The expanded Fairfax City water treatment plant will be finished within 1 1/2 to 2 years, not longer as incorrectly reported in yesterday's editions of The Post.

Some Fairfax City residents got an unanticipated preview last weekend of the water shortage that some local officials believe will hit the Washington area this summer.

Residents in the southeastern section of the city found that their taps began to sputter Saturday and Sunday, although no one experienced a total water cutoff. The sputtering taps were due to low pressure which occurred when Fairfax City's treatment plant was unable to meet peak demand for water.

The plant located on Goose Creek in Loudon Coutny, serves not only Fairfax City but the fast-growing communities of Herdon in northwestern Fairfax County and Sterling Park in Eastern Loudon.

To head off more problems next weekend, Fairfax City's acting manager, Robert C. Norris Jr., called on city residents to eliminate all outdoor uses of water and cut down on other nonessential uses. Norris also said people abserved wasting water would receive notices from city police officers urging them to conserve. To notices are not summonses.

The Fairfax City water systems' problem is not finding enough water to treat - there's plenty of that - but a treatment plant whose capacity is being overwhelmed by additional customers. The plant can ordinarily treat six million gallons daily, temporarily increasing that capacity by about 65 per cent during peak hours. "But it's like a racehorse," city spokesman Robert Becker said. "You can't keep racing him at top speed."

The city's voters have approved a $6 million bond issue that would upgrade the treatment capacity to 15 million gallons a day, but the project won't be finished for two to 7 1/2-years.

The Fairfax City water system experienced supply strains last year and in previous years but never as early as mid-May. Heaviest demand in the past came in July and August.

In another development with broader implications, representatives of Washington-area jurisdictions met yesterday with officials of the Army Corps of Engineers on the controversy over who get how much water from the Potomac River when demand is high but flow is down.

The Corps has refused to sign an allocation agreement reached late last year by Virginia; the Washington Suburban Sanitary Commission, which serves Prince George's and Montgomery counties, and Maryland. The Corps has claimed the District's future water needs are not protected in the agreement. The Corps is in charge of the District's Dalecarlia water treatment facility.

While no agreement was reached at the meeting, which was closed to the public and news media, Corps officials spelled out "one option" for a low-flow agreement that would be acceptable to them.

Under the option, the District's percentage share of Potomac water at low-flow would not fall below its unspecified share during this past winter. This freeze on allocation would last until the Bloomington Dam, now being constructed up river in Garrett County, Md., is completed in 1980 or 1981.

Under the agreement reached by Maryland, the WSSC and Virginia, the growing suburban jurisdictions would get a bigger percentage of the Potomac's flow than D.C. which is losing population. This is why the Corps opposes the agreement as now drawn.

An agreement soon between the Washington-area jurisdictions and the Corps is critical to the Fairfax County Water Authority and the WSSC. Neither agency can proceed with plans to increase its capacity until the Corps, which must approve all construction on the Potomac and other national waterways, gives its assent. The Corps has said it won't approve any plan until a satisfactory low-flow allocation agreement has been reached.

Anticipating water supply problems this summer, the WSSC last week approved a four-stage conservation plan that could lead to a water cutoff affecting schools, public agencies and water-intensive industries.

The conservation plan begins with voluntary steps, but if customers don't cooperate or the Potomac's flow reaches some historic low flows, mandatory conservation may be necessary, WSSC officials said.