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Plan for new treatment plant, lowering Pit water level is scrapped

Plan for new treatment plant, lowering Pit water level is scrapped

Mark Thompson


A new water treatment plant proposed last year to further lower the Berkeley Pit’s water level is no longer in the works.

Cameron Nazminia, Atlantic Richfield director of state and local affairs, wrote on behalf of the company in an email that the former oil company now owned by BP “proposed to EPA in June 2019 to construct a new water treatment plant to lower water levels in the Berkeley Pit.”

Mark Thompson, MR’s vice president of environmental affairs, said the new water treatment facility that Atlantic Richfield proposed last year would’ve had the same function as the existing Horseshoe Bend Water Treatment Plant.

But the plan for the second water treatment plant proposed by Atlantic Richfield has been scrapped.

“Since that time, Atlantic Richfield and Montana Resources have considered whether the process of lowering the water levels in the Berkeley Pit may destabilize portions of the pit walls,” Nazminia’s email said. “The companies will not seek to lower water levels in the pit unless further evaluations are performed that show this could be accomplished in a safe manner.”

The Berkeley Pit, a former copper mine, began filling with contaminated water after Atlantic Richfield turned off underground pumps in 1982. The 1994 Record of Decision and 2002 Consent Decree for the Butte Mine Flooding Superfund Site requires Atlantic Richfield/BP and Montana Resources, the two companies responsible for the Pit, to maintain the contaminated water below the level at which it would enter Butte’s groundwater.

Last year, Atlantic Richfield and Montana Resources launched a pilot project that aimed to pump enough water from the Berkeley Pit to stop water levels from rising.

The pilot project maintains the water level pumping it out and treating it, then discharging it.

Water pumped from the Berkeley Pit goes to a precipitation plant, which separates copper from the water and allows the company to recover the leftover metal. The water is then sent to the Horseshoe Bend Treatment Plant, where it is treated with lime to separate out other metals. The water then goes to the Yankee Doodle tailings impoundment, where it’s mixed with lime before it’s piped down to Atlantic Richfield’s polishing plant for further treatment to meet state and federal water quality standards. The treated water is then discharged to the creek.

Thompson said it would have been fine “if Atlantic Richfield wanted to build a water treatment plant outside of the specified remedy in the mine flooding CD.” However, he said, “MR had never sought to lower the pit water.”

The proposed water treatment facility was projected to lower the pit's water levels to roughly between 50 feet and 150 feet.

But lowering the Pit’s water to those levels “may have effects on the established remedy, on our operation and on safety concerns,” according to Thompson. “Our concern was, if you're going to use that water treatment plant to lower the Berkeley Pit water, make sure it's done in a responsible manner that doesn't destabilize the walls,” he said.

“The whole premise of the Record of Decision for the Butte Mine Flooding unit was to allow the bedrock aquifer to re-saturate most of the 10,000 miles of underground mine workings under Butte,” Thompson said. “Reducing the water in the Pit at the rate ARCO proposed impacts the pit wall stability.”

Thompson explained any reduction of the water level in the Pit exposes the sulfide-ore minerals mined from the underground work to oxygen and water. “Exposing sulfide minerals to oxygen and water is what generates the acids and the metals that show up in the Berkeley Pit,” he said.

Saturating the underground workings with water takes oxygen out of the equation and prevents acid generation and metal separation, Thompson added.

“So the whole idea of the remedy was to let it (the Pit) fill up with water to stop the acid generation,” Thompson explained. “And to prove that's working, around 2013, the pH of the Berkeley Pit was 2.5. The pH of the Pit right now is 4.3.”

Since the pH scale is logarithmic, a pH jump from 2.5 to 4.3 means the water is roughly a hundred times less acidic now than it was in 2013.

In addition to reducing the acidity, Thompson said saturating the Pit with water preserves the strength of the pit walls by preventing it from weathering, or degrading into less stable forms of rock.

“Flooding the underground workings by allowing the pit level to rise helps stabilize potential subsidence in the Butte area,” Thompson explained. “Atlantic Richfield’s proposal to draw down the water level in the Berkeley Pit  would likely result in destabilizing slopes in the Pit.”

Destabilizing the slopes of the Berkeley Pit, Thompson explained, “would not only have an effect on Montana Resources’ operations but also on the remedy in place for the Butte Mine Flooding Unit.”

“I think to not pursue a water treatment plant to drop the Berkeley pit water level is the status quo. That's what the (mine flooding) C.D. says. That's what MR and Atlantic Richfield are doing jointly,” said Thompson. “I don't think it causes a problem to not do it. It just maintains the remedy that we have in place right now.”


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Kristine de Leon is a reporter for the Montana Standard, her focus in on natural resource stories.

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