State of Environment Reporting

Metal Leaching and Acid Rock Drainage

Metal Leaching/Acid Rock Drainage Mine Sites Requiring Mitigation or Further Assessment

SOURCE: British Columbia Ministry of Energy and Mines, Mines Branch, 2002.

Metal Leaching (ML) and Acid Rock Drainage (ARD)

• Metal leaching (ML) and acid rock drainage (ARD) are caused when sulphide minerals are weathered and exposured to air and water.
  
  
• Sulphide weathering produces acidic compounds that can become dissolved in water if they are not neutralized by other minerals on the site.
  
  
• Many metals become highly soluble under acidic conditions; significant metal leaching can also happen in neutral or alkaline drainage conditions.
  
  
• ML/ARD is a concern at most metal mines and some coal mines, where there are high concentrations of the sulphide minerals and other trace metals. This is because mining greatly increases the amount of rock surfaces exposed to the weathering processes that result in ML/ARD.
  
  
• There are more than 60 mines in British Columbia with the potential to generate sufficient ML/ARD to significantly affect the receiving environment. Most of these are no longer in production.
  
  
• Once conditions conducive to ML/ARD have been established, significant impacts can persist for hundreds of years and mitigation efforts can be very expensive. Therefore, most ML/ARD mitigation measures need to be designed, constructed and operated in a manner that allows them to perform indefinitely.

Why is it important?

• Dissolved metals such as copper, zinc and cadmium can be toxic to fish and animals and can adversely affect ecosystem health. Metals can also be absorbed and accumulate in plant
  and animal tissue.
  
  
• ML/ARD mitigation can be very expensive with capital costs of over $10 million and operating costs up to $1.5 million per year at a single site.
  
  
• A number of historic mines (e.g., Britannia and
  Mt. Washington), which closed prior to the enactment of modern day ML/ARD regulations, have caused significant environmental impacts due to unmitigated ML/ARD.
  

What is being done?

• In British Columbia, regulations for managing mine wastes are included in the Waste Management Act, Mines Act and federal
  Fisheries Act.
  
  
• Under the Mines Act, metal and coal mines are required to predict the ML/ARD potential of all the wastes produced. Where mitigation is necessary, mines must provide reasonable assurance of environmental protection and their ability to cover future mitigation costs.
  
  
• Under the Waste Management Act, the Province can set conditions for discharges to the environment and for pollution abatement. It also authorizes the Province to include previous owners of mines in a cleanup order and for pollution abatement and prevention.
  
  
• Many mines with the potential for significant ML/ARD, flood mine wastes to limit oxidation or apply compacted soil covers to reduce leaching.
  
  
• Large-scale drainage collection and chemical treatment can be very effective in removing metals from drainage. However, due to on-site impacts and high costs it tends to be a mitigation strategy of last resort.
  
  
• There are a large number of processes that control ML/ARD and many of these continue to change and evolve over time. Even where mitigation measures have been implemented, the full extent and potential cost of ML/ARD mitigation may not be known. Therefore, conservative design criteria, detailed monitoring, regular maintenance, ongoing review and adaptive management are key components of successful ML/ARD mitigation.