The Rocky Mountain Arsenal, once a cauldron of poisons, home to chemical weapons and pesticides manufacturing, is making another major shift toward its emerging status as an urban wildlife refuge.

Federal environmental regulators want to remove the biggest piece yet of the site northeast of Denver from the Superfund list of the country's most polluted places and hand it over to the U.S. Fish and Wildlife Service, the result of significant cleanup progress in recent years.

In all, the Environmental Protection Agency is proposing that 7,399 acres of what was once a 17,000-acre site be turned over to wildlife managers.

If the agency proceeds, it would mark the third section of the arsenal removed from Superfund status, for a total of 13,392 acres taken off cleanup rolls since 2003.

"It's our biggest piece of land to date that we've been able to certify as complete," said Charlie Scharmann, program manager for the U.S. Army, which, along with Shell Oil Co., is responsible for decontaminating the site.

"It is a significant milestone for us."

The effort to de-list the parcels comes after years of work mopping up a number of fairly small, scattered areas of contamination. The projects included eradicating old landfills containing asbestos and PCBs, moving soil tainted with mercury and chemical weapons residue and digging up old canals and sewers that transported contaminated water.

Major cleanup work at the arsenal dates to 1989, with completion expected in 2011. All told, the cost of the job is expected to reach $2.2 billion, making the arsenal the state's second-costliest Superfund site. It's still far less than the more than $6 billion closure of the Rocky Flats nuclear weapons plant west of Denver.

One of the most important components of the recent arsenal cleanup work included what EPA project manager Laura Williams called a "scrubdown" of the entire site to ensure no remaining unexploded weapons remained.

The survey followed unexpected discoveries in 2000 of small, unexploded bombs filled with sarin gas, an event that drew headlines and stirred public worries about what other hazards might still lurk at the site.

"We did it to give us assurances that what we are deleting (from the Superfund list) is where it needs to be," Williams said.

"Any potential explosive items have been scrutinized yet again to ensure we don't have any future surprises."

The EPA has opened a 30-day public comment period through May 25 on its proposal to remove the 11.5 square miles from Superfund status. Barring any major objections, the property could be off federal cleanup rolls by September, Scharmann said.

If so, that would leave just 3,608 acres at the arsenal under the Army's and Shell's jurisdiction.

Remaining cleanup work in that area, at the center of the arsenal, includes capping large landfills created to hold most of the contaminated debris moved from other areas.

"The last few years of cleanup are actually clean construction, where we're moving clean solids over the tops of landfills . . . to provide that cap and cover for rain and snow to move over the top of, instead of moving through" the landfills, Scharmann said.

Gradually, officials say, all the steady cleanup progress is making the public view the old arsenal site more as a wildlife refuge, a place to see deer, bald eagles, burrowing owls and prairie dogs, and less for its fading notoriety as a home to military nerve gas production decades earlier.

"The public has seen the transition and I think their perception of the site is following it," said Dean Rundle, refuge manager for the Fish and Wildlife Service.

"They know some cleanup is left to do out here, but they know the vast majority of the site wasn't contaminated, or that the areas that were have already been cleaned up."

hartmant@RockyMountainNews.com or 303-892-5048