State regulators partially approved Thursday an expansion for the coal mine that feeds the Colstrip power plants.
The Rosebud Mine will be allowed to add 6,746 acres to its existing mine in a spot known as Area F, which is about 12 miles west of Colstrip, the Montana Department of Environmental Quality announced Thursday.
The agency excluded 74 acres in its permit for the expansion because of the potential risk to groundwater.
The expansion would extend the operational life of the mine by about eight years, meaning Rosebud could run through 2038. The expansion would give miners access to an additional 70.8 million tons of recoverable coal.
The area disturbed within the permit would be about 4,260 acres, and roughly half of that would be mined. The remainder would be disturbed by highwall reduction, soil storage, road construction and other mine-related activities.
Mine owner Westmoreland Coal, which emerged from bankruptcy last month, has posted a $13.75 million reclamation bond to cover five years of cleanup costs, according to DEQ.
The Rosebud coal mine has had an exclusive agreement to supply Colstrip for four decades. After Westmoreland finished its Chapte 11 reorganization, its new managers stated they would honor the existing agreement through the end of 2019 but did not indicate if they would continue next year.