (EPRI), prepared a Protocol for the collection and analytical requirements needed to obtain lake sediment core data that can be adequately interpreted. A critical requirement is radionuclide age dating with 2J°pb and 137Cs. Such dating is essential to ensure that the surface layer of a core has been collected and that the sedimentation rate is known. This, in turn, allows accurate historical dates to be assigned to the sediment layers analysed (EPRI, 1996). In Canada, concentrations of Hg can exceed food consumption guidelines in specific native food items and in freshwater fish in many remote areas (Allan, 1996). Mercury concentrations in the blood of many aboriginal Canadians, especially the Inuit, sometimes exceed 20 ppb, the lower limit for increasing risk (less than 20 ppb is the normal acceptable range)
98
R. Allan/Journal of Geochemical Exploration 58 (1997) 95-100
(Wheatley and Paradis, 1995). An unquantified component of this Hg may come from long-range, anthropogenic atmospheric emissions such as those from coal-fired power stations and smelters. There is always far more non-organic Hg present, for example in lake sediments, than present in aquatic biota where virtually all of the Hg is in the methylated form. The processes and factors controlling Hg methylation in aquatic ecosystems in Canada have been researched for many years. Reduction or removal of point source inputs has resulted in clear reduction in Hg concentrations in downstream biota in cases of extreme Hg pollution of aquatic ecosystems (Allan et al., 1984). However, when contamination of aquatic ecosystems with anthropogenic Hg is less severe, for example from long-range atmospheric transport and deposition of Hg from anthropogenic sources, the results of this on Hg concentra':ions in aquatic biota are not so clear. The proportion of annual emissions of natural and anthropogenic Hg to the atmosphere is a crucial question. Early estimates of natural Hg emissions in lhe 1970's relied on analyses of deep glacier ice from Greenland (Weiss et al., 1971) and from sites around the world (Jaworowski et al., 1981). New clean techniques for sample collection and analyses of metals in water and ice were implemented in the late eighties. Although the earlier estimate of Weiss et al. (1971) continues to be cited in recent publications (IPCS, 1976, 1989), estimates since 1990 give the ratio of global annual natural/anthropogenic emissions as roughly 5 0 / 5 0 (OECD, 1994). The most recent and widely accepted estimate of natural ttg emissions arose from an expert panel assembled in 1993 by the EPRI in the United States. Their conclusions were that annual pre-industrial emissions of Hg to the atmosphere were in the order of 1600 tonnes and that anthropogenic emissions contributed 5:0 to 75% of the total annual natural plus anthropogenic emissions of some 5000 tonnes (EPRI, 1994). Anthropogenic Hg emissions in Canada in 1990 were 38.8 tonnes of which 30 tonnes came from base-metal smelters and associated plants, 4 tonnes from coal-fired power stations, 3.3 tonnes from municipal waste incinerators and 1.5 from other sources (Doiron et al., 1996). This 1990 Canadian total anthropogenic emission of some 39 tonnes of I-[g can be compared to the United States and Euro-
pean totals of 301 and 626 tonnes, respectively (Doiron et al., 1996). In Canada, a program for Voluntary Emission Reductions by mines and smelters has since resulted in dramatic reductions in metal emissions to the atmosphere. For example, an 83% reduction in Hg emissions (from 25.2 to 4.4 tonnes) occurred between 1988 and 1995 (Mining Association of Canada, 1996), with plans in place to result in further reductions of this and other metals such as Pb and Cd. When estimating anthropogenic emissions of Hg, consideration should also be given to sources such as former railings of abandoned gold mines, even centuries old ones. This volume contains several papers on Hg releases from present-day gold mining operations in the Amazon River basin of Brazil. Emissions from Hg-contaminated waterways, such as those downstream of former chlor-alkali plants may be an important source of anthropogenic Hg to the atmosphere. Emissions of anthropogenic and natural Hg from flooded soils, for example due to reservoir construction, also need to be better assessed.
4. Natural metal anomalies
The fact that metals are natural in origin and that background concentrations for metals vary across terrestrial, aquatic and atmospheric ecosystems, must always be taken into account in relation to all of the above issues concerning metal contamination or pollution by mining-related activities. It is therefore of great importance to continue to try to complete national and global geochemical surveys of natural metal concentrations in a variety of media. For smaller countries, metal surveys using sufficial media such as lake or stream sediments or soils have been completed (Darnley et al., 1995). For larger countries, only certain districts have been surveyed geochemically. In Canada over the last twenty-five years, federal and provincial agencies have extended geochemical mapping over vast areas using aquatic sediments (Painter et al., 1994). The maps reveal global-scale metal concentrations of several metals, extending over areas of tens to thousands of square kilometres in area. These natural concentrations of metals in aquatic sediments vary widely and can often exceed chemistry-based sediment quality
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guidelines associated with metal pollution. Even when aquatic systems have been contaminated or polluted with metals, it is still possible to derive natural historical metal concentrations, for example bv collecting deep lake sediments or by sampling buried layers of overbank sediment as found in some of the papers contained in this special volume. Determining background metal concentrations for soil surface horizons can be more difficult but could involve samples of the same soil types from very remote m-eas or soils that have been buried along with their natural metal content, for example by historical excavations. For natural inputs of metals to the atmosphere, we require more sampling of emissions from both terrestrial and sub-aqueous volcanoes, forest fires, windblown dust, and ocean spray as well as other sources.
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Koval, P.V., Plant, J.A., Steenfelt, A., Tauchid, M. and Xuejing, Xie, with contributions by Garrett, R.G. and Hall, G.E.M., 1995. A Global Geochemical Database for Environmental and Resource Management: Recommendations for International Geochemical Mapping. Final Report of IGCP Project 259. UNESCO Publ. Earth Sci., 19, 122 pp. Doiron, C.C, Whalen, P.J. and Novaczek, I., 1996. Background Information Paper for a Heavy Metals Protocol under the United Nations Economic Commission for Europe Convention on Long Range Transboundary Air Pollution. Pub. EPS, Environment Canada, Ottawa, Vol. 1., 133 pp. EPRI (Electric Power Research Institute), 1994. Mercury Atmospheric Processes: A Synthesis Report. Publ. Electric Power Research Institute, Palo Alto, Calif., EPRI/TR- 104214, 31 pp. EPRI (Electric Power Research Institute), 1996. Protocol for Estimating Historic Atmospheric Mercury Deposition. Publ. Electric Power Research Institute, Palo Alto, Calif., EPRI/TR-106768 3297, 53 pp. 1CME (International Council on Metals and the Environment), 1995. Persistence, Bioaccumulation and Toxicity of Metals and Metal Compounds. Publ. ICME, Ottawa, Ont., 95 pp. IPCS (International Programme on Chemical Safety), 1976. Environmental Health Criteria for Mercury, 3.1. Natural Sources. World Health Organisation, Geneva, p. 42. IPCS (International Programme on Chemical Safety), 1989. Environmental Health Criteria (86) for Mercury - - Environmental Aspects. World Health Organisation, Geneva, p. 13. Jaworowski, Z., Bysiek, M. and Kownacka, L., 1981. Flow of metals into the global atmosphere. Geochim. Cosmochim. Acta, 45: 2185-2199. Landers, D,H., Gubala, C , Verta, M., Lucotte, M., Johansson, K. and Lockhart, W.L., 1996. Using lake sediment mercury flux ratios to evaluate the regional and continental dimensions of mercury deposition in arctic and boreal ecosystems. Atmos. Environ. (in press). Lockhart, W.L., Wilkinson, P., Billeck, B.N., Hunt, R.V., Wagemann, R. and Brunskill, G.J., 1995. Current and historical inputs of mercury to high-latitude lakes in Canada and to Hudson Bay. Water Air Soil Pollut., 80: 603-610. Mining Association of Canada, 1996. Voluntary Emissions Reduction (The Mining Industry in Canada and the ARET - Accelerated Reduction/Elimination of Toxics - - Program). Publ. Mining Association of Canada, Ottawa, 56 pp. OECD (Organisation for Economic Co-operation and Development), 1994. Mercury Risk Reduction. OECD, Monograph No. 4, pp. 37 and 38. Painter, S., Cameron, E.M., Allan, R.J. and Rouse, J., 1994. Reconnaissance geochemistry and its environmental relevance. J. Geochem. Explor., 51: 213-246. Reynoldson, T.B., Mudroch, A., Rosa, F. and Azcue, J.M., 1995. Studies of effects of metal mine discharges on benthic organisms. In: T.P. Hynes and M.C. Blanchette (Editors), Sudbury '95 - - Mining and the Environment. CANMET, Ottawa, Catalogue No. M39-66/1995E, 3: 703-714. Schroeder, W.H. and Markes, J., 1994. Measurements of atmospheric mercury concentrations in the Canadian environment near Lake Ontario. J. Great Lakes Res., 20(l): 240-259.
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Sturges, W.T. and Barrie, L.A., 1989. Stable lead isotope ratios in Arctic aerosols: evidence for the origin of Arctic air pollution. Atmos. Environ., 23:2513-2519. UNECE (United Nations Economic Commission for Europe), 1994. Task Force on Heavy Metals Emissions: State-of-the-Art Report. Publ. EGU Prague, PLC, CZ 190 11 Prague 9, 266 pp. UNESCO (United Nations Educational, Scientific and Cultural Organisation), 1988. International Hydrological Programme 1988, Workshop on Metals and Metalloids in the Hydrosphere; Impact Through Mining and Industry, and Prevention Technology. Unesco, Paris, 192 pp. UNESCO (United Nations Educational, Scientific and Cultural
Organisation), 1989. International Hydrological Programme, 1989. Metals and Metalloids in the Hydrosphere; Impact Through Mining and Industry, and Prevention Technology in Tropical Environments. Asian Institute of Technology, Bangkok, 191 pp. Weiss, H.V., Koide, M. and Goldberg, E.D., 1971. Mercury in the Greenland icesheet: evidence of recent input by man. Science, 175: 692-694. Wheatley, B. and Paradis, S., 1995. Exposure of Canadian aboriginal peoples to methylmercury. Water, Air Soil Pollut., 80: 3-11.