Acid mine drainage

Posted on 18 Mar 2022

Acid mine drainage (AMD) is a prominent environmental problem which is associated with the mining industry where impacts are present in groundwater, rivers and dams. AMD occurs when sulphide minerals found in mining waste oxidises (reaction with water and air) resulting in an adverse impact on surface and groundwater sources posing a threat to humans and wildlife. The impact comprises typically of a low pH level, high salinity and elevated concentrations of sulphate content and heavy metals. Once AMD occurs it can be expensive and difficult to mitigate and control.

 

The conventional methods to treat AMD include physiochemical methods which involve an excessive use of chemicals. However, newer technologies that include biological treatment have been viewed as an efficient, cost-effective and eco-friendly alternative. Due to inherent variability between mines and the environmental conditions, a one treatment technique may not be effective and may require a combination of methods. The most common AMD treatment in South Africa is to aerate the water which will, in turn, oxidise the iron followed by added lime in order to increase the pH level. This treatment enables iron (and most heavy metals) to precipitate and settle out. However, generally the water contains a high sulphate content after treatment which exceeds the World Health Organization’s recommended limit. Some other common methods for AMD treatment include:

 

  • Water migration – Restricting water contact with acid-generating waste. This method includes properly designed diversion trenches, grout curtains and slurry walls.
  • Reduction of water inflow – Includes encapsulation, capping and sealing. Caps, seals and covers are utilised to isolate/encapsulate sulphate bearing waste from coming into contact with oxygen or water.
  • Exclusion of oxygen – Limit exposure to oxygen by disposing of mining waste underwater or flooding.
  • pH control – The pH level can be controlled by segregating waste and blending. For example, material containing a high levels of Sulphate or Sulphide can be disposed of in a specifically designed storage area. Waste can also be disposed of with benign material. In addition, lime or other alkaline materials can be added in order to neutralize the acidity.
  • Control of bacterial action – The formation of AMD can be increased by certain common bacteria. In order to prevent AMD, bactericides can be utilised to eliminate these bacteria.

 

The implementation of control measures should be implemented throughout the life cycle of the mine. Monitoring measures must be included to gauge the actual performance against the predicted performance of the control measures. Water quality monitoring should be implemented during all phases including post-closure to ensure that any effects can be detected, and mitigation measures can be implemented.

 

Environmental Assurance (Pty) Ltd. (ENVASS) offers a wide range of water monitoring services, which can be utilised on your next project. If you may require a proposal for water monitoring services please do not hesitate to contact our highly qualified and experienced specialists at ENVASS on 012 460 9768 or info@envass.co.za