An overview of GenOn's coal-fired power plant on September 1, 2011. Old Town Alexandria is in background. The power plant is scheduled to close Oct. 1, 2012. (Tracy A Woodward/THE WASHINGTON POST)

The city and plant owner GenOn had announced an agreement to shutter the 62-year-old coal-fired plant a month ago. But the October 2012 shutdown date was dependent upon the decision, which came Thursday night, about whether the loss of the 485 megawatts of power it generates would adversely affect the region.

PJM Interconnection, a regional transmission organization that coordinates the movement of electricity in the Mid-Atlantic, affirmed that finding in a letter to GenOn on Thursday. The city announced the decision Friday.

“The plant doesn’t generate power for the city of Alexandria at all, it never has,” Mayor William D. Euille said in an interview Friday. “By closing this plant, they will have improved the air quality, health and safety of our residents. It’s been a long time coming.”

The multiple-smokestack plant was once considered the largest single source of pollution in the Washington area. GenOn, the current owner of the plant, retains ownership of the building, and it’s in the 10th year of a 99-year lease on the land. The city of Alexandria plans to work with them about what happens to the 25 acres of prime riverfront property where the building sits. About 120 employees work there.

Local activists began fighting for the closure in 2001, and the city joined the struggle several years later. D.C. Mayor Vincent Gray, concerned over the high level of sulfur dioxide pollution in Washington’s Ward 8 district — a byproduct of the Potomac River generating plant -- had asked PJM to investigate whether it was necessary to keep the plant open.