Responding to citizen concerns about a new trash incinerator that will begin operating in the next few weeks, the Alexandria City Council agreed yesterday to monitor smoke from the facility for evidence of toxic substances.

The council voted unanimously to establish a task force, which will keep tabs on the $80 million incinerator, after a public hearing in which representatives of several community groups questioned whether the facility posed health hazards for nearby residents. The plant will be located at the west end of Eisenhower Valley, and will be used to dispose of trash from Alexandria and Arlington County.

Officials said they believe the facility poses no threat, but agreed to conduct tests for pollutants as a safeguard.

"We think data concerning emissions at the plant should be recorded at least quarterly," Sue Patterson of the Alexandria League of Women Voters told the council at the hearing. "And you should require comprehensive stack testing annually. This should be part of a broader plan for dealing with waste disposal in the community."

The council did not act on requests from several groups that the city equip the plant with devices called scrubbers that would reduce the amount of some potentially harmful substances contained in the smoke. An official of the firm that will operate the plant said that the scrubbers would cost $3 million to $5 million.

"We have a chance now to fit this facility with devices that are state of the art and that are already required in other states," said Richard Dennison, a toxic scientist with the Environmental Defense Fund.

Council members said they would consider adding scrubbers only after more plant data becomes available.

The incinerator is essentially complete, and test burning is scheduled to begin there this month. Officials hope it will be fully operational by year's end when it will be able to burn 975 tons of garbage daily. The ash will buried at a landfill near Lorton in Fairfax County.