After talking to the Ghoul leader beneath the streets of Necropolis, travel to the watershed. Enter the sewer manhole next to Larry. Locate a pile of junk in the eastern part of this sewer system, in a dead end.
With the junk in the inventory, approach the water pump in the north-eastern part of the watershed. Use the Repair skill or a tool on the pump. A successful skill check will fix the pump and remove the junk from inventory. A critical failure will deal a single point of damage to the player character and remove the junk as well.
Then the player can rest easy that they have saved the ghouls from death. For a while, anyway.
Set offers an alternative method of solving this quest. If the player approaches the nasty ghoul with junk in their inventory, they will be sent to the watershed with Garret, to repair the pump at gunpoint. Garret's shotgun might be threatening. It might be not.
The game only checks if the player has the generic junk item in their inventory, allowing them to use any such item (such as from Jake's house in the Hub or from the Boneyard deathclaw hunting ground) to complete the quest.
If the player returns to the leader of the underground ghouls before fixing the pump and talks to him, they will receive three skill books.