Although they agreed Northern Virginia is "facing a very, very grave situation," leaders of Alexandria and Fairfax and Prince William counties said yesterday they would continue to ask for voluntary water conservation to prevent the Occoquan Reservoir from going dry.

Fairfax County Executive Leonard Whorton acknowledged that "consumption has not dropped at all" since the entirely voluntary first stage of the regional water emergency was declared last Friday.

Whorton, speaking on behalf of the three jurisdictions served by the Fairfax County Water Authority, said the localities would wait seven days to see of voluntary efforts reduce comsumption. If they don't, he said, "It means we will have to go to mandatory measures."

The localities had been hoping the 612,000 customers of the authority in Alexandria, most of Fairfax and part of Prince William would conserve enough to reduce total consumption by 10 million gallons.

But instead water use had been running at the average rate of about 65 million gallons a day. Based on that level of consumption, the fact that 12 million to 13 million gallons is lost daily through evaporation and the extended forecast calling for an abnormally dry August, the Occoquan Reservoir would be unusable as a water source in about 50 days.

The water authority is almost wholly dependent on the reservoir for its water supply.But, with cooperative efforts, those supplies could be augmented.

For example, Lake Manassas, owned by the City of Manassas, contains about five billion gallons. According to City Manager C. M. Moyer Jr., the city could safely divert six million gallons daily into the Occoquan.

According to Moyer, James J. Corbalis Jr., engineer-director of the Fairfax Water Authority, has discussed that possibility with city officials, and the City Council may soon act on such a plan. Prince William County Executive Clinton B. Mullen said at yesterday's press conference that such a water loan would be "a last ditch effort."

Officials of the jurisdictions served by the Fairfax authority are reluctant to impose mandatory conservation restrictions because such measures would require extensive policing to insure compliance. Violations of such restrictions - which include drastic curtailment of outside residential and most commercial water use, including water-cooled air conditioning - could result in $500 fines.

Whorton said at yesterday's press conference that if the second stage of the region's water emergency is invoked, the commercial restrictions, which would require many businesses to close, would be reserved for a "last phase."

The mandatory restrictions could not be imposed without approval from Gov. Mills E. Godwin, but such clearance, if it is sought, is expected to be only a formality, local officials said.

Though voluntarism is still the byword, Whorton said of the situation, "It's deadly serious. The way we project if now, there won't be a drop of water in the reservoir by Oct. 1 . . . at the current rate of usage."

Actually the bottom 700 million gallons wouldn't be usable because they are beneath the lowest of the three intake beneath the lowest of the three intake pipes leading to the authority's filtration facilities.Even if it could be siphoned out, the water couldn't be used because it is too contaminated from eutrophication (lack of oxygen)

The reservoir, which can hold 9.8 billion gallons, contained only 4.2 billion gallons yesterday - the lowest level in 10 years and only 200 million gallons short of the all-time low, which was reached Nov. 9, 1968.

While the reservoir can be quickly replenished, the steady rainfall necessary for that happen isn't likely to occur during normaily dry August.