Since mining began, the water resources in the Butte area have changed drastically. Butte’s surface waters form the headwaters of the Clark Fork River, which is part of the Columbia River system that flows into the Pacific Ocean. Butte’s surface waters begin in small drainages high in the mountains and meet in larger creeks flowing through the center of town.
In the late 1800s, historical mining imprinted itself on the local landscape and dramatically altered the area’s groundwater and surface water systems. The map below illustrates historic water flows through the urban area and generally depicts stream flows prior to mining. The illustration shows historic Silver Bow Creek flowing from the northeast and through what is now the east wall of the Berkeley Pit.
Map of Butte that shows historic Silver Bow Creek. Photo courtesy of the Montana Bureau of Mines and Geology (MBMG).
Since open pit mining began in the Berkeley Pit, the headwaters of the upper watersheds have been severed from lower stream reaches below the Berkeley Pit. The headwaters of some watersheds still flow into the Berkeley Pit site and are captured and treated similar to Berkeley Pit water.
Butte’s groundwater system consists of two aquifers, the bedrock aquifer (hard rock) and alluvial aquifer (sandy gravel near the earth’s surface and below the soil layer). Water found in alluvial aquifers collects in the spaces between the grains of sand and gravel. Water in bedrock aquifers collects in naturally occurring fractures. Bedrock aquifers are typically found deep below the surface under layers of soil and sandy gravel. Alluvium is a mixture of sand and gravel found on top of bedrock and below the soil layer. A good example of alluvium are the sands that lie on the banks and beds of rivers and streams.
This illustration depicts the bedrock aquifer and alluvial aquifers in Butte. The Berkeley Pit is within the bedrock aquifer and captures and contains acid mine drainage from the mine shafts and tunnels located in the bedrock aquifer. Area surface waters pass through the alluvial aquifer and are separate from contaminated groundwater so long as the water in the Berkeley Pit remains below the protective water level. Rendering courtesy of Clark Fork Watershed Education Program for PitWatch.
When it comes to mining, water is both an asset and a liability. In Butte, the presence of groundwater relatively close to the surface created an imposing challenge for underground mining operations. In order to gain access to ore bodies, a complex system was developed to route groundwater to centralized pumps for discharge to the surface. This prevented groundwater from flooding the underground system and allowed miners to sink shafts and expand mining operations underground. Meanwhile, surface waters were used in mining operations like drilling, ore refining, and smelting.
Historic underground mining relied on a network of vertical shafts and horizontal tunnels (known as drifts) to access mineral rich ore veins. The tunnels connected large void spaces where miners could access ore veins, remove the ore, and bring it to the surface for processing. It is estimated that over 10,000 miles of underground workings were created by mining of the Butte Hill.
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.
The pumping stations in the High Ore and Kelley Mines moved water throughout the system for use in ore processing and ultimately release into Silver Bow Creek. Some estimate water pumping on the Butte Hill occurred continuously for over 100 years.
Today, groundwater moves through the mine tunnels and fractured bedrock and collects in the Berkeley Pit, much like how groundwater flows through karst (cave) systems in limestone.
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.
Open Pit Mining
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, allowing for the expansion of the Berkeley Pit.
After Open Pit Mining
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.
Since the pumps were turned off in 1982, roughly 49 billion gallons of water have filled the Berkeley Pit. That’s the equivalent of about 74,000 Olympic sized swimming pools. Water continues to flow into the Berkeley Pit through the alluvial and bedrock aquifer systems. To prevent Berkeley Pit water from contaminating Butte’s surface water systems, it must be
Is all Groundwater in Butte Contaminated with Berkeley Pit Water?
No, groundwater in the bedrock aquifer collects in the Berkeley Pit. The water is intentionally kept below the protective water level to ensure contaminated groundwater always flows toward the Berkeley Pit rather than toward the alluvial aquifers and area streams. Water in the Berkeley Pit cannot flow uphill and does not contaminate groundwater outside the bedrock aquifer. For comprehensive information about the protective water level, please visit the Protective Water Level page.
Did Silver Bow Creek Disappear?
The historical headwaters of Silver Bow Creek lie northeast of the Berkeley Pit. Water from the creek had historically been used in mining operations. After the Berkeley Pit operations began, Silver Bow Creek was severed from its headwaters. Today, the water from the headwaters of Silver Bow Creek collect in the Yankee Doodle Tailings Pond. The former channel remains at Texas Avenue and consists of seasonal stormwater until it reaches its confluence with Blacktail Creek. Silver Bow Creek flows westerly receiving water from additional drainages to warm springs ponds where Silver Bow Creek ultimately becomes the Clark Fork River.
This map illustrates historic underground mining activities. Each color represents a different subterranean depth where mining occurred, and depicts the concentration of shafts and tunnels accessing orebodies in the district. The outline of the Berkeley Pit indicates where open pit mining occurred following the underground era. Note the concentration of shafts and tunnels in the area. Photo courtesy of the Montana Bureau of Mines and Geology (MBMG).