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The Science of Acid Mine Drainage

What is Acid Mine Drainage (AMD)?
AMD is a term used to describe drainage flowing from or caused by surface mining, deep mining or coal refuse piles that are typically highly acidic with elevated levels of dissolved metals.

Formation of acid mine drainage
Acid mine drainage (AMD) forms when sulfide minerals in rocks are exposed to oxidizing conditions in coal and metal mining, highway construction, and other large-scale excavations. There are many types of sulfide minerals, but the iron sulfides, pyrite and maracasite (FeS2), common in coal regions are the predominant AMD producers. Upon exposure to water and oxygen, pyritic minerals oxidize to form acidic, iron and sulfate-rich drainage.

The drainage quality emanating either from underground mines or backfills of surface mines is dependent on the acid-producing (sulfide) and alkaline (carbonate) minerals contained in the disturbed rock. In general, sulfide-rich and carbonate-poor materials are expected to produce acidic drainage. In contrast, alkaline-rich materials, even with significant sulfide concentrations, often produce alkaline conditions in waterw.

Acidity in AMD is comprised of mineral acidity (iron, aluminum, manganese, and other metals depending on the specific geologic setting and metal sulfide) and hydrogen ion acidity. Approximately 20,000 km of streams and rivers in the United States are degraded by AMD. About 90% of the AMD reaching streams originates in abandoned surface and deep mines. Since no company or individual claims responsibility for reclaiming abandoned mine lands (AML), no treatment of the AMD occurs and continual contamination of surface and groundwater resources results.

Problems associated with acid mine drainage

  • Contamination of drinking water
  • Harmful affects on aquatic plants and animals
  • Corrosion of infrastructure such as bridges, monuments, and buildings

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