BOX 5.4 Acid Mine Drainage
The United States has a backlog of almost 50 billion tons of old mining and mineral processing wastes (Kleinmann and Hedin, 1990). Therefore it is not surprising that more than 12,000 miles of rivers and streams and 180,000 acres of lakes and reservoirs are adversely affected by mining in the United States (Kleinmann and Hedin, 1990). Acid mine drainage—a fluid generally 20 to 300 times as acidic as acid rain—is responsible for at least a third of this ecological damage (Kleinmann and Hedin, 1990).
In sufficient concentrations, acid mine drainage (AMD) coats stream bottoms with a rust-colored iron precipitate, adds enough sulfuric acid to acidify the water, and kills aquatic life (Kleinmann and Hedin, 1990). Formed from the oxidation of iron pyrite, AMD is associated with coal mining in the eastern United States and with metal mining in the West.
In the past 20 years, the number of rivers and streams adversely affected by AMD has reportedly dropped by about a third, primarily due to perpetual chemical neutralization of mine water before discharge (an expensive process) and by reclamation of abandoned mines (Kleinmann and Hedin, 1990). Some of the improvement, however, has come from natural amelioration by gradual oxidation of the iron pyrite and some by intentional flooding of deep mines to prevent the pyrite from oxidizing.
Still other improvements have been gained by construction of cattail wetlands to purify mine wastewater, usually by bacterial action; more than 400 such wetlands have been constructed in recent years (Kleinmann and Hedin, 1990). Anionic surfactants are also used to inhibit iron-oxidizing bacteria in mine waste piles. Another technique to control AMD caused by fractured streambeds that leak into underground mines is to seal the streambeds by injecting them with polyurethane grout beneath the sediment-water interface to minimize pyrite-water contact.