Montgomery County's lawmakers introduced legislation today that would allow the county to dump its garbage at an incinerator under construction in downtown Baltimore.

The bill, backed by County Executive Charles W. Gilchrist, would enable Montgomery to join Baltimore and Baltimore County in using the new incinerator, which is scheduled to be built by 1985 on the site of the abandoned pyrolysis gas-burning plant.

The new facility, with a total capacity of 587,000 tons a year, could extend the life of the new county landfill at Laytonsville and negate the need for a costly new county incinerator proposed for Shady Grove Road and Rte. 355. That proposed incinerator has drawn fire from area residents, who fear the chemical fallout will pollute the air.

Finding a place to dispose of the 372,000 tons of garbage Montgomery County generates each year has been a mounting problem. Faced with opposition to the incinerator, officials have been exploring a plan to haul garbage by rail to West Virginia, where it would be dumped in strip mines.

General Assembly approval is needed for Montgomery to join the regional authority and begin dumping its garbage in Baltimore. The enabling legislation was introduced by Del. Ida Ruben and Sen. Sidney Kramer.

The legislation would not oblige the county to use the new facility, but it gives county officials an added option for disposal of the county's garbage.