The beleaguered Fairfax County Water Authority, whose Occoquan Reservoir has shrunk to record low levels this week, now faces another roadblock in its efforts to tap the Potomac River as an additional water source.

The Loudoun County Planning Commission, in a 3-to-2 vote Wednesday night, deferred action on a permit for the authority to build a water intake pipe off the Loudoun shoreline of the Potomac. The commission whose approval is necessary, said it won't act until an environmental impact statement on the project is completed.

Some of the commissioners asked Fairfax Board of Supervisors Chairman John F. Herrity why the authority couldn't dig wells to serve new customers. Herrity told them Fairfax's population - now approaching 600,000 - makes such a suggestion impractical.

The commission's stipulation on the impact statement - unless overturned by the Loudoun Board of Supervisers - could delay construction for up to six months. The water authority says it must begin work early next year if the Potomac plant is to be ready to meet an expected water shortage in 1980.

Alread clouding that prospect is the Army Corps of Engineers' decision not to approve the Potomac pipe corps. Virginia and Maryland that protects the District of Columbia's share of river water.

Because the planning commission deferred action, the issue does not go directly to the Loudoun supervisors, a majority of whom have already indicated their support for the Potomac project. Supervisors supporting the facility are believed to be considering a special ordinance that would take the issue out of the planning commission's jurisdiction and bring it before the Board.

While the commissioners did not explain their reasoning at Wednesday's meeting, at past sessions some of them expressed apprehansions that a big water plant on the Potomac would accelerate development in eastern Loudoun. Loudoun has been trying to put the brake on growth pushing westward from Fairfax.

While the Potomac plant would treat only 24 million gallons a day when it opens, it is being engineered to filter 200 million gallons daily - or almost twice the capacity of the Occoquan plant, which serves 600,000 persons.

It is the possibility of such growth and its impact on low water flows in the Potomac during dry periods that has led the Corps of Engineers to put conditions on any construction permit it might grant the water authority.

After the decision in Loudoun, the authority received no better news yesterday on the state of its Occoquan Reservoir. While voluntary water conservation actually increased, the reservoir dropped to another record low level - 105 feet, 7 inches. At the present rate of consumption, including normal evaporation, the Occoquan has a little more than 50 days' supply remaining.

A heavy, extended rain could raise the reservoir to its normal level, but such a downpour is not likely in this ordinarily dry period.

Heavy rains did fall this week through extensive areas of southern and central Virginia, ranging from Tidewater to Richmond to the mountainous Southwest, but not within the Occoquan's drainage basin. That basin extends through northern Prince William and western and southern Fairfax.

Even if the drought continues in the Occoquan watershed, its present supplies can be stretched another 24 days with 1.5 billion gallons of water that could be released from Lake Manassas upstream.

The water authority is negotiating with the city of Manassas - the lake's owner - on the purchase of the water. Area officials caution, though, that the lake can only meet short-term area needs.TThe Fairfax proposal for a Potomac River plant won support yesterday from Virginia Lt. Gov. John N. Dalton, the Republican Party's nominee for governor. During a campaign stop at the Occoquan Reservoir, Dalton suggested that Fairfax should sue the Army Corps of Engineers to obtain a permit for the proposal Potomac plant.

But Herrity, who was accompanying Dalton, quickly passed the proposed suit back to the candidate. "The state has a primary cause of action" against the Corps, not the county, he said.

Herrity said he was referring to a 1785 compact between Virginia and Maryland that granted Virginia rights to its Potomac shoreline even though Maryland owns the riverbed.

"I believe the Corps is in a serious conflict of interest situation" because it both controls permits to take water from the Potomac and supplies Potomac water to .D.C., Dalton said.