According to the US Army:
"The Blue Grass chemical weapons stockpile comprises more than 500 tons of blister and nerve agent in weapons."
The Assembled Chemical Weapons Alternatives program’s mission is to destroy the last two remaining U.S. chemical weapons stockpiles – 2,611 tons of mustard agent in projectiles and mortars at the U.S. Army Pueblo Chemical Depot in Colorado, and 523 tons of mustard and nerve agent in rockets and projectiles at the Blue Grass Army Depot in Kentucky.
Here is the US Army's map of its stockpiles, including chemical munitions still in storage and those already destroyed. It is not like the old days, when you simply put unwanted chemical weapons on board ships, sent them offshore and scuttled the boats and the chemical weapons. Check out some of the many places the US admits it simply dumped chemical weapons in the ocean. Reportedly 32 dumps were off US shores and 42 were off foreign shores. Consider the alternate method for getting rid of chemical agents at Rocky Mountain Arsenal, a home of chemical weapons since 1942. A 12,000 foot deep well was drilled in 1961 and unwanted chemical agents flushed down the drain. After earthquakes (710 earthquakes in 3.5 years) started appearing, the army had to pump the chemical stew back up and find another way to get rid of it.