drastic times call for drastic measures.. hahaha sure, it may be a bit cruel, a better question to ask is this...
What happens to a firecracker and a bottle placed 4" apart in the air? (say m80 type)
What happens to the same bottle and firecracker same distance under water?
The bottle breaks in the water scenario as water conducts kinetic energy much much better.
But yeah, gunpowder contains sulfur and other crap that I'm sure will remain and effect the inhabitants for a long long time no matter how "unaffected" they look.
Excerpted from the link on water changes...