A chemical that will be added to D.C. and Northern Virginia drinking water next week will produce a "noticeable reduction" in lead within a year, but it could be longer than that before some homeowners can stop using filters, an Environmental Protection Agency official said yesterday.

The colorless and tasteless chemical, phosphoric acid, coats the inside of plumbing to prevent lead from leaching into drinking water. It has been tried since June in a section of Northwest Washington. The EPA considers the test successful because there were no major problems with rusty water or elevated bacteria readings, two possible side effects.

On Monday, two treatment plants run by the Army Corps of Engineers will add the chemical to water that goes to 1 million customers -- everyone in the District, Arlington, Falls Church and Vienna, as well as parts of northeast Fairfax County that receive water from Falls Church. It will take several days to disperse.

The EPA will sponsor two public meetings in the District to explain the change. They warned homeowners that they might temporarily see reddish water, and urged D.C. homeowners with lead pipes to run water for 10 minutes before using it for drinking or cooking. Pregnant women, nursing mothers and children younger than 6 should drink only filtered water. All other District residents should flush the taps for one minute.

The widely used chemical, a type of orthophosphate, is intended to counter high lead levels detected in thousands of D.C. homes since 2002 but only made public in January. Scientists believe the contamination began after the treatment plants changed disinfectant chemicals in November 2000. The drinking water in Northern Virginia does not have elevated levels of lead, but there is no practical way to treat the water separately from the city's.

"We are changing the treatment process to ensure the public gets the highest-quality water delivered to their tap," Thomas Jacobus, the Corps of Engineers official who runs the treatment plants, said at a news conference. "We're confident that the effect of this will be seen."

The EPA, which oversees the city's water system, approved the chemical change this month. Yesterday, Rick Rogers, the drinking water chief of EPA's regional office in Philadelphia, called it a "major step." He said his agency -- which was criticized for not acting on the lead problem until this year -- pushed to implement, in a matter of months, a chemical change that typically takes two or three years.

Rogers said that though phosphoric acid should achieve noticeable results within a year, it could be longer before contamination subsides enough to be below the federal safety levels that trigger mandatory replacement of lead pipes and other actions. "We're not sure we'll get compliance with action levels in a year," Rogers said. "It may be a little longer."

Laboratory tests using sections of D.C. lead pipe have found that phosphoric acid does reduce lead, but more slowly than in other cities, he said. That may be because the scale inside D.C. pipes has "noticeably different" mineral makeup than in other cities, but scientists "are just not sure what that means," Rogers said.

The chemical will be fed continuously into drinking water at a cost of $600,000 a year, which Jacobus said would have a small impact on water rates. Backed up by city health officials, he said phosphoric acid is so safe that it is added to processed foods and drinks. The EPA also released a report saying that phosphate from the chemical would not hurt sewage plants or pollute the Chesapeake Bay.

Still, Rogers said the chemical could temporarily wash rust from pipes into drinking water. Homeowners should not drink or cook with discolored water, and should run it until it is clear, he said.

EPA officials have scheduled two public meetings in the District, one for tonight at Congress Heights United Methodist Church in Southeast and the second for Tuesday at the Martin Luther King Jr. Memorial Library in Northwest. Each includes an informal open house from 6 to 7:30 followed by a one-hour formal presentation and question-and-answer session.

No public meetings are scheduled for Arlington or Falls Church, but those jurisdictions are sending notices to customers and posting information on their Web sites.

Staff writer Annie Gowen contributed to this report.