While mercury poisoning has affected major ecosystems (it's not a good idea to eat fish you catch in the Great Lakes), it's limited to where the chemical actually is. I suspect the problem with radioactive waste is that it's ranged: you don't have to touch the radioactive source to get a fatal case of acute radiation syndrome.
But it gets spread just about everywhere. Mercury only affects you where it actually is, but it's widespread enough to have significantly contaminated all seafood at this point.
Nuclear waste might get you if you're not actively ingesting it, but you still have to be within, what, a few dozen feet of a large chunk of it?
