Montana Earth Science Picture of the Week

At one point in the late 1800s 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."

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.

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 . . . Groundwater will continue to flow into the pit until the level of water in the pit reaches the natural water table. The pit will not overflow. However, when the water reaches this level, toxic water from the pit will begin to seep into another aquifer located near the surface. To prevent this from happening, water will be pumped from the pit and treated for future industrial use. A treatment plant is currently under construction. This treatment will have to be done continuously (forever) to prevent the pit water from reaching a "critical level". Millions of dollars will be spent to insure that the groundwater flow is always toward the pit. . . not away from it.

Terms: water table, aquifer