Engineers have found no adverse side effects from a chemical added to the drinking water in part of the District's distribution system six weeks ago and intend to expand its use system-wide early next month, federal officials said yesterday.

Authorities at the Washington Aqueduct added orthophosphate June 1, saying they expect the chemical to form a protective coating on lead service pipes and stem the leaching of the toxic metal into the water supply. Although some water-quality experts had warned that reddish, discolored water could be produced by the chemical, no such problems have been discovered, said Jon M. Capacasa, director of water protection at the U.S. Environmental Protection Agency's mid-Atlantic office.

Tests at several houses in the limited distribution area in Northwest Washington have shown that the lead levels remain above the federal limit, Capacasa said. But officials have cautioned that the orthophosphates will need six months to a year to show results.

Therefore, aqueduct officials, in consultation with a technical advisory panel established by the EPA, will go forward with a plan to add orthophosphates to the water in the remainder of the distribution system about Aug. 9, he said.

"The aqueduct reported to EPA this week that this date is still achievable," Capacasa told D.C. Council member Carol Schwartz (R-At Large) during a hearing on the matter at city hall yesterday. "We want to remind the council and the public that reductions in lead levels at the tap will likely only be accomplished after six months or more have passed, possibly up to a year or longer."

Lead levels above the federal limit of 15 parts per billion have been found in tap water at thousands of D.C. homes. Capacasa said yesterday that elevated levels were found recently at two locations operated by the U.S. Navy: the Washington Navy Yard and the Nebraska Avenue Complex.

Although the two campuses receive water from the Washington Aqueduct, the Navy conducts separate tests from the ones taken throughout the city by the D.C. Water and Sewer Authority. This is the first time the Navy has found excessive levels in the two years it has been conducting the tests, Capacasa said.

Navy Lt. Cmdr. Edward Zeigler, a spokesman for the Naval District of Washington, said only a few residences are on the two bases, which mostly contain offices. The Navy shut some taps and posted signs over others warning workers not to drink from them, Zeigler said.

The orthophosphate treatment has been applied only to the water that serves 20,000 homes in a slice of Northwest, including parts of Chevy Chase, Cleveland Park and Tenleytown. There is no guarantee the chemical will solve the lead leaching problem, but it has worked in some other cities.

In the District, engineers report that tests on a laboratory-controlled "pipe loop," which speeds up the effects of the orthophosphates by stimulating them with electrolytes, have shown that the chemical is effective in preventing lead from leaching after a sufficient amount of time, said Rick Rogers, the EPA's regional water-quality chief.

Capacasa recommended that District residents with excessive lead levels continue to use water filters and flush their pipes for 10 minutes daily if their homes have lead service lines.

"Local agencies and EPA will notify the public when these measures are no longer needed," he said.

WASA announced this week that General Electric has donated 12,500 water pitchers designed to filter out lead. Those pitchers will be distributed free to residents who have excessive lead in their water. About 30,000 filters, donated by Brita and PUR, already have been distributed, the agency said.

Jon M. Capacasa, regional director of water protection for the EPA, left, shown with Thomas C. Voltaggio, deputy regional administrator, says the use of orthophosphate should reduce lead leaching within about six months.