The Loundoun County Board of supervisors yesterday approved construction of the water pipe the Fairfax County Water Authority wants to build on the Potomac RIver as a supplement to the Occoquan Reservoir. The pipe would be built in Loundoun and extend into Fairfax, where a treatment plant would be built.

While the action, approve on a 7-to-1 vote, moves the long-delayed project ahead, the water authority still must get approval from the Armu Corps of Engineers before it can begin construction, which would take about 30 months.

The corps has indicated it will not grant approval until Virginia - along with Maryland, which wants to increase its present use of the potomac as a water source - agrees to guarantee the District of Columbia its present share of Potomac water.

As the metropolitan area becomes more populous, allocation of Potomac water could become a critical regional problem during the occasional periods of low flow, when demand might exceed supply.

The Loundoun action came after the county planning commission had pigeon-holed the water authority's request dor a permit until the Corps of Engineers completes an environmental impact statement on the proposal. To circumvent the commission's action, the Loundoun supervisors had passed a special ordinance that permits them to hear cases deferred by the planning body.

In granting approval to the project, the supervisors required the water authority to shift the route of the last 1,000 feet of pipe leading from the river to the treatment plant.

Authority engineer-director James J. Corbalis Jr. said the shift would increase the cost of the $53 million project by $500,000.

When completed, the Potomac facility would be able to treat 50 million gallons of water daily, and could be expanded to 200 million gallons. Howere, the authority initially will only be able to take 24 million gallons from the river under the terms of another permit - this one granted by Maryland, which actually owns the river.