Lead levels in D.C. drinking water fell significantly after the city's water treatment plants switched to chlorine for annual pipe-flushing this spring, providing the first concrete evidence of the cause of excessive lead levels in thousands of homes.

Experts have wondered for months whether the city's lead problem could be blamed on a change in disinfectants, from chlorine to chloramines, at the water treatment plants in 2000. Lead levels in household tests exceeded the federal standard in 2002 and have continued to do so.

The treatment plants stopped using chlorine because it creates byproducts that are linked to cancer with chronic exposure. Water experts say further study is needed before they decide whether chlorine will play a role in solving the city's lead problem.

From April 2 to May 8, utility officials switched back to chlorine, a yearly change intended to rinse bacteria from the pipes before summer. During that time, officials said yesterday, lead level test results in homes with lead service lines were 25 percent to 30 percent lower than they would have predicted.

Even more dramatically, the D.C. Water and Sewer Authority had collected before and after results from seven homes in the city. Lead levels fell tenfold in one home and plummeted in most of the others. "We think that this is a major finding," said WASA's general manager, Jerry N. Johnson.

"We thought during the switch to chlorine we might see a small change," said Rick Rogers, the regional water-quality chief for the U.S. Environmental Protection Agency, which regulates WASA. "Hearing about such a drastic change was quite surprising."

WASA announced the test results yesterday to a committee of scientists and other experts convened by the EPA to fix the city's lead problem, and the group talked about them for an hour during a conference call. For now, utility officials and regulators said, they are sticking to the lead-reduction plan that EPA approved April 30.

The water treatment plants, run by the Army Corps of Engineers, already have switched back to chloramines, a combination of chlorine and ammonia. On June 1, a chemical that has reduced lead levels elsewhere, zinc orthophosphate, will be added to the drinking water that goes to a section of the city. If no major problems result, the treatment plants will add it to water throughout the city and in parts of Northern Virginia in mid-July, but officials say it could take months to show results.

Customers who live in about 1,800 homes without lead service lines and who had low lead-level test results during the time that chlorine was used will be offered free retesting, Johnson said. The utility does not see the need to retest homes with lead service lines because they are being given free water filters, he said.

Government officials cautioned that although chlorine appears to have countered high lead levels, it is not a quick fix. For one thing, many homes still had high lead levels in tests during April and the first week of May.

Of 1,683 D.C. homes with lead service lines tested in March, 52 percent had excessive lead levels. But in another testing sample -- 478 homes with lead service lines -- 26 percent tested high from April 20 to May 8, when chlorine's effect would have been strongest.

"We don't want to say that this is the silver bullet," Johnson said. Experts also are concerned that chlorine could have unexpected harmful effects on the water system -- much as chloramines are suspected of having -- and there needs to be extensive study before any change is made.

Because chlorine creates byproducts that are linked to cancer, the EPA recently tightened permissible levels of those byproducts in drinking water.

The surprising drop in lead levels announced yesterday not only feeds the theory that chloramines drove lead levels up but also shows that chlorine might lower them. Some scientists believe that chlorine may offer protection against lead. "This is the first time there is real good data from a real-system situation" on that theory, Rogers said.

"It's going to mean an awful lot more head scratching and a lot more work on the research side," he said. "This is groundbreaking science in the water industry."

"We don't want to say that this is the silver bullet," WASA's Jerry Johnson warns.