Over 31 years ago economic factors led the Atlantic-Richfield Corporation, or ARCO, now a subsidiary of British Petroleum, to cease mining operations at the Berkeley Pit in Butte, Montana. Underground mining had come to an end seven years earlier, but the underground pumps had continued to operate, pumping groundwater out from the mines and the Berkeley Pit.
The 1982 suspension of mining coincided with the stoppage of pumping, allowing groundwater to begin rising in the underground mines and eventually into the Berkeley Pit.
With ARCO’s suspension of mining in the neighboring East Berkeley Pit (now known as the Continental Pit) on July 1, 1983, the future of mining on the Butte Hill was uncertain at best.
Soon after, the Berkeley Pit was classified as a federal Superfund site by the United States Environmental Protection Agency (EPA). According to the EPA, a Superfund site is an uncontrolled or abandoned place where hazardous waste is located, possibly affecting local ecosystems or people.
The end of mining at the Berkeley also marked the beginning of the Berkeley Pit lake we see today. 3,900 feet deep underground in the Kelley Mine , the pumps used to dewater the underground mines and the Berkeley Pit ran until April 23, 1982. Without pumping, the Berkeley Pit began to fill with water flowing in from both surface runoff and groundwater. Due to the natural geochemistry of the area and mining activities, the water is highly acidic and contains high concentrations of dissolved heavy metals.
By 1985, ARCO had sold a portion of its holdings to Montana businessman Dennis Washington. Mining operations in the Continental Pit, as well as heap leaching of old Berkeley Pit leach pads, were resumed by his new company, Montana Resources.
Yes. One involves changes in bedrock Well H. The water level of this well is usually about 10 feet above the level of the Berkeley Pit, just like other bedrock wells nearby. But one July, Well H’s level started falling, and by September it had dropped about 3 ½ feet. Meanwhile, the Pit kept rising. From December through February, the water levels of the Pit and Well H were just inches apart. Well H is now gaining on the Pit again.
What is the significance of the surprise reading in Well H? It appears to be an isolated incident caused by underground subsidence (shifting of dirt) adjacent to the well. Well H is southeast of the Pit in the middle of the old Pittsmont Mine workings, where the ground is known to be unstable. The water levels in all of the surrounding bedrock wells remain high, indicating that flow is still toward the Pit. It is important to look at the monitoring well system as a whole, rather than focus solely on the performance of a single well.
And Well H is not the only surprise to date. In 1989, the water level in the Kelley shaft dropped two feet in one month, but it, too, recovered. Well DDH-5 also occasionally fluctuates. The monitoring program was set up specifically to detect changes like these. When something unusual turns up, monitoring is heightened at that spot, and scientists determine what action, if any, needs to be taken. At Well H, water levels have been checked weekly since October. On February 18, a special camera was lowered 927 feet into the well to look for abnormalities, and none were found.
Future years will bring many more monitoring well changes, especially in the area between the Berkeley and Continental Pits.
No. Pumping and treating of Pit water must begin when the 5,410-foot level is reached at ANY of the water-level monitoring compliance sites on the Hill, in what is called the East Camp. These sites include the Anselmo, Granite Mountain, Kelley, Belmont, and Steward mine shafts, as well as several other monitoring wells established to the south and the east of the Pit.
Presently, the water level at these sites is about 20-40 feet higher than the level in the Pit. So, the 5,410-foot level should be reached sooner at these sites than in the Pit, which means water pumping and treating would begin that much sooner.
Tsunamis, volcanoes and earthquakes in recent months have created an increased interest in seismic activity. Many readers have written, called, or stopped by questioning what will happen to the Berkeley Pit if an earthquake occurs in Butte, Montana. To help answer these questions, local experts were asked to explain the likelihood of an earthquake and what effect it would have on the Berkeley Pit.
Probability of an Earthquake in Butte
Mike Stickney, Director of the Earthquake Studies Office at the Montana Bureau of Mines and Geology says that Butte is not likely to suffer a severe earthquake anytime soon. Large earthquakes are certainly possible in western Montana as demonstrated by the 1959 Hebgen Lake earthquake (magnitude 7.3), but are most likely to occur in the more seismically active regions located to the north, east, or southeast of Butte. The state of Montana is unlikely to experience earthquakes larger than the 1959 earthquake because the faults are not large enough to produce earthquakes greater than magnitude 7.5.
Stickney also explained that Butte has been monitored closely for seismic activity over the past 25 years. There has never been any significant seismic activity recorded that suggests the nearby faults to be active enough to cause a large earthquake. Most seismic activity that registers in the Butte area is caused by blasting at the open mine site, and very minor underground subsidence, especially near the old block caving zones under the Kelley Mine.
Effects of an Earthquake
Even assuming a worst-case earthquake scenario, the Berkeley Pit would not overflow. Experts suggest that there would be far more damage to buildings and other structures in Uptown Butte than would be caused by adverse impacts from the waters in the Berkeley Pit.
Studies show that the Yankee Doodle Tailings Pond dam would withstand at least a 6.5 magnitude quake. It can also be assumed from similar studies that such a quake could cause some sloughing on the pit walls, but the resulting movements would not discharge enough rock and materials to cause the water in the pit to overflow.
Sloughing and Landslides
Although earthquakes are not likely to be a problem, landslides and sloughing of the Pit could occur. The majority of the Berkeley Pit walls are made of “solid” bedrock. However, the southeast wall is composed of “loose”silts, sands and gravels, and this is the area where sloughing is most likely to occur, with or without a major earthquake.
In September 1998, about 1.3 million cubic yards of “loose” alluvium on the southeast wall sloughed into the Pit. This event caused a 3-foot rise in the water level and surface waves greater than 20 feet.
The water rise associated with any pit wall sloughing would ultimately depend on the volume of material that breaks free and displaces the water. But it should be noted there is enough space for more significant events. For example, there is more than 150 feet between the current Pit water level (5,252′ above sea level) and the Critical Water Level (5,410′), and there is another 100′ feet up to the rim of the Pit.
If an earthquake were to occur, the effects of seismic activity at the Berkeley Pit would be the least of Butte’s worries. Since a large earthquake is not likely anytime soon, and because landslides are relatively manageable, the public should not be overly concerned. There will probably continue to be some sloughing on the benches and old roads, but not enough to cause the Pit water to rise more than a few feet.