Globally, mining contributes to erosion, undermining, deforestation, loss of biodiversity, significant use of water resources, dammed rivers and standing water, sewage disposal problems, acid mine drainage and contamination of soil, ground and surface water, all of which can lead to local health problems. The biggest physical disturbances in a mine are the mine workings themselves, such as open pits and associated waste rock disposal areas. Mine facilities such as offices, shops and mills, which occupy a small part of the disturbed area, are usually reclaimed or demolished when the mine closes. Open pits and waste rock disposal areas are the main visual and aesthetic impacts of mining.
Underground mining often results in relatively small waste rock disposal areas, ranging from a few acres to tens of acres (0.1 km). These areas are usually located near underground mine openings. Open-pit mining disturbs larger areas than underground mining and therefore has greater visual and physical impacts. As the amount of waste rock in open-pit mines is often two to three times greater than the amount of ore produced, huge volumes of waste rock are removed from shafts and deposited in nearby areas.
Processing waste heaps, such as tailings impoundments, leach heaps and slag heaps, vary in size but can be very large. The impoundments associated with some of the largest mills, such as open-pit copper mines, can cover thousands of acres (tens of kilometres) and be several hundred feet (about 100 m) thick. Leach heaps can cover tens to hundreds of acres (0.1 to 1 km) and be a few hundred feet (about 100 m) high. They resemble waste rock heaps in location and size, but are more precisely engineered.
Slag is a vitreous by-product of smelting; slag piles can cover tens to hundreds of acres (0.1 to 1 km) and be more than 100 feet (30 m) high. In most places, concentrations of copper, gold and other elements are too low to be extracted profitably. But in some places they are found in seams of highly concentrated, mineable minerals called ores. The economically viable concentration of an ore depends largely on its market price.
Gold ore can be viable at concentrations as low as 0.0001%, while copper becomes uneconomic below 0.5%.