The town of Somerset, two families, the Mazza Gallerie shopping mall and civic groups sued Metro in federal district court Monday, charging it has filled a local stream and the mall's basement pumping system with diesel fuel.

The suit complains that diesel fuel from two aged underground storage tanks at the Metro bus yard at 5248 Wisconsin Ave. has seeped into the Friendship Heights Metro tunnel. The fuel, along with drilling slurry and cement dust, has been discharged by Metro without state permission into the Little Falls Branch creek, its neighbors charge.

The suit claims the leaks from the tanks could be "several hundred thousand gallons" in size.

That estimate was termed "completely unreasonable" by Frank R. Filiatreau, assistant general counsel for Metro. He said the suit was unnecessary since Metro already has acknowledged responsibility for the leaks and is trying to correct the problem.

The residents and businesses say the suit was needed because past complaints to Metro and government agencies have gone unheeded.

The fuel has seeped into the basement pumping system of the Mazza Gallerie, causing "noxious" odors in one elevator and forcing the mall to pay $10,000 to haul some 10,000 to 15,000 gallons of fuel away in tank trucks, the suit contends. The mall has been required to install an extra, $1,400 sump pump system and to replace $3,000 worth of electrical equipment damaged by the fuel, the suit adds.

The fuel spills result from routine bus washing, oiling and refueling at the bus yard, its neighbors complain.

Little Falls Branch runs from Chevy Chase Country Club by the Somerset swimming pool on its way to the Potomac River. Residents say the stream has been turned into a foul-smelling mess by the pollution.

"When you looked out of your back window you felt you were in the country," said Eleanor Fink, a freelance film researcher, and one of the plaintiffs. The rear of Fink's Dorset Avenue home is about 75 yards away from the creek. "Now, the stream is a dead stream because of the pollutants. Nothing can live in it."

Eugene Miller, whose Uppingham Street home is 50 yards away from the stream, says the main concern of the 13 civic associations represented by one plaintiff, the Citizens Coordinating Committee on Friendship Heights, is safeguarding the community's health and the neighborhood's appearance.

"The kids used to be able to go down to the creek and find crayfish and waterbugs," said Miller, vice president of the Somerset town council. "It used to be a haven for birds . . . it's a major down-county asset."

In serious cases, the court can fine violators of federal clean water laws up to $10,000 a day until the pollution is corrected.

Filiatreau said that leaks were discovered in two old D.C. Transit storage tanks last June and the tanks then were drained and filled with sand. Filiatreau said that Metro is digging test wells and attempting to pump the leaked fuel out of the ground, adding that a new drainage system and oil separators to handle surface runoff should be in place by late April.

Metro applied last week for a discharge permit from the Maryland Department of Health and Mental Hygiene, Filiatreau added, and hopes to settle the case before trial.