Nearly one in five public water systems tested was contaminated by chemicals, many of them toxic, a group affiliated with consumer advocate Ralph Nader said yesterday.

The study, for the Center for Responsive Law, said 2,110 contaminants had been found in some concentration in water systems. Of those contaminants, 2,090 were organic, or carbon-containing, compounds and most of the rest metallic compounds; 190 of the compounds are known or suspected to be harmful to health, it said.

Up to 90 percent, by weight, of chemical contaminants in water remain unidentified, the center said.

Center researcher Duff Conacher, an author of the study, said 3,422 or 19 percent of 18,157 water systems that tested their water for unregulated compounds had found some of those compounds. There are about 79,000 public water systems in the United States.

Conacher and coauthor Walter Hang, a staff scientist with the New York Public Interest Research Group, combined the results of previous studies and surveyed state authorities to update work begun in the 1970s to identify chemicals in drinking water. Post-1977 results were available from 35 states, Puerto Rico and the Virgin Islands.

A 1984 study by the National Cancer Institute identified 1,565 chemical contaminants, of which 117 were known or suspected causes of cancer or cell mutations. The Environmental Protection Agency in 1985 published a survey of drinking water in seven cities in the 1976-80 period that identified 1,338 compounds.

The Safe Drinking Water Act of 1974, amended in 1986, requires the EPA to set maximum permissible concentrations for any substance known or expected to be harmful to health. So far, the agency has produced 30 standards, of which 22 are in effect and eight take effect next December.

The EPA has been "derelict in its duty" to set standards and monitoring requirements, the study group said.

The EPA's assistant administrator for water, Lawrence J. Jensen, said the agency had been addressing the contamination problem effectively since the 1986 amendments.

"Even Nader points out that fewer than 200 are ones we would suspect of being adverse to human health. Immediately you get to a narrower focus. And he doesn't make much effort to tell you the levels . . . . Sure, we can detect it, but that doesn't mean we should regulate it," Jensen said.

The 1986 amendments require the EPA to review its standards and adopt 61 more by June 1989; publish a list of possible contaminants needing regulation and adopt standards for 25 substances from that list by 1991, and set standards for 25 more every three years after that.

Treatment with activated carbon filters can remove at least 87 percent of the organic chemicals in water at a cost of 21 cents per thousand gallons or $27 a year per household, according to data from Cincinnati, the group said. Aeration treatment also can be effective. The EPA in 1981 withdrew a proposal to require aeration or carbon treatment of contaminated supplies.