Repairs to an Arlington water main that ruptured under the Potomac River two weeks ago will cost between $150,000 and $250,000, county officials said yesterday. At the time of the break, officials said the repairs could cost as much as a million dollars.

County Public Works Director Henry S. Hulme Jr. said the repairs to the 48-inch wide pipe below Chain Bridge could be finished by mid-September. Water restrictions that limit lawn watering to alternate days will remain in effect until the work is completed.

The county has retained North and Parker Inc., a Stevensville, Md., engineering and construction company, to repair the damaged sections of the pipe. Work may begin by Aug. 13, said John Hummel, Public Works' chief engineer.

"This is not an easy job at all," said North and Parker Vice President Howard Parker, adding that pipes similiar to the one that supplied Arlington with much of its water do not break often. "They usually are noted for reliability and longevity." The pipe was installed in the late 1960s and had a life expectancy of 50 years.

The 6-by-8-foot hole in a 16-foot section of the pipe was the result of erosion caused by storms that disturbed the riverbed. A diver for North and Parker found the break two days after it disrupted service to the eastern part of the county.

County officials said a section near the hole was weakened and also must be replaced before water can be pumped through the main.

In the meantime, two 20-inch pipes suspended below the bridge are providing the county with 85 percent of its water needs.

Hulme said the repairs will be paid for with funds from the regular Public Works budget and from increased revenue earned in the two months preceding the rupture.

"The ironic thing is that during the dry spell, people used more water and we got more revenue," Hulme said. "Much of the cost will come from that."