In what would be one of the biggest givebacks of Indian land in U.S. history, the government has agreed to return 84,000 acres to the Northern Ute tribe as part of a deal to clean up millions of tons of uranium waste along the Colorado River.

Energy Secretary Bill Richardson announced the agreement today at the tribe's headquarters in Fort Duchesne, about 110 miles southeast of Salt Lake City.

The deal, which the Energy Department called the largest return of Indian land in the lower 48 states in a century, is subject to approval by Congress.

The land, which is believed to contain oil-rich shale deposits, was given to the Utes in 1882. But in 1916, on the eve of the nation's entry into World War I, the federal government took it back to create a reserve supply of oil for the Navy fleet. The reserve was never tapped.

"The land is not needed for national security anymore," Richardson said. "The right thing to do is turn it back."

Under the agreement, the Indians can open the land to oil and gas drilling. But they will have to pay a percentage of the royalties to the government.

That money will, in turn, help the government cover the $300 million cost of relocating 10.5 million tons of radioactive rock and soil left over from the mining of uranium during the Cold War.

The Energy Department estimates that the land, which is next to the 4.4 million-acre Ute reservation, holds 6 trillion cubic feet of natural gas, or about 30 percent of the natural gas used in the United States during 1998. There are no estimates of how much oil is there. The royalty percentage is being negotiated but is expected to be around 8 percent.

The radioactive waste sits 750 feet from the Colorado River just outside Arches National Park. A study by the U.S. Fish and Wildlife Service found that toxins such as arsenic and ammonia are leaching from the pile and contaminating the river, threatening endangered species of fish.

Rep. George Miller (D-Calif.) also has raised concern that the river contamination is tainting the drinking water for 25 million people in Arizona, Nevada and Southern California.

Denver-based Atlas Corp., the company that operated the mine from 1962 to 1984, has declared bankruptcy, leaving the Nuclear Regulatory Commission to clean up the pile. However, the cleanup bond Atlas left behind was woefully inadequate.

The NRC planned to cover the tailings with an earthen cap and drain 500 million gallons of water from the pile. But state and federal officials and environmentalists argued that would not stop contamination. They pressed the Energy Department to take control of the pile and move it 18 miles north of Moab.

As part of the agreement, the Utes pledged to protect a quarter-mile corridor along 75 miles of the Green River.