Talen Energy is proposing to install more wells to flush out and capture underground water reserves that were polluted by coal ash dumped into leaky holding ponds at eastern Montana's Colstrip power plant.
State regulators will decide on the proposal following an upcoming public comment period. It applies to ponds that have taken ash from Colstrip Units 3 and 4.
The leaking ash ponds have contaminated groundwater in the area with boron, sulfate and other potentially harmful materials.
Roughly 100 wells designed to capture contaminated water already are in place, said State Department of Environmental Quality scientist Sara Edinberg. Talen is proposing to install at least 27 new ones to begin pumping in 2021. It also would install 54 injection wells that would pump clean water into the underground reserves to help flush out contaminants.
A separate cleanup proposal for Colstrip Units 1 and 2 is pending. Those ponds include a large one that is unlined, meaning contaminated water is in direct contact with groundwater reserves.
"That's one gets more complicated. ...It's an ongoing source" of contamination, Edinberg said.
Most residents and businesses in the city of Colstrip get water from the Yellowstone River. But some ranches rely on the underground water reserves for their livestock.
The ash pond cleanup at Colstrip could cost an estimated $400 million to $700 million and was expected to last decades. It's being carried out under a 2012 legal agreement reached by Talen's predecessor, PPL Montana, and state officials.
The plant was built in the 1970s and 1980s.
Units 1 and 2 are scheduled to shut down this year, as coal has become a less desirable fuel for electricity due to competition from natural gas, tighter restrictions on air pollution and increased use of renewable energy.
Colstrip is operated by Talen but co-owned by a group of utilities. Once the cleanup plans are finalized, the owners will be asked to post bonds to make sure the work is completed.