Urban sources of water could be dangerous because of chemical contamination, and ponds could have the same problem. Ponds and even small lakes are sometimes treated for algae control, and the runoff water feeding ponds may contain pesticides or other chemicals. Algicides and pesticides are particularly dangerous because they may have little or no taste or smell in toxic quantities, where oil or fuel contamination would likely be noticeable. For any of these, a carbon filter may reduce the effect, but that might not be enough.

If you have rivers or lakes nearby, you could check to see what the fishing restrictions are. In this area, the Hudson river is now clean enough that you can eat fish caught from it, but some local lakes are heavily contaminated with various chemicals and are unsafe. Pretty much if the fish are safe to eat, I believe the water is safe from chemical contamination.

This doesn't mean that a disaster couldn't make the water worse, and it probably would in terms of the "bugs" in it, but it seems unlikely that a disaster would make the water better.

I've heard that boiling water for a few minutes will burn off most petrochemicals, PCB's and that kind of stuff, and that 0.2 micron filters and then chlorine dioxide tablets will generally take care of the rest. Is this true or myth?

Of course I could build a still, (and that could provide some interesting side benefits ), but that would be hard to carry in an EDC kit.