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Two aquifers feed into the Pit

Water from two different underground areas, or aquifers, affects the Berkeley Pit. The illustration above illustrates the difference between these aquifers, the alluvium and bedrock.
Water from two different underground areas, or aquifers, affects the Berkeley Pit. The illustration above illustrates the difference between these aquifers, the alluvium and bedrock.

Aquifers are places where water is found in permeable rocks and soils underground. The area around the Berkeley Pit contains two main underground aquifers – the alluvial aquifer and the bedrock aquifer. The alluvial aquifer is closer to the surface. Water flows freely through the layer of ground called the alluvium, a porous mixture of sands, gravels, and clays. Near the east wall of the Pit, the alluvium is saturated with water from this aquifer.

This image from the Montana Bureau of Mines & Geology illustrates the connections between historic underground mining tunnels and the Berkeley Pit. After groundwater pumping ceased in 1982, the tunnels, and eventually the Pit, began to fill with water.
This image from the Montana Bureau of Mines & Geology illustrates the connections between historic underground mining tunnels and the Berkeley Pit. After groundwater pumping ceased in 1982, the tunnels, and eventually the Pit, began to fill with water.

The bedrock aquifer runs deep below the ground. It is a “confined aquifer” trapped within fractured bedrock which water cannot easily pass through. In areas adjacent to historic mining activities, this aquifer was dewatered by large pumps located underground to allow for underground mining. Up to 5,000 gallons of water per minute were pumped from the underground mines to allow for mining, including the Berkeley Pit.

The large stainless steel pumps located underground in the Kelley Mine were turned off in 1982, and since that time the dewatered area has been filling back up.

These two aquifers are independent systems, separated by a thick layer of clay-rich weathered bedrock that hinders water from the alluvial aquifer from seeping down into the bedrock aquifer. Instead, water from both aquifers is flowing toward the Pit because it is the lowest spot in the area.

The workings of a typical monitoring well in the Berkeley Pit system are shown in the illustration above.
The workings of a typical monitoring well in the Berkeley Pit system are shown in the illustration above. Click on the image to view a larger version.

Monitoring wells installed throughout the area are used to closely track the water levels and the water quality of both aquifers. Since monitoring began, the alluvial aquifer has remained fairly constant, fluctuating only a few feet here and there depending on seasonal precipitation.

In contrast, the water levels of the bedrock aquifer in areas of historic dewatering have been steadily rising to pre-mining levels. Water levels in the bedrock system have risen hundreds of feet and show minimal seasonal trends. The monitoring wells also allow scientists to measure the pressure differential between the two aquifers, expressed in pounds per square inch, or psi.

Have the monitoring efforts revealed any surprises?

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.

Butte, Montana, mine flooding west camp wells, shafts and area of 1960s flooding. The west camp groundwater system is monitored and maintained separately from the Berkeley Pit and connected east camp mines.

Learning About The Westcamp

Butte, Montana, mine flooding west camp wells, shafts and area of 1960s flooding. The west camp groundwater system is monitored and maintained separately from the Berkeley Pit and connected east camp mines.
Butte, Montana, mine flooding west camp wells, shafts and area of 1960s flooding. The west camp groundwater system is monitored and maintained separately from the Berkeley Pit and connected east camp mines. Click on the image to view a larger version.


PitWatch Issue Volume 3, Number 2

Previous issues of PITWATCH have been devoted almost exclusively to the Berkeley Pit and surrounding “East Camp” underground wells and mine workings. Another area of the underground water system called West Camp also deserves our attention.

The West Camp lies southwest of the Berkeley Pit/East Camp drainage and includes the Travona, Emma, and Ophir mine workings. Like in the East Camp, the groundwater in this area has been closely monitored since 1982 to make sure the water does not rise above a certain level—in this case 5,435 feet. Since November 1989, pumping operations have kept the water below this level.

In the late 1950s, the West Camp mine workings were sealed off from the rest of the shafts and drifts on the Butte Hill by a series of barriers, or bulkheads—some made of wood, some cement. Three main cement bulkheads block the connections between the Emma and Original mines at the 1,600-foot level and the Emma and Colorado mines at the 1,400- and 1,000-foot levels.

Anaconda Company crews installed the bulkheads for two main reasons: 1) they were finished mining in the West Camp and 2) they wanted to increase the efficiency of continued operations in the other mines and the Berkeley Pit. The bulkheads allowed them to eventually reduce both the volume of groundwater pumped and the area underground that required fresh air. However, even after the bulkheads were installed, they continued to pump water out of the Emma shaft until 1965.

Over the years, leakage has occurred through the bulkheads, but according to monitoring data, it appears that the West Camp water system remains mostly independent. The groundwater levels in its shafts are several hundred feet higher than those in the other mine workings, indicating that the bulkheads still separate the two areas.

After studying the West Camp in the late 1980s, the U.S. Environmental Protection Agency (EPA) ruled that the water in the Travona shaft could rise to an elevation of 5,435 feet without threatening human health or the environment. However, if the water were to rise above this level, EPA believes it could eventually flow untreated into Silver Bow Creek, and ARCO would face daily fines starting at $5,000 and increasing to $10,000 after 10 days.

To ensure that the West Camp water stays below 5,435, groundwater is pumped from the Travona shaft into a county sewer line and on to the Metro Sewer plant. ARCO pays Butte-Silver Bow about $30,000 a month to treat this water, depending on the volume received. Treatment mainly involves reducing the water’s arsenic content.

Recently, ARCO installed a larger main pump south of the Travona near Centennial Avenue. It can handle 100 more gallons per minute than the current pump (330 compared to 230), and it should go on line sometime this fall. The Travona pump will then become the back-up, used only when needed.

The old pump is due for some downtime, as it has been working at near full capacity for more than a year to keep up with rising water. For example, in September 1997, the Travona water hit 5,432 feet—just 3 feet below the critical mark. The current level, last measured on September 30, is 5,422 feet.

The Berkeley Pit in 1982. The water seen here is surface runoff flowing into the Leonard mine shaft to the right at the Pit bottom.

Active Mining in Butte and the Berkeley Pit


PitWatch Issue Volume 3, Number 1 (1998)

We can explain some, but not all, aspects of the relationship between the Berkeley Pit and Montana Resources (MR), the corporation actively mining here in Butte. As you may recall, last year MR and ARCO hammered out an agreement on Berkeley Pit clean-up responsibilities, but the details were not made public. Here’s what we do know:

The Present

Horseshoe Bend. For the past two years, MR and ARCO have been diverting and treating the stream of water that once flowed directly into the Berkeley Pit from the northeast. The Horseshoe Bend diversion project has reduced by about half the amount of water entering the Pit. MR reuses this water in its concentrating operations. MR and ARCO share the project’s annual operating cost of about $2 million. Around $1.6 million of that goes toward buying and transporting lime, which is used to treat the water.
Continental Pit Dewatering. From March through October, MR pumps groundwater out of the south end of the Continental Pit to keep the area dry for future mining. Starting this summer, the company may begin pumping additional water out of the north end of the pit, with all water routed to the MR concentrator. This pumping diverts water that could otherwise flow into the Berkeley Pit from the east.

The Future

Monitoring Program Expansion. Over time, the monitoring well system will be expanded south and east to cover the entire Berkeley Pit/Continental Pit Complex, rather than just the area around the Berkeley Pit. This expansion will be necessary because groundwater pumping at the Continental Pit will eventually take place at an elevation lower than the water level of the Berkeley Pit. Pumping now occurs about 90 feet above the Berkeley Pit water level, but over the next 20 years, MR plans to expand the Continental Pit eastward and southward and mine it down to 4,986 feet—166 feet below the current Berkeley Pit water level (all USGS elevations).

When this new low spot is created, some water that would otherwise flow west toward the Berkeley may instead start flowing east toward the Continental. Water levels in the monitoring wells between the two pits (such as Well H) will be affected by this change, although exactly how they will react is unknown. Most importantly, however, all water will still be confined to the mining area with flow going toward the two pits.

Central Zone Mining. MR may also expand active mining westward into the Central Zone area between the two pits. Five bedrock monitoring wells (H, C, D1, D2, and DDH-2) lie within this zone and would be mined out. The Continental Fault, which runs along the Continental Pit’s current west border, would also be mined out, eliminating what is now a partial groundwater barrier between the two pits. MR officials said the Central Zone is rich in copper, but they also admitted that a great deal of research must be done to determine the feasibility of mining there.