BUTTE, Mont. (AP) — Atlantic Richfield has scrapped a proposal to build a second water treatment plant to lower the level of acidic, metal-laden water that has collected in a former copper mining pit in Butte.

There were concerns that reducing the water levels in Berkeley pit could destabilize portions of the walls of the pit, Cameron Nazminia, director of state and local affairs for Atlantic Richfield, told The Montana Standard.

Atlantic Richfield, which is owned by BP, suggested the additional water treatment last summer after after a tailings dam failed in Brazil in January 2019, killing 270 people. The idea was that if the tailings dam at Berkeley Pit failed, the pit would have enough room to hold the additional water.

Berkeley Pit began filling with water after Atlantic Richfield turned off underground pumps in 1982. Atlantic Richfield/BP and Montana Resources are responsible to maintain the water level in the pit below the level at which it would enter Butte’s groundwater.

Last year, Montana Resources began a pilot project to pump and treat enough water from Berkeley Pit to stop the water levels from rising.

Atlantic Richfield had proposed lowering the pit’s water levels by between 50 feet and 150 feet (15 to 46 meters). The water is about 1,000 feet (305 meters) deep.

In addition to pit wall stability, Thompson said reducing the water level would expose sulfide-ore minerals to oxygen, generating acid that would increase the acidity of the water in the pit. Leaving the minerals covered with water takes the oxygen out of the equation, Thompson said.