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Water in the Berkeley Pit rising over time, 1979-2013. Photos from the Montana Bureau of Mines & Geology, Justin Ringsak, and Fritz Daily.

1982-2013: 31 years since pumps stopped

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.

Water in the Berkeley Pit rising, 1979-2013. Photos from the Montana Bureau of Mines & Geology, Justin Ringsak, and Fritz Daily.
Water in the Berkeley Pit rising, 1979-2013.

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.

EPA LogoSoon 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.

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.

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.

The Yankee Doodle Tailings Pond, part of the active Montana Resources mine that borders the Berkeley Pit, in 2008. Photo by Justin Ringsak.

Above the Pit: The Yankee Doodle Tailings Pond

Looking west from Rampart Mountain over the Yankee Doodle Tailings Pond, located north of the Berkeley Pit, in 2007.

Looking west from Rampart Mountain over the Yankee Doodle Tailings Pond, located north of the Berkeley Pit, in 2007.

North of the Berkeley Pit stands one of the largest earthen dams in the United States. The dam, constructed from waste rock mined out of the Berkeley Pit and, in more recent years, the Continental Pit, stands over 650 feet (200 meters) tall. It holds back the Yankee Doodle tailings impoundment, also known as the Yankee Doodle Tailings Pond. As part of active mining operations, Montana Resources pumps tailings and water to the Yankee Doodle Pond. Lime rock is also added, resulting in a non-acidic pH (above 7.0) tailings slurry, thus mitigating or avoiding the phenomenon of acid mine drainage.watch T2 Trainspotting 2017 film now

The Yankee Doodle Tailings Pond, part of the active Montana Resources mine that borders the Berkeley Pit, in 2008. Photo by Justin Ringsak.

The Yankee Doodle Tailings Pond, part of the active Montana Resources mine that borders the Berkeley Pit, in 2008.

Tailings particles settle out on the south portion of the ponds. Snowmelt runoff from upper drainages also mixes with the water at the north end of the pond. These factors result in clear water with an alkaline (or non-acidic) pH and very low concentrations of dissolved metals at the north end of the pond.

When mining operations were suspended from 2000 through 2003, water was no longer pumped to the Yankee Doodle site, and the tailings deposited there began to dry out. In response to concerns from the community over dust clouds blowing in the vicinity of the tailings pond, Montana Resources spread about 1.5 million tons of rock, approximately 18 inches deep, over about 506 aces at the tailings impoundment site to keep the dust down. Since the mine reopened, the tailings deposit has remained wet, resulting in no further instances of tailings-dust clouds on Butte’s northern horizon.

43 billion gallons and counting: Where does it come from?

Water in the Berkeley Pit rising over time, 1979-2013. Photos from the Montana Bureau of Mines & Geology, Justin Ringsak, and Fritz Daily.

When ARCO suspended underground pumping operations in 1982, groundwater levels on the Butte Hill began to rise. Nineteen months later the water level in the underground workings and surrounding bedrock reached the bottom of the Pit, allowing bedrock groundwater to start filling the Pit void.

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.
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.

Prior to that time alluvial groundwater seeped into the Pit from the east and south walls, beginning to fill the Pit lake. ARCO also diverted water from its mining operations (leach pad water, Continental Pit, Horseshoe Bend, etc.) into the Pit following the 1983 shutdown of their entire Butte operations.

When Montana Resources began operations in 1986 a number of these surface water sources were diverted away from the Pit, however, the Horseshoe Bend water continued to flow into the Pit until April 1996 when it was incorporated in Montana Resource’s mining operations for treatment and disposal in the Yankee Doodle Tailings Dam.

When Montana Resources suspended mining operations from 2000 through 2003, about 7.5 billion gallons of water, or an average of 6 million gallons per day, went into the Pit. Of this total, an average of 3.4 million gallons per day came from rising groundwater flows in the underground mine workings and surface stormwater flow. An average of 2.6 million gallons per day came from the Horseshoe Bend drainage. Montana Resources also diverted water from the Continental Pit into the Berkeley Pit for containment during their suspension.

Since the Horseshoe Bend Water Treatment Plant began operating in 2003, water flows from the Horseshoe Bend drainage have been diverted to the treatment plant. After treatment, this Horseshoe Bend water is entirely recycled or consumed in mining operations, or, in other words, no water is discharged off of the site.

About 2.6 million gallons per day from groundwater and stormwater still flow into the Pit, contributing to the rising level there. Eventually, when the water level approaches the Critical Level of 5,410 feet above sea level, water will be pumped from the Berkeley Pit and treated at the Horseshoe Bend facility. Present projections put this date around 2023. Having the plant in place provides assurance that the capability to manage Berkeley Pit water levels is there when it becomes necessary to treat Pit water.

This 2006 image from the NASA Earth Observatory shows the Berkeley Pit and surrounding area after the construction of the Horseshoe Bend Water Treatment Plant and after the resumption of mining at the Continental Pit.
This 2006 image from the NASA Earth Observatory shows the Berkeley Pit and surrounding area after the construction of the Horseshoe Bend Water Treatment Plant and after the resumption of mining at the Continental Pit.

Is water from the Berkeley Pit expected to flow to the Continental Pit?

Presently there is no evidence that water is moving from the Berkeley Pit to the Continental Pit or that there is any underground connection between the two pits. Right now, the mining level in the Continental Pit is below the water level in the Berkeley Pit. In the future, by the end of the mine life, the bottom of the Continental Pit may be as much as 400 feet below the water level in the Berkeley. But even then, there is no expectation that Berkeley water will enter the Continental Pit.

What is the water quality of the Continental Pit compared to the Berkeley Pit?

Current sampling indicates that the water quality is significantly different in the two pits. The pH of the water in the Continental Pit is about 6.5-7.0, which is much more neutral than the water in the Berkeley Pit, which has a pH of about 2.5. Also, the levels of arsenic, copper and cadmium are many times less in the Continental Pit water.

In the future, as part of the reclamation effort, the Continental Pit water will likely require some management to sustain water levels and treatment to remove metals. However, that treatment should be less costly and less complicated compared to the Berkeley Pit.

Have there been any major rock slides along the Pit walls?

Yes. Wet weather may have also played a role in the 1998 Pit wall slough that sent about 3 million tons of rock and dirt into the water. Rising groundwater saturated and gradually weakened that section of the southeast wall, eventually causing it to break away.

Montana Resources, Inc. (MR) is taking steps to stabilize the piles of waste rock that form sections of the Pit walls. Two options are available: The first is to remove material from the tops of the dumps (the crests) to relieve pressure, and the second is to add material to the bottoms of the dumps (the toes) to bolster their foundations.

MR is employing both strategies to minimize future problems. Using a bulldozer, crews shaved the crest of what is called the “Bird Watch Dump” along the Pit’s south wall. And to shore up the underwater toe of another dump in the Pit’s southeast corner, crews pushed in material from the Bird Watch Dump, plus additional waste rock and dirt from the active Continental Pit.

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.

The Berkeley Pit, Continental Fault, and the two wells that showed water level changes after a July 2005 earthquake.

Earthquakes did not affect Pit

The Berkeley Pit, Continental Fault, and the two wells that showed water level changes after a July 2005 earthquake.
The Berkeley Pit, Continental Fault, and the two wells that showed water level changes after a July 2005 earthquake.

A 5.6 magnitude earthquake centered near Dillon on July 25, 2005 did not affect the Berkeley Pit. There was no Pit wall sloughing or change in the water levels in the Berkeley Pit, the underground mine shafts, the alluvial aquifer wells, or the majority of the bedrock monitoring wells.

However, two bedrock monitoring wells (A&B) showed changes. Well A showed an initial water level decline of about one (1) foot after the earthquake, and the level stayed lower for a number of days before rising again. Well B, which is located in an area that wasn’t dewatered as extensively by historic mining activities as other portions of the bedrock aquifer had a 9-foot drop in water levels in the month following the earthquake. Recently, the water elevation in Well B is rising again.

One possible explanation for the lower water level in these wells is that the earthquake opened up existing fractures in the bedrock surrounding the wells. Water then flowed into these fractures until the bedrock adjacent to them became saturated. When that happened, the water levels began to rise again.

Since the July earthquake, there have been two additional quakes in the region, one of which was centered in the Butte Basin. Both of these other quakes were considerably smaller in magnitude, and no effects were noted in the Berkeley Pit or bedrock monitoring wells.