Environmental groups have filed a federal lawsuit over a proposed Montana coal mine expansion they say threatens the water supplies of nearby property owners.

The lawsuit was filed Tuesday in U.S. District Court in Helena by attorneys for the Montana Environmental Information Center and Sierra Club.

The case targets a proposal by Westmoreland Coal Company to expand its Rosebud mine near Colstrip.

The groups say state Department of Environmental Quality Director Richard Opper has shown a "pattern of neglect," by approving at least 10 prior mine proposals without ensuring protection of water supplies.

They want a judge to order the state to require Westmoreland to address water quality issues before permits are issued. The 25,000-acre mine currently produces about 12 million tons of coal a year.