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What is acid rock drainage? Image: G. Plumlee, USGS Metals-7-1 Rocks that contain useful metals commonly contain sulfide minerals minerals that combine metal with sulfur. Image: Travis Hudson Sulfide minerals in a rock sample Metals-7-2 The most common sulfide mineral is pyrite - or “Fool’s Gold” - a combination of iron and sulfur. Image: R. Busch Metals-7-3 Pyrite occurs widely in rocks including disseminations through this entire mountain. Image: G. Plumlee, Metals-7-4 When pyrite comes in contact with oxygen in air or water it “oxidizes” or turns into rusty iron oxides and sulfuric acid. Certain bacteria make this process go faster. Oxygen Pyrite Bacteria Sulfuric Acid Images: USGS, DeAtley Design Metals-7-5 The sulfuric acid produced by oxidation of pyrite makes nearby soil and water acidic this is acid rock drainage. Acid rock drainage can harm habitat. Image: Travis Hudson Metals-7-6 Acid rock drainage occurs naturally and is an environmental concern where sulfide minerals are mined. There are several ways to control or prevent acid rock drainage at mines. Image: Travis Hudson Metals-7-7 Controlling Acid Rock Drainage Storing waste rock underwater: Storing wastes underwater reduces the rate of sulphide mineral oxidation by air to almost zero, effectively stopping ARD formation. Underwater storage is considered to be the most reliable and effective ARD prevention strategy. Flooding and sealing underground mines: The flooding of underground tunnels is used where it is possible to isolate the mine by sealing all the entrances. Sealing the mine prevents water moving in and out, which could result in surface water or groundwater contamination. Mixing acid-producing materials with acid-buffering materials: Combining sulphide wastes with limestone or calcite can result in ARD neutralization on site. Controlling Acid Rock Drainage Covering waste rock: Installing a cover of clay, plastic, or soil over piles of waste rock prevents rain and other precipitation from contributing to ARD formation and transport, and reduces the amount of oxygen available to react with the sulphide minerals. Chemical treatment of sulphide wastes: Organic chemicals designed to kill sulphide-oxidizing bacteria have been applied to sulphide wastes in order to slow the rate of ARD. However, there is concern that some of these chemicals may kill beneficial microorganisms in the environment, thus becoming pollutants themselves.See more at: http://www.miningfacts.org/Environment/Whatis-acid-rock-drainage/#sthash.aQbelzxj.dpuf