Fairfax has informed the federall Environmental Protection Agency that the county "may be required to impose a (sewer) moratorium as early as August" in the fast-growing Dulles-Herndon-Reston area.

In a letter to EPA, County Executive Leonard L. Whorton said that if flows to the Blue plains treatment plant in the District continue to be as heavy as they were in January, the county could run out of capacity in a few months. January flows were about 15 1/2 million gallons daily. The county's capacity is 16 million gallons.

However, county officials said in interviews that the prospects for a moratorium are remote. The urgent tone of Whorton's letter, they said, was part of the grantsmanship ploys that are used with EPA.

In his letter to EPA Regional Administration Jack J. Schramm, Whorton said the possible moratorium was one of the "emergencies" facing Fairfax as it tried to comply with a judge's order that the county provide adequate sewage treatment for the northwestern part of the county. The emergencies, Whorton said, entitled the county to go ahead on the first stage of the $20.4 million Difficult Run pumpdown before a federal decision on full funding of the project. The pumpdown is Fairfax' solution to providing more capacity.

"You use the figures and the tactics to make your point," Public Works Director glen Ehrich said. "But we're not ready to wave the flag" and declare a moratorium.

While Fairfax flows to Blue Point in January were near the 16-million-gallon limit, January, as Ehrich pointed out, was "an unusually wet period."

In wet weather, hundreds of thousands and even million gallons of water from rain or snow can infiltrate leaky sewer pipes. All that water gets charged as part of the sewage flow.

But wet weather doesn't last. The 12 month average ennding in January of Fairfax flows to Blue Plains was slightly more than 13 million gallons daily. Even if future commitments - worth about 1 1/2 MGD - were added, the county would still be safely under its 16 MGD ceiling at Blue Plains.

In fact, according to Deputy County Executive J. Hamilton Lambert, the county should have enough capacity at Blue Plains to last until the Difficult Run pumpdown - the project for which the county is seeking EPA funds - is completed in about 2 1/2 years.

He added, though, "We're operating in a very tight time frame." The pumpdown would permit the county to divert sewage to its Lower Potomac treatment plant when there is no more capacity at Blue Plains.

The pumpdown has been embroiled in controversy ever since the county and the State Water Control Board tried to give it higher priority on the funding schedule.

EPA, saying the county and the board violated federal rules on project priorities, refused to fund the pumpdown during the current fiscal year.