A 30-year buildup of approximately four gallons of spilled mercury has been discovered in a drainage area adjacent to a nitroglycerin plant at the Naval Ordnance Station in Indian Head, Md., the Navy announced.

Officials at the sprawling munitions depot said the mercury poses a health threat, and they have placed a 10-acre area of tidal wetlands off limits.

"It is the biggest case involving mercury that this office has ever dealt with," said Art Capla of the Maryland Waste Management Administration. But he said the preliminary findings indicate that aquatic life in the area has not been affected.

Mercury, which causes blindness, deafness, birth defects, paralysis or death when ingested in large amounts, gets into the food chain through the soil and water. Even minor spills must be cleaned up thoroughly, state environmental officials said.

Navy spokesmen said that none of the station workers routinely tested for mercury exposure have been found to have hazardous levels in their blood. The laboratory is still operating.

Earlier this year, the Navy spent more than $120,000 to dispose of 20 tons of soil that had been contaminated by a half pint of mercury when a drainpipe burst at the same laboratory. Ditches and holes at that site were lined with sulfur to render any remaining mercury insoluble, and then paved over. About 60 drums of contaminated soil were shipped to an out-of-state hazardous waste dump.

Navy spokesman Michael Ward said the recently discovered concentrations, estimated to amount to nearly 500 pounds, were noted in a report issued by a Reston engineering firm as part of an investigation of hazardous waste sites at Naval facilities.

The mercury apparently was spilled over a 30-year period, during tests to determine the purity of the nitroglycerin. It accidentally was flushed into a 500-foot drainage ditch outside a laboratory that produces propellants, Ward said. From there, the mercury washed into a wetland area that drains into the Mattawoman Creek, a tributary of the Potomac River.

"It wasn't until the lab reports started coming back that we knew the mercury out there posed a more serious problem than we first thought," said Lawrence M. Sparks, an environmental engineer with the Chesapeake Division of the Naval Facilities Engineering Command.

Since 1984, the Navy has taken precautions to ensure that spilled mercury and silver are recovered and used again, Ward said.

But officials in the nearby town of Indian Head said that they still worry about the safety of their well water, which is fed by a network of underground rivers, or acquifers.

Indian Head Town Manager David L. Spinney said that the town periodically monitors its water supply, but has not noticed an increase in heavy metals.

James G. Sanders, director of the Academy of Natural Sciences laboratory in Benedict, Md., said he had not read the report, but that the problem with any heavy metal such as mercury is "that it evaporates very rapidly into a mercury vapor which, inhaled, is very toxic to humans . . . . If it stays in the environment, it may change form and show up in fish and oysters . . . . That may take years, but that's when we really have environmental disasters."

Sparks said the area was roped off "as an added precaution, mostly because of the possibility that on a very, very hot, humid day, right at the place of discharge, you could get some mercury vapors there . . . . It was near the occupational safety limits."

The U.S. Fish and Wildlife Service tested fish, sediment and water lilies from the area and found that mercury did not appear in them in significant amounts, said Glenn Kinser, regional field supervisor for the federal wildlife service.

The Environmental Protection Agency will make final recommendations on how to deal with the site after consulting with officials at the Maryland Waste Management Administration, Yates said.