What is Acid Mine Drainage & How to Identify It
Acid mine drainage forms when sulfide minerals are exposed to oxidizing conditions in coal and metal mining, highway construction and other large-scale excavations. There are many types of sulfide minerals. Iron sulfides common in coal regions are predominately pyrite and marcasite (FeS2), but other metals may be complexed with sulfides like chalcopyrite (CuFeS2), covellite (CuS), galena (PbS) and sphalerite (ZnS). Pyrite commonly occurs with these other metal sulfides thereby causing AMD where copper, lead and zinc are mined.
Upon exposure to water and oxygen, sulfide minerals oxidize to form acidic, sulfate-rich drainage. Metal composition and concentrations in AMD depend on the type and quantity of sulfide minerals present. The drainage quality emanating 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 produce acidic drainage. In contrast, alkaline-rich materials, even with significant sulfide concentrations, often produce alkaline conditions in water.
Acidity in AMD is comprised of mineral acidity (iron, aluminum, manganese and other metals depending on the specific metal sulfide) and hydrogen ion acidity. Approximately 20,000 kilometers (or roughly 12,400 miles) 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, no treatment of the AMD occurs and continual contamination of surface and groundwater resources results.