A bicycle and pedestrian bridge over the George Washington Memorial Parkway that will provide a hub for the area's three major bike trails is nearing the end of seven years of planning and is expected to be completed next summer.

The bridge will link the I-66 bike trail, the Mount Vernon trail, the Four-Mile Run trail and, by crossing Memorial Bridge into the District, the Rock Creek Park trail. It will also link the trails with the existing footbridge to Theodore Roosevelt Island and allow pedestrian access to Arlington County's waterfront.

Construction has been delayed because of the routine process of getting permission from a long list of government agencies, and because of an old water main that, if ruptured, would have cut off the water supply to the Pentagon and National Airport.

"Someone remembered seeing the water main on some old plans" last year, said Arlington County Engineering Design Supervisor Danny Matthews. "I checked through old drawings of the Washington Aqueduct, in which I discovered a 16-inch cast iron water main that was installed in the 1940s. If we hadn't discovered the water main, we would have gone merrily along until disaster struck."

Matthews verified the water main's existence in February, and the National Park Service issued a construction permit for the bridge in late March. Bids for the bridge are expected back June 12, said Arlington's chief planner, John Hummel.

"If the bids are acceptable and . . . if funds are sufficient," Hummel said, "the plan is to get the [Arlington] County Board to award the contract at its meeting of June 28."

In 1983, the Virginia Commission on Outdoor Recreation provided a $225,000 grant for construction of the bridge, whose cost was estimated the following year to be about $750,000.

If the bidding process goes well, construction should begin in August or early September, and it is expected to take 10 months to complete, Matthews said.

"We've been waiting a long time for this bridge," said Linda Keenan, director of the Washington Area Bicyclist Association. "It's a terrible hazard getting from Arlington Memorial Bridge to the I-66 bike path," she said.

The bridge will also offer access to the hiking trail between Roosevelt Island and the Cabin John Bridge along the Potomac River that was blazed last year by the Potomac Appalachian Trail Club.

In addition, the park service is planning to extend the George Washington Memorial Parkway bike trail, which now dead-ends at Arlington Memorial Bridge, to Roosevelt Island, said parkway assistant superintendent Margaret Osborne.

Because the bridge crosses the federally owned parkway and will affect the view of motorists along the scenic route, an old water main was not the only cause for delay. Construction plans had to be approved or reviewed by a number of local, state and federal authorities, including, by Matthews' count:

*The National Park Service, which has jurisdiction over the parkway.

*The U.S. Commission of Fine Arts, which approved plans in September 1984.

*The Federal Highway Administration.

*The National Capital Planning Commission, which signed off on the project in October 1984.

*The Virginia Commission on Outdoor Recreation.

*The Virginia Department of Highways and Transportation, because part of the project is to rest on the I-66 right of way.

*The Roosevelt Renaissance Commission.

*The D.C. Department of Parks and Recreation.

*The D.C. Historic Preservation Commission.

"The problems haven't been that many," said Matthews. "It was just a matter of all the hurdles, commissions, associations and so forth we had to go through."

Each new commission involved the drafting of new drawings and a presentation, said Matthews. And with some reviews, changes had to be made in the plans, such as moving the location of the bridge to preserve trees and lowering the bridge height to make it less obtrusive.

"Anything in this region is complicated because there are so many interests," said Osborne, who added that coming up with a concept, preliminary designs and final designs routinely can take three years. In the case of the bridge, "it's pretty complicated to cross that many lanes of traffic," she said.

"It is unusual that it would take us as long as it did to get National Park Service permission," said Hummell. "It got somewhat involved with the study to widen the parkway. The service wanted, logically, to be sure that what they gave us permission for wouldn't affect plans to the widen the parkway. That process probably in total delayed us a year, maybe."

The park service recently reached a compromise with Rep. Frank R. Wolf (R-Va.) that provides for parkway improvements at key entrances and exits between the Roosevelt Bridge and Spout Run, where there are frequent rush-hour tie-ups.