Rolling prairie where defense workers for four decades produced deadly sarin, mustard gas and napalm is now home to more than 300 species from white pelicans to foxes to bald eagles.

Interior Secretary Gale Norton, preparing for Saturday's opening of the first phase of the refuge, said Friday the project will serve as both a habitat and an oasis of open space in the rapidly urbanizing Denver suburbs.

"It is a wonderful legacy," Norton said.

Not everyone is convinced that opening the site to the public is a good idea.

The arsenal is a federal Superfund site, still undergoing a cleanup expected to cost $2.2 billion.

Ten grapefruit-sized sarin gas bomblets were found during cleanup in 2000 and 2001, four years after historic tours of the facility had begun. Just six weeks ago, another canister designed to hold sarin in a weapon called an Honest John rocket was found, though it contained only water.

"That site should never have been and should never be open to the public," said Adrienne Anderson, a University of Colorado professor and environmental activist.

"We are worried. I think it just doesn't make sense to have people near a hazardous waste cleanup site while the cleanup is going on," said Sandra Horrocks, chairwoman of the arsenal committee for the Colorado branch of the Sierra Club.

However, other environmental groups support the conversion.

Stephen Torbit, direction of the Rocky MountainResource Center of the National Wildlife Federation, said the eagles and other wildlife are the best possible barometer of the ecosystem at the arsenal site. "And they are doing just fine."

"Our assessment is that this is the best and highest use for that land; a wildlife oasis in the middle of the concrete jungle of Denver," said Torbit.

The arsenal sits on 17,000 acres of rolling grassland about 10 miles northeast of downtown Denver. Perched at the foot of the Rocky Mountains with commanding views of the Denver, the area is dotted with small manmade lakes.

As reporters toured the area, deer ambled across empty roads and tumbleweeds blew past.

Opening to limited public use this weekend are 5,000 acres of the refuge. Walking trails will give access to about 2,000 acres, and tram tours will take visitors to the other 3,000 acres.

An additional 10,000 acres are slated to become part of the refuge by 2011, though some of that area will be closed to the public as eagle habitat. The most seriously contaminated area, about 1,000 acres, will be permanently controlled by the Army.

The Fish and Wildlife Service is working on similar plans to make the former nuclear weapons plant at nearby Rocky Flats into a wildlife refuge.

Norton, who as Colorado attorney general pursued a lawsuit against the federal government that resulted in state health standards being applied at the site, said the open areas are safe for the public.

Officials noted that more than 500,000 people have been through the area on tightly controlled public tours for historical purposes since 1996 and there have been no problems.

Dean Rundle, who manages the refuge for the U.S. Fish and Wildlife Service, said the area had been closely studied by experts, who even compared 1938 photos of every prairie dog town with current photos to look for signs of contamination.

"I'm just really confident we are not going to have any issues with chemical agents," Rundle said.

"Obviously if there were any problems the government would need to respond to those," said Norton.