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Abandoned‐mine drainage (AMD) is drainage flowing from or caused by surface mining, deep mining, or coal refuse piles that is typically highly acidic with elevated levels of dissolved metals. AMD results from the interactions of certain sulfide minerals with oxygen, water, and bacteria. Passive treatment systems have been used to remediate AMD at numerous sites throughout the United States. The theory behind passive treatment is to allow naturally occurring chemical, biological, and physical reactions that aid in AMD treatment to occur in the controlled environment of the system, not in the receiving water body. The advantages of passive treatment over active treatment include lower operation and maintenance costs, virtually no use of chemicals, and minimal energy consumption. The disadvantages are that smaller volumes of water are treated than with active systems, and discharges with high concentrations of dissolved metals and extremes of pH may have to be treated several times within one system to treat the discharge. AMD passive treatment systems include aerobic treatment systems and anaerobic treatment systems. It is estimated that it will take 50 years and between $5 billion and $15 billion to remediate all AMD problems in Pennsylvania. © 2001 John Wiley & Sons, Inc.

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