Inside a fenced-off complex in Northwest Washington and behind the locked door of a concrete-block building, a chemical pump no bigger than a toaster oven waits to play a key role in fixing the problem of excessive lead in the District's drinking water.

When it is switched on tomorrow at the Fort Reno Reservoir, the pump will begin sending orthophosphate, a chemical that forms a protective coating inside lead pipes, to the water supply for one section of the city. At the Dalecarlia Water Treatment Plant a few miles away, where operators will monitor the pump remotely, an alarm will go off if water-quality numbers head in the wrong direction.

Government officials say they are confident that their solution will work, but experts warn that any new chemical added to a water system could disturb its delicate balance.

Officials want to know more about the basic science of how orthophosphate stacks up against other possible remedies. So in a makeshift basement laboratory in another building at the reservoir, scientists are testing a variety of treatments, using D.C. water and pipes.

They also want to proceed slowly to ensure that there are no persistent problems with orthophosphate. For now, the treatment will be applied only to the water that serves 20,000 homes in a slice of Northwest. The area, which includes parts of Chevy Chase, Cleveland Park and Tenleytown, is isolated from the rest of the city's water system.

If all goes well, water treatment officials say, they will begin using orthophosphate July 15 citywide and in parts of Northern Virginia served by two Army Corps of Engineers water treatment plants.

"We think this is a very good thing," said Richard A. Rogers, water quality chief in the Environmental Protection Agency's Philadelphia regional office. "We're moving forward as fast as possible, as safely as possible."

Officials warn that it could take months or even years to reduce lead to acceptable levels. One reason, some scientists say, is that the District's water and its problems are unlike those they have seen anywhere else.

At Fort Reno, a smell of sawdust infuses the room, the size of a large walk-in closet, where the chemical pump is installed. A 1,000-gallon chemical storage tank, which holds about a two-week supply, arrived last week. The pump will send a slow two-gallon-an-hour dose into the water supply. If orthophosphate is used systemwide and is successful, it would be employed permanently year-round.

Orthophosphate is widely used and so safe, EPA officials point out, that it is an additive in food and soft drinks.

Orthophosphate will not make the water look or taste different, officials said. But the phosphate might soften rust inside water mains enough that bits break loose and temporarily discolor water in some homes. Red water should not be used, officials said, and customers who notice it should call the D.C. Water and Sewer Authority's emergency line so crews can flush the pipes.

Tiny lead particles could break loose from pipes before the phosphate begins to coat them, Rogers said, so residents should continue to follow flushing and filtering recommendations. Bacteria also could be shaken loose from the pipes, and although Rogers said the bacteria are harmless, the EPA will require WASA to increase its water-quality monitoring during the trial.

Orthophosphate has lowered lead levels in Philadelphia and other older cities that have large numbers of lead service lines.

The Washington Suburban Sanitary Commission began adding orthophosphate in the fall to try to stop tiny leaks in copper plumbing pipes that have plagued thousands of homes. Spokesman Chuck Brown said there have been no reports of red water, and pinhole-leak complaints have declined. Fairfax County, which began using zinc orthophosphate in 1998, did not have problems with red water, according to spokeswoman Jeanie Bailey.

A decade ago, a consultant recommended that the Army Corps and the customers of its water plants -- the District, Arlington County and Falls Church -- consider using orthophosphate to prevent lead problems. But the Army Corps and its customers rejected the proposal, citing the cost and impact on their sewage plants. Arlington and Falls Church officials, although they do not need orthophosphate for lead reduction, now say they have no objections.

"Our lead levels are not a concern, but whatever lead levels we have, it will lower them, which is a good thing," said Robert Etris, Falls Church's public utilities director. He said the change will have a "barely noticeable" impact on rates.

Water engineers who are not involved in trying to fix the District's problem say orthophosphate is the first remedy that should be tried. But the chemical has not worked everywhere, and some communities have had to look for other solutions.

Experts say the extent of lead contamination in the city, where tests found excessive levels in the drinking water of thousands of homes, exceeds what they have seen elsewhere. Its cause and cure remain somewhat mysterious.

Scientists suspect that the high levels might have been triggered when the water system switched from chlorine, which produces cancer-causing byproducts, to another disinfectant, chloramine, in 2000.

EPA regulators began considering how to fix city lead levels detected in 2002, but pressure built for a quick decision only after the disclosure in January of the extent of the lead problem. The official approval came April 30.

Last week, the EPA approved a last-minute switch in the type of orthophosphate that will be tried. Water treatment officials, relying on the recommendation of an outside advisory panel, were set to go with zinc orthophosphate until Arlington raised concerns that the zinc could overload its sewage plant. So the EPA approved the use of orthophosphate, which water treatment officials originally wanted to use.

D.C. Council member Carol Schwartz (R-At Large) said she was unsettled by the sudden change, especially when she learned that phosphoric acid costs 30 percent less than zinc orthophosphate. At a hearing last week, she demanded, and received, assurances from city and federal officials that science, not money, was behind the substitution.

EPA officials will not allow members of the advisory panel to talk to the news media, so they could not be reached to comment on the switch.

The Army Corps of Engineers will spend about $3 million by Sept. 30, the end of the fiscal year, for engineering studies, hardware and chemical costs related to the use of orthophosphate, said Thomas Jacobus, water plants manager. Annual chemical costs after that will be about $700,000. Water customers will pay the bill.

The use of orthophosphate will add about $1.4 million a year in phosphorus-removal expenses at the regional Blue Plains Wastewater Treatment Plant, which is run by WASA, according to the utility's chief engineer, Michael Marcotte.

To answer lingering questions about the use of orthophosphate with hard science, contract scientists and WASA officials will rely on short-term experiments at Fort Reno. Scientists are conducting similar work at a lab at the Dalecarlia plant, but it will take months to show results.

"Our goal is to get this information to the scientists before they go full steam ahead," said Rich Giani, WASA's water-quality director. That means results by mid-July.

On the left side of the laboratory are six tabletop "pipe loops," assemblages of plastic tubing attached to short sections of lead or copper drinking water pipe. By measuring how each pipe reacts to an electrical charge, scientists can compare how rapidly they corrode in response to different chemical blends. Reducing the city's lead problem, scientists say, comes down to making the water less likely to corrode lead plumbing.

On the right side of the room are six larger experiment stations that incorporate three-foot sections of city lead service pipes, also being fed differing corrosion-control blends. There, the objective is to measure how much lead dissolves into the water when the plumbing is not used overnight.

"I do think this is a unique test of orthophosphate's capabilities," said Marc Edwards, a nationally known water expert at Virginia Tech who did not participate in the decision to use the chemical but endorses it. "This combination of factors to cause this perfect storm of lead corrosion has to my knowledge never been replicated elsewhere."