The main fear with the Berkeley Pit is that its toxic water will rise high enough to reverse the natural flow of groundwater, pushing billions of gallons of highly acidic, metal-laden water out into the surrounding aquifer and eventually into Silver Bow Creek. The critical threshold is 5,410 feet above sea level. If the water reaches that point without intervention, contamination could spread beyond the pit and into the drinking water and waterways of Butte, Montana.
Why the Water Keeps Rising
The Berkeley Pit is a former open-pit copper mine that flooded after mining operations shut down in 1982 and the pumps that had kept groundwater out were turned off. Without those pumps, water from underground mines, surrounding bedrock, surface runoff, and a large acidic spring near a nearby tailings dam began filling the pit. It hasn’t stopped since.
As of July 2025, the water sits at roughly 5,356 feet above sea level. That’s about 54 feet below the protective water level of 5,410 feet. If all pumping and treatment were to stop today, projections from the Montana Bureau of Mines and Geology estimate the water would reach that critical level in about 4.4 years.
What Makes the Water So Dangerous
This isn’t ordinary floodwater. The rising water comes into contact with leftover ore rich in iron sulfide minerals that have no natural ability to neutralize acid. When those minerals react with air and water, they produce sulfuric acid and release dissolved metals. The result is a lake with a pH of about 2.6, which is roughly as acidic as stomach acid or lemon juice. For comparison, most natural lakes have a pH between 6 and 8.
The concentrations of dissolved metals are staggering. The pit water contains more than 1,000 milligrams per liter of iron, over 600 mg/L of zinc, and around 150 to 190 mg/L of copper. It also carries arsenic, cadmium, and lead. These aren’t trace amounts. Copper levels in the pit, for instance, are roughly 190 times higher than in the surrounding groundwater, where copper typically measures less than 1 mg/L.
The Groundwater Reversal Problem
Right now, the situation is contained because of a simple principle of physics: water flows downhill. The Berkeley Pit sits at a lower elevation than the surrounding water table, so groundwater flows inward toward the pit from all directions. The EPA has confirmed through ongoing monitoring that the flow in the affected bedrock aquifer still points toward the pit, not away from it. That inward flow acts as a natural barrier, keeping contaminated water trapped inside.
The fear is what happens if the pit fills high enough to change that equation. Once the water level reaches or exceeds the level of the surrounding aquifer, the hydraulic gradient flips. Contaminated water would begin flowing outward from the pit into the alluvial aquifer, which is the shallow groundwater layer that connects to surface water. From there, it could reach Silver Bow Creek and spread downstream through the Clark Fork River watershed, one of the largest river systems in western Montana.
This isn’t a hypothetical concern dreamed up by environmentalists. It’s the central scenario that drove the EPA to designate the area as part of the Silver Bow Creek/Butte Area Superfund Site and to require ongoing water management as a legal obligation.
How the Water Is Being Managed
The primary line of defense is the Horseshoe Bend Water Treatment Plant, which was built specifically to keep the pit’s water level from reaching the critical 5,410-foot mark. The plant can treat up to 7 million gallons per day of combined water from the Berkeley Pit, the nearby Continental Pit, and the Horseshoe Bend spring that feeds into the system. Treated water is cleaned of metals and neutralized before discharge.
The treatment plant doesn’t drain the pit. It manages the rate of rise, pulling out enough water to keep the level safely below the threshold while the pit continues to fill from below and around its edges. This means the system has to run indefinitely. There is no scenario in current planning where the pit stops filling on its own. If the treatment infrastructure fails, loses funding, or can’t keep pace with inflow, the water level climbs toward the danger zone within a few years.
What Would Contamination Look Like
If acidic water did escape the pit and enter the alluvial aquifer, the effects would compound quickly. Silver Bow Creek already has a long history of mining contamination, and the Superfund cleanup has spent decades restoring it. A reversal of flow from the Berkeley Pit would reintroduce the same metals (copper, zinc, arsenic, cadmium, lead) at concentrations far higher than anything currently in the surrounding groundwater. Aquatic life in the creek would face acute toxicity. Nearby wells drawing from the alluvial aquifer could become unsafe.
The deeper concern is that once contaminated groundwater spreads into a porous aquifer, it’s extraordinarily difficult and expensive to clean up. Containing it inside the pit, where it can be pumped and treated in a controlled setting, is orders of magnitude simpler than chasing a plume of acidic water through miles of underground rock and gravel. That asymmetry is why maintaining the water level is treated as a non-negotiable priority by the EPA and the responsible parties managing the site.