The Occoquan Reservoir, which supplies drinking water to about 600,000 Northern Virginians, is down to one of its lowest levels in recent times.

The reservoir held 5 billion gallons of water yesterday - 4.2 billion gallons less than at the same time a year ago.

Officials of the Fairfax County Water Authority, which owns the reservoir, said they would wait a week before considering any conservation measures.

"We think we have reason to be concerned," said James Warfield, the authority's public affairs director. "When the reservoir is down this low in the summer, it can't replenished that easily."

On the other hand, the Potomac River, the water supply for the District and a large part of suburban Maryland, is in better condition. While the drought in Virginia has lowered the Occoquan, close-to-normal rainfall in the Maryland part of the Potomac basin has maintained the flow of the river above critical levels.

If the dry spell in Virginia continues, the Occoquan's supply will shrink at an accelerated rate because the reservoir is shaped like an inverted pyramid.

The present demand from the reservoir is about 65 million gallons daily. At that rate of usage, and with little or no rainfall, the reservoir would theoretically go dry in about 60 to 75 days. The normal dry season can continue as long as three more months.

A water supply committee appointed by the Fairfax Board of Supervisors is exploring alternative sources of water supply. One possible source is the Vulcan quarry adjacent to the Occaquan. It could impound about 1.4 billion gallons. The quarry could become available in a couple of years when stone extraction there is finished.