#31 - Where did water in the Berkely Pit come from?

Photo credit: KRTV

Copper boom . . .
At one point in the late 1890s approximately one fourth of the world's copper was coming out of the underground mines beneath Butte (The Butte Hill). Prior to WWII, Butte as populous as modern-day Billings (about 100,000 people) with over 15,000 men working the underground mines to supply the copper that "electrified the nation."

Right: Photo of Berkeley Pit from when it was being mined. Credit: Pitwatch.org, Butte-Silver Bow Public Archives, Smithers Collection

Richest Hill on Earth . . .
To facilitate the underground mining, groundwater seeping into the shafts and drifts had to be pumped out. The pumping kept the shafts and drifts from flooding by lowering the water table in the shape of a funnel around the underground workings. In 1955 the Anaconda Copper Mining Company began phasing out the underground mining, and started open-pit mining. The "Richest Hill on Earth" became the Berkeley Pit. Pumping of the groundwater was continued to keep the floor of the pit dry. When the Anaconda Company stopped mining the Berkeley Pit in 1982, the pumps were shut off. Since then groundwater around has been flowing toward the pit and the water table in the pit area has been rising toward its natural pre-mining level. During the first year it rose 460 feet, but then slowed considerably. In the early 1990s it was rising 26 feet per year.

A groundwater mess . . .
The problem with the groundwater in the pit area is that it becomes very acidic as it flows toward the pit through fractures, shafts, and drifts. The openings created by mining provide an opportunity for air and water to react with sulfide minerals found in the rock. This causes the water to become acidic (sulfuric acid). As this "acid mine drainage" makes its way toward the pit, it is able to dissolve metals still contained in the rock. So, not only is water in the pit very acidic (pH of 2.5), but it also contains toxic levels of dissolved metals.

Future of the Berkeley Pit . . .
If allowd to do so, groundwater would continue to flow into the pit until the level of water in the pit reaches the natural water table. The pit will never overflow, however if the pit water were allowed to reach the protective water level (see diagram below), toxic water from the pit would begin to seep into another aquifer located near the surface. To prevent this from happening, water is pumped from the pit, run through a treatment plant, and then piped over for use at a nearby active mine. This treatment will have to be done continuously (forever) to prevent the pit water from reaching the protective water level. Millions of dollars have been spent to insure that the groundwater flow is always toward the pit - not away from it.

Terms: water table, aquifer

For much more about the Berkeley pit, go to Pitwatch.org.

Below: This illustration depicts the bedrock aquifer and alluvial aquifers in Butte. The Berkeley Pit is within the bedrock aquifer and captures and contains acid mine drainage from the mine shafts and tunnels located in the bedrock aquifer. Area surface waters pass through the alluvial aquifer and are separate from contaminated groundwater so long as the water in the Berkeley Pit remains below the protective water level. Rendering courtesy of Clark Fork Watershed Education Program for Pitwatch.org.