Are there recognized symbols for vomiting, diarrhea, hair loss, weight loss reaching near skeletal stages and finally death? Along with some kind of indicators for passage of time that seems more useful at least for high exposure areas.
Of course there is no helping if the future people are especially dismissive of your warnings, but they should suffer from debilitating radiation sickness before they get too far away, so at least the leak would be somewhat contained.
Interesting story idea for a post-apocalyptic agrarian society where some guy wipes out his rival's towns by lightly burying preapocalyptic nuclear waste in the town square to kill off the inhabitants while preaching about curses and gods smiting the inhabitants for their sins.