The ghouls of Necropolis
If you talk with the doctor in vault 13 and use the "ask me about" option, and ask him about the "water chip" he tells you the water chip is used to purefy the water that comes from the underground river. The manual says that all vaults have underground rivers. The water chip, therefor, does not pump the water. It is possible that during the years the water became so contemintated with both radiation and poison that the ghouls cant drink it.
