A new regional park proposed for Alexandria would be created out of what one park official calls "a piece of blank paper" - the largely barren wasteland along the city's southern border and the Beltway.

The site which will encompass 35 to 45-acres, depending on how much land the authority decides to purchase, was approved conditionally last week by the Northern Virginia Regional Park Authority. Lying in the center of the city's largest industrial area, the future parkland is now zoned for warehouses and high-rise commercial and residential development.

It is surrounded by railroad tracks, the Beltway and a huge concrete and stone ditch, which is the new flood-proof channel for Cameron Run. In addition, the city plans to build a two-lane industrial road - Eisenhower Avenue - through the future park along the edge of the new Cameron Run channel. Metrorail tracks will soon go up along the other side of the tract.

A divided Park Authority board approved the Alexandria site. The majority called the project a "challenge" to attempt to turn the desolate, bulldozed area into scenic parkland and an apropriate place to spend the $1 million the Park Authority programmed two years ago for its first designated urban park."

However, the four Fairfax County and Fairfax City members voted against it, alleging it is not an appropriate park site and that Alexandria is again "blackmailing" the park agency into approving a park in the city.

Several Alexandria City Council members earlier this year threatened to withold funds from the park agency if the new urban park isn't located in their city although the Alexandria council has taken no such position officially.

Alexandria similarly suggested in 1969 that the Park Authority by and restore Carlyle House and the Alexandria National Bank building, at Fairfax and Cameron streets, as an unofficial condition to the city's joining the Park Authority. Shortly after Alexandria joined, the park agency spent $2.7 million on Carlyle House, including state and federal grants. The bank is now being restored with private funds.

Alexandria Mayor Frank Mann last week protested it is not "blackmail" but the city simply wanting "to get back dollar for dollar" what it puts into the Park Authority. "And we think it (Cameron Run) is a good site for an urban park . . . The authority shouldn't always be going out in the middle of farmer Jones' pasture and making a park" but should create parks near city dwellers as well as suburbanites.

More than 95 percent of the Park Authority's 8,000 acres of parkland are in rural or until-recently rural sections of Fairfax and Loudoun counties. But Arlington's Potomac Overlook Park, a small nature sanctuary near Spout Run, can be considered an urban park as can two of the authority's newest parks, Upton Hill at Seven Corners on the Arlington-Fairfax border and the 42-mile W&OD railroad right-of-way, now being converted into a biking and hiking trail from Washington to the Blue Ridge Mountains.

Some of the antagonism to the Alexandria site appears to be due to the Carlyle House acquisition and Upton Hill, which is the authority's most expensive park. Its 26 acres in the heart of Seven Corners' prime development area cost about $80,000 an acre. About $1 million is being spent there to build a major public swimming facility.

Final Park Authority approval of the Alexandria over what kind of park should be created along Cameron Run and on a compromise now being negotiated with the city over the extension of Eisenhower Avenue through the park.

A major public tennis complex was proposed two years ago when the site was first considered, as Northern Virginia has no large public tennis facility.

But agreement over what goes into the park does not appear to be a major stumbling block, since Alexandria officials and board members have expressed confidence that planners will come up with something they can agree upon.

The major objections raised by board members was the city's plans to extend Eisenhower Avenue through the park. The City Council this summer reduced the size of the proposed road from six lanes to two and moved it from the center of the park to one side, along the Cameron Run channel.

But even Arlington County Board member Dwight Rettie, whose urban committee studied and proposed the Alexandria site, and calls it a great "conservation challenge," said he too is concerned about plans for any road through the park. A condition of the Park Authority approval is that the need for the road be restudied.