Coal ash pond D at Possum Point Power Station in Dumfries, Va., shown in June, 2015. Ash Pond D is the only lined coal-ash pond at Possum Point Power Station. Coal ash from other ponds has been put in Pond D, which is supposed to be drained and capped with nonpermeable material. (Kate Patterson /The Washington Post)

Environmental groups are mounting opposition to the final phase of a Dominion Virginia Power plan to bury nearly 1 million tons of coal ash at the utility company’s Possum Point plant near the Potomac River.

The state Department of Environmental Quality has scheduled a hearing for Thursday on Dominion’s application for a permit to permanently seal the coal ash with vegetation, soil and synthetic membranes as part of the company’s efforts to comply with a nationwide federal mandate to safely dispose of all forms of the pollutant.

Dominion, which stopped burning coal at the Prince William County site in 2003, had been storing the ash in five retention ponds.

Last year, the state water board granted the company permission to discharge about 215 million gallons of treated coal ash water into a Potomac tributary, allowing Dominion to dredge the ash sediment and consolidate it into one pond that will also be drained before it is sealed.

Environmentalists, who fought to block the earlier permit, say that evidence of some groundwater contamination at the Possum Point site makes the plan to keep the coal ash there dangerous to nearby residents and wildlife.

Trucks excavate and carry coal ash from drained coal ash pond E at Possum Point Power Station in Dumfries, June, 26, 2015. (Kate Patterson /The Washington Post)

Monitoring wells have shown elevated levels of nickel, boron and other metals in the groundwater, prompting Dominion to offer local households with private wells financial help to connect to Prince William’s public water system.

Environmental groups and some local elected officials argue that the state should require the ash to be carted away and disposed of elsewhere, or recycle it into a cleaner form of ash that is used in cement, silica and other building materials.

Dominion already recycles coal ash produced at some of its other sites in Virginia, selling about 1.4 million tons commercially during the past two years, according to a company spokesman.

State Sen. Scott Surovell (D-Fairfax), whose district includes Possum Point, said the company should not be allowed to bury the coal ash on site if other alternatives are available. He sponsored legislation that passed the state Senate last week and would require state environmental officials to assess other options before approving a permit allowing coal ash to be buried on site.

“The process we’re undertaking now is a decision we may have to live with for the next 100 or 500 years,” Surovell said. “It’s important that we make the right decision first.”

DEQ officials say the permit under consideration has multiple safeguards against potential leaks and appears to comply with the state’s solid-waste requirements.

“As long as the option they choose meets the regulations, then DEQ is obligated to approve,” said department spokesman William Hayden.

Dominion has agreed to cover the 64-acre site with a mixture of grass and other plants, a total of 24 inches of soil and several synthetic liners meant to keep as much as 30 inches of rainwater in a 24-hour period from seeping into the ash below. A one-foot-thick clay wall already lines the bottom and sides of the site.

A draft of the permit requires Dominion to maintain the site for at least 30 years after it is sealed, and to dig extra monitoring wells to check for signs of increased groundwater contamination.

Prince William County Supervisor Frank J. Principi (D-Woodbridge) said he worries that the permit doesn’t do enough to guard against a breach in the ash site caused by a major flood or some other natural disaster in the distant future.

Currently, the permit requires quality-control testing of the liners for 25 years. Principi, who represents the area, said that leaves the area vulnerable afterward.

“It may not happen in my lifetime, but that coal ash isn’t going to go anywhere if they cap it in place,” he said. “This is about permanently protecting the environment.”

Environmental groups argue that Dominion’s plan is not in the spirit of the 2015 federal rule that sought to guard against groundwater contamination by requiring coal ash ponds to be closed.

Dean Naujoks, an environmentalist with the Potomac Riverkeeper Network, noted that the latest federal standards call for a two-foot-thick clay wall liner, instead of a wall that is one foot thick.

The standards apply to new and existing coal ash ponds. The Possum Point liner met federal requirements when it was installed in 1988, a report by a Dominion consultant group says.

Naujoks argues that the stricter rule should still apply to coal ash that remains on site.

“We know this is showing up in people’s drinking wells,” he said about coal ash contaminants. “There is more than enough evidence to consider alternatives.”

Hayden said that since the pond will be closed when the ash is sealed, the federal liner requirement doesn’t apply.

Dominion spokesman Robert Richardson said the clay liner is still sound. He added, however, that Dominion is open to altering its disposal plan if other alternatives prove to be cost-effective.

“We are open to still considering all the options and the options that we consider will factor in the environment, our neighbors and the cost,” Richardson said. “We’re keeping all those things in consideration.”