Three years ago the Interior Department informed Nellie Edwards that it intended to buy the 27 acres of land she owned along Bull Run in western Fairfax County to expand historic Manassas National Battlefield Park.

Edwards is still waiting for a check. And, at age 84, the Arlington resident is getting impatient.

Congress enacted the 1,500-acre expansion of the park in 1980, but the purchase never occurred. The investment property Edwards and her late husband bought "for our old age" remains in limbo. "Who's going to touch it now that the government's interested?" Edwards asks.

She is not alone. When Interior Secretary James G. Watt in 1981 ordered a temporary moratorium on all national parkland purchases and then a review of purchase policy, nearly all planned acquisitions were frozen. Nellie Edwards is one of the policy's more vocal critics.

"That Jimmy Watt, he stopped it," snaps Edwards. "He thinks he's the lord of all . . . . I can't sit here willy-nilly forever. I'm old, I'm a widow, and I can't wait while they dither."

The National Park Service has a different view. "It's just good housekeeping . . . ," says Park Service spokesman George Berklacy. "We've got a backlog of about $900 million in unpurchased properties. We can't go out and buy everything. The new plan is designed to help us decide what properties are most important, the most deserving of immediate attention, and the most creative ways to protect them."

The Watt-imposed moratorium on parkland purchases was lifted a few months after it was levied, but since then Watt, arguing that the federal government already owns more parkland than it can properly maintain, has ordered a change in purchase policy.

Instead of buying outright the backlog of thousands of acres Congress has ordered added to national parklands, Interior has instructed its park managers to draw up alternate "land protection plans" that identify different, cheaper means of preserving properties--options such as partial leasing, acquiring development rights or arranging for life tenancy of the current owner.

Environmentalists have attacked the change in policy as a sophisticated stalling tactic, a de facto moratorium. The Park Service says the new plans are expected to be in place by September, but that doesn't necessarily mean buying will begin. That leaves Edwards and thousands of landowners nationwide paying taxes on land they have the legal right, but limited chance, to sell.

Even before the change in policy, federal land acquisition often took years. Once the federal government has included land in a park's boundaries, it has the legal right from now until doomsday to decide to purchase it.

Edwards, of course, can't wait that long. "Do they want it or don't they want it?" she asks. "I'm too old to sit around and play games. If they don't want it, let them say it so and be done with it."

Manassas Battlefield Park was established in the 1940s on the site of two bloody Civil War battles, and expanded by Congress in 1954, and again in 1980, to its present 4,500 acres. It's not the only park in the Washington area affected by delays (Harper's Ferry, the C&O Canal and Antietam National Battlefield are others) but its location, about 30 miles west of Washington near the intersection of Interstate 66 and Rte. 234 in rapidly developing Prince William County, makes it attractive to private developers.

The 1980 expansion of the park came only after years of disputes among Northern Virginia politicians, haggling so vehement that it was dubbed the "Third Battle of Manassas." The final compromise overrode the wishes of the Prince William Board of Supervisors, who wanted part of the land designated for the park's expansion to be set aside for commercial development--exactly what the expansion was designed to prevent.

Environmentalists and others worry that the delays may damage permanently the park's chances for expansion. "That land isn't going to be there forever," warns former 8th District Democratic representative Herbert E. Harris II, who introduced the initial legislation to expand the park and who worries that development nearby might drive the price of the land higher than the government will pay.

Battlefield superintendent Rolland Swain disagrees. "It hasn't hurt the park, not with building as slow as it's been, and interest rates so high," he says. If anyone were to buy the land to develop it for use incompatible with the park, Swain says, the government has the ability to make a preventive purchase.

"Well, of course that doesn't help me," fumes Nellie Edwards, whose numerous conversations and correspondence with the staffs of Northern Virginia's elected officials have proven fruitless. "They run their hips slim trying to convince you to put 'em into office," she says, "then they get there and they don't want to know you. I'm a Democrat, but I voted for Mr. Watt's boss last time--like a fool. I won't repeat that mistake again."