Montgomery County Executive James P. Gleason has narrowed the number of possible garbage landfill sites in the county from four to two, choosing a site in Laytonsville, east of Route 108, and a location in Potomac, at Persimmon Tree Road and Bradley Boulevard.

"It is clear that these two sites are suitable in terms of environment, health and community aspects," said Gleason in notifying the state Department of Health and Mental Hygiene of his choices.

Gleason indicated that his decision was based on evidence contained in a six-volume, 2,000-word report prepared by the consulting firm of Dames and Moore. The study, in progress for the past 18 months, has cost the county $2 million.

"This is the most comprehensive landfill siting (study) done in the nation," said county landfill planner Andrea Weirich.

In submitting the two proposed sites to the state, Gleason was complying with state orders to develop a new county landfill. The present landfill on Gude Drive in Rockville will be filled to capacity in three years. The choice of a new location has been the subject of bitter controversy for almost a year as the county has narrowed the possible sites from 24 to two. Residents of both Laytonsville and Potomac vigorous opposed the idea of putting the landfill in their communities.

If the state approves the sites, Gleason is expected to choose one of them after holding a public hearing. The site selected would then operate from five to six years if all wastes are dumped there, according to Weirich. But the county hopes to build a resource recovery system to recycle garbage by 1984, Gleason said. If so, the landfill would be used twice as long but only for non-recyclable wastes. The county could acquire both sites and use them sequentially, according to Weirich.

The state is expected to make its decision on the landfill permits by Oct. 19.

The consultants stated in their report, and Weirich said she agrees, that the Laytonsville site (known as site 55-B) is the "most suitable" for a landfill. The Potomac site, according to the consultants, is the "second most suitable." But, said Weirich, "I'm not saying my mind is totally closed on this. I don't know which one Gleason will pick as the final site."

The Laytonsville site, according to the consultants, is "the least exposed to present and prospective development," meaning there are fewer residential homes nearby than at other locations. The site also would be the least expensive to develop and operate, according to the report, although Weirich said cost is not a primary factor in picking a landfill.

The Laytonsville site would cost an average of $3.5 million a year over a 14-year period, according to Weirich. This would include the cost of land and operations. The Potomac site would cost an average of $4 million a year over an 11-year period.

The Potomac site was considered suitable because the access roads to the site are sufficient to carry the garbage truck traffic that comes into a landfill. Road modifications needed are minimal but there is greater community impact, the report continued. Weirich noted that the Potomac area near the site "is very low density compared to other sites that we excluded earlier - like the one in Gaithersburg off Rt. 28."

In narrowing down the sites, Gleason left out another site in Laytonsville (called 55-A), West of Route 108, and a small proposed landfill site, intended only for construction debris, near the Montgomery Airpark. According to the letter Gleason sent to the state, both sites were unsuitable.

The letter said the airpark site would be too expensive to develop and the Laytonsville site would have attracted scavenger birds. The county would have run the risk of birds colliding with airplanes flying into the Montgomery Airpark, according to Gleason.

Collisions of birds and airplanes have been known to cause fatal airplane crashes, according to the Federal Aviation Administration. The FAA, in a study of all four sites, concluded that Potomac would present bird and plane collision hazards.However, the county's consultants did what Gleason's office called a "far more exhaustive analysis," which concluded that the Potomac site would not produce such hazards.

Weirich said the county "laid out very conservative, very stringent criteria (for picking landfill sites). We wanted to find - environmentally and health-wise - the most suitable sites in the county. As you start becoming more and more careful on environmental controls, you find the best land for landfill is in developed areas because development also follows good land. It's easiest to build on good soil."

Laytonsville resident and landfill opponent Priscilla Benner said she was pleased the county eliminated the site west of Rt. 108.

"But I am very unhappy the other (Laytonsville) site was picked," she said. "(A landfill there) will have a heavy impact on the Mount Zion community - a historic area - there."

Potomac residents who are fighting against the landfill site echoed her words of disappointment. "We'll contest that it should be placed there for reasons of safety and health," said Alfred Scanlan, a lawyer who is representing Potomac residents and civic groups which banded together into the Committee to Save Our Land.

Scanlan said the citizens would be granted a hearing by the state department of health to hear their opposition to the county's applications for permits. "If they decide against us, we'll appeal to a board of review, an agency within the department. Then, if necessary, we'll take it to the appropriate Circuit Court and probably upwards."