General Information on Metal Leaching and Acid Rock Drainage

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FROM: BC Ministry of Energy, Mines & Petroleum Resources

What is Metal Leaching and Acid Rock Drainage?

  • Metal leaching and add rock drainage are naturally occurring processes that are caused when minerals containing metals and sulphur (called sulphides) come in contact with both air and water.
  • When sulphides are exposed to water and the oxygen from air, they net (or oxidize). This oxidizing of sulphides can also produce acid. If this acid is carried by streams or other natural watercourses it is called acid rock drainage (ARD).
  • The acid in ARD can leach metals from surrounding rocks causing drainage that has high amounts of dissolved metals (such as iron, aluminum, copper, lead, silver, zinc). This is called metal leaching (ML). Other metals can also be leached from react in non-acidic drainage (such as selenium, zinc, molybdenum, nickel, arsenic and antimony).
  • Not all rocks that contain sulphide minerals will become acid generating. It depends on the amount of neutralizing minerals and materials (such as limestone) that are present in the rocks. If there is balance, or if there is an excess of neutralizing minerals, the rocks may not generate metal leaching and add rock drainage (ML/ARD). If there is excess sulphide minerals, then ML/ARD will typically develop after all of the neutralizing minerals have dissolved. This can result in a significant time delay to the development of ML/ARD in rocks.
  • The rocks at most metal and some coal mines contain high amounts of sulphide minerals. The excavating and crushing of ores during mining greatly increases the amount of rock surfaces that can be exposed to oxygen and water. Therefore mining activities can have a high potential for leaching acid and metals.
  • ML/ARD can occur from sulphide bearing mining wastes or from open pit or underground mine surfaces. Mining wastes often include mineralized rock that is not of ore grade (called waste rock) and tailings which is sand sized material left over from processing ore.
  • Most mining operations leach metals to some degree. The potential for environmental impacts depends on many factors including the amount of metals in the mine drainage, the amount of acid-neutralizing ability in nearby rocks and water, the amount of dilution available in streams and how sensitive the receiving environment is.
  • Mining is not the only industry that can cause ML/ARD. Forestry, road construction and other industrial activities that dig up sulphide bearing rocks have caused significant ML/ARD. For example, the Halifax airport had a major problem with ARD when it constructed a new runway in sulphide bearing rocks.

Why is ML/ARD Significant?

  • ML/ARD can have significant negative impacts on the environment if not adequately managed. High levels of metals and/or acid can be harmful or toxic to living organisms. Metals that are absorbed by plant and animal tissue can also be passed through the food chain.
  • Once ML/ARD has been initiated it can persist for hundreds of years until the sulphides are completely oxidized and the acid and metals are leached from the rocks.
  • Historical mining practices did not recognize or manage ML/ARD. A number of older mines in BC (e.g., Britannia and Mt. Washington) have caused significant environmental impacts due to ML/ARD.
  • ML/ARD can be very expensive to manage once it has developed. For example, in British Columbia water treatment plants to treat ML/ARD have cost more than $10 million to put in place and have had operating costs of up to $1.5 million per year. Other management strategies can also be very expensive.
  • ML/ARD associated with mining activities is a globally significant issue. There are several organizations world-wide that are dedicated to improving mining practices and developing new technologies to reduce the environmental effects and liabilities associated with ML/ARD.

How Can the Effects of ML/ARD be Mitigated?

  • Proper planning of new mining developments can reduce the risks, liabilities and the costs associated with ML/ARD.
  • Testing of the chemistry of the rocks before they are mined can predict whether ML/ARD will be an issue that needs to be prevented or managed. If the potential for leaching of acid and metals is identified through testwork, there are number of strategies that can be used to prevent and manage ML/ARD.
  • Each mitigation strategy has strengths and weaknesses and not all strategies are applicable for all mine sites and their environments. Each strategy also has unique monitoring and maintenance needs.
  • Many management strategies rely on preventing oxygen contact with sulphide minerals or reducing the amount of water that comes in contact with ARD generating wastes to minimize the amount of leaching that occurs. The sulphide minerals can stay in the rocks for hundreds of years, so mitigation strategies must often be designed to last forever.
  • Often mines use a combination of different strategies to ensure environmental protection. Sometimes additional measures are provided for back-up or contingency safeguards.
  • Mine sites and their environments are dynamic and continue to change after mining is finished. For example, changes can occur to water movement to a mine through climate variations and non-mining water users. Changes can also happen to the ecology of a mine site as nature reestablishes itself, and to the chemistry of the site from a variety of factors. These changes can significantly influence the effectiveness of mitigation strategies over time.
  • Many ML/ARD mitigation technologies are relatively new and have performance uncertainties and limited performance histories.
  • Due to changes that can happen after mining and uncertainties with the performance of many mitigation technologies, regular monitoring, maintenance and responsive management are key to long-term success in preventing impacts from ML/ARD.
  • The main mitigation strategies that are used to prevent and manage ML/ARD are avoidance, flooding, covers, blending, and drainage treatment.


  • Sometimes it is possible to avoid mining some, or all, of the sulphide bearing rocks that could generate ML/ARD problems. If this is not practical, then other mitigation strategies may be needed to ensure protection of the environment.

Flooding of Mine Waste Materials

  • Flooding sulphide bearing mining wastes with water limits their contact with oxygen and effectively prevents the formation of ARD.
  • Flooding can be done behind a constructed dam, in old mine pits or underground workings or in natural water bodies such as ponds or lakes.
  • The timing that flooding occurs is important. Mine wastes should be flooded before they become acid generating or metal leaching, otherwise some metals and acid will dissolve into the water and this will have to be managed.
  • Although flooding can effectively prevent sulphide oxidation and the acid generation process, there can sometimes be residual metal leaching issues. One way that this can be prevented is by isolating the mining wastes from the overlying water by placing a barrier of clean rock or other material.
  • Flooding has to be maintained in the mine wastes at all times, even during extreme climatic conditions (dry and wet). The flooding of sulphide bearing mine waste behind dam structures also leaves a legacy which requires long-term inspection, monitoring and maintenance to ensure that they last forever.
  • Flooding of mine wastes is the most common mitigation strategy used at newer mines. Flooding has been used successfully at many mines in BC including Equity Silver, Eskay Creek, Goldstream, Huckleberry, Island Copper, Johnny Mountain, Kemess South, Mount Polley, Myra Falls, Premier, QR Gold, Samatosum, and Snip.


  • Covering mining wastes can limit the amount of oxygen and/or water that reaches the wastes. This can decrease the amount of drainage from the wastes and the quantity of acid and metals in the drainage that has to be managed.
  • Covers can be simple (such as one made of soil or glacial till) or complex engineered systems.
  • Although covers can reduce the quantity and severity of ML/ARD, they are susceptible to break down over time from wind and water erosion, cracking, burrowing by animals, and plant roots.
  • Soil covers are also used at most mines to re-establish plant growth at the end of mining.
  • Covers to mitigate ML/ARD have been used with varying degrees of success at several BC mines including Cirque, Equity Silver, Gibraltar, Myra Falls and Sullivan.

Blending of Materials

  • Mining wastes that have the potential to generate ARD can be mixed with materials that have acid-neutralizing qualities, such as limestone or other mine waste materials.
  • Although blending is a potentially effective mitigation strategy, it can be very difficult to get enough mixing to prevent all acid generation and there can sometimes be residual metal leaching that occurs.
  • Blending is generally more successful for small volumes of mining wastes and/or wastes that have low amounts of sulphide minerals.
  • Blending has been used at several BC mines with varying degrees of success including Elk, Samatosum, Quinsam and Quintette.

Drainage Treatment

  • Mine water that contains leached acid and metaIs can be treat. in water treatment plants. Lime is used to neutralize the acid and precipitate out the metals. Other chemicals are used to treat drainage with non-acidic metal leaching.
  • Drainage treatment does not prevent ML/ARD. Mine lands that have ML/ARD and treatment plants generally have on-site environmental impacts and the land cannot be used for other purposes. Treatment must go on for as long as the drainage is impacted by leached acid and metals, often for centuries.
  • The treatment process usually produces large amounts of other wastes (called sludge) which have to be stored and managed to make sure they do not release metals in the future.
  • Water treatment is very expensive and requires a lot of resources to maintain. The mine operator must exercise great care and watchfulness, often for hundreds of years.
  • Drainage treatment plants can reliably produce acceptable drainage and can be a very effective Iiabilities,means of protecting the environment. However, due to its high costs and significant environmental it is usually viewed as the mitigation strategy of last resort.
  • Drainage collection and treatment is the most common strategy used for managing ML/ARD at older mines. Water treatment is currently used at the Brenda, Equity Silver, Island Copper, Myra Falls, Nickel Plate, Premier, Samatosum and Sullivan mines.

How is ML/ARD regulated in British Columbia?

  • The challenge faced by the Ministry, of Energy, Mines and Petroleum Resources is how to regulate mining and exploration activities in a way that supports the Provincial goal of sustainable resource development while ensuring the environment is protected, mining lands are reclaimed and the risk and environmental liabilities are minimized.
  • The Provincial government's regulatory approach is guided by several over-riding principles:
    • A mining company has to demonstrate the ability and intent to operate a mine in a way that protects the environment.
    • Every mine and its environment are unique so mines are evaluated on a site-specific basis.
    • If significant disturbance of bedrock is proposed, then a ML/ARD assessment program is required before mine approval. This program involves predicting the potential for ML/ARD from all mine materials, developing prevention/mitigation strategies, and outlining monitoring programs that will be used to assess the performance of proposed ML/ARD management strategies.
    • Where there is a high degree of uncertainty or environmental risk involved, back-up plans am required.
    • The prevention of ML/ARD is preferred. When ML/ARD cannot be prevented, mines are required to reduce the quantity and improve the quality of drainage to levels that protect the environment. An important secondary objective is to minimize the impacts to mining lands and mine site water courses that restrict their future productive use.
    • The Ministry will exercise a cautious approach when the level of information or understanding is deficient.
    • The Provincial goverment requires reasonable assurance that environmental risks will be minimized and that taxpayers are not going to have to pay for the costs of reclaiming mines and managing ML/ARD.
    • The Ministry requires a financial security that covers the cost of reclaiming a mine and any ongoing costs for managing ML/ARD. This security is raised and lowered throughout the life of a mine to correspond to the level of land disturbance and the cost of reclamation and any mitigation.
  • The Ministry uses the Mines Act and the Health Safety and Reclamation code to regulate mining. As well, the Ministry of Energy, Mines & Petroleum Resources and the Ministry of Environment have a joint policy on metal leaching and acid rock drainage at minesites in BC. The Ministry also has a guideline document for assessing and mitigating ML/ARD plus a manual of some ML/ARD prediction techniques.
  • The available scientific tools, combined with a well-informed, cautious approach, should allow mines with the potential for ML/ARD to extract the province's mineral and coal resources, while protecting the environment and minimizing risks.

Additional References

Prepared by: Kim Bellefontaine
Senior Mine Review Geologist
Mining & Minerals Division