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Open-pit mining or opencast mining refers to a method of extracting rock or minerals from the earth by their removal from an open pit or borrow. It is a form of surface mining. The term is used to differentiate this form of mining from extractive methods that require tunneling into the earth, like longwall mining.
Open-pit mines are used when deposits of minerals or rock are found near the surface; that is, where the overburden (surface material) is relatively thin or the area is structurally unsuitable for tunneling (e.g. sand, cinder, and gravel). For minerals that occur deep below the surface—where the overburden is thick—underground mining methods are commonly used.
The process begins with the top layers of the land being removed. After that, layer after layer is taken away until the coal is visible. The coal is taken out, processed, and sold. Taking away layer after layer of land creates a gigantic, open hole or pit. Explosives like dynamite are used when miners want to remove large blocks of materials. Open-pit mines are typically enlarged until either the coal is exhausted, or an increasing ratio of overburden makes further mining uneconomic. When this occurs, the exhausted mines are sometimes converted to landfills for disposal of solid wastes. However, some form of water control is usually required to keep the mine pit from becoming a lake.
Acid mine drainage (AMD) refers to the outflow of acidic water from coal or metal mines, where mining has exposed rocks containing the sulphur-bearing mineral pyrite. Pyrite reacts with air and water to form sulphuric acid and dissolved iron, and as water washes through mines, this compound forms a dilute acid, which can wash into nearby rivers and streams.
When large quantities of rock containing sulphide minerals are excavated from an open pit or opened up in an underground mine, it reacts with water and oxygen to create the sulphuric acid. When the water reaches a certain level of acidity, a naturally occurring type of bacteria called Thiobacillus ferroxidans may kick in, accelerating the oxidation and acidification processes, leaching even more trace heavy metals from the wastes. The acid will leach from the rock as long as its source rock is exposed to air and water and until the sulphides are leached out – a process that can last hundreds, even thousands of years. Acid is carried off the minesite by rainwater or surface drainage and deposited into nearby streams, rivers, lakes and groundwater. AMD can severely degrade water quality, and can kill aquatic life and make water virtually unusable.
Erosion and sedimentation mineral development disturbs soil and rock in the course of constructing and maintaining roads, open pits, and waste impoundments. In the absence of adequate prevention and control strategies, erosion of the exposed earth may carry substantial amounts of sediment into streams, rivers and lakes. Excessive sediment can clog riverbeds and smother watershed vegetation, wildlife habitat and aquatic organisms.
Water pollution problems caused by mining include acid mine drainage, heavy metal contamination, and increased sediment levels in streams. Sources can include active or abandoned open-pit mines, and their associated processing plants, coal sludge and coal waste-disposal areas, and haulage roads. Sediments, typically from increased soil erosion, cause siltation or the smothering of streambeds, affecting fisheries, swimming, domestic water supply, irrigation, and other uses of streams.
- "Open-pit mining" ThinkQuest, accessed March 2011.
- "Coal mining and the environment: Acid Mine Drainage" World Coal Institute, accessed June 2010.
- "Mining and water pollution," Safe Water Drinking Association, accessed March 2011.
- "Mining" Pollution Issues, accessed March 2011.
Related SourceWatch Articles
- Coal mine reclamation
- Health effects of coal
- Longwall mining
- Mine Safety and Health Administration
- Phulbari Coal Project
- The Hidden Costs of Clean Coal: The Environmental and Human Disaster of Longwall Mining, Center for Public Integrity website
- "Protection of Water Resources From Longwall Mining Is Needed in Southwestern Pennsylvania," Schmid & Company Inc., Citizens Coal Council, July 2010
- "Longwall Mining," Energy Information Administration, DOE/EIA-TR-0588, March 1995