Final approval is near on the design of a controversial $3 million-plus interchange and bridge over the George Washington Memorial Parkway south of National Airport, officials say.

The design calls for a four-lane bridge with double- and triple-lane ramps, and traffic lights. The plan recently was approved by the National Park Service and the Commission of Fine Arts. It still must be approved by the National Capital Planning Commission.

Now that final approval of the design is near, the case has stirred up a decade-old controversy over a land swap that made the interchange possible and opened the possibility of large-scale development on part of the railroad yards south of Crystal City and opposite the Washington Sailing Marina.

In response to those concerns, the Alexandria City Council last week voted to appraise the railroad yard property with an eye toward possibly buying it, perhaps in conjunction with Arlington County, the Park Service, and the U.S. Army Corps of Engineers. The idea was suggested by Rep. Stan Parris (R-Va.), who attended the council meeting to urge that the city prevent the railroad yard development.

Parris said that even the scaled-down development now proposed for the railroad yard "is a very, very large development and its impact on the parkway will be awesome. The parkway is totally saturated with traffic and this will have a very serious transportation impact on the city and much of southern Fairfax County, including Mount Vernon."

The interchange is designed to handle more than 7,000 cars at rush hour, almost exactly the number of cars now using the parkway during the 7-to-9 a.m. period.

Parris suggested that the vacant railroad land might be used--as much of it is already--to store silt dredged from adjacent Four Mile Run and the Potomac River. The two waterways require dredging every four or five years to prevent flooding and to keep channels clear.

That idea was criticized, however, by Richard Beadles, a vice president of the Richmond, Fredericksburg and Potomac Railroad, which owns the railroad yards and most of the undeveloped land in the city. He dismissed the suggestion offered by Parris as "unrealistic," saying the land is too valuable to be used to store silt. The railroad would pay for construction of the interchange.

The controversy began when the late Charles Fairchild, who had obtained a lease from the railroad to develop the land, arranged in 1970 to give 29 acres of marsh to the Park Service in return for parkway access to the railroad land.

The access agreed to in 1970 called for an interchange that would handle a maximum of 4,000 cars, which Park Service officials said at the time would have a "minimal" impact on the parkway.

But the plans of Fairchild, who lost the lease last year shortly before his death, soon were enlarged. He ended up proposing a "Potomac Center" that would have consisted of a $300 million high-rise office and hotel complex with parking for 18,800 cars. To handle this traffic, Fairchild asked the Park Service to approve a giant cloverleaf, parallel service roads and half a dozen ramps into the complex.

The Park Service rejected the request but agreed last year to a smaller "diamond" interchange that could handle 7,000 cars in rush hour.

The impact on the parkway was the major issue in a citizen suit brought against the Park Service in 1978. It was temporarily dismissed by U.S. District Court Judge Oliver Gasch as premature because the Park Service was not then considering any specific interchange plans.

However, Gasch said that a comprehensive environmental impact statement "may well be justified as soon as the Park Service approves any plan for construction of the access structure."

The Park Service contends that it complied with the environmental regulations when it approved the exchange and now is obligated to provide access to the railroad yard.

Park Service assistant regional director John Parsons said the project would provide a new, safer entrance to the sailing marina at virtually no cost to the Park Service.

Beadles said last week that RF&P did not want to be associated with Fairchild's plans, which he called "grandiose" and full of "Buck Rogers skyscrapers."

Beadles, who said that the railroad has no immediate plans to develop its yards or even to build the interchange, said the company wanted to reassure Alexandria that "we care about the city and are committed to doing the right thing."