After 10 years of negotiation, the National Park Service has approved a controversial interchange on the George Washington Memorial Parkway south of National Airport as the entrance to a major Crystal City-type development. Opponents of the interchange claim it will aggravate serious traffic problems.

The interchange bridge and ramps will be the main entrance for Potomac Center, a development proposed to be built on 17 acres of railroad land next to the parkway opposite the Daingerfield Island Marina. The park service agreed to grant access to the parkway in 1970 in exchange for a donation of 29 acres of Potomac marshes known as Dyke Marsh along the parkway south of Alexandria.

The latest plans for the development, somewhat smaller and on half the acreage previously proposed by developer Charles M. Fairchild, call for a 1,300-room hotel, 835 condominiums or apartments, 2.6 million square feet of office space and 700,000 square feet of commercial-retail space. s

Potomac Center would contain parking for about 11,500 cars, almost three times the maximum 4,000 cars Fairchild predicted when the 1970 agreement was signed. However, it is significantly less than the maximum 18,800 cars he estimated four years ago, when he was proposing a complex that would contain 29-story buildings and officers for more than 22,000 workers.

The present "scaled-down" development would increase rush-hour traffic on the parkway by at least 50 percent, adding 7,129 cars to morning and evening rush-hour Slater's Lane, which connects the parkway with Rte. 1, according to a traffic consultant for Fairchild.

Alexandria's chief transportation official, Charles Kenyon, said yesterday that rush-hour traffic on the parkway already is above the saturation level for the four-lane road, and Potomac Center would have a major impact that will concern the city. Alexandria would have to approve construction in the railroad yards.

Last November, traffic counts showed 7,900 cars use the parkway during the 7-9 a.m. rush hours, a 36 percent increase since 1971 when the park service was predicting Potomac Center would have little impact on the parkway. Kenyon said the parkway has a safe operating capacity of about 4,800 cars an hour in one direction and is now carrying more than 6,400 in the morning rush hour.

Park service spokeman George Berklacy said yesterday, "We have had a legal obligation to grant access to the parkway since 1970 and we are in effect now honoring that commitment."

The present plan calls for only one entrance to the development from a bridge over the parkway, which will need one to two acres of parkland, according to the park service.

Previous Potomac Center interchange proposals, rejected by the park service, called for up to four lanes of parallel service roads within 10 feet of the parkway, a four-lane bridge and seven separate entrance and exit ramps into the development. That would have needed five to eight acres of parkland.

Berklacy said yesterday, "The scaled-down development will offer a minimal intrusion on the parkway and at the same time enhance safe access to Daingerfield Island."

The traffic impact and the size of the Potomac Center buildings near the parkway were the major reasons for a citizen suit against the park service in 1978. The suit was temporarily dismissed by U.S. District Court Judge Oliver Gasch. Gasch ruled that the issue was "not yet ripe for review" because the park service had not at the time approved any plans for the bridge.