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The last of the bulk mustard gas stored at Deseret Chemical Depot in Tooele County was destroyed Monday, a milestone for the facility that has been incinerating aging munitions since 1996.

Deseret Chemical Depot, about 15 miles south of Stockton, was once the nation's largest stockpile of chemical weapons. Only a small amount of those munitions remain, according to spokeswoman Alaine Grieser.

"This has been a long time coming. This chemical agent has been around for a long time," she said.

The chemical weapons stockpiled in Tooele County are decades old, some dating back to the early 1940s.

About 350 mustard agent projectiles and mortars are scheduled to be detonated in coming weeks at a special facility at the depot. Technicians were unable to safely separate the chemical agent from the explosives to incinerate them, Greiser said.

"These munitions were aging; the canisters were deteriorating," she said.

In addition, the depot also plans to destroy in coming months lewisite, a blistering agent, and GA, a World War II-era nerve agent recovered from German stockpiles.

The weapons disposal is mandated by the Chemical Weapons Convention treaty, which promised to destroy by 2007 chemical weapons outlined in the agreement. The United States requested an extension and Grieser said that deadline will be met.

The remaining stores of lewisite and GA are expected be destroyed by February. Much of the chemical depot will then be decommissioned and transferred to the Tooele Army Depot. The chemical depot employs about 350 people. But areas known as "solid waste management units" will remain. Those are old munitions dumps from an era when arsenals were destroyed in open-air pits, Grieser said.

"We don't know what the final disposition [of those areas] will be," she said.