• How is water managed and treated in mining?

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    In order to comply with regulations and ensure that the quality of water leaving mine sites is not adversely affecting water users downstream, mining companies develop water management plans to minimize the potential for water contamination, and to prevent the release of polluted water into the environment. Surrounding surface and groundwater quality is monitored, and a number of treatment processes can be used to ensure mine water meets regulatory standards prior to being discharged.

    Environmental management strategies

    Awareness of environmental impacts and concern for the environment has increased in recent decades, resulting in more stringent environmental regulations worldwide and a corresponding response from the international mining industry. Environmentally responsible practices, especially relating to water, have become central to the viability and acceptance of a modern mining operation. [12] For example, if other stakeholders believe a mine is using too much water, or polluting the water, it can lead to social conflict and discontent. [11] Water management strategies are used to minimize the environmental impact of mining operations, and are now at the heart of mine development, operation, and restoration activities. [19] Increasingly, companies exceed regulatory compliance in order to provide clean water to surrounding communities. [11]

    Water control techniques

    Water is carefully managed to prevent the release of contaminated water into the environment. [14] Various control techniques can be used to reduce the potential for water contamination and minimize the volume of water requiring treatment. These techniques include: [12]

    • Intercepting and diverting surface water (rain and snowmelt runoff, streams, and creeks) from entering the mine site by building upstream dams to reduce the potential for water contamination from exposed ore and waste rock
    • Recycling water used for processing ore in order to reduce the volume of water requiring treatment
    • Capturing drainage water from precipitation at the mine site through the use of liners and pipes and directing the water to tailings dams in order to prevent potentially contaminated water from entering groundwater or flowing off site
    • Allowing the water to evaporate in ponds to reduce the volume of contaminated water; in dry regions, enough water may be evaporated that no water needs to be discharged, resulting in the containment of contaminates at the mine site
    • Installing liners and covers on waste rock and ore piles to reduce the potential for contact with precipitation and contamination of groundwater

    Different combinations of strategies can be applied, and the selection of strategies is site-specific. [12] For instance, the interception and diversion of surface water is a more prominent concern in environments with high rates of precipitation, whereas more emphasis is placed on water recycling in arid regions with little water availability. [11]

    Regulations governing mine water discharge

    In cases where the mine water is discharged into the environment, it is expected to meet the mine licence and regulatory requirements of the responsible jurisdiction. [4] Compliance with regulations is usually enforced with some sort of financial penalty such as a fine, and in extreme cases, mining operations may have their licences revoked. The mining industry in Canada has had a high level of compliance over the last fifteen years. [6] Although the extent and effectiveness of environmental regulations varies considerably among developing nations, the international mining industry has recognized that environmental responsibility has become essential to the acceptance of a mining project by a community, and has developed a number of best practices which are in use by many mining companies around the world. [17]

    Water treatment

    There are a number of different treatment technologies available to clean contaminated water; these technologies can be described as either active or passive. Active treatment technologies require the input of energy and chemicals, and passive treatment uses only natural processes such as gravity, microorganisms, and/or plants in a system, any one of which requires infrequent but regular maintenance. [1, 19] The treatment technology used at a mine depends on how polluted the water is, what chemicals need to be removed, how much water requires treatment, and the required discharge water quality standards. [14]

    Active water treatment

    Active water treatment is the most common form of water treatment at working mines. [18] Sophisticated software is used to predict the chemistry of the effluent prior to mine construction, and appropriate water treatment technologies are chosen accordingly. [12, 18] Mine waters are often acidic, and require the addition of lime, limestone, or caustic soda to raise the pH. [3] Once the pH has been elevated, dissolved metals precipitate out of solution and sink to the bottom of settling or sedimentation ponds, where they can be removed. [3] Chemicals known as coagulants or flocculants may be added in order to combine smaller particles into larger clumps which settle out of the water more quickly. [3]

    Depending on the desired final water quality, a mine may also use other technologies including ion exchangers, membrane filters, and reverse osmosis. [14] The semi-solid portion or sludge removed from the water is dried and disposed of either underground in the mine, or in a waste disposal facility. [18]

    Because of the high maintenance and disposal costs of active treatment, ways to recycle the sludge are being explored, such as the production of marketable iron concentrate from acid rock drainage from coal mines. [9] However, the majority of sludge has little economic value and is handled as waste.  In extreme cases where the sludge is rich in cadmium or arsenic, it may be classified as hazardous waste and require special handling and disposal. [19]

    Passive water treatment

    Passive water treatment is usually combined with water monitoring programs, and takes advantage of natural physical, chemical, and biological processes that remove water contaminates without additional physical or chemical inputs. [18] Examples of these processes include bacteria-controlled metal precipitation, contaminate uptake by plants, and filtration through soil and sediments. [3] Although passive water treatment has been used successfully to treat municipal wastewater and urban runoff, its use in the mining industry is still under development, largely because of the challenge of treating highly acidic waters. [18] Despite these challenges, low operation and maintenance costs continue to generate interest in passive water treatment, especially for use after mine closure. [18]

    Constructed wetlands are the most common form of passive water technology. [1] Constructed wetlands act as natural purification systems which are capable of filtering and removing many contaminates before they are transported into marine or freshwater environments. [3] However, it is difficult to achieve ideal contaminate removal conditions, and passive water treatment currently requires ongoing maintenance and treatment. [3]

    Water management following the closure of a mine

    Mining is a transient activity, and once mining operations finish mine closure activities are carried out to minimize the environmental impact of a mine site. These activities can include rinsing any ore in heap leach piles to remove any leftover chemicals, draining and capping tailings ponds with dense clay to prevent water infiltration and groundwater contamination, and removing or stabilizing any contaminated material on site.  [2, 10] Ongoing water monitoring and treatment is often a component of mine closure plans, especially where acid rock drainage forms. [18]

    Since most mines operate in the ground below the water table groundwater often is pumped from wells to dewater the mine (i.e., clear it of water) and gain access to the minerals during active mining. [5] Once operations have ended, pumping usually stops, which results in the flooding of any open pits and underground workings as the water table rises. Pit lakes are a common feature at closed mine sites. [13] However, mine flooding can potentially decrease the water quality of surrounding areas where runoff and groundwater dissolve minerals in the exposed ores and waste rock. [7, 19] Continued dewatering past the closure of the mine can be the most cost-effective way of dealing with mine water pollution in these cases, and treated water from dewatering wells can be used for drinking water and other applications. [1] For instance, in 1996 water from an inactive surface coal mine in Yorkshire, U.K., was treated using reverse osmosis, and contributed to the public water supply during a drought. [19]

     

    Case Study: Wheal Jane tin mine, Cornwall, U.K.

    The Wheal Jane mine is located in the Carnon River Valley of Cornwall, U.K., and produced tin, copper, lead, and silver from the early 18th century until 1991. [8] Modern mine workings reached a depth of 450 metres, and 10 million litres of water were routinely pumped out every day to dewater the mine. [1] The dewatering of the mine was stopped when the mine closed in 1991; the groundwater rise was monitored, and the water was found to be very acidic with a high dissolved metal content due to the dissolution of exposed minerals in the mine. [16] Initially, mine owners Carnon Consolidated Ltd. provided a treatment system to pump and treat the water using lime and settling tanks, but this was considered to be a temporary solution because of high operating costs and the large volume of sludge produced. [3] Following a spill as a result of the unexpected failure of a concrete plug in 1992, environmental impact and remediation studies were carried out on behalf of the National Rivers Authority, now the Environmental Agency, for both active and passive water treatment systems. [7]

    A pilot passive system was constructed in 1994, and consisted of three combinations of passive lime treatment with and without air, artificial reed beds, biological reactors, and rock filters. [7] The passive system was found to work well, but was less controllable and consistent than an active system, and would have required too much land to treat the large volume of water coming from the mine. [16]

    An active water treatment system using oxidation, lime additions, and settling was found to be the most cost-effective method for treating the mine water. [16] A water treatment plant was commissioned by the Environmental Agency and began operations in 2000. [7] The treatment system was quickly optimized and is capable of removing over 99.2% of the dissolved metals present in the water. [7]

    Although the passive system was not considered to be a viable solution for the treatment of mine water at the Wheal Jane mine, it provided a valuable research opportunity for the exploration of passive water treatment systems. In addition, its construction resulted in the formation of a natural wetland area which has now been converted into a recreation area and business park. [15]



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    References

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    2Blight, G., Chapter 5: Mine Waste: A Brief Overview of Origins, Quantities, and Methods of Storage, in Waste: A Handbook for Management, T. Letcher and D. Vallero, Editors. 2011, Academic Press: Burlington, M.A. p. 77-87.

    3Brown, M.p., Minewater treatment : technology, application and policy\par. 2002(Book, Whole\par).

    4Canada, Environment Canada. Guidance Document for the Sampling and Analysis of Metal Mining Effluents 2/MM/F/B. 2001 [cited 2012 May 30]; Available from: http://www.ec.gc.ca/Publications/default.asp?lang=En&xml=30EE2CC9-5118-4D47-A14B-B109294FCABA.

    5Canada, 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.

    6Canada, Natural Resources Canada. Mining Sector Performance Report 1998-2008. 2010 [cited 2012 May 31]; Available from: http://www.nrcan.gc.ca/minerals-metals/publications-reports/3398#es.

    7Contaminated Land: Applications in Real Environments. Mine Water Treatment at Wheal Jane Tin Mine, Cornwall. Case Study Bulletin 2004 [cited 2012 June 22]; Available from: http://www.claire.co.uk/index.php?option=com_phocadownload&view=file&id=6:case-study-bulletins&Itemid=25.

    8Cornwall in Focus. The Mines of the Carnon Valley -- Wheal Jane. 2012 [cited 2012 July 5]; Available from: http://www.cornwallinfocus.co.uk/history/whealjane.php.

    9Hedin, R.S., Recovery of Marketable Iron Oxide From Mine Drainage in the USA. Land Contamination and Reclamation, 2003. 11(2): p. 93-97.

    10Hutchison, I.P.G. and R.D. Ellison, Mine Waste Management. 1992, Chelsea, Michigan: Lewis Publishers, Inc.

    11International Council on Mining & Metals. Water Management in Mining: A Selection of Case Studies. 2012 [cited 2012 June 12]; Available from: http://www.icmm.com/www.icmm.com/water-case-studies.

    12Lottermoser, B., Mine Wastes: Characterization, Treatment and Environmental Impacts, 2012, Springer: New York. p. 400.

    13McKenna, G. and R. Dawson. Closure Planning Practice and Landscape Performance at 57 Canadian and U.S. Mines. Proceedings of the 21st Annual British Columbia Mine Reclamation Symposium 1997 [cited 2012 Aug. 1]; Available from: https://circle.ubc.ca/bitstream/handle/2429/9873/1997+-+McKenna,+Dawson+-+Closure+Planning+Practice.pdf?sequence=1.

    14Rankin, W.J., Minerals, metals and sustainability : meeting future material needs. 2011, Collingwood, Vic.: CSIRO Pub.

    15The Wheal Jane Group. The Wheal Jane Sustainability Project. 2010 [cited 2012 July 5]; Available from: http://www.whealjanemasterplan.co.uk/.

    16UniPure. Case Study: Wheal Jane: A Clear Success. 2012 [cited 2012 July 5]; Available from: http://www.unipure.com/case_studies/casestudy4.html.

    17Warhurst, A., Environmental degradation from mining and mineral processing in developing countries : corporate responses and national policies. 1994, Washington, D.C.: OECD Publications and Information Centre.

    18Wolkersdorfer, C., Chapter 11: Mine Water Treatment and Ground Water Protection in Water Management at Abandoned Flooded Underground Mines: Fundamentals, Tracer Tests, Modelling, Water Treatment, A. International Mine Water and I. ebrary, Editors. 2008, Springer: Berlin. p. 235-277.

    19Younger, P.L., S.A. Banwart, and R.S. Hedin, Mine Water: Hydrology, Pollution, Remediation. 2002, Dordrecht, The Netherlands: Kluwer Academic Publishers.

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