The water main break that has disrupted service throughout Arlington County was located last night, and an official estimated that it would be repaired in a month to six weeks.

County public works director Henry S. Hulme Jr. described the discovery of the 6 foot by 4 foot break in the pipe under the Potomac River as "good news." Earlier it had been estimated that repairs would require from two to six months.

Discovery of the break by a diver sent through the 48-inch diameter pipe came only hours after the county board officially declared a state of emergency. The board called on residents and businesses to comply with water restrictions, almost all of them on outdoor use.

If those restrictions are followed, county officials said, they believe the backup water system will continue to provide an adequate supply of water to the county's 155,000 residents.

The supply is now near its previous capacity.

"It is a community emergency," said Board Vice Chairman Albert C. Eisenberg. "We're all in it together. We all share in its effects, and we all share in its solution."

The emergency began about 5 a.m. Friday when the pipe carrying water from the District's Dalecarlia Water Treatment Plant to Arlington ruptured just downstream from Chain Bridge.

County Manager Larry J. Brown declared a state of emergency, which was ratified yesterday by the County Board at its scheduled meeting.

In the early evening, Hulme said, a diver from a private firm found the oblong shaped break in the pipe, which is buried in concrete beneath the river bottom.

He said the cause of the rupture, which was in the upper part of the pipe, about 135 feet from the Virginia shore, could not be immediately determined.

"It blew out almost like a tire," Hulme said.

By the end of this week, he said, authorities hope to have enough information to solicit bids on repairs.

Actual repairs, during which one of the 16-foot-long sections of the pipe would be replaced, "shouldn't take more than two or three weeks," he said.

Hulme said the county first called on Navy divers Friday to locate the leak. When they were unsure if they could continue, the firm of North and Parker Inc. of Stevensville, Md., was retained.

The 48-inch pipe was built in the late 1960s, with technology still used today, and it was expected to last about 50 years.

It supplied about 150 million gallons of water a day to the county. He said pipes older than 10 years do not usually have warranties and that it would be "very difficult to prove the problem is a pipe failure."

Board members said yesterday that they were grateful the county had a backup system -- three 20-inch pipes suspended from Chain Bridge. They can carry about 85 percent of the water that the 48-inch pipe downstream carried. But officials said that adherence to the new water restrictions should decrease the demand by 15 to 20 percent, allowing the county to supply adequate water for firefighting, bathing, cooking, drinking and similar needs.

Because of the break, Arlington, which buys its water from the Army Corps of Engineers, suspended its practice of piping 1.5 million gallons a day to help ease Fairfax County's water shortage.

The Pentagon is augmenting the Crystal City area's water supply.

"There's a great comfort in recognizing that it's only because Arlington has this backup in its system that we are able to use water in our homes and businesses as we're used to," said board member Ellen M. Bozman.

The restrictions on watering lawns and gardens continues during off-peak hours, said board Chairman Mary Margaret Whipple, because that time is needed to refill storage tanks.

The board, she said, "is concerned about the many lovely plants and gardens in the community," but the constant flow of water could deplete the supply. Residents may water plants and trees or wash cars if they use three-gallon buckets, officials said.

Arlington residents shopping at the Farmer's Market beside the courthouse where the board meets said they did not expect much difficulty in coping with the restrictions.

Many said they thought the breakdown was connected to work done on a part of the distribution system earlier in the week, but Hulme said they were unrelated.

Shopper Dave Voorhees said, "The board is going about it in the right way by asking for cooperation. The main problem will be neighbors calling police about other people who are watering when they shouldn't be. It needs a cooperative effort more than policing. If you don't get the cooperation, you can't really police it."

"That's bad news," Ron Thornhill, who rents one of seven county-owned gardens, said of the water restrictions. "If this continues, with the drought conditions, we'd have no fall garden at all. Unless we have rain, there's going to be an adverse effect."