In the Appalachian region coal mines, such as in West Virginia, the problem of acidic water is significant. A chemical reaction occurs with water and coal mines. Coal usually has pyrite, commonly called "fool's gold," intermixed with it. According to the United States Geological Survey (USGS), pyrite becomes oxidised, or corroded. This corrosion dissolves in with the water washing over it. The end effect is the water becomes acid. The level of naturally occurring iron eating bacteria also affects how acid the water is. The USGS stated several treatments are used to control acid water.
In the Pinal creek watershed near Globe, Ariz., acidic water is caused from a different source. Copper is mined in open pit mines. Open pit means a huge hole is dug in the ground, big enough for dozers and other earth moving equipment to operate in. Copper is never pure, but has sulfide containing minerals intermixed with it. When exposed to air and water, these minerals turn into sulfuric acid. This acid becomes dissolved in the water drainage. The USGS states the water is treated with calcium carbonate to reduce its acidity.
The Silverton mines in northwestern Colorado are abandoned as of 2011. The primary metals mined were gold, silver, zinc and lead. The problem is iron and aluminum and other trace metals were intermixed with the other metals, and a metal "soup" was left behind when the precious metals ran out. Water reacts with these trace metals and as a result becomes acidic.
The Boulder River mines in southwestern Montana has similar problems to the Silverton mines. Gold, silver, copper, and lead were mined until the 1970s. Peak mining occurred roughly from 1895 to 1907. The problem is back then, pollution was not well understood. as a result, mine waste (called tailings) were simply dumped anywhere. The intermixed metals combined with rain water runoff to form acid water. In 2011, monitoring and cleanup efforts are ongoing.