Fairfax County, which has encountered problem after problem in trying to tap the Potomac River, yesterday decided to take another look at an old and rejected proposal - buying water from the District's underutilized system.

The Board of Supervisors, endorsing a motion by Audrey Moore (D-Annandale), will meet with officials of the Fairfax County Water Authority and the Army Corps of Engineers to find out if the District could provide a supplementary source when the water authority's Occoquan Reservior won't be able to meet demand in its service area that probably will be by 1981.

Until now, the supervisors have pursued a single-minded course in trying to find a supplement to the Occoquan Reservoir. That course calls for water being taken from the Potomac River and being treated at a plant in the northwestern corner of the county.

While the Fairfax Water Authority, in 1971, rejected using the District's Dalecarlia plant for supplementary water, that possible solution is attracting attention again because the proposed Potomac water pipe is mired, once more, in problems.

The Environmental Protection Agency has called the Potomac plan - along with an agreement on how to share river water during drought - "environmentally unacceptable." Objections have been raised as well by the Interior Department.

The Fairfax Water Authority hopes to get a permit to build the Potomac pipe by July so the project will be ready when the Occoquan can't meet all demand. But the Corps of Engineers has indicated that the EPA and Interior objections will have to be settled first.

Although Moore said in a memo that "Dalecarlia could be a much cheaper source of water for Fairfax County than the Potomac intake," her colleagues are not enthusiastic, even thoughthey supported her motion calling for another assessment.

Supervisor Marie V. Travesky (R-Springfield), said at yesterday's meeting: "I think we have . . . endangered our intake on the Potomac. If anyone was looking at any way to cast doubt on the intake in favor of a makeshift solution, this is it."

On Traversky's suggestion, the supervisors passed another motion which reaffirmed the board's support for the Potomac pipe even while Dalecarlia was being analyzed again.

The Fairfax Water Authority rejected a Dalecarlia solution because of the agency's "inability to obtain firm commitments" from the corps on water supply according to engineer director James J. Carbalis.

But according to Washington Aqueduct officials, the Fairfax authority "came to us with a proposal that was unacceptable."

In a letter dated March 16, 1971, Corbalis said the Fairfax authority's requirements would hit a peak of 70 million gallons daily in 1988, then taper off to zero over succeeding years, after the agency acquired its own long-term permanent supply.

Harry C. Ways, chief of the aqueduct, said the District system would need a long-term contract involving a steady amount of water in order to justify spending on expansion.

Corbalis said his March 16, 1971 letter was just one proposal, but according to Ways and his deputy chief, Perry Costas, the Fairfax authority made no other proposals.

Since 1971, all Fairfax analyses have been predicated on the assumption that it was Dalecarlia that couldn't make a long-term commitment. But that assumption is disputed by Ways, who says "it is technically feasible" for Dalecarlia to supply water to the Fairfax authority for a long term.

If Fairfax bought Dalecarlia water, it would have to build transmission lines from the District to the country. The cost of the project was estimated at $14.1 million in 1975. The figure was far cheaper than the $19.2 million estimate given for the Potomac facilities, whose cost were revised upward to more than $53 million last year.

In a memo to the supervisors, Corbalis claimed Dalecarlia is "unacceptable" not only because "the corps was not able to give us any firm commitments" but because it is not as "cost-effective" as the Potomac intake. He also said a transmission line to Dalecarlia would involve a series of time-consuming applications for permits - and that this "would entail intolerable delays."