The Fish and Wildlife Service has backed down from its earlier demands for Atlas Corp. to move 10.5 million tons of uranium tailings from its defunct plant on the banks of the Colorado River.

The agency released its final opinion Thursday, conceding that capping the tailings on site will suffice if steps are taken to ensure pollutants don't seep into the river.Draft opinions issued by the agency said Atlas' preferred option of capping the tailings in place was inadequate to protect the four species of endangered fish in the river just 750 feet away from the tailings, which stand 40 feet high spanning 150 acres.

But those opinions were harshly criticized by Atlas and the Nuclear Regulatory Commission. The commission said the Fish and Wildlife Service did not have the legal authority to force Atlas to move the tailings, and both Atlas and the commission were critical of the scientific basis for the Fish and Wildlife conclusions.

The final draft maintains that simply capping the pile, as Atlas has proposed, would jeopardize the Colorado squawfish, razorback sucker, bonytail chub and humpback chub.

But the Fish and Wildlife Service devised a "reasonable and prudent" alternative to minimize the threat to the fish, including steps to reduce leaching from the pile, imposing standards for allowable emissions of ammonia - a primary toxin released from the pile - and future studies to refine the ammonia standards.

The final opinion also requires contributions to a program to recover the endangered fishes.

"Originally our biggest concern was for making sure we remove the threat to the fish, and I think we're still focused on that," said Reed Harris, field supervisor for the service's Salt Lake City office.

"I think what we're saying is we're buying into the capping as long as it is sufficient for groundwater remediation to take place," he said. "Overall, I do feel good about the opinion."

Richard Blubaugh, Atlas vice president for environmental and governmental affairs, was also pleased with the changes and the final product.

"We worked very hard . . . to ensure the opinion is based on facts and good science, and I think it's come a long way toward that end," he said.

The biological opinion will be incorporated into the environmental impact statement to be prepared by the Nuclear Regulatory Commission. Blubaugh said the EIS could be issued by the end of the year, allowing Atlas to begin capping the pile with sand, clay and boulders.

Those plans could be affected by the threat of a lawsuit by the Grand Canyon Trust. The group filed an intent to sue Atlas last April after Fish and Wildlife initially backed away from its demand that the pile be moved. The trust alleged the Atlas violated the Clean Water Act by emitting excessive ammonia into the Colorado River, which provides water for seven western states.

Cullen Battle, the attorney representing the trust, said his client has not seen the entire final biological opinion, and the wording of the provisions setting the allowable ammonia emissions would be crucial to whether to pursue the lawsuit.

He said the trust remains convinced the best way to handle the tailings is to move them away from the river, then proceed with ground-water cleanup.