BILLINGS, Mont. (AP) — Montana regulators have given conditional approval to plans to inject clean water underground around the city of Colstrip to flush out water reserves polluted by coal ash from the small community’s namesake power plant, state officials said Wednesday.

Some details are still in the works, but pumping water into the 54 injection wells could begin by next year, said Sara Edinberg, a hydrogeologist with the Montana Department of Environmental Quality.

Leaking ash ponds at the plant operated by Talen Energy have contaminated groundwater in the area with boron, sulfate and other potentially harmful materials.

Contaminated water seeps from the ponds at a rate of about 240 gallons (908 liters) per minute. That translates into more than 126 million gallons (477 million liters) of polluted water annually.

The ash pond cleanup at Colstrip could cost an estimated $400 million to $700 million and is expected to last decades. It’s being carried out under a 2012 legal agreement reached by Talen’s predecessor, PPL Montana, and state officials.

Most Colstrip residents and businesses get water from the Yellowstone River. But some ranches rely on the underground water reserves for their livestock.

The cleanup plan covers two of the power plant’s four units. Talen is required to post a $107 million bond to ensure the work is completed.

Some of the pollution that’s deepest underground would not be pumped out under the plan approved Tuesday, and instead is expected to naturally decrease in concentration over time.

Talen would monitor the plume of pollution to make sure it does not become a risk to human health or the environment.

If Talen can’t show the plume is decreasing, state officials said they would require additional work to remove the pollution.

Company representatives did not immediately respond to email and telephone messages seeking comment.

Cleanup plans are still pending for the plant’s other two units, where the pollution is ongoing and expected to be more complicated to resolve.

“Part of the pond is unlined and will remain in contact with groundwater long-term, meaning the ash in this area acts as a continuing source of impacts to groundwater,” Edinburg said.

The state currently holds about $170 million in bonds for cleanup and closure work. State lawmakers have raised concerns over whether enough money will be available in the long term.

The Colstrip plant was built in the 1970s and 1980s. Units 1 and 2 shut down earlier this year after Talen and the plant’s five other co-owners said they were not longer economical to operate.

Coal has become less desirable among utilities over the past decade. There’s been rising concern over the pollution it emits even as coal’s share of the power market has shrunk because of competition from cheap natural gas.

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