Fix the Necropolis water pump.

Fix the Necropolis water pump is a side quest in Fallout.

Find the junk
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.