Alexandria’s coal-burning power plant, scheduled to shut down Oct. 1 after years of local opposition, must pay a $280,704 fine for violating air-quality laws, the largest such fine ever imposed against the plant.

The GenOn Potomac River power plant, whose five stubby smokestacks loom over the north end of Old Town Alexandria, exceeded its nitrogen oxides limit six times between June 28 and July 18, 2011, according to a consent degree issued by the Virginia Air Pollution Control Board last week and agreed to by GenOn.

Asked why the penalty was so high, Sarah Baker, regional enforcement manager for the Virginia Department of Environmental Quality, said “Compliance history is taken into account when we’re assessing fines.”

The company has been fined before, and the most recent penalty was $5,000 more than the largest previous fine, assessed in May.

The 482-megawatt plant has been operating intermittently this winter, due to low natural gas prices and the mild weather, said GenOn spokeswoman Misty Allen. In the past week, however, several of the plant’s units were fired up after PJM Interconnection, a regional transmission organization that coordinates the movement of electricity in the Mid-Atlantic region, said additional electricity was needed.

The GenOn coal-fired power plant in Alexandria. The plant was fined for violating air-quality laws the Virginia Air Pollution Control Board has ruled. (Gerry Martineau/The Washington Post)

The most recent fine comes after the plant, which was operated until December 2010 by Mirant Energy, was found to have no water-fogging system in place to control its bottom fly ash, which contributes to the formation of ozone, said William Skraback, ­Alexandria’s deputy director of transportation and environmental services. The Washington ­region has exceeded the federal air-quality compliance standard for ozone in the past several years.

“The [GenOn] violation was just over the threshold, it wasn’t off the charts,” Skraback said. “While it’s important the region stays in compliance with the ozone standard, bottom ash . . . is not the fine particulate matter which people breathe in.”

GenOn also informed the city in the past week that it will now use trucks, instead of railroad cars, to remove its waste ash. Allen said GenOn’s railroad contract expired and since the plant is not operating regularly, using trucks is more economical.

The plant has operated on the Potomac riverfront since 1949 and was once considered the single biggest source of air pollution in the region. Alexandrians take pains to point out that it never generated electricity for the city, but they lived with the pollution until about 10 years ago when two citizens, Elizabeth Chimento and Poul Hertel, began investigating the chalky residue that dusted their neighborhood.

What a GenOn consultant dismissed as “common dirt,” turned out to be ash from the generating plant. Fine particulates from the plant could be affecting the health of its neighbors, the federal Agency for Toxic Substances and Disease Registry warned, and both the city and the state Department of Environmental Quality began policing the plant.

GenOn, which owns the building itself, has 89 years to go on a 99-year lease from landowner Pepco. Pepco operates a transmission substation and below-ground lines on the 25 acres; a railroad line runs through the property, and the National Park Service controls the running-and-biking trail along the water’s edge.

The city, which plans to redevelop its waterfront, won’t even begin the planning process for the future of the GenOn property until at least 2013, Mayor William Euille (D) has said.