The long-awaited permit from the Army Corps of Engineers that will allow the Fairfax County Water Authority to tap into the Potomac River - and by 1981 end the annual summer threat of a water shortage in Northern Virginia - should be issued about July 1, according to Rep. Herbert E. Harris II (D-Va.)

Completion of an environmental impact statement last week by the Corps sets to begin construction, expected to take 30 months, of a $35 million intake pipe and pumping station on the river bank at the Loudoun-Fairfax county line, Harris said.

The station will have the capacity to pump 50 million gallons of water a day from the river to the Water Authority's customers in Fairfax and Prince William counties and Alexandria.

The 700-page environmental impact statement is scheduled to be published tomorrow in the Federal Register.Interested parties have 30 days from that date to raise objections, but Harris said completion of the document indicated that "major obstacles have been overcome."

Fred C. Morin, chairman of the Water Authority, said, "we don't have the permit yet," but he agreed with Harris that filing of the document was a major step toward construction. The authority will finance construction with bonds previously sold, Morin said.

Morin said the authority also is still trying to get permission from the Corps to buy water from its Dalecarlia Reservoir in the District. But the two projects are not inter-related, Morin said, and construction of the pumping station and treatment plant upstream will proceed regardless of what happens at Dalecarlia.

Water authority officials believe there could be minor shortages again in 1980, before the new facility is completed, because of increasing demand and problems with distributing available water.

Harris, however, said another agreement signed this year between the authority and the city of Falls Church should assure adequate water until the new station is in operation.

"We don't have a water shortage problem, we have a water management problem," said Harris, who criticized management of the area's water supply during last summer's drought.

The agreement permits the county water agency to draw between eight and 15 million gallons a day from Falls Church any time the level of the Occoquan Reservoir, which is the chief source of the authority's water now, falls two feet below normal.

"We won't have to wait until the end of the summer, as we did last year," said Harris.

Last summer, the county water authority did not begin drawing water from Falls Church until emergency drought conditions existed. Harris feels the new crisis water management.

Completion of the intake valve and pumping station in 1981 should assure Northern Virginia an adequate water supply at least through 1990, Harris said.

"We'll still have long-term problems to cope with," which, if unresolved, could result in frequent water problems by 2030, Harris said.

Hugh Calkin, a staff member of the House District Subcommittee on Regional Affairs, which Harris chairs, said the Army engineers indicated that early in the 21st century there will be a need for a second dam on the Potomac, similar to the one now being built upstream at Bloomington, near the Virginia-West Virginia border.

The remaining unresolved issue regarding the pumping station is establishing the minimum amount of water that must be allowed to go beyond the intake pipe to preserve aquatic life downstream.

representatives of the U.S. Department of Interior, which manages the C & O Park that runs parallel to the river on the Maryland side, and the Environmental Protection Agency, were planning to meet this week on that question. But both agencies have agreed that work on the pumping station can begin without settlement of the issue.