Although it still doesn't have a permit to go ahead with construction, the Fairfax County Water Authority has decided to spend another $1.4 million to plan its proposed Potomac River water pumping-treatment facilities.

The new appropriation has raised to $4.8 million the amount the water authority says it has had to spend on the $53 million project, which could be killed by an unfavorable decision by the Army Corps of Engineers.

THe water authority applied to the Corps for a permit in September, 1976, but a protracted controversy over how Potomac water would be allocated among local jurisdictions during a drought has delayed action. The corps is expected to decide on the permit by July.

The nearly $5 million expenditure covering administration, engineering and legal fees - was unavoidable, according to authority engineer-director James J. Corbalis Jr. "If we waited until we got the permit, work in progress would have to be suspended," he said "This would cause delays in completion and would increase the costs." He added that "you can't get a permit without spending a certain amount of money."

While approving more spending on the proposed Potomac facilities at its Thursday-night meeting, the water authority board also directed Corbalis to aget information from the Washington Aqueduct on how much water it could provide and at what cost. The aqueduct serves the District and part of Northern Virginia.

Withe the water authority's proposed Potomac intake encountering new opposition - this time from the Environmental Protection Agency and the Interior Department - the Washington Aqueduct may be a better solution for Northern Virginia's future water needs, according to Fairfax Supervisor Audrey Moore (D-Annandale).

Moore has virtually no support on the Fairfax Board of Supervisors fo her position, although the board did vote last week to hold a workshop on the possibility of buying Washington water Engineers, which operates the aqueduct.

According to 3 spokesman for the Corps of Engineers, which operates the aqueduct, its Dalecarlia treatment facility has a surplus capacity of more than 100 million gallons a day. While a sizable portion of that surplus probably could be made available to Fairfax, the cunty would have to build a transmission line from the District.