The GenOn plant is seen in Alexandria in this file photo. (Katherine Frey/The Washington Post)

For more than 60 years, two 25,000-gallon tanks buried underground at the coal-fired power plant on Alexandria’s Potomac River have been slowly leaking heating oil into the ground and into the river’s sediment.

An estimated 17,000 gallons had contaminated the soil, new owner NRG discovered in 2013.

This week, the Virginia Department of Environmental Quality approved the company’s plan to clean up and contain the spill over the next three years, and to monitor the soil and groundwater for two years after that.

“We don’t see this as any direct risk to human health,” said Alex Wardle, a remediation specialist in the state’s regional petroleum tank program.

Bill Skrabak, deputy director of Alexandria’s transportation and environmental services department, said the contaminated groundwater is not near any drinking-water wells and the company’s plans for containment should keep the pollution from affecting residents.

NRG is working with the city; the National Park Service, whose land adjoins the site; and the District’s Department of Environment, which is responsible for the Potomac riverbed, to make sure all issues are addressed, company spokesman David Gaier said.

The company will be required to make periodic reports to the state.

NRG says it will use a combination of techniques, involving injections of air and water into the soil and sediment, to extract the pollutants and allow them to break down through natural biological processes.

Paul Smedberg (D), an Alexandria city council member who is on a local monitoring committee set up years ago to oversee environmental concerns involving the plant , said NRG has worked quickly to address the contamination it found after taking over the plant from previous owners.

Pepco owns the land where the plant is located, but the facility itself was owned and operated by several other firms, including Mirant and GenOn, before NRG merged with GenOn at the end of 2012.

“The biggest question now is what will they do with the building,” Smedberg said.

The city is starting to create a land-use plan for North Old Town, where the plant is located, and leaders have expressed interest in encouraging a mixed-use development of some sort on the 25-acre site.

Nearby residents and the city fought for years to close the power plant, a relic of the 1940s, when coal was the preferred source of energy.

Once considered the largest source of air pollution in the region, the plant provided power to the District, but never to Alexandria.

Even when the turbines were turned off in 2012, residents predicted that cleanup was going to be a long, arduous affair.

Several submitted comments on the cleanup plan last month and urged the state to strictly regulate NRG’s efforts.