About 500 Montgomery County residents filled the Rockville High School auditorium this week to hear their fellow citizens spell out opposition to a plan to construct a garbage landfill on one of three sites.

The opponents, who testified at a public hearing held by the County Council, are residents of communities near the landfill sites tentatively selected for study by county officials: Rt. 1o8 and Dorsey Road in Laytonsville: Rt. 108 and Riggs Road in Laytonsville, and Persimmon Tree Road and Bradley Boulevard in Potomac.

County Executive James P. Gleason must now approve the final site. Temporarily, all three sites are being placed in the county's 10-year solid waste management plan for study.

Citizens urged county officials to concentrate their energies on looking for ways to avoid the need for landfills. Several residents suggested they consider construction of a resource-recovery plant which separates metals, glass and other recoverable materials from trash that could be burned as fuel material.

Charles Allmon, of the Potomac Committee to Save Our Land, told the council that technology for the resource-recovery plants is "now so reliable that a back-up dump is not needed."

Priscilla Benner of the Greater Laytonsville Civic Association suggested that the council expand the capacity of the present Gude-Southlawn landfill, which will soon be completely filled. Others echoed her words.

"I would remind you that the roads (for the garbage trucks) are already built at Gude," said Frank Blunda of the Olney Mill Civic Association. " . . .the character of the community has already been changed at Glude through the zoning process. I would remain you that the smells and the birds and varmints already exist at Gude. The least impact of a dump would be that which affects the least amount of new citizens of Montgomery County. It would seem that the expansion of Gude would meet the criteria of least impact."

All of the speakers urged the council to consider the impact of a landfill on communities.

"We are concerned that a long-term landfill site located just two miles from our homes will irreparably damage our environment," said Susan Lemkin, chairman of the Olney Mill Landfill Committee. "The people who moved to Olney came here not for convenience, but for tranquility. The Olney master Plan . . .envisions a small residential community surrounded by green wedges and corridors - not by landfills and industry".

They reminded the council of their fears that real estate values near landfills would plummet. The speakers also said they feared contamination of underground water that eventually would flow into the many wells of Laytonsville residents.

The location of one of the sites near the Montgomery County Airpark could be the source of another problem, said several citizens.

Birds attracted to landfills might collide with low-flying airplanes , an event that has resulted in fatal airplane crashes, according to United Airlines pilot John Ruddy.

"Potomac has been listening to Laytonsville tonight," said Sally Kanchuger of the West Montgomery Citizens Association, "and Laytonsville has been listening to Potomac. But the problem is, are the council members listening? We're afraid we may have closed minds here."

"We'll look at the report (on tentantive sites, made by county executive staff members) to make sure it's solid, to make sure they've been operating efficiently," said Council Member Neal Potter. "I think they have."

Council Member John Menke said he hoped the council could quickly pursue development of a resource-recovery plant. "I thing it's obsence that we have to have landfills in this day of technology."

The county is now ready to talk to private owners and operators of resource-recovery plants, said Andrea Weirich, the planner in charge of landfill site collection. Weirich said the county has always been interested in resource recovery but only in the last few months have they decided that it should be handled by a private firm. She added that a staff person will soon be hired to coordinate taking bids from companies.