Arlington's major water main, which burst last July and forced county officials to impose water use restrictions that lasted into the fall, is expected to be back in working condition by the end of the month, said H.T. Angell, chief of the county's water and sewer division.

The July 11, 1986, rupture of the four-foot-diameter main beneath the Potomac River about 100 yards south of Chain Bridge cut off the major source of water for Arlington residents for several hours before a backup system -- three 20-inch pipes suspended under the bridge -- could be activated. Backups are still in use, and water demand is down because of a rainy spring, Angell said.

The cost of repairing the 48-foot-long damaged section of the concrete main has grown from an initial $200,000 estimate to more than $700,000.

Rough water conditions and poor visibility in the Potomac have hampered work on the main, which is buried in six feet of bedrock in the river's 35-foot-deep channel.

On Wednesday, a barge used by repair crews was swept downstream by a sudden rise in the river that broke the cables linking it and two boats to shore. Five men were aboard, but they were unharmed. The engines were started in calmer water and the barge was pulled to Fletcher's Boat House in the District.