The Interior Department has been ordered to buy a chunk of land for the Golden Gate National Recreation Area near San Francisco, in what would be the largest addition to the federal parks system outside of Alaska since the Reagan administration took office.

The word came in the form of an amendment to an appropriations bill, placed there by Sen. Pete Wilson (R-Calif.), who accused Interior Secretary James G. Watt of "thwarting the will of Congress" by failing to buy Sweeney Ridge, a thousand-acre parcel of scenic coastal land from which Spanish explorers first sighted San Francisco Bay in 1769.

"Sen. Wilson has long supported the acquisition of Sweeney Ridge, which has long been noted for its beauty and scenic value," said an aide to the Californian.

Interior officials, however, suspect that they've been done in by an outside agitator: the San Francisco-based Trust for Public Land, a small nonprofit group that negotiates for privately held lands with the idea of turning them over to the government.

The trust has held an option on Sweeney Ridge for three years, after persuading its owner, West Aspen Co. of Oklahoma City, that the land properly belonged in the public domain.

Since then, it has been trying to persuade Congress.

"They have been active in pushing it in Congress," said a former aide to the late representative Phillip Burton (D-Calif.), who championed the cause of Sweeney Ridge until his death in April.

But Interior officials say the trust didn't ask the Reagan administration if it wanted Sweeney Ridge. If the trust had asked, said Interior official William Hartwig, "We would have said, 'Forget it.' "

Even more galling to Interior officials is that the trust stands to make $100,000 or more on the deal, and says it intends to plow the money right back into a revolving fund to acquire more land for the government to buy.

"If nonprofits are going out there without talking to federal agencies, and then going to Congress to lobby for this land, they will control the budget," said Hartwig, who chairs an internal policy group on land acquisition. "We have a hard enough time without somebody coming in and generating additional needs."

Watt, of course, has argued that the government doesn't need any new land at all. But as Interior officials see it, that's beside the point.

They argue that Sweeney Ridge is not a high-priority addition to the recreation area. In the most recent "land protection plan" for Golden Gate, the ridge ranked 14th among 81 possible additions, partly because the area was presumed to be protected from development under a "no-growth" ordinance passed by the city of Pacifica.

"There is no threat, no urgency," said Hartwig.

But supporters of the acquisition contend that the protection plan does not take into account recent changes in the "no-growth" ordinance, including zoning changes that will open part of Sweeney Ridge to residential development.

They also note that a year earlier, the National Park Service had described the ridge as a key addition to Golden Gate and a "missing link" that will open public access to city and county park lands and watershed areas that lie north and south of the site.

Martin J. Rosen, president of the trust, said that the organization was not trying to do an end run around Interior.

"We are not spectators, but we haven't lobbied this transaction," he said. "It just happens to be one they've dug their heels in on."

He acknowledged, however, that the trust hopes to come out $100,000 to $200,000 ahead on the deal.

In any other business, that's called a profit. "We don't have profit," Rosen said. "We have gain." The "gain" is the difference between the price the trust has negotiated with West Aspen, plus its own legal and surveying expenses, and the price the government ultimately pays for the property.

Interior is conducting a new appraisal to determine the land's market value, but Wendell O. Steward, president of West Aspen, said he figured the property was worth $14 million as is, or "somewhere in the $15 million to $20 million range" if it were developed.

West Aspen, a subsidiary of Texas International Co., an oil and gas firm, signed an option to sell the site to the trust for $8.5 million. Congress has authorized Watt to spend up to $9.6 million for the property.

Rosen contends that that's a good deal for all concerned: The trust gets its "gain," the government gets the land at less than its fair-market value, and West Aspen gets cash, plus a tax writeoff for the difference between the selling price and the land's value.

West Aspen, which at the moment owns an attractive piece of land with an uncertain future, is clearly eager to end the standoff one way or another.

"Congress wants it. The Interior Department doesn't. And that's not my battle," Steward said. "If they don't want it, we want to develop it."