When you first meet him, Walter in Megaton asks you to fix three leaking pipes found around the town. A Repair skill of 30 is required to fix the pipes. After the pipes in Megaton are fixed, Walter will offer to buy scrap metal from you in order to maintain them. Scrap metal is a very common sight around the Capital Wasteland, often found in tool boxes, inside robots and in merchant inventories.
Walter gives you ten bottle caps per scrap metal; if you decline payment, you will receive +10 Karma instead. In addition, you get a five experience points reward per scrap metal you give Walter.
Although Walter mentions that without the scrap metal the pipes will eventually burst again, the pipes will never again break after fixing them for the first time, even if you never give him any scrap metal.
Xbox 360 If you bring Walter some scrap metal (tested with more than five scrap metal units) there is among the possible statements: "Eden wanted me to contaminate the water with a virus", which is something you can tell to Elder Owyn Lyons when coming back from Raven Rock.