Purified water is water that is completely free of radiation and as such can be safely consumed without causing harm. In addition, it also heals far more than dirty water does.
In a random occurrence, you may encounter two groups fighting over a fridge stocked with 10 purified water bottles. You can leave them alone, kill them and take the water, or with a successful speech check, convince them to split the water with you.
There is also another random encounter which involves two wastelanders requesting you kill a radscorpion guarding a refrigerator with about 7 bottles of purified water. They will then allow you to take all the water.
Notes[]
Purified water can be given to Micky outside Megaton, Carlos outside of Rivet City, or Willy in front of Tenpenny Tower for positive karma. This introduces the possibility of infinite good karma, as they will never stop asking for more water.
There is a random occurrences in the wastes of water beggars, if you deny certain ones water they request to be shot in the head (for negative karma, of course... just for denying them sustenance).
It is important to only give purified water to water beggars and not aqua pura, as regardless of story-line decisions aqua pura still kills water beggars.
Each bottle of purified water appears featuring the text "'H²O" hand written on the label on the side of the bottle. This format is incorrect (according to IUPAC nomenclature), as chemical formula numbers are written in subscript (Xy) and not in superscript (Xy). If read literally, the chemical formula H²O would mean there is one hydrogen and one oxygen isotope that contains two neutrons (instead of the standard eight). This would leave the oxygen extremely unstable and radioactive with a half life that would be so small that it would not be measurable.
