The Naval Ordnance Station here spent more than $100,000 to get rid of 20 tons of soil contaminated in August after seven pounds of poisonous mercury spilled out of a ruptured sewer, Navy officials said last week.

Ammunition, propellants and explosives are produced for the Navy at this sprawling facility 20 miles south of Washington.

The mercury is believed to have spilled into a sink in the laboratory and and to have drained into the sewer pipe, but officials did not know when it occurred. It was discovered in August when a construction contractor ruptured the 30-year-old line. About a half pint, or seven pounds, of the silvery metal trapped in the pipe went spewing into a nearby trench, bursting into thousands of minuscule balls.

Mercury, which in extreme cases may cause birth defects and paralysis in humans when ingested directly in large amounts, gets into the food chain through soil and water, so even small spills must cleaned up thoroughly, state environmental officials said.

State officials say the spill poses no danger to Charles County's water supply because a thick, impervious clay barrier separates the contaminated surface soil from the deep, water-rich acquifers that the county taps for its drinking water.

County officials said they had not been told about the mercury spill.

"Obviously, we would've appreciated being told," said Marland Deen, chairman of the county commissioners. "It's an issue of environmental concern to everyone. But we've never had reason to doubt [the ordnance station's] ability or willingness to monitor its operations."

James Story, head of the county's environmental health office, said that in recent years the station has been "extremely open" and maintained a good safety record. "I don't know why we weren't notified this time, except that they are only required to tell EPA and state health officials," he said.

Although the August spill was considered minor, state environmental officials say the incident illustrates the time and cost of cleaning up even small accumulations of hazardous materials.

"It's highly unlikely that anyone will eat this soil, but we consider it an industrial chemical out of place. Ultimately, the mercury might migrate to an area where it could enter the food chain, and we are concerned about overloading the entire ecosystem," said John Koontz, chief of enforcement with the Maryland Waste Management Administration.

"It's not a large spill, but mercury is a metal and, like lead, depending on its form and concentration, can be hazardous. . . . A spill like this becomes significant when you consider that seven pounds of mercury contaminated at least 20 tons of soil," Koontz said. He said the accident was the first involving mercury at the ordnance station.

The Navy did not begin to scrape away contaminated soil around the spill until Oct. 17 and did not notify the regional office of the federal Environmental Protection Agency or Maryland health officials until 30 days after the incident, Koontz said.

"Normally we are notified while an incident is going on or shortly after, so we can evaulate the extent of the risk. That's why we have a 24-hour phone: 30 days is a long time," he said. On Nov. 27, Koontz said, his office finally sent the Navy a complaint order directing that the spill be cleaned up.

Such a formal notice is necessary in only about 5 percent of the 200 to 300 hazardous incidents investigated each year in Maryland, he said.

Since the accident, all discharges from the Biazzi Chemical Plant Laboratory where the spill occurred have been stopped and the floor drain in the building has been plugged to prevent additional mercury from leaving the lab and getting into sewer pipes or the soil.

Toxicity tests done by Versar Inc. of Reston last week showed 0.075 milligram of mercury per liter remaining, said Hugo Lopez, head of station control, environmental oversight and security. Lopez said 1 milligram of mercury per liter is normal for this area of Charles County.

Ditches and holes at the site will be lined with sulfer, which will render any remaining mercury insoluble, and they will be paved over, he said.

The Defense Property Disposal Office in Columbus, Ohio, will transport the drums of mercury-contaminated soil to an as-yet-unidentified storage facility for hazardous materials, said Michael Ward, director of public information at the ordnance station.

Meanwhile, an independent study ordered by the Department of Defense of all Navy, Air Force, Army and Marine bases has identified three potentially hazardous sites where chemicals may have leeched into the soil or been improperly dumped at the 90-year-old Indian Head facility.

One includes a 500-foot drainage area running from the lab where the August mercury spill occurred down to the Mattawoman Creek, which empties into the Potomac River. Versar is testing water samples and doing deep-ground borings of the creek bed and nearby drainage ditches, but it would not comment on its findings.

The other two potentially hazardous sites are outside two X-ray buildings here where chemicals used to develop the pictures may have contaminated the soil, Lopez said. Final results of the federal study are due March 31.

Lopez said the three sites were identified after federal investigators interviewed longtime employes at the base, "asking them if they ever remembered anyone dumping any waste materials anywhere. Then they looked for stressed vegetation. If there are very toxic materials in the earth, flora and fauna on top will be dead or discolored."

In addition to the federal findings, state environmental officials identified the Naval Ordnance Station last August as one of 168 potentially hazardous sites in Maryland as a first step toward creating the Superfund cleanup program ordered by lawmakers during the 1984 General Assembly.

Activity at the facility here, which station officials described as having a good safety record, reached its peak during the Vietnam War, when there was production around the clock. Now engineers and officers work to produce and test nitroglycerin-based explosives, rocket motors and propellents -- the explosive charges that propel a projectile from a gun or a rocket from its pad.

The naval base, which has about 3,000 civilian and military employes, has a school to train personnel from all military branches in munitions disposal. It also produces and tests several cruise missile simulators, including the Tomahawk, and it makes cartridges that aid the ejection of pilots and propellants for the Poseidon nuclear submarine series.

Since 1975, the Navy has taken steps to make the station pollution-free, including oil separators for its hydraulic presses, a $4 million contract to rid the station of aging septic tanks and install a consolidated sewer treatment plant, Lopez and Ward said.

Most recently, the Navy commissioned a major engineering study to determine the most effective way to treat the myriad of industrial effluents produced on the base, Lopez said.