Virtually all purchases of new federal parkland have been halted while the Interior and Justice departments try to reduce a huge backlog of condemnation cases for land the National Park Service has sought in the past.

The policy is an outgrowth of Interior Secretary James G. Watt's goal of consolidating existing parks and managing them better before adding new areas. To implement it, Interior has had to win the informal approval of House and Senate Appropriations subcommittees on three occasions to shift $11.7 million in parkland acquisition funds to condemnation purchases.

As a result, Watt has been able to move closer to his goal of ending outright purchases of parkland, even though Congress last June told him he couldn't do that.

Originally, Congress appropriated $10.3 million to buy land expected to be acquired through condemnation proceedings, but because the departments have been speeding up the cases, Interior projects that a larger portion of the estimated $42 million bill will come due this year.

But, congressional sources say, congressmen have become annoyed that authorized purchases have stopped in the meantime.

Caught in the middle are about 30 park areas that Congress wants to acquire, including segments of the Appalachian Trail, Delaware Water Gap National Recreation Area, New River Gorge (W. Va.) National River, Valley Forge (Pa.) National Historic Park, the Santa Monica Mountains (Calif.) National Recreation Area, the new Biscayne (Fla.) National Park and Channel Islands (Calif.) National Park.

The crux of the issue seems to be Sweeney Ridge, a 1,064-acre tract in California from where San Francisco Bay was first sighted by the Spaniards in 1769. Interior has stopped moving to acquire the property, even though its cost is likely to rise as a result. Meanwhile, said park service spokesman Duncan Morrow, "we do not intend to make any major purchases until we have sorted out what our highest priority for land acquisitions ought to be. Condemnation cases have to be the higher priority."

G. Ray Arnett, assistant secretary for fish, wildlife and parks, wrote to Sidney R. Yates (D-Ill.), chairman of the House Appropriations subcommittee on the interior, on Feb. 12 that "it is the question of priorities that has led us to the decision to forgo acquisition of Sweeney Ridge and to reprogram the funds to higher priorities condemnations ."

Before the "freeze," Interior did spend $26.6 million of $93.2 million appropriated for acquisitions this year, adding to Cumberland Island (Ga.) National Seashore, Chattahoochee (Ga.) River Park and the Virgin Islands National Park.

Parkland is generally acquired in three ways: a buyer sells property to the government, called a "fee-purchase"; the land is condemned, then purchased at fair market value, or the land is donated. Watt has no objections to the donations, but has worked assiduously to cut off "fee-purchase" arrangements.

Pending condemnation cases, however, have proceeded at a record rate. Since Watt took over, the number of pending cases--many of which have been around for as long as five years--have been reduced from 7,279 to 4,789, effectively adding 25,057 acres to 73-million-acre national park system. Officials could not provide a figure on the cost of those acquisitions.

However, only two of hundreds of new condemnation cases that the park service has wanted to file since President Reagan took office have been cleared by Arnett's office.

Condemnations generally involve small parcels of land in, or adjacent, to existing parks. The government can acquire the land through eminent domain procedures. The cases wind up in court when the seller doesn't have a clear title or can't agree on a price or when an owner doesn't want to sell.