Last week when the Army Corps of Engineers issued its long-expected decision allowing the Fairfax County Water Authority to tap the Potomac River for future water supplies, the action didn't suffer for lack of patrons.

There was ReP. Herbert E. Harris, the Northern Virginia Democrat, ready to take credit for the decision at an 11 a.m. press conference.

And there was Fairfax County Board Chairman John F. Herrity, Republican challenger for Harris' 8th Congressional District seat in the Nov. 7 election, who was also poised to take credit for the Corps action at a 1:30 p.m. press conference.

As it turned out, neither man's press conference went off as planned, but that didn't stop them - or Virginia Gov. John N. Dalton - from saying who should be credited with winning the permit.

Harris had to cancel his planned Capitol Hill press conference because of the Metro transit strike and Herrity had to settle for an assistant water authority engineering director as his technical expert when his two announced conference participants failed to show up.

Undaunted, each man took some credit for the permit, which will allow the county water authority to build a $52 million water treatment and pumping facility on the Potomac and avoid a serious water shortage like the one that forced imposition of strict water controls last summer.

"It's a credit to everyone involved that after meeting with me last October, (Army) Secretary (Clifford) Alexander promised to try to get the permit issued in July and he did it," Harris said in a press release. "This is a remarkable example of how government can work when people get together in a spirit of trust and cooperation."

In a telephone interview, Harris said he had been involved in the county's attempt to get the permit as early as June, 1976, when he held hearings on the Washington area's water supply as chairman of the House Subcommittee on Washington Regional Affairs.

Harris said subsequent to that June public hearing, he held meetings and other hearings with the Army Corps of Engineers and state and local officials on the permit.

The Army Corps told the county water authority in January 1977, that it would not approve the permit until there was a regional agreement on how the Potomac's water would be shared during drought periods when the river's flow was low. That agreement was reached last January, but then the Interior Department said the agreement threatened the fish and plant life that depends on the river.

Harris said he became an intermediary between the Interior Department and the Army Corps of Engineers to resolve the dispute. Th Corps agreed to consult with the Interior Department to avoid any damage to the ecology of the river, he said.

Later, Herrity said at his conference that he, too, was respondible for meetings on the matter.

Herrity said he became involved "on an intense basis" on January 31, 1977, when he and other members of the county Board of Supervisors, asked Col. G. K. Withers of the Army Corps to explain why the Corps would not issue the permit.

Herrity said he also talked with former Virginia Gov. Mills Godwin and requested that Godwin, acting Maryland Gov. Blair Lee Ill, D.C. Mayor Walter Washington and Secretary Alexander meet to devise a low-flow allocation plan for the three jurisdictions.

Harris also took responsibility for this meeting, which Alexander also requested.

"There's no question that I called for that summit meeting with the governors," Herrity said.

At his press conference, Herrity praised the regional cooperation that made the issuance of the permit possible. "There has been involvement of an awful lot of people," he said as he mentioned officials of the Army Corps of Engineers, the Interior Department, the State Water Control Board, the county water authority, and others.

Asked about Harris' participation, Herrity said later, "Mr. Harris' involvement is speculative at best."

Asked about Herrity's involvement, Harris said, "I think after we got it started the (Herrity) was helpful."

A spokesman for the Army Corps of Engineers took a middle ground. "We've had a number of meetings with the two of them. They were kept abreast of what was going on," he said.

Construction of the new pumping station on the Potomac is scheduled to take about three years, and the authority has warned that if the facility isn't ready by 1981 there could be another water shortage in Northern Virginia. The county water authority supplies water to more than 600,000 persons in the city of Alexandira, and the counties of Fairfax and Prince William.

Many of those water users live in the Eighth Congressional District and both Herrity and Harris last week were probably as concerned with what the water users thoughts will be this fall as they will be in 1981, when the pumping station is completed.

In Richmond, Dalton, who had pushed for another scheme to tap the Potomac when he ran for governor, singled out fellow Republican Herrity for having "pursued a solution vigorously."