Coal-ash pond D at the Possum Point power station in Dumfries, Va. (Kate Patterson /The Washington Post)

Opponents of Dominion Virginia Power’s plan to release treated coal-ash water into a tributary of the Potomac River have found a new ally: Maryland Gov. Larry Hogan.

The Republican governor’s administration notified the office of Virginia Gov. Terry McAuliffe (D) this week of its intent to appeal a permit that was approved last month for Dominion to release about 215 million gallons of treated coal-ash water into Quantico Creek.

“The fact is, Virginia’s decision to dump millions of gallons of polluted wastewater into the Potomac River could adversely impact both human and aquatic life,” chief Hogan spokesman Matthew Clark said Tuesday. “Ignoring the risk simply isn’t an option.”

The Maryland governor’s opposition to Dominion’s plan is the latest development in a mounting legal battle over what to do with the coal-ash residue held in five ponds at the utility company’s power plant at Possum Point in Northern Virginia.

A spokesman for McAuliffe declined to comment, citing the pending litigation.

Virginia’s water-control board overwhelmingly approved Dominion’s plan to release the treated coal-ash water into the creek as part of a larger effort to comply with a nationwide Environmental Protection Agency mandate to safely dispose of all forms of coal ash.

Dominion, which has not burned coal at the Possum Point plant since 2003, intends to permanently seal all five coal-ash ponds at Possum Point once they have been drained of water, company officials said.

On Tuesday, the company argued that misinformation is being spread about its plan for the coal-ash water. Dominion estimates that executing th e plan will take three to four years.

“The permit is appropriate, properly issued and consistent with federal and state regulations,” Dominion spokesman David Botkins said in a statement. “As part of the process to close coal ash ponds, water will be filtered, treated, monitored and released in a controlled manner with ongoing testing. This process meets the very stringent limits imposed by the Virginia [Department of Environmental Quality] and ensures the safety and health of the public and aquatic life.”

Maryland’s appeal, to be heard at the Circuit Court in Richmond, will be handled by the office of state Attorney General Brian E. Frosh (D).

Virginia’s Prince William County and the Potomac Riverkeeper Network, an environmental group, have filed similar notices of their intent to fight the permit. Those groups argue that the permit’s standards for water treatment are insufficiently stringent to protect fish and other wildlife.

They also contend that Dominion has been allowed to release contaminated water into Quantico Creek for years.

“Once the coal-ash water is released, the potential damage is irreversible,” said Corey A. Stewart, chairman of the Prince William Board of County Supervisors. “We just want more time so this can be evaluated scientifically.”

Maryland officials said they also intend to lobby McAuliffe’s administration to change some of the permit’s requirements.

“Any time we are releasing potentially toxic substances into the watershed, we need to ensure that we are doing our utmost to protect the ecosystem, the environment and — most importantly — human health, especially in the Chesapeake Bay watershed,” Mark J. Belton, Maryland’s secretary of natural resources, said in a statement. “We are confident that additional efforts by our two states working together will benefit the river and reduce any risks to the people, fish and wildlife that depend on its health.”