Conservation groups filed a formal notice of an intent to sue Arch Coal Inc. over failure to reduce pollution from its West Elk mine in Colorado.

After providing their notice of intent to sue, the groups must wait 60 days before filing their lawsuit in a U.S. district court. WildEarth Guardians, High Country Conservation Advocates, the Center for Biological Diversity and the Sierra Club comprise the group filing the lawsuit.

Arch's West Elk mine vents "massive amounts of methane, a potent greenhouse gas" from wells drilled above the mine, the conservation groups wrote in a Dec. 17 news release. Matt Reed, public lands director at High Country Conservation Advocates, said in the release that the mine contributes more to climate change from methane emissions than any other single industrial source in Colorado.

Volatile organic compound emissions vented with methane during Arch's mining operations react with sunlight to form ground-level ozone, the groups wrote. Arch does not have a permit to release the compounds, they added.

"The reality is that the West Elk mine has been needlessly polluting the environment while the state did nothing," said Nathaniel Shoaff, a senior attorney with Sierra Club. "There are legal protections in place that call for reasonable steps to limit air pollution and protect public health, and we intend to ensure these provisions work for Coloradans."

Representatives of Arch Coal did not immediately respond to a request for comment.