The Berkeley Pit in Butte, Montana
The Berkeley Pit was an open pit for copper mining located in Butte, Montana. It has a length of 1,600 meters and width of 800 meters with an estimated depth of 540 meters. The water in Berkeley Pit has an approximate depth of 270 meters and is highly acidic with a pH level of 2.5, about the same acid strength as lemon juice. The pit is contaminated with heavy metals and toxic chemicals including copper, arsenic, cadmium, zinc, and sulfuric acid.
The Anaconda Copper Mining Company opened the mine in 1955 until it was closed in 1982 on Earth Day. When the pit was closed, the adjacent water pumps in the Kelley Mine were turned off, and the Berkeley Pit was slowly getting filled with groundwater from the surrounding aquifers with a rate of about 1 feet per month. The water level in the pit has risen to about 46 meters of the natural water table since its closure.
The Berkeley Pit present a significant environmental problem because the water has dissolved oxygen allowing pyrite and sulfide minerals in the ore and wall rocks to decay making the water in the pit more acidic. If the pit water level reaches the natural water table, it will reverse flow back into surrounding groundwater and will pollute the nearby Silver Bow Creek which has a connection to the Columbia River. The Berkeley Pit’s water contains so much heavy metal that at one point, it is possible to mine copper directly from the water.
Around the 1990s, the first solution for the groundwater problem was planned. The Horseshoe Bend Water Treatment Plant diverted the water flowing into the pit to decrease the rate of rising water level. Water treatment facility was constructed in September 2018 to avoid contaminating the groundwater of Butte when the pit’s water level rises. The Berkeley Pit is considered to be one of the expensive project site.
Despite the environmental issue, the Berkeley Pit is currently a tourist spot with a nearby gift shop, and it charges a small fee to see the pit on a viewing platform.
Related Topic: Butte, Montana
Images property of Chromoscience.
Edwin Dobb. “New Life in a Death Trap”. Discover, 2000.
Influence of copper recovery on the water quality of the acidic Berkeley Pit lake, Montana, U.S.A
The Berkeley Pit lake in Butte, Montana, formed by flooding of an open-pit copper mine, is one of the world’s largest accumulations of acidic, metal-rich water. Between 2003 and 2012, approximately 2 × 10(11) L of pit water, representing 1.3 lake volumes, were pumped from the bottom of the lake to a copper recovery plant, where dissolved Cu(2+) was precipitated on scrap iron, releasing Fe(2+) back to solution and thence back to the pit. Artificial mixing caused by this continuous pumping changed the lake from a meromictic to holomictic state, induced oxidation of dissolved Fe(2+), and caused subsequent precipitation of more than 2 × 10(8) kg of secondary ferric compounds, mainly schwertmannite and jarosite, which settled to the bottom of the lake. A large mass of As, P, and sulfate was also lost from solution. These unforeseen changes in chemistry resulted in a roughly 25-30% reduction in the lake’s calculated and measured total acidity, which represents a significant potential savings in the cost of lime treatment, which is not expected to commence until 2023. Future monitoring is needed to verify that schwertmannite and jarosite in the pit sediment do not convert to goethite, a process which would release stored acidity back to the water column.