Bacteria levels in D.C. tap water exceeded federal health standards this month for the first time since 1996, city and Environmental Protection Agency officials announced yesterday, but they said most people are not at risk.

The sudden rise in bacteria, detected in routine testing this month, probably stemmed from use of a new water treatment chemical intended to reduce lead levels in the water at thousands of city homes, officials said during a news conference.

They said the chemical, orthophosphate, may have shaken off a layer of rust and bacteria inside city water pipes.

Officials from the D.C. Water and Sewer Authority, city Health Department and the EPA said tests did not find specific disease-causing bacteria. Therefore, they said, they would recommend only that people in certain at-risk groups consult doctors about whether to boil water before drinking it.

The at-risk groups include people with weak immune systems, some elderly people and infants.

"The water is safe, but some individuals in those three groups may have some increased risk," said Gregg A. Pane, acting director of the D.C. Department of Health.

Testing has not turned up bacteria problems in Northern Virginia, where hundreds of thousands of customers drink water from the same treatment plants that serve the District. The areas served in Northern Virginia include Arlington County, Falls Church, parts of Fairfax County that receive water from Falls Church and the city of Vienna.

Testing on 179 water samples from sites across the city found 19 that registered positive for total coliform, a general indicator of bacteria. That 10.6 percent positive rate exceeds the federal limit of 5 percent for bacteria. That violation requires city water officials to notify customers, which they said they would do next week.

Pane said city officials checked for and found no recent increase in symptoms of waterborne or foodborne illness that could be linked to the tap water test results. The city monitored emergency-room visits and sales of some over-the-counter drugs.

Rick Rogers, the chief of the EPA's regional water branch, said the city's high bacteria levels "could go on for a few months."

He said it is "not an option" to stop using orthophosphate. EPA officials say they are confident the chemical will bring down lead levels, though it could take a year to do so.

When the EPA approved use of orthophosphate throughout the city in August, after a three-month trial in a section of Northwest, it ordered WASA to undertake an aggressive flushing program to wash bacteria from the pipes. But it agreed that the flushing could begin after the Army Corps of Engineers, which runs the two city treatment plans, began adding orthophosphate.

Rogers said "unidirectional flushing," which is more forceful than simply opening hydrants to clear the system, "did not happen as early as we had hoped." But he said "in terms of this issue, it wouldn't have made any difference."

WASA began the flushing this month, and General Manager Jerry N. Johnson said the utility will finish before the onset of freezing weather. He said WASA will add more crews next week to expand the work. He said 10 to 15 three-person crews will be on the job, compared with eight or nine now.

Erik Olson of the Natural Resources Defense Council blamed WASA for not pursuing a more aggressive flushing program years ago, as consultants had recommended. "It is too bad it would take a violation of the health standard for WASA to really move this forward," he said.

The coliform problem is the city's first violation of federal bacteria standards since 1996, when one high test result prompted the city's health director to issue a quickly rescinded boil-water order for at-risk populations during the July 4 weekend. That was the fourth violation in a year, part of a regular pattern of problems in hot weather, when bacteria are more likely to flourish.

EPA officials blamed poor pipe maintenance for the problem. Rogers said the current problem, stemming from the use of orthophosphate, is unusual.

"It hasn't happened all that often in other systems" that use orthophosphate, he said. "I underestimated the amount of coliform that is out there."

Rick Rogers of the EPA's regional water branch said the high bacteria levels "could go on for a few months." Gregg A. Pane of the D.C. Department of Health said city officials found no increase in symptoms of illness.