THANKS TO A STRING OF unconscionable decisions made by the National Park Service some years ago, a monstrous proposal for a monster commercial development now threatens the George Washington Parkway in the vicinity of the marina north of Alexandria. There are plans to erect a $300 million, ever-so-high-rise office complex along the parkway just south of National Airport.

Developer Charles Fairchild insists that he hasn't settled on the exact number and heights of buildings for his proposed "Potomac Center." But the last information obtained by the Park Service called for 16 buildings ranging from eight stories to - look out - 29 stories, with parking for up to 18,800 cars. All of this - or however much Mr. Fairchild really contemplates doing - would be cramped into a narrow strip of undeveloped land in the Potomac Railroad Yard 200 feet from the parkway.

As to access, Mr. Fairchild has submitted plans for an interchange entrance with a four-lane bridge over the parkway, ramps on the Potomac River side and, on the other side, a pair of two-lane service roads with a total of seven separate motor entrances and exists.This concrete tarantula, moving cars in and out of a road already choked at rush hour, would be built right at the site of the Daingerfield Island marina.

Fortunately, a group of citizen is suing to prevent construction of the bridge-interchange. That would probably kill the project, since the proposal hinges on the parkway being the major entrance to the complex. The citizens claim that the Park Service got into this entire deal without ever giving sufficient public or legal notice of a land swap in 1970 that traded access to the parkway opposite Daingerfield for acreage held by Fairchild & Co. near Dyke Marsh, a wildlife area south of Alexandria - and without the proper environmental reviews.

If the citizen's suit doesn't work, there's another possibility: To their credit, the current National Capital Parks director, Jack Fish and other Park Service officials are having second thoughts about the original land swap. Mr. Fish has proposed buying out Fairchild's access rights. It may just take such a move to kill this project. But kill it someone must - and the best place to start would be to deny the proposed access. Anyway you look at it, the Potomac Center project should be knocked off the drawing boards.