Where did water exist in the Butte Hill and valley before mining?
What happened with water in Butte during underground mining?
How did the Berkeley Pit change the water resources in Butte?
Why is the Berkeley Pit filled with water and what happened to the historic water resources?
What metals and minerals are in the Berkeley Pit and why are they there? Why is the composition of the water so harmful to life?
Want to learn more about the Berkeley Pit than is covered in this primer?
Butte’s Historic Water Sources
Since mining began, the water resources in Butte and the Summit Valley have changed drastically. Butte is located just east of the Continental Divide, water flows from drainages throughout the area which collect in surface waters in the center of Butte. Historically, this central area of Butte was comprised of small creeks and wetland areas. Today, Basin, Blacktail, and Silver Bow Creeks and various wetlands remain. In the late 1890s, historic mining imprinted itself on the local landscape and dramatically altered the area’s groundwater and surface water systems.
Water was both an asset and a liability to mining. The presence of groundwater relatively close to the surface created an imposing challenge for underground mining operations. Over time, the Anaconda Company engineered a sophisticated drainage system within the vast underground mining systems to route groundwater to centralized pumps to allow for underground operations. Without this pumping system, groundwater would flood the shafts and drifts used to access and mine the area’s vast ore bodies. Despite the liabilities of groundwater, the surface waters throughout the area proved an important asset as they became central to the viability of the area’s mining operations and facilitated smelting and refining processes throughout the area. Surface waters were even used to convey contaminated byproducts of these processes downstream.
The map below shows water in Butte before mining took place, as well as the outline of the Berkeley Pit. The former Silver Bow Creek channel runs directly through East wall of the Berkeley Pit.
Before underground mining began on the Butte Hill and in the Summit Valley, Butte’s groundwater system contained groundwater from two sources: bedrock (hard rock) and alluvium (sandy gravel near the earth’s surface and below the soil layer).
Water found in bedrock collects in naturally occurring fractures and spaces between the hard rocks. Bedrock aquifers are typically found deep below the surface under layers of soil and sandy gravel.
Water found in alluvium collects in the spaces between the grains of sand and gravel. Alluvium is a sandy gravel material that is found on top of bedrock near the surface and below the soil layer. Think of this as the sands that lie on the banks of rivers and streams.
The graphics below show a comparison of the groundwater systems before and after mining took place (*these are renditions and not actual representations of Butte’s system). Notice how the Berkeley Pit sits lower than the rest of Butte and also how most of the bedrock groundwater flows towards the Berkeley Pit, as it is the lowest point in bedrock system, similar to how water collects in the bottom of a sink.
How has water in Butte changed?
Historically, Butte had a landscape of natural streams, wetlands, and two groundwater systems. Mining in Butte disturbed some of these natural systems including Silver Bow Creek and the bedrock groundwater system. The Berkeley Pit exists where Silver Bow Creek used to flow. This historical alluvial groundwater system is visible in the upper benches of the Berkeley Pit.
Groundwater and Underground Mining
Prior to mining, groundwater naturally flowed through fractures in granite bedrock and through the pore spaces between sand and gravel grains in the overlying alluvial deposits laid down by upper Silver Bow Creek. Underground mining created a network of vertical shafts and horizontal tunnels to access ore veins and large void spaces where veins were mined out (stopes). The 10,000 miles of underground workings now functions as a large drain system for the fractured bedrock aquifer. Think of it as a karst (cave) system in limestone. See above for more information about Butte’s groundwater system.
This graphic from the Montana Bureau of Mines and Geology shows the vast network of underground workings as they currently exist in the Butte Hill. Notice how the tunnels are spread out underground. It is estimated that there are over 10,000 miles of underground workings in Butte.
Historically, groundwater was pumped out of the mining network to keep the mines from flooding and allow mining to continue. The mine groundwater was pumped directly to Silver Bow Creek. The High Ore Kelley Mines had massive pump stations. It is estimated that water was pumped from the Butte Hill for over 100 years.
This graphic illustrates how water was pumped from underground mine shafts and up to the surface.
Open pit mining creates a series of benches that are dug into the ground to extract all of the rock (ore containing or not) out of an area. Over time, this creates a large void or an open pit mine.
In the case of the Berkeley Pit, the benches were dug straight into both bedrock (hard rock) and alluvium (sandy gravel near the earth’s surface and below the soil layer). Open pit mining the Berkeley Pit further disturbed the groundwater system of the Butte Hill. When mining began on the Berkeley Pit, groundwater was still being pumped from the nearby Kelley Mine shaft up to the surface and this activity allowed the expansion of the Berkeley Pit.
Where did Silver Bow Creek Go?
The east portion of the Berkeley Pit dug into the historic headwaters of Silver Bow Creek. Water from the former creek channel was used in mining operations and was pumped to the Yankee Doodle Tailings Pond during operation of the Berkeley Pit. When mining on the Berkeley Pit stopped, there was no creek channel left and the headwaters of Silver Bow Creek were gone.
Water from the upper Silver Bow Creek watershed collects in the Yankee Doodle Tailings Pond and is used in active mining operations within the Continental Pit. Today, Silver Bow Creek receives all of its water from Blacktail and Basin Creeks at the confluence near the Butte Chamber of Commerce.
When open pit mining at the Berkeley Pit stopped in 1982, the pumps at the nearby Kelley Mine were shut off for the first time in over 100 years. This decision caused the groundwater to return to the fractures within the bedrock and the spaces between sands and rocks in the alluvium as well as the underground mine tunnels and the void left by the Berkeley Pit.
The graphic below illustrates how groundwater is returning to both the Berkeley Pit and the old mine shafts.
Is all Groundwater in Butte Contaminated with Berkeley Pit Water?
No. The contaminated water in the Berkeley Pit is groundwater from the bedrock aquifer. It is important to note that the Berkeley Pit receives water from both the bedrock groundwater system and the alluvial groundwater system. The alluvial groundwater system that feeds into the pit, sits above the bedrock system. This is where the protective water level comes from. The Berkeley Pit is the lowest point in the groundwater system, and therefore all groundwater will always flow to the Pit and be controlled. The water will not be allowed to rise to the level where it could contaminate the alluvial groundwater system. For comprehensive information about the protective water level, please see the protective level page.
Another important note is that the Berkeley Pit exists in a bedrock system that is separate from the bedrock system that the Summit Valley sits on. A groundwater divide (a divide separates one water system from another system) exists near Continental Drive that keeps the groundwater flowing toward the Berkeley Pit and away from the groundwater system that feeds Blacktail and Silver Bow Creek.
For information about how long the Berkeley Pit will fill for and why the Berkeley Pit was allowed to fill, please visit the Protective Water Level page.
Underground mining in Butte left a network of tunnels and shafts that contained exposed rock. Much of this exposed rock is composed of Iron Pyrite (fools gold). In a naturally occurring bedrock system, iron pyrite remains out of contact with air. However, when iron pyrite is exposed to air and water it creates sulfuric acid.
In the case of the Butte Mines, nearly 10,000 miles of tunnels were left exposed to air. Furthermore, when the Berkeley Pit closed, the groundwater pumps were shut off and groundwater returned. The returning groundwater mixed with the air-exposed iron pyrite creating groundwater contaminated with sulfuric acid. Sulfuric acid also helps breakdown other metals and minerals found in the bedrock in the Butte Hill, adding even more contamination to the water.
The network of underground mines is directly connected to the Berkeley Pit, meaning that all of the contaminated water from the mines is now filling the Berkeley Pit.
The Berkeley Pit contains water contaminated with over 21+ metals and minerals as well as sulfuric acid.
Why is the water harmful to life?
Water in the Berkeley Pit has a low pH that has historically measured between 2.5 and 4.5. pH is a measure of the acidity or basicity of a liquid. The scale below illustrates the pH of various materials, including Berkeley Pit water. Recent data has shown an increasing trend in the pH of the water in the Berkeley Pit to at or above pH 4.0. The reasons for this trend have been discussed but not yet fully evaluated.
The pH of the water is too acidic to support aquatic life like fish or small macroinvertebrates, but researchers have discovered extremophilic organisms surviving in the Pit. Drinking water has a pH value of 7.0, the Berkeley Pit has a pH that ranges between 2.5-4.5. The graphic below illustrates the pH levels that aquatic life (other than extremophiles, such as those found in Yellowstone National Park) need in order to survive.
High concentrations of metals in the Berkeley Pit water are not suitable for human or aquatic life. The chart below shows the number of copper concentrations (in parts per billion) found in the Berkeley Pit vs the aquatic and human life standard.
|Human Drinking Water Standard||1,300|
|Aquatic Life Standard||12|
|Berkeley Pit Water||71,174|
For more information about the water in the Berkeley Pit, please contact us at firstname.lastname@example.org. The information in this section is intended to be a primer for the basic complexities of the Berkeley Pit. More complex and in-depth information is available. Please see the Academic Research Page for a full list of publications on the Berkeley Pit.