Destroy the Super Mutants at the Watershed

If you talk to Set, the leader of the ghoul gang, he'll tell you to destroy the Super Mutants in the Watershed map in Necropolis. You can go there and fight them, or you can talk to one of the mutants named Harry who will take you to his superiors at the Military Base. If you get past the mutants in this building you will come to the broken water pump and then to a hatch that leads you underground to the vault (Vault 12). You'll find the water chip on the third level of this vault, but if you want to be nice and get some extra experience you can also use the pile of Junk you find in the sewers to repair the water pump and keep water flowing to the city.