A state board upheld the denial of a key water quality permit for the Millennium Bulk Terminals project — a proposed $680 million facility in Longview, Washington, that would be the largest coal shipping terminal in North America.

Senior Vice President of External Affairs Wendy Hutchinson said Wednesday Millennium plans to appeal the decision, The Daily News reported .

The State Department of Ecology in 2017 denied Millennium’s application for a Clean Water Act certification, citing “unavoidable and significant adverse environmental impacts associated with construction and operation of the project.”

Ecology’s decision was based on Millennium’s projected train traffic, not impacts to water quality, Hutchinson said.

“We feel it was an improper application of the Clean Water Act,” she said. “The question the state was asked was does this harm water quality. It does not.”

The project is proposed at the former site of an aluminum smelter. It has been the subject of a permitting battle for six years and is currently tied up with state and federal legal challenges.