• What is acid rock drainage?

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    Acid rock drainage (ARD) or acid mine drainage refers to the acidic water that is created when sulphide minerals are exposed to air and water and, through a natural chemical reaction, produce sulphuric acid. ARD has the potential to introduce acidity and dissolved metals into water, which can be harmful to fish and aquatic life. Preventing and controlling ARD is a concern at operating mine sites and after mine closure. Advances continue to be made in research and the development of technology to improve ARD prediction, prevention, and treatment.

    Acid rock drainage formation

    Acid rock drainage (ARD) is formed by the natural oxidation of relatively common sulphide minerals when they are exposed to water and air. [1, 2] Like the rusting of iron nails, sulphide oxidation is a spontaneous chemical reaction where oxygen is present. [2] The oxidation of pyrite (iron sulphide or ‘fool’s gold’) produces sulphuric acid and ferric sulphate, and is responsible for the majority of ARD formation. [2] More information on the chemical reactions involved in ARD is available from the International Network for Acid Prevention. [2]

    Results of acid rock drainage

    Because ARD contains sulphuric acid, the pH of the water decreases once sulphide oxidation starts. [2] Under low pH conditions, ferric sulphate may be oxidized to ferric iron, which is capable of oxidizing other minerals such as lead, copper, zinc or cadmium sulphides. [2] As a result, ARD frequently contains high concentrations of toxic dissolved metals. [2]

    Occurrence of acid rock drainage

    ARD formation occurs naturally where sulphide minerals are exposed to the atmosphere. [2]  In fact, the detection of ARD in surface or groundwater is a powerful exploration tool for prospectors looking for mineral deposits. [3] Sulphide minerals make up a major proportion of rock in some geological environments, and exposed deposits will generally have a red or yellow cap of oxidized material known as a gossan. [3, 4]

    ARD has taken place over millions of years, and the names of rivers such as the Rio Tinto in Spain, the Cornish Red River, the Norwegian Raubekken, and the Iron Creek in Colorado reflect the historical nature of ARD. [3, 5] Human excavation activities have the potential to accelerate the ARD process by exposing sulphide minerals to air and water. [2] These activities include highway construction, quarrying, civil engineering works, logging, and metal and coal mining. [2]

    The rate of ARD production depends on a number of factors such as: [4]

    • Surface area of sulphide minerals exposed: Increasing the surface area of sulphide minerals exposed to air and water increases sulphide oxidation and ARD formation.
    • Type of minerals present: Not all sulphide minerals are oxidized at the same rate, and neutralization by other minerals present may occur, which would slow the production of ARD.
    • Amount of oxygen present: Sulphide minerals oxidize more quickly where there is more oxygen available. As a result, ARD formation rates are higher where the sulphides are exposed to air than where they are buried under soil or water.
    • Amount of water available: Cycles of wetting and drying accelerate ARD formation by dissolving and removing oxidation products, leaving a fresh mineral surface for oxidation. In addition, greater volumes of ARD are often produced in wetter areas where there is more water available for reaction.
    • Temperature: Pyrite oxidation occurs most quickly at a temperature around 30°C.
    • Microorganisms present: Some microorganisms are able to accelerate ARD production.

    The role of microorganisms in acid rock drainage

    There are a number of common microscopic bacteria which oxidize sulphur minerals as a source of energy; these bacteria can accelerate the rate of sulphur oxidation, and are an important factor in ARD formation. [2] Sulphur-oxidizing microorganisms have even been found living in sub-zero degree temperatures, contributing to ARD in permafrost regions. [4]

    Environmental impacts of ARD

    ARD has the potential to decrease the water quality of receiving waters by lowering the pH and increasing the dissolved metal content of surface and groundwater. [2] The environmental impact of ARD depends on the size and sensitivity of the water body affected, and the amount of neutralization and dilution that occurs. [6] For example, the same volume of ARD would have a much greater impact on the water quality in a small lake than it would have in the ocean, as the ocean has a higher dilution capacity and salt water has stronger acid-buffering capacity than fresh water. [4]

    Where ARD enters surface water, it can have adverse impacts on the health of aquatic animals, insects, and plants. [7] The dissolved metals associated with ARD are often more toxic to fish and aquatic organisms than is the acidity. [6]

    Acid rock drainage in mining

    ARD is a major concern for the mining industry because mining activities tend to increase the amount of rock surface exposed to air and water. [6] In addition, most metal deposits and some coal deposits are relatively rich in sulphide minerals. [6] ARD contributes to the pollution of mine water, which must then be treated at great cost before being discharged into the environment. In addition, ARD may prevent the successful reclamation of a mine site by inhibiting plant growth. [6]

    Advances in acid rock drainage management

    The understanding, prevention, and treatment of ARD are the subject of a substantial research effort by government agencies, the mining industry, universities, and research establishments. [2] Organizations that work to promote and coordinate developments in ARD research include the International Network for Acid Prevention (INAP), Mine Environment Neutral Drainage (MEND), Partnership for Acid Drainage Remediation in Europe (PADRE), and the Acid Drainage Technology Initiative (ADTI). Today, the scientific community has achieved a detailed understanding of sulphide oxidation and ARD formation, and has developed best practices and protocols to treat ARD and protect the environment. [4] It has been estimated that the Canadian organization MEND has reduced the risk and liability associated with ARD by nearly half a billion Canadian dollars since its inception through innovative research, development and technology transfer. [8]

    Prediction of acid rock drainage

    Advances in ARD prediction include the development of computer programs, chemical evaluations, and acid-base accounting which are used to anticipate whether ARD is likely to form at a mine site. [9] Mining operations that expose sulphide-bearing rock do not always lead to ARD because the ore may contain a high proportion of “acid-buffering” minerals such as lime, calcite, carbonate or bicarbonate, which are able to neutralize acidic waters. [4, 6]. For example, when baking soda (soda bicarbonate, a buffer) is added to vinegar (a weak acid), it reacts with the vinegar and neutralizes it. Because the mineralogy of sulphuric wastes and ores is highly varied and is unique to each deposit, the prediction of ARD requires a good understanding of the physical, geological, geochemical, and mineralogical characteristics of the mine site. [4]

    Mining companies are increasingly required to evaluate the ARD potential at future mine sites and provide detailed plans to prevent or suppress ARD at all phases of mine operation as part of the environmental impact assessment (EIA) process practiced in more than 100 countries. [2, 10] In Canada, major mining projects generally require EIA approval from both provincial and federal regulators. [11]

    Prevention of acid rock drainage at mine sites

    The most cost-effective and low-risk ARD management strategy is the prevention of ARD formation through prediction and mine planning. [6] For instance, mine design can be done in a way that minimizes the excavation of sulphide minerals. [6] Where sulphide mineral excavation is unavoidable, a number of ARD prevention strategies have been developed: [6, 12, 13]

    • Storing waste rock underwater: Storing wastes underwater reduces the rate of sulphide mineral oxidation by air to almost zero, effectively stopping ARD formation. Underwater storage is considered to be the most reliable and effective ARD prevention strategy.
    • Flooding and sealing underground mines: The flooding of underground tunnels is used where it is possible to isolate the mine by sealing all the entrances. Sealing the mine prevents water moving in and out, which could result in surface water or groundwater contamination.
    • Mixing acid-producing materials with acid-buffering materials: Combining sulphide wastes with limestone or calcite can result in ARD neutralization on site.
    • Covering waste rock: Installing a cover of clay, plastic, or soil over piles of waste rock prevents rain and other precipitation from contributing to ARD formation and transport, and reduces the amount of oxygen available to react with the sulphide minerals. 
    • Chemical treatment of sulphide wastes: Organic chemicals designed to kill sulphide-oxidizing bacteria have been applied to sulphide wastes in order to slow the rate of ARD. However, there is concern that some of these chemicals may kill beneficial microorganisms in the environment, thus becoming pollutants themselves.

    Preventative measures have been most effective when used in combination, and adapted to the situation at the specific site. [7] In addition, the groundwater and surface water surrounding mines are monitored in order to provide an early warning system for the detection of ARD formation.

    Treatment of acid rock drainage

    Where the complete prevention of ARD formation is unsuccessful, acidic mine water can be captured and treated using a number of water treatment processes. [6] The most widespread method used to treat ARD is by actively adding alkaline material (often lime) to the water in order to neutralize the acid and precipitate the dissolved metals. [12] Although this method is effective, it can have high operating and materials costs, and produces sludge with a high metal content. One way to offset the costs of water treatment being explored by Hedin Environmental is to concentrate the sludge into a marketable resource. [14] Hedin (2003) added bicarbonate to ARD from an abandoned coal mine in Pennsylvania to produce an iron oxide sludge which was comparable to iron oxide mined for use in paint pigments. [14]

    The development and use of passive treatment technologies to treat ARD has received a lot of interest because of low operation and maintenance costs. [12] However, only constructed wetlands and bioreactors have been used in full-scale treatment systems so far. [12, 15]

    Innovations in ARD treatment continue to be explored. A few examples include:

    • Small-scale microbial fuel cells developed by researchers at Pennsylvania State University were used to generate electricity while removing and recovering dissolved iron from ARD. [16]
    • Filtration and ion exchange is being pioneered by Earth Metallurgy Solutions in South Africa to treat ARD and recover valuable resources which can be used for fertilizer, explosives, and concentrated solar power salts. [17]
    • Microbial reactors are being developed by the South African Council of Scientific and Industrial Research (CISR). These reactors use microbes found in cow guts to treat ARD. [17]
    • Red mud bauxite is an alkaline waste product from aluminum refineries, and has been used successfully in Australia to neutralize acidic tailings and waste rock. [4, 18]

     

    Case Study: Britannia Mine, near Squamish, B.C., Canada

    Britannia Mine is a historic copper mine located on the coast of British Columbia approximately 51.5 kilometers north of Vancouver. [19] The mine started producing copper in 1904, and by 1929 it was the largest copper producer in the British Empire. [20] A total of 53,630,983 tonnes of ore was mined, and 516,743 tonnes of copper, 125,291 tonnes of zinc, 15,563 tonnes of lead, 445 tonnes of cadmium, 180 tonnes of silver, and 15.3 tonnes of gold were extracted before the mine was closed in 1974. [19] In 1988, Britannia Mine became a National Heritage Site, and the following year, the Britannia Mine Museum was designated a B.C. Historical Landmark. [20]

    The ore at Britannia Mine is largely in the form of metal sulphides, which were probably producing ARD naturally before the construction of the mine. [21] However, the excavation of 210 kilometers of tunnels in the mine greatly increased the surface area available for oxidation and ARD generation. [22] In 1930, it was known that the water coming out of the mine was acidic and contained metals, but the environmental impacts of the mine water were not well understood, and the pollution was not tackled during the life of the mine. [22]

    When the mine was closed, the owners at the time followed the environmental standards of the day, and took steps to divert the mine water away from the most sensitive environments. [22] A dam was built to prevent water from entering Britannia Creek, and untreated mine water was released into Howe Sound via an outflow pipe 50 meters underwater. [22] However, these measures did not adequately control the ARD, and the dam failed sometime in the 1980s or 1990s, resulting in the severe pollution of Britannia Creek and associated impacts on aquatic life in the area. [22]

    With the introduction of the Contaminated Sites Regulation in 1997, the Province of B.C. began the remediation of Britannia Mine. [22] In 2001, 30 million Canadian dollars was recovered from past mine owners, and consultants were hired to manage the remediation. [22] In December 2001, the University of British Columbia Centre for Environmental Research in Minerals, Metals and Materials (UBC-CERM3) installed an earth plug at an outlet, successfully stopping mine water from flowing into Britannia Creek. [23] All the remaining portals and shafts were blocked, diverting all the mine water to a water treatment plant. [22] The water treatment plant began operations in 2005, and treats all the water coming from the mine with lime to raise the pH and precipitate metals out of the solution. [21] The resultant sludge is used to refill the mine, and to further prevent water entering the tunnels. [21]

    Prior to the remediation of Britannia Mine, Britannia Creek was discharging acidic waters heavily laden with dissolved metals into Howe Sound, which made the creek and the area around it uninhabitable for fish and other aquatic organisms. [24] Within three years of the installation of the earth plug, salmon fry were observed swimming at the mouth of the creek, and in 2011, pink salmon had returned to the creek to spawn. [23, 24]



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    References

    1INAP: The International Network for Acid Prevention. Global Acid Rock Drainage Guide Summary, 2012 [cited 2012 June 27]; Available from: http://www.gardguide.com/index.php/Summary.

    2Mills, C. An Introduction to Acid Rock Drainage, 2012 [cited 2012 June 26]; InfoMine. Available from: http://technology.infomine.com/enviromine/ard/Introduction/ARD.HTM.

    3Downing, B. and C. Mills. Natural Acid Rock Drainage, 2012 [cited 2012 June 26]; InfoMine. Available from: http://technology.infomine.com/enviromine/ard/Introduction/Natural.htm.

    4Lottermoser, B., Mine Wastes: Characterization, Treatment and Environmental Impacts, 2012, Springer: New York. p. 400.

    5Banks, D., et al., Mine-water chemistry: the good, the bad and the ugly. Environmental Geology, 1997. 32(3): p. 157-174.

    6Price, W.A. and J.C. Errington. Guidelines For Metal Leaching and Acid Rock Drainage at Minesites in British Columbia, 1998 [cited 2012 June 27]; B.C. Ministry of Energy and Mines and Responsible for Housing. Available from: http://www.empr.gov.bc.ca/Mining/Permitting-Reclamation/ML-ARD/Pages/Guidelines.aspx.

    7Jennings, S.R., D.R. Neuman, and P.S. Blicker. Acid Mine Drainage and Effects on Fish Health and Ecology: A Review, 2008 [cited 2012 June 27]; Reclamation Research Group. Available from: http://reclamationresearch.net/publications/Final_Lit_Review_AMD.pdf.

    8Canada, Environment Canada. 9. Land-Use Practices and Changes - Mining and Petroleum Production, Threats to Water Availability in Canada. NWRI Scientific Assessment Report Series No. 3 and ACSD Science Assessment Series No. 1. 128 p., 2004 [cited 2012 July 3]; Available from: http://www.ec.gc.ca/inre-nwri/default.asp?lang=En&n=0CD66675-1&offset=14&toc=show#table1.

    9G.A. Tremblay and C.M. Hogan, eds. MEND Manual Volume 3 -- Prediction, MEND 5.4.2c, 2000 [cited 2012 July 13]; Available from: http://www.mend-nedem.org/reports/files/5.4.2c.pdf.

    10Noble, B.F., Introduction to Environmental Impact Assessment: A Guide to Principles and Practice, 2006, Don Mills, Ontario: Oxford University Press.

    11British Columbia, British Columbia Environmental Assessment Office. Frequently Asked Questions, 2012 [cited 2012 July 13]; Available from: http://www.eao.gov.bc.ca/FAQ.html.

    12Johnson, D.B. and K.B. Hallberg, Acid Mine Drainage Remediation Options: A Review. Science of The Total Environment, 2005. 338: p. 3-14.

    13Eba Engineering Consultants Ltd. Heavy Metals and Acid Rock Drainage: A Select Literature Review of Remediation and Recommendations for Applied Research, 2004 [cited 2012 June 27]; Mining Environment Research Group. MERG Report 2004-2:[Available from: http://www.geology.gov.yk.ca/pdf/MPERG_2004_2.pdf.

    14Hedin, R.S., Recovery of Marketable Iron Oxide From Mine Drainage in the USA. Land Contamination and Reclamation, 2003. 11(2): p. 93-97.

    15Sobolewski, A. Wetlands for the Treatment of Mine Drainage, 1996 [cited 2012 June 26]; InfoMine. Available from: http://technology.infomine.com/enviromine/wetlands/Welcome.htm.

    16Cheng, S., B.A. Dempsey, and B.E. Logan, Electricity Generation from Synthetic Acid-Mine Drainage (AMD) Water using Fuel Cell Technologies. Environmental Science & Technology, 2007. 41: p. 8149-8153.

    17Kardas-Nelson, M. The Acid Mine Drainage Solution Bandwagon, 2010 [cited 2012 July 10]; Mail & Guardian. Available from: http://mg.co.za/article/2010-12-10-the-acid-mine-drainage-solution-bandwagon/.

    18Paradis, M., et al. Using Red Mud Bauxite for the Neutralization of Acid Mine Tailings: A Column Leaching Test, 2006 [cited 2012 June 27]; Canadian Geotechnical Journal. 43: 1167-1179:[Available from: http://www.nrcresearchpress.com/doi/pdf/10.1139/t06-071.

    19Mullan, M. Britannia -- The Story of a British Columbia Mine -- From Mining Resource to Heritage Resource, n.d. [cited 2012 June 26]; B.C. Ministry of Energy and Mines and Responsible for Housing. Available from: http://www.empr.gov.bc.ca/MINING/GEOSCIENCE/PUBLICATIONSCATALOGUE/OPENFILES/1992/1992-19/Pages/Britannia.aspx.

    20Britannia Mine Museum. Britannia Story, 2012 [cited 2012 June 26]; Available from: http://www.bcmm.ca/history/britannia_story.html.

    21Britannia Mine Museum. Environment, 2012 [cited 2012 June 26]; Available from: http://www.bcmm.ca/history/environment.html.

    22BC Museum of Mining. Britannia's Environment, 2009 [cited 2012 June 26]; Available from: http://www.bcmm.ca/pdfs/Info%20Zone%20Britannia%27s%20Environment.pdf.

    23Meech, J. UBC Mining Expertise Helps Pink Salmon Return to Britannia Creek, 2011 [cited 2012 June 26]; University of British Columbia. Available from: http://apsc.ubc.ca/apsc/news/2011/09/ubc-mining-expertise-helps-pink-salmon-return-britannia-creek.

    24Burke, D. Britannia Creek in the Pink -- Salmon, That Is, 2011 [cited 2012 June 29]; The Chief. Available from: http://www.squamishchief.com/article/20110914/SQUAMISH0101/110919981/-1/squamish/britannia-creek-in-the-pink-8212-salmon-that-is.

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