The last major stumbling block to closing the Alexandria coal-burning power plant was removed Friday after a regional power organization determined that its operation is not needed to keep the area’s electrical grid reliable.

The 62-year-old Potomac River generating plant, once considered the single biggest source of air pollution in the region, is now set to close no later than Oct. 1, 2012, a spokeswoman for owner GenOn confirmed.

The city of Alexandria and the company had announced an agreement to shutter the 62-year-old coal-fired plant in late August. But the shutdown was dependent upon whether the loss of the 485 megawatts of power the plant generates would adversely affect the region’s power grid.

PJM Interconnection, a regional transmission organization that coordinates the movement of electricity in the Mid-Atlantic, concluded in a letter to GenOn Thursday it would not.

“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, [GenOn] will have improved the air quality, health and safety of our residents. It’s been a long time coming.”

City residents began fighting for the plant’s closure in 2001, and the city joined their struggle several years later. National environmental organizations joined the fray about a year ago, and D.C. Mayor Vincent C. Gray (D), concerned over the high level of sulfur dioxide pollution that came from the plant into the District’s Ward 8, had asked PJM in August to investigate whether the plant was still needed. About 120 people work there.

“I feel very sure this plant will go down, maybe before Oct. 1, 2012,” said Elizabeth Chimento, one of the citizens who started the movement to close the facility. “There are still a lot of questions. . . . There will have to be cleanup of the plant ground — there’s asbestos in the building and definitely pollutants in the ground. But there is a feeling of delight.”

The five-smokestack plant, commonly known by the names of its former owners, Pepco and Mirant Energy, sits astride 25 acres of prime riverfront property that the city views as a linchpin in its efforts to turn the waterfront into public lands, from Daingerfield Park to Jones Park.

GenOn owns the plant and could choose to let it stand or demolish it. But the land itself is owned by Pepco, and GenOn has 89 years to go on a 99-year lease. Pepco also has transmission lines and a transmission-relay substation there.

“The fate of the property forward is not only going to be decided by Pepco, the owner, but it’s going to be sometime before [any change] happens,” said Mary Beth Hutchinson, a Pepco media representative. “There is no suggestion of moving that transmission substation. It is an essential component of our system and supplies a significant service to the District of Columbia.”

A rail line also runs through the property, serving GenOn and the Robinson Terminal, owned by a subsidiary of The Washington Post Co. A bike trail on National Park Service land also passes by the power plant.