Plans to head off a water shortage in Northern Virginia in 1980 were thrown into confusion yesterday when the Army Corps of Engineers said there was "a possibility" that it might not approve withdrawal of water from the Potomac River on the Fairfax County side.

Col. G. G. Withers, chief engineer with the corps' Baltimore District, told the Fairfax County Board of Supervisors that in any case, a decision would take at least two years. Withers said the decision would require an environmental impact statement.

Although Withers gave no indication that the two-year time table could be accelerated the supervisors voted unanimously to call a meeting with the Northern Virginia congressional delegation to see how the review process could be speeded up.

Withers' expectation of a two-year study caught most of the Boad members, as well as some officials of the Fairfax County Water Authority, by surprise.

"This is preposterous," Fairfax Board Chairman John F. Herrity told Withers. "Two years is not a reasonable period of time. The county is going to fight to see that this thing is handled in a reasonable period of time. The county is going to fight to see that this thing is handled in a reasonable period of time."

The two-year time table means that the water authority, which supplies drinking water to 600,000 Northern Virginians, couldn't begin construction on a new treatment plant in time to head off the daily deficit of 9.9 million allons it expects to encounter by 1980.

The authority had hoped to begin construction of the $52 million plant to be located in northwest Fairfax County, next January. Under the authority's tight schedule, the plant would have been done barely in time to meet the peak summer demand of 1980.

However, based on Wither's assessment, construction couldn't begin sooner than about January, 1979, too late to meet the 1980 peak demand.

Peak demand - which is about 160 per cent of average demand - lasts from about May 30 to mid-September, a period when the Potomac is flowing at its lowest level because of a fall off in rain and high consumption.

The water authority has approval to build the treatment plant to meet new demans, but the facility cannot take water from the Potomac unless it has approval of the Corps of Engineers, which oversees construction affecting the nation's waterways.

The corps' concern is that additional withdrawal of water from the Potomac, which already supplies all of the District's and much of suburban Maryland's needs, could cause the river to run dry when peak demand occurs on days of record low flows.

At present, most of Northern Virginia, including virtually all of Fairfax County, get most of its drinking water from the Occoquan Reservoir, but the Virginia Health Department has put a ceiling on the water authority's treatment at the reservoir of 96.4 million gallons daily.

Peak demand in 1980 is, expected to reach 118.3 million gallons daily - 9.9 million gallons more than the Occoquan plus water bought from Falls Church, can supply.

One alternative the water authority is expected to explore is raising the height of the dam at the Occoquan Reservoir by five feet. The addition, which is estimated to cost more than $32 million, would be able to supply an additional 30 million gallons daily.

The newest member of the water authority, David G. Russell, said, "I'm not going to support building a new treatment plant on the Potomac," until approval from the Corps of Engineers is in hand.