View of the northeast corner of the Pueblo Chemical Agent-Destruction Pilot Plant’s Biotreatment Area where the Brine Reduction System (BRS) resides. (Provided by the Program Executive Office, Assembled Chemical Weapons Alternatives)

PUEBLO — The Army has destroyed about half of the 2,600 tons of mustard agent contained in decades-old shells stored at a southern Colorado chemical depot.

Walton Levi, the site project manager of the Pueblo Chemical Depot, made the announcement Wednesday, calling the milestone a “tremendous achievement and one that I am very proud to announce and one we all can celebrate.”

The plant started operating in September 2016 and has eliminated more than 220,000 munitions.

The depot is eradicating 780,000 shells filled with thick liquid mustard agent — many of them dating to the Cold War — under an international treaty banning chemical weapons.

Mustard agent, which was first used in World War I, can maim or kill, blistering skin, scarring eyes and inflaming airways.