One of the three remaining gaps in the Washington area's largest park, Bull Run-Occoquan Park, was filled recently with the Northern Virginia Regional Park Authority's purchase of 53 acres along the Occoquan Reservoir on the southern edge of Fairfax County.

The two tracts of land still to be acquired are 46 acres of woodland near Bull Run Marina, which the park authority hopes to purchase next year, and 26 acres of woods on Sandy Run Creek, near the Occoquan Reservoir dam, to be acquired in 1979. Together they will complete an uninterrupted 26-mile park along the Fairfax County side of Bull Run and Occoquan Creek, and conclude the park authority's decade-long struggle to assemble the 5,000-acre recreation and conservation area.

"THis is NVRPA's first land purchase since the successful park bond referendum last June," and probably would have been lost to development had the bond referendum failed, park authority executive director Darrell Winslow said this week.

The 53 acres, which includes a half mile of river frontage, was bought for $176,000 from Mr. and Mrs. Allan T. Chanel, who purchased the land 50 years ago.

The bond referendum provided funds for two other projects in Bull Run-Occoquan, completion of the Sandy Run crew rowing facility, expected to become Northern Virginia's major crew practice area, and the Overlook Environmental Studies Center, being built by high school students under work-training programs. It will provide a permanent camp for groups to study wildlife along the creeks and woodlands. Deer and fox and other small game abound in the park, as do many less common species of birds such as egrets, hawks and even an occasional bald eagle.