Tap water in thousands of District houses has recently tested above the federal limit for lead contamination, a new phenomenon that has baffled the D.C. Water and Sewer Authority and forced the agency to begin replacing service pipes.

Two-thirds of the 6,118 residences that WASA tested last summer, or 4,075 homes, had water that exceeded the lead limit of 15 parts per billion set by the Environmental Protection Agency in 1991. This is the first time the city's water has shown significant lead contamination since the late 1980s, officials said.

WASA officials said they are not sure what has caused the spike in lead levels. They are investigating whether changes in the way water is treated at the Washington Aqueduct could have a corrosive effect on lead pipes.

Lead, which can be ingested by drinking contaminated water or inhaling lead paint fumes, can cause serious damage to the brain, nervous system, kidneys and red blood cells, particularly in children, babies and fetuses. Health officials said it is difficult to quantify how much danger lead contamination in water poses. A person whose blood has more than 10 micrograms of lead per tenth of a liter should be concerned, but how much contaminated water a person must drink to reach that level varies.

Although the extent of the water problem and its public health implications are just coming to light, WASA officials have been aware of the contamination since random tests on a small number of houses revealed a problem in 2002. Although agency officials discovered a more extensive problem last summer, they did not begin to notify homeowners about the results until November. WASA held a public meeting about the issue in December, but its advertisements did not reveal the lead problem. Instead, they simply stated that the purpose of the meeting was "to discuss and solicit public comments on WASA's Safe Drinking Water Act projects."

Tony Bullock, spokesman for Mayor Anthony A. Williams (D), said he was unaware of the lead problem and believed that the mayor had not been informed. Several D.C. Council members said they, too, were unaware.

D.C. Council member Adrian M. Fenty (D-Ward 4) said he first heard about the situation yesterday when two constituents sent his office e-mails about the lead contamination.

It is not just the number of houses that registered above the EPA limit that has alarmed experts, but also the amount of lead found in the water. Although the federal government requires that cities begin a pipe replacement program when lead levels exceed 15 parts per billion, 2,287 D.C. houses had lead levels exceeding 50 parts per billion, including 157 residences with more than 300 parts per billion.

Erik Olson, an analyst for the Natural Resources Defense Council, an environmental group that monitors public health issues, said he was shocked.

"I've never heard of anything like that. This is a really big deal," said Olson, who has surveyed the drinking water in more than 100 cities. "If schools go over 20 parts per billion, they immediately take the water out of production."

Federal authorities said it is unusual for a city that did not have lead contamination problems to suddenly exceed the level above which the EPA requires corrective action.

"The District is the only one in our region -- this is pretty rare," said George Rizzo, an environmental scientist for the EPA's mid-Atlantic office, which oversees five states and the District.

Cynthia Dougherty, head of the EPA's drinking water office, said some cities have exceeded the action level. Most of those, however, were above the limit in 1991 and are still trying to get below it, she said.

"It's shocking," said Charles Eason, whose home in Georgetown had water that registered 36 times the EPA's lead limit. "It's a particular risk for young people, and I have a 4-year-old grandson in my house regularly."

There are 130,000 water service lines for residential customers in the District. About 23,000 of those are made of lead, while the rest are made of copper, officials said. The lead pipes are spread throughout the city, mostly servicing older single-family homes.

Now that WASA has discovered widespread lead contamination, EPA guidelines require the agency to replace 7 percent of its lead pipes annually, which is estimated to cost $10 million to $20 million a year. The agency will focus on neighborhoods where lead contamination is the highest, said Michael Marcotte, WASA's chief engineer.

"Where we are aware of a situation where someone has a particular health concern and young children, we'll work with them as quickly as we can in the process," Marcotte said.

WASA is responsible only for pipes in public space. The portion of pipe that runs through private property and into a house is the responsibility of the homeowner. Thus, owners must decide whether to replace those pipes, a proposition that could cost as much as several thousand dollars, WASA officials said.

WASA recommends that residents whose water is contaminated flush their taps by allowing the water to run for 30 seconds to one minute before drinking it or using it for cooking, although that process is not always successful in clearing the lead. Residents also can purchase a home treatment device or use bottled water. Boiling the water or using a standard pitcher with a filter will not help protect against lead, officials said.

Lynette Stokes, a physician who oversees the D.C. Department of Health's lead testing program for children, said that in general, dust or lead paint poses a far greater risk than contaminated water. Parents of children younger than 6 can bring them in to have their blood tested for free, Stokes said.

"That will help us identify whether or not any of that lead in the water has dosed that child to a degree where we need to be concerned," she said. The District does not offer a screening program for adults or for children 6 and older.

WASA gets its water from the Washington Aqueduct, which also services Arlington and Falls Church. The water at the aqueduct has long been treated with chlorine to kill bacteria. But the chlorine was combining with organic materials in the pipes and creating new, harmful chemicals, officials said. Four years ago, scientists added ammonia to balance the chlorine, creating a compound known as chloramine.

It's possible, officials said, that the chloramine is more corrosive to lead pipes. Falls Church has no lead pipes; Arlington has a few but has discovered no lead contamination problems, officials said.

"It's definitely in the 'not sure' category," said Lloyd Stowe, chief of operations at the Washington Aqueduct. "The whole idea of corrosion control is more of an art than a science."

In the late 1980s, lead was found in tap water in thousands of D.C. homes, particularly in the Palisades area. The city replaced some lines, and the problem diminished in subsequent years. WASA has been conducting random sampling of water since the EPA began requiring it in 1991.

WASA first noticed problems with lead contamination during routine testing of about 50 houses from July 2001 to June 2002, Marcotte said. The agency noted the problem in its August 2002 report to the EPA and began to comply with the EPA's guidelines, which required WASA to replace lead pipes and to inform the homeowners of the dangers of lead.

Marcotte said WASA replaced 400 pipes at a cost of $3 million last year. But last summer, the agency also began conducting more widespread tests. If the sample size increased but the number of homes with lead contamination remained small, WASA would have met the EPA standard without having to replace many service lines.

Instead, the results from more than 6,000 homes last summer revealed widespread problems. WASA then began mailing results to homeowners who participated in the sampling program, Marcotte said.

"We fully disclosed, in our view, what the situation was," Marcotte said. "We let people know there was an issue."

As an independent agency, WASA is autonomous in its budgeting. But its performance is overseen by the D.C. Council's Committee on Public Works and the Environment, headed by Carol Schwartz (R-At Large). Her office knew little about the situation when contacted this week.

"Of course, I'm concerned," Schwartz said in a statement released by her office. "I hope WASA will continue the studies and rectify the problems found."

Georgetown resident Janet Stone heard about the lead contamination from her neighbor. Stone, who has two infant daughters and is pregnant, decided to spend more than $100 to have her water tested by an independent company. She also had her two daughters' blood tested for lead poisoning, even though she thinks her house has copper pipes.

"I'm concerned," said Stone, who is waiting for the results. "I'm trying not to panic."

Marilyn Lashley, whose home in Bloomingdale had water that registered 12 times the EPA's limit, put a notice about the problem on a neighborhood Internet mailing list. Cleopatra Jones, a neighborhood advisory commissioner in Bloomingdale, east of Howard University, said some neighbors played down the risks because they buy bottled water.

"I said, 'Don't you brush teeth, shower and cook?' " Jones recalled. "It's got to be alarming."

Four-year-old Michael Joseph often visits at the Georgetown home of his grandfather Charles Eason, whose tap water registered 36 times the EPA limit in the random testing. "It's shocking," Eason said of the test results.