Saturday, May 4, 2013

Toxic Berkeley pit

Presently known as one of the most toxic man-made lakes on earth, the Berkeley pit in Butte, Montana used to be one of the largest open pit copper mines. Several underground mines were combined to form the open pit in 1955 by Anaconda Copper (the company also owned the largest copper smelter in the world in the nearby town of Anaconda). Imagine that under the city of Butte around 78 km of vertical and 9000!! km of horizontal underground shafts are located. (See: These mines produced not only 9.6 million tons of copper, but also 2.1 million tons of lead, 22,000 tons of silver and 90 tons of gold between 1880 and 2006. Since Butte lies in the headwaters of the Clark Fork River, the groundwater level had to be lowered as the... underground mining went deeper.

The open-pit mine was in operation until 1982 when the dewatering pumps were turned off and the pit filled with water. Well, you can almost not call it water since it is so saturated with copper that miners are able to mine copper directly from the water. The acidic water does not only contain high concentrations of copper but also sulfuric acid, iron, cadmium, arsenic and zinc. With a pH between 2.5-3.0 it is as acidic as vinegar. If you would drink a bottle of the water it would form a corrosion layer in your digestive system meaning basically that you die of rust.

The pit is 540m deep and filled with acidic water to a depth of 270 meters. It is thus remarkable that bacteria, fungi and algae manage to survive in the lake. More than 100 types of microbes were discovered in the lake by Don and Andrea Stierle. It is natural selection at its height, since some of these organisms are solely found in the lake.

A flock of 345 snow geese landed in the lake in 1995 and died. The custodian of the pit first denied this was due to the toxicity of the lake and claimed it was due to a ‘grain infection’. However, the state of Montana revealed that the insides of the geese were filled with burns and sores caused by high concentrations of arsenic, copper and cadmium. Nowadays gunshots and loud speakers are supposed to keep the birds away from the lake.

In 2003 a water treatment plant was built near the northeast rim of the pit to prevent water levels from rising and contaminating the groundwater. Drinkwater is being pumped from high land reservoirs, since historic mining has already contaminated the valley aquifer. For now the Berkeley pit remains the only toxic lake in the world you have to pay (2 dollars) to visit.

Image: Courtesy of Barbara Oosterwijk. The Berkeley pit as seen from the viewing platform. If you look close you can see the reddish color of the water.


Dobb, E. 2000. New life in a death trap. Discover

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