I know that. The problem is, we dont know what is the type that irradiated the soil. But, in the light of previous games, it is relatively safe to say that bethesda over-radiated wasteland.
Fallout 2 had drinkable water in relatively "large" amounts,otherwise, places like NCR couldnt have formed, so water purification shouldnt be a problem, atleast 200 years after the war.
It just seems that the war ended 30/40 years ago,in fallout 3,with all of the ghouls, food and ammo lying around, working unmaintaned generators, unrobbed places, no development or farms etc.