The leak was discovered during routine monitoring operations in a storage igloo.
Depot workers in protective clothing used equipment to contain the leak and removed the leaking container to a different structure.
The small amount of crystallized agent residue from the leaking munition will be cleaned up and the plugs and valves will be replaced later this week, the depot said in statement.
The storage igloo is covered by earth and the air inside is filtered. No vapor escaped, and there was no danger to the surrounding community, the news release said.
The depot is about 40 miles west of Salt Lake City.
A similar leak was reported by the depot earlier this month on Dec. 8.