How good is the drinking water in the nation's capital? To find out, The Washington Post commissioned an independent laboratory to analyze samples from 21 taps in the metropolitan area, ranging from White Flint Mall in Rockville to a Senior Citizens Center in Anacostia to Fairfax City Hall.

The water samples were collected in the morning, usually with the first drawn water of the day, in case water might be picking up excess lead or copper after sitting overnight in the pipes. WaterTest Corp. of Manchester, N.H., and Gascoyne Laboratories in Baltimore jointly analyzed the water for 51 different qualities. Among them:

*Metal content, from arsenic to zinc.

*Total coliform, an analysis of how much bacteria is present.

*pH, a measure of how acidic or basic the water is -- an important test, because acidic water can leech lead or copper out of pipes.

*Hardness, an analysis of how much calcium, mangesium and other ions are in the water. The higher the number, the harder the water. Such water is likely to wear out pipes and water heaters faster due to a buildup of these ions. There is also some evidence that harder water may help reduce the risk of heart disease.

*Total dissolved solids, a measure of how much filterable material is present.

*Volatile organic chemicals, including EDB (ethylene dibromide), a pesticide used on many grains; vinyl chloride, one of many chemicals used in manufacturing plastics; and tetrachloroethylene (TCE), a compound that is used by dry cleaning establishments and sometimes contaminates water.

On the positive side, the water tests showed no contamination from water-borne bacteria or viruses and safe levels of potentially toxic metals, including lead and mercury.

But the tests also found a high level of chloroform and other chlorine byproducts -- called trihalomethanes -- in the water. While most of these levels still met Environmental Protection Agency standards, they occurred at amounts many authorities believe are too high for long-term safety.

On average, chloroform levels ran 0.053 milligrams per liter. EPA does not set a separate standard for chloroform, but permits a total trihalomethane count of 0.100 milligrams per liter. In the Washington area, the average thihalomethane level was 0.067 milligrams per liter, the tests found.

The highest chloroform count came from a two-family house in Dupont Circle where the level reached 0.135 milligrams per liter -- over the EPA standard. The lowest was from a home in Reston, where the chloroform ran just 0.017 milligrams per liter. Most readings were in the 0.050- to 0.060-milligrams-per-liter range, regardless of whether the water originated from the Potomac, the Patuxent or the Occoquan Reservoir, and regardless of which water authority treated and distributed it.

The Post's findings are not unusual. Selected health department and water authority records show they closely match the levels of trihalomethanes measured regularly by the water systems themselves. In fact, in summer months, when bacteria grow faster, trihalomethane levels can run even higher, because water systems add more chlorine to kill water-borne organisms.

For instance, on June 12, 1985, total trihalomethanes in Falls Church ran 0.103 milligrams per liter. On Sept. 24, they were 0.093. In Herndon on Aug. 23, trihalomethanes reached 0.093 milligrams per liter, and in Vienna on Aug. 13, they ran 0.127 milligrams per liter.

High levels of trihalomethanes have been a persistent battle for Vienna. After two quarterly readings above the EPA limit occurred in 1983, (0.138 milligrams/liter and 0.114 milligrams/liter), the town was warned by Virginia's State Department of Health that it was at the limit of state regulation governing trihalomethanes and was in danger of exceeding it.

The concern about trihalomethanes centers on chloroform, one of the most common chlorine byproducts. Chloroform is a carcinogen, or cancer-causing agent. It forms in water as a result of chlorination, the process used to kill water-borne pathogens, including those that produce typhoid fever, hepatitis and diphtheria. EPA allows chloroform and other trihalomethanes to remain in the drinking water because chlorination is an important -- and economical -- way to control water-borne diseases.

But for other carcinogens, EPA sets a recommended contaminant level of zero. Then it usually establishes a mandatory standard called a maximum contaminant level, or MCL. The MCL is what can be achieved technically and what is economically feasible.

Most MCLs are set by calculating the average person's exposure to that chemical over a lifetime. EPA estimates that the average American drinks about two liters of water a day. Based on that rate and a 70-year life expectancy, EPA allows a level that could produce cancer in one person in 1 million over a lifetime.

For trihalomethanes, however, the agency uses a more liberal calculation: a one-in-10,000 chance of cancer developing. The agency set the standard at its present level, said Arnold Kuzmack, acting deputy director of EPA's Office of Drinking Water, because the agency worried "how low a moderately small water system could get the trihalomethanes without interfering with the chlorination system" and thus producing outbreaks of water-borne diseases.

EPA is considering tightening the standards for trihalomethanes, Kuzmack said, since at the time they were instituted, the agency did not calculate the risk of exposure from inhaling chloroform fumes -- which occurs when people shower and bathe.

An EPA study of the Philadelphia metropolitan area, where, as in Washington, concentrations of chloroform in tap water run high but within EPA limits, found that "chloroform in drinking water contributed over 70 percent to the total estimated cancer risks from air and drinking water."

"The compound responsible for most of the estimated excess cancer incidence associated with ingestion of drinking water is chloroform," EPA's Elaine Haemisegger, Alan D. Jones and Forest L. Reinhardt reported in a paper on the study, published in August in the Journal of the Air Pollution Control Association. "We should also note that the city of Philadelphia is well within EPA's Maximum Contaminant Level for trihalomethanes of 0.100 milligrams per liter." In fact, they said, concentrations of chloroform in the finished water were less than 0.050 milligrams per liter.