Berkeley Pit
The Berkeley Pit is a former open pit copper mine in the western United States, located in Butte, Montana. It is one mile (1.6 km) long by one-half mile (800 m) wide, with an approximate depth of 1,780 feet (540 m). It is filled to a depth of about 900 feet (270 m) with water that is heavily acidic (2.5 pH level), about the acidity of Coca-Cola, lemon juice, or gastric acid. As a result, the pit is laden with heavy metals and dangerous chemicals that leach from the rock, including copper, arsenic, cadmium, zinc, and sulfuric acid.
| Silver Bow Creek/Butte Area | |
|---|---|
| Superfund site | |
Berkeley Pit (center) and Yankee Doodle Tailings Pond (upper left) with terraced levels/access roadways. The city of Butte is at lower right. | |
| Geography | |
| City | Butte |
| County | Silver Bow |
| State | Montana |
| Coordinates | 46.02°N 112.51°W |
Butte Location in the United States Butte Location in Montana | |
| Information | |
| CERCLIS ID | MTD980502777 |
| Contaminants | Arsenic, cadmium, copper, zinc, lead |
| Progress | |
| Proposed | 30 December 1982 |
| Listed | 8 September 1983 |
| List of Superfund sites | |
The mine was opened in 1955 and operated by the Anaconda Copper Mining Company, and later by the Atlantic Richfield Company (ARCO), until its closure on Earth Day in 1982. When the pit was closed, the water pumps in the nearby Kelley Mine, 3,800 ft (1,200 m) below the surface, were turned off, and groundwater from the surrounding aquifers began to slowly fill the Berkeley Pit, rising at about the rate of one foot (30 cm) per month. Since its closure, the water level in the pit has risen to within 150 feet (46 m) of the natural water table.
The Berkeley Pit is currently one of the largest Superfund sites. The water, with dissolved oxygen, allows pyrite and sulfide minerals in the ore and wall rocks to decay, releasing acid. The acidic water in the pit carries a heavy load of dissolved heavy metals. A water treatment plant has been operating since October 2019.
The Berkeley Pit is a tourist attraction.