Acid and Metalliferous Drainage

Acid and Metalliferous Drainage (AMD) is formed by the natural oxidation of sulfide minerals, when exposed to air and water. Activities that involve the excavation of rock that contain sulfide minerals, such as mining, accelerate the process. The drainage produced from the oxidation process may be neutral to acidic, with or without dissolved heavy metals, but always containing sulfate. AMD results from a series of reactions and stages that typically proceed from near neutral to more acidic pH conditions. When sufficient base minerals are present to neutralize the AMD, neutral mine drainage or saline drainage may result from the oxidation process. AMD is also known as Acid Mine Drainage or Metal Leaching/Acid Rock Drainage (ML/ARD) and it encompasses contaminant mobility under acidic or neutral/alkaline pH conditions

Stopping AMD formation, once initiated, may be challenging because it is a process that, if unimpeded, will continue (and may accelerate) until one or more of the reactants (sulfide minerals, oxygen, water) is exhausted or excluded from reaction. The AMD formation process can continue to produce impacted drainage for decades or centuries after mining has ceased.

Proper mine characterization, drainage-quality prediction, and mine-waste management can prevent AMD formation in most cases, and minimize AMD formation in all cases. Recognizing the potential of sulfide rock to oxidize, mine operators can isolate these rock types and prevent or minimize the environmental impact of these weathering processes.  To assist mine operators and associated collaborators, INAP developed and maintains a leading practice guide for the prevention and mitigation of AMD (See GARD Guide).