Mercury-filled devices installed in water-supply systems in the Washington area and many other cities may pose "a significant threat to health," a new Naval Research Laboratory report says.

The study urges water-supply officials to consider removing water-flow measuring devices that contain mercury, a toxic substance, and replacing them with mercury-free equipment to avoid any risk of contaminating drinking water.

The Washington Aqueduct, which supplies water to the District of Columbia and parts of Northern Virginia, and the Fairfax County Water Authority both employ meters containing mercury to measure the flow of water at major plants and pumping stations, officials said yesterday. No mercury leak has ever been detected from these devices in either system, the officials noted.

The Washington Suburban Sanitary Commission, which supplies water in Montgomery and Prince George's counties, does not use the mercury-filled instruments, an official said.

The study was prompted by a mercury-contamination incident four years ago in the water system at the Naval Research Laboratory's station at Chesapeake Beach, Md., on the Chesapeake Bay about 25 miles southeast of Washington. The system provided water to lab buildings and homes of about 250 residents, including lab employes and their families.

A faulty valve improperly allowed between 7 and 14 1/2 pounds of mercury to be sucked back from the meter into the drinking water supply, setting off an intensive cleanup effort. Researchers found concentrations of mercury in the drinking water about 50 times higher than permissible under government safety standards.

The report also disclosed an incident of improper dumping by Navy personnel during the cleanup effort. "Acting in spite of impending plans for proper disposal, and in direct disregard for environmental consequences, these individuals discharged the waste cleaning solution onto the ground near the eastern shore of the Potomac River," the report says, terming the incident "unfortunate."

Lab officials said yesterday they did not believe any disciplinary action was brought against Navy personnel because of the incident. Neither the water-supply contamination nor the improper waste disposal resulted in health damage to residents of the lab complex, officials said.

In urging government water-supply officials to remove mercury-filled meters or take precautions to avoid contamination, the Navy Lab report warns that similar incidents elsewhere "are likely since mercury-containing flow meters are in common use."

Drinking-water specialists at the Environmental Protection Agency and the Denver-based American Water Works Association said they knew of no other mercury spills from water-flow meters in the last decade. Industry officials also said the mercury-filled meters are gradually being replaced by mercury-free devices in American water systems, partly because mercury has become increasingly expensive.