The Berkeley Pit, located in Butte, Montana, is a water-filled former open-pit copper mine. This excavation, over a mile long and a half-mile wide, holds one of the largest volumes of hazardous water in the United States. Its water is a highly concentrated, toxic mixture of sulfuric acid and heavy metals, resulting from over a century of mining activity. The pit’s danger stems from the specific geological and chemical processes unleashed when mining ceased.
The Geological Foundation and Mining History
The Butte mining district earned the nickname “The Richest Hill on Earth” due to its deposits of copper and other metals. The ore body, the focus of intense mining operations, was naturally rich in sulfide minerals. These minerals, primarily iron pyrite, were locked deep within the earth’s crust, isolated from oxygen for millions of years.
The extraction of copper, which began in the Berkeley Pit in 1955, involved removing large amounts of rock. This process of blasting, digging, and crushing created the perfect conditions for a toxic reaction. It exposed the sulfur-bearing rock to the air and water, setting the stage for the pit’s toxicity. Mining operations ceased in 1982, and the dewatering pumps were shut off, allowing the pit to begin filling with groundwater and surface runoff.
The Chemical Engine: Acid Mine Drainage
The pit’s extreme toxicity is driven by Acid Mine Drainage (AMD). This continuous chemical process begins when exposed iron pyrite reacts with oxygen and water. When these elements combine, the reaction generates sulfuric acid.
The sulfuric acid acts as a powerful solvent, dissolving heavy metals naturally present in the surrounding rock. Metals like copper, arsenic, cadmium, and zinc are chemically stripped out and dissolved into the water. The continuous influx of groundwater ensures the reaction never stops, constantly generating fresh acid and leaching more metals.
The result is water with an extremely low pH level, historically measured as low as 2.5, comparable to battery acid. This profound acidity characterizes the water as highly corrosive and toxic. The low pH keeps the dissolved heavy metals in solution, creating a toxic chemical soup that accumulates in the pit.
The Toxic Inventory and Environmental Impact
The water in the Berkeley Pit is a potent cocktail of contaminants, making it immediately dangerous to life. Specific heavy metals found in high concentrations include copper, arsenic, cadmium, and zinc. For perspective, the concentration of cadmium has been measured at levels exceeding 2,000 parts per billion, far surpassing the safe drinking water standard of five parts per billion.
This high metal concentration, combined with the extreme acidity, poses a severe localized environmental threat. The most visible impact has been the death of migratory waterfowl, such as snow geese, that land on the water. Necropsies on deceased birds have shown severe internal burns and sores from ingesting or being exposed to the acidic, metal-laden water.
Managing the Rising Threat
Since mining stopped in 1982, the Berkeley Pit has been steadily filling with groundwater and precipitation. This rising water level represents the primary ongoing environmental challenge. A threshold, known as the Protective Water Level, exists where the toxic water would begin to contaminate the surrounding shallow groundwater and the nearby Clark Fork River system.
To prevent this ecological disaster, an extensive mitigation effort is underway. The central strategy involves operating a water treatment plant, which pumps water out of the pit for processing. This plant uses lime treatment, which raises the pH of the acidic water by adding a neutralizing agent.
The increase in pH causes the dissolved heavy metals to precipitate, or solidify, out of the water, forming a metal-rich sludge. The treated water, which meets discharge standards, is then safely released into local waterways, while the sludge is managed separately. This continuous pumping and treatment process is designed to keep the water level permanently below the threshold, ensuring the toxic inventory remains contained.