The Northern Virginia Regional Park Authority has purchased the missing link in its 25-mile belt of park land along the Occoquan Reservoir stream valley in Fairfax County.

With the purchase of the 43 1/4-acre parcel four miles south of Bull Run Marina, the park authority will be able to finish by late spring a 21-mile, unbroken hiking and bridle path from Bull Run to Fountainhead regional parks.

"This is the culmination of a 20-year dream," said Elisabeth Hartwell, a Fairfax member of the park authority. "Preserving the 25 miles of shoreline . . . was planned years ago to provide a buffer for development that is already pushing toward the (stream valley's) borders."

"It's a beautiful property, almost untouched, and very secluded," said Barbara Hildreth, Fairfax's other representative on the park authority. "We plan to keep it that way, but we will give the public access to it via trails."

The regional park authority, which is financed by Fairfax, Arlington and Loudoun counties and the cities of Alexandria, Fairfax and Falls Church, owns 5,000 acres of park land along the Fairfax side of the Bull Run-Occoquan River stream valley, which includes the reservoir serving more than 600,000 Northern Virginians.

On the other side of the reservoir, in Prince William County, most of the land is in private hands and is being developed.

There are fears that uncontrolled development in the stream valley will lead to serious pollution of the reservoir. While the Prince William shoreline is not set aside for park land, officials of that county point out that most of the development in the affected watershed is in Fairfax, even though its shoreline is protected.

The 43 1/4-acre parcel purchased by the regional park authority cost $261,684. Funds came from a park bond referendum approved in 1977.