U.S. oil refiner Tesoro Corp said on Monday it has reached a $10.4 million settlement with the United States for air quality violations at refineries in six Western states.

Under the settlement, Tesoro will also invest $400 million in technology to better control air emissions at operations in Alaska, Northern California, North Dakota, Utah, Hawaii and Washington.

The settlement also requires Tesoro to pay an additional $1 million to fund purchase of natural gas-powered school buses in Northern California.

“We are pleased to have reached agreement on this consent decree that allows us to fully implement the required procedures and investments to further improve our environmental performance,” said Tesoro’s executive vice president for operations, Keith Casey.

The Department of Justice and Environmental Protection Agency are expected to announce the terms of the settlement at 3 p.m. EDT (1900 GMT).

The U.S. attorney general, EPA administrator, attorneys general for Alaska and Hawaii, and the Northwest Clean Air Agency filed the complaint against Tesoro for Clean Air Act violations at their refineries in six Western states.