Montgomery County's environmental experts have discovered an unusually high level of oil and grease in five residential wells near the county's controversial new landfill in Laytonsville.

County officials were unsure of the source of the oil and grease, but insisted that it posed no danger to the residents, who depend on those wells for drinking water. A new series of tests, described as a precautionary, was ordered to make certain the oil and grease contained no harmful or toxic substances.

On Thursday, about a dozen Laytonsville residents picketed the County Office Building in Rockville, claiming they were not informed about the test results. They also said the high oil and grease levels did not exist before the landfill opened June 1.

The $40 million, 550-acre garbage dump at Rte. 108 and Riggs Road opened after a long battle that that pitted a small but highly organized community group against the county executive and the County Council ended in a court decision.

David Sobers, the county's director of environmental and energy planning, said the test results posed no threat since residents would be able to smell and see oil and grease in their drinking water before it reaches a dangerously toxic level, and they would never drink it.

"We're still testing to make sure, but we're confident enough to think that remedial measures are not needed," said James Baker, director of the office of environmental protection. "If there is some foreign substance getting into those wells, it's getting there from some source other than the landfill."

Sobers suggested the substance could be caused by poor construction of some domestic wells or by a septic tank or a spill somewhere nearby not associated with the landfill.