Puget Sound Energy today reached a settlement with Sierra Club to dismiss all of the Clean Air Act allegations against the company’s Colstrip Generating Station in eastern Montana.

The Western Washington utility provider also agreed to retire the two oldest Colstrip units within six years — no later than July 1, 2022 — in part because coal fuel is now more expensive than natural gas for generating electricity.

“Our customers expect PSE to be good stewards of the environment and to keep energy costs reasonable,” said Kimberly Harris, PSE’s president and CEO. ”The eventual closure of Units 1 and 2 at Colstrip without the risk of further legal proceedings or additional significant investments in the units to meet regulatory requirements enables us to accomplish both of these goals.”

Today’s settlement does not impact Units 3 and 4 at Colstrip, which are co-owned by PSE and five other utilities. These two are newer, have more capacity and are more efficient, and will remain in operation.

Advocates for the environment have long targeted these 40-year-old Colstrip coal-fired generating plants as harmful to air quality and global warming. Gov. Jay Inslee issued a statement saying today’s action “puts us on a path that is good for Washington ratepayers and good for clean air.”