Federal and local government officials said yesterday that they have decided to change the type of phosphate chemical that will be added to D.C. drinking water next week in hopes of reducing lead contamination, citing Arlington County's concerns about the impact on its sewage plant.

The Environmental Protection Agency signed off on a plan late last month to put a commonly used pipe-coating chemical into drinking water that goes to part of Northwest Washington, beginning June 1. If no major problems develop, its use would be expanded in mid-July to the entire city and to parts of Northern Virginia served by two Army Corps of Engineers treatment plants.

The EPA had approved a chemical called zinc orthophosphate because it was favored by an advisory panel of experts. Yesterday, EPA and local water officials said they have decided to go with another orthophosphate, phosphoric acid, a change they described as a minor tweak.

"We are completely confident that the orthophosphate . . . is going to achieve the results in due course," said Thomas Jacobus, manager of the treatment plants, told a D.C. Council oversight hearing. "We just changed the version of the way the phosphate is delivered."

But council member Carol Schwartz (R-At Large) said that she was uneasy about the sudden switch and that "I need a comfort level here." When she learned that the new chemical would cost 30 percent less than the old one, she demanded reassurance that money was not the motivation.

Jacobus and two other officials -- EPA Deputy Regional Administrator Thomas C. Voltaggio and D.C. Water and Sewer Authority Chief Engineer Michael Marcotte -- said cost was not the reason for the change. All originally favored phosphoric acid over zinc orthophosphate but had accepted the advice of the advisory panel.

Later, Schwartz said in a statement, "I can only hope and pray that these experts are doing the right thing this time around."

Arlington County and Falls Church do not have the widespread lead contamination problem found in the District, so they do not need an orthophosphate, the type of chemical most widely used in corrosive water nationwide to control the leaching of lead from plumbing. But they have a voice in water treatment decisions because the water they buy from the Corps of Engineers comes from the same treatment plants as the District's.

Last week, Arlington officials learned that new calculations showed that zinc from the zinc orthophosphate might overload the county's sewage plant. "We're not sure," Dave Hundelt, the county's acting bureau chief for water, sewer and streets, said in an interview. With a tight deadline and not enough information, "we expressed the concerns and looked to the guidance from EPA."

"We don't believe it will cause a wastewater problem," Voltaggio told the D.C. Council hearing, "but we are sympathetic to Arlington's wish."

Marcotte said WASA officials have asked the EPA to seek an opinion on the chemical change from the advisory panel, which included two consultants, an Illinois water official and a utility official. EPA has not released a report from the advisory panel, but EPA officials say it recommended zinc orthophosphate because it is so widely used, including in nearby Fairfax County, which also draws water from the Potomac River.

Also speaking at the hearing was a D.C. Health Department official, Thomas Calhoun, who reported on follow-up blood tests done on public school students after it was reported late last month that lead contamination had been found in some fountains and sinks at 29 schools. School officials say the 43 outlets with high lead levels have been turned off.

Calhoun, administrator of the department's emergency health and medical services, said 454 students have been screened so far at 10 schools. Two students at Wilkinson Elementary School in Southeast had blood lead levels above the federal standard, he said. One is enrolled in the city's lead-abatement program, and the other was referred to a private physician, he said.