Generally speaking you don't have to worry that much about chemical weapons waste. Anything toxic enough and potent enough to incapacitate people on any militarily useful timeline needs to be pretty reactive with some subset of organic molecules. That pretty much always means that it readily makes bonds with something in the top few rows and right half of the periodic table. Likewise it will quickly find something in the environment to react with and become much less of a problem if you let it loose.
Heavy metals and exotic engineered chemicals that are only mildly toxic and/or are mostly nonreactive that can pass through the food chain or will not affect you immediately but build up is what you should worry about (agent orange, mercury, etc).
> Generally speaking you don't have to worry that much about chemical weapons waste...
This whole paragraph is gibberish. Some chemical warfare agents are vulnerable to hydrolysis, but only if the munition casing is compromised. However mustards are quite robust against environmental degradation; they have low volatility, they're hydrophobic, and they resist hydrolysis. Thus they appear to persist indefinitely even when the bare chemical is exposed to water. Read the news stories about fishermen being burned by blobs of mustard that were accidentally trawled from the sea floor, or people on beaches injured by random enounters with wandering bits of mustard.
And of course none of this matters as long as munitions are still intact, which will be the case for millions of hundred-year-old shells dumped at sea and buried on land for the foreseeable future.
> Heavy metals and exotic engineered chemicals...
Mercury is indeed a persistent pollutant that bioaccumulates. When you mention agent orange, I'm guessing you're talking about the dioxins that are supposed to have contaminated some batches of pesticides. However none of these are exotic engineered chemicals, and they certainly aren't "mildly toxic". While many old weapons dumps present a mercury hazard, they're dwarfed by the mercury pollution caused by coal-fired power plants. Dioxins aren't really relevant to weapon dumps; the vast majority of dioxin pollution (and chlorophenols in general) comes from burning of waste and industrial accidents.
Oh, we've got quite some of that to go around, too. Wars are good for creating further problems down the line.
In the closing months of WWII, the German sub U-864 was sunk just off the island of Fedje in south-western Norway; among its cargo was 67 tons of mercury and scientific personnel - it was on its way to Japan to aid in the war effort.
The quicksilver is still there, slowly seeping out of the wreckage; good thing (as such things go...) quicksilver is heavy and insoluble in water. With any luck it seeps into the seabed before any marine life ingests it to find it is not insoluble in fat and tissue.
Another problem-- probably a much bigger one-- is the huge amount of mercury fulminate used in munitions throughout much of the 20th century. It's much more soluble and dispersible than native mercury, and there's a lot more of it in weapons dumps.
Heavy metals and exotic engineered chemicals that are only mildly toxic and/or are mostly nonreactive that can pass through the food chain or will not affect you immediately but build up is what you should worry about (agent orange, mercury, etc).