Environmentalists and local officials opposed a Dominion proposal to drain about 215 million gallons of coal ash-contaminated water into Quantico Creek. (Astrid Riecken/For The Washington Post)

A state regulatory board on Thursday approved a permit allowing Dominion Virginia Power to divert treated water from coal-ash ponds in Prince William County into a nearby creek that links to the Potomac River.

The decision, which state officials said was approved 5 to 1, is part of an effort by the utility company to permanently seal five coal-ash ponds at the Possum Point Power Plant, near Quantico Creek.

Dominion, which stopped burning coal to provide electricity at Possum Point in 2003, is seeking to comply with a nationwide Environmental Protection Agency mandate to safely dispose of all forms of coal ash.

“This approach complies with all current federal and state regulations, including the newly promulgated EPA rule,” Cathy Taylor, Dominion’s director of Electric Environmental Services, told the state water board before the vote, according to a transcript of her comments provided by Dominion. “It protects human health and the environment while being a prudent use of our customer dollars.”

The power company plans to drain the coal-ash water at Possum Point and three other industrial sites in Virginia.

Dominion must seek additional permission to then permanently seal the remaining toxic residue with protective layering, soil and vegetation.

The company’s $325 million plan has sparked widespread opposition from environmentalists and state officials in Northern Virginia and Maryland, who complain that the standards for water treatment in the permit are not stringent enough to protect fish and other wildlife in the area.

Phillip Musegaas, legal director for the Potomac Riverkeeper Network environmental group, said his organization plans to file a lawsuit seeking to have the permit revoked.

“We think the law is clear that the permit does not comply with the Clean Water Act,” Musegaas said.

Prince William County is also leaning toward legal action, said Corey A. Stewart (R), chairman of the Board of Supervisors. Earlier this week, the county board voted to hire its own water-quality consultant to test Quantico Creek and smaller streams nearby, Stewart said. The board also directed the county attorney to begin seeking court action to keep Dominion from diverting any coal-ash water into the creek.

“I just can’t say in stronger terms how disgusted we are,” Stewart said.