Well, it has to do with the ability of the water to block radioactive elements which are photons, electrons, quarks, and who no's what. Four feet of water or more is equal to lead encasement, much concrete, and a lot of dirt and rocks, plus...gave you ever seen the movie, "A Boy and His Dog?".

Anyways I have camped in a place where where atomic bombs were tested with all of my camouflaged friends and equipment and it royally sucks the big bajonga. That was life 50 years after the fact.