A construction crew error yesterday left a 5-square-mile section of Northern Virginia, including the entire city of Falls Church, with water everwhere but not a drop fit for an estimated 25,000 residents to drink.

Worried residents deluged Falls Church officials with telephone calls after an unappetizing brown sediment turned up in tap water in hundreds of homes, restaurants and stores in the area early yesterday morning. Health officials began analyzing it, but their tests take a minimum of a day to complete.

The muddied water -- and an official warning against using it before boiling -- touched off a minor boom in the sale of bottled water and other liquid alternatives. Several restaurateurs rushed to grocery stores to buy up packaged ice and canned drinks to placate customers.

"Spring water, distilled water -- this is all gone," said Russ fitzgerald, manager of the Grand Union supermarket on Rte. 7 in Falls Church, at midafternoon. "They've been buying anything liquid they can get their hands on."

Fitzgerald said many of his customers had turned to canned juices and were helping themselves liberally to his apple juice special.

The washing machines stood silent at the "Superclean" laundromat on West Broad Street, and owner Clarence Ellis, without a customer in the store, was out back shoveling the last of the snow out of the alley.

"It's a 24-hour laundromat, you know," Ellis said. "I'm sure there'll be plenty of people here tonight always are."

Sam Habibi, manager of the Wooden Nickel restaurant in Falls Church, said he was boiling cauldrons of water at half-hour intervals to meet his customers' needs.

To those who complained about its taste, Habibi said, "I've been giving people free Perrier. Usually I sell it for 75 cents. But I don't want people to suffer."

Fairfax County Director of Environmental Heath John Clayton said it was unlikely tests on the water would trun up dangerous bacteria, but urged residents not to drink unboiled water before this morning.

Officials said tap water in the affected area was running clear by late yesterday afternoon.

A spokesman for Falls Church department of public utilities said the incident occurred when workers installing a 20-inch water main at the intersection of I-66 and Haycock Road north of Falls Church, failed to flush the pipe of contaminants properly.

As a result, a large quantity of dirt entered the area's water supply, which serves Falls Church's approximately 9,000 residents as well as about 16,000 people in neighboring Fairfax County.

The affected area extended south to Rte. 50 west to the Capital Beltway and east to Willston-seven Corners, water department officials said. Water supplies in McLean, Tysons Corner,Dunn Loring and Merrifield were not affected.

Samples of the dirty water were rushed to the Fairfax County division of enviromental heath for analysis. Tow tests, one lasting 24 hours and the other several days, will be performed in the division's laboratory to be certain the water contains no bacteriological contaminants, officials said.

The water alert caused a flurry of excitement at Falls Church City Hall, where bureaucrats huddled to discuss the day's crisis in the unaccustomed glare of television news cameras. Office workers fielded dozens of inquiries, advising callers, for examples, on the safety of adding tap water to hydrated milk.

"It's been a big headache," said 'howard Smith, head of the city's department of public utilities. "Actually, I've been drinking the tap water myself -- but I wouldn't advise it."