GRAND JUNCTION, Colo. (AP) — Conservation groups have filed a federal lawsuit against St. Louis-based Arch Coal over methane and other toxic air emissions at the company's West Elk Mine in Colorado's North Fork Valley.

WildEarth Guardians, the Sierra Club, the Center for Biological Diversity and High County Conservation Advocates filed the suit this week in U.S. District Court in Denver, The Daily Sentinel reports.

The West Elk Mine, near the unincorporated community of Somerset in Gunnison County, is Colorado's largest coal mine. It produced more than 4 million tons of coal in 2019, according to the Colorado Division of Reclamation.

The suit contends the mine's operator is violating the federal Clean Air Act by not sufficiently restricting emissions of methane, a greenhouse gas, and smog-forming volatile organic compounds. It also alleges that Arch Coal failed to obtain federal permits required for venting of air pollution sources from underground.

Arch Coal, based in St. Louis, didn't immediately respond to a Sentinel request for comment.

The suit seeks to require the mine to use the best available technology to control organic compound and methane emissions, claiming the mine has the potential to emit about 300 tons a year.

Environmentalists previously have challenged the mine's efforts to expand beneath 1,700 acres (663 hectares) of roadless area in national forest. The operation must build temporary roads on the surface to drill methane-venting wells for the mine expansion.