Montgomery County officials believe they have discovered something precious amid the offal of fish heads, cheese rinds and trash down at the county dump in Rockville.

Lurking in the bowels of the Gude landfill, where county garbage has been dumped for a dozen years, are pockets of swamp gas that officials estimate could bring Montgomery as much as $1 million a year for the next decade.

Swamp gas is the foul-smelling mixture concocted by bacteria that live in the landfill and decompose its organic material without the presence of oxygen. But swamp gas is about 50 percent methane, an odorless fuel gas that sells for around 40 cents a cubic foot, according to County Energy Coordinator Myles McGrane.

This week Montgomery officials signed a contract with a civil engineering firm in Reston to determine the size and quantity of the methane gas field and to assess its commercial prospects. SCS Engineers will drill holes and extract gas for 14 months under the $50,000 agreement with Montgomery, and the county Office of Environment and Energy Planning will study the potential market for the gas.

The county is studying the possibility of using the gas to generate electricity or piping it to customers located near the landfill. If the operation proves economically feasible, the county may either run the gas recovery operation itself or lease it and collect a royalty.

The landfill, which is scheduled to to closed next year, currently receives more than 1,300 tons of garbage a day. After 12 years of use, the dump now contains roughly 3.5 million tons of garbage. The garbage is an average 80 feet deep and covers 250 acres, much to the delight of thousands of Washington area seagulls and scavenging birds who frequent it for meals.

Engineers from John Hopkins University first tested the dump for gas in 1978. McGrane says the;y estimated that the landfill could contain 150 million cubic feet of methane.

Gas is currently being vented by wells at the edge of the fill to prevent it from seeping through the ground and into nearby homes.

A number of landfills in California are producing up to one million cubic feet of methane, McGrane said. And the National Park Service is currently warming greenhouses with methane produced from the old Kenilworth dump along the Anacostia River in Northeast Washington.