Everytime this comes up the practical answer is to just not mark it and bury it as deep as you can.

If your mark it as poison, or honestly and descriptively, people will try to use it as a weapon, or a manhood test, or whatever dumb shit might happen. The patronizing warnings won't be legible to survivors and they're, quite frankly, embarassing.

My money is on descriptive warnings buried with the waste that only get exposed if it's exposed. You still run the risk of people trying to weaponize it but at least they aren't being attracted to the site by the warnings themselves.