As an airport baggage handler I often refilled 733s with a hose connection which was out in all weathers with no protection. Myself and others would handle all manner of bags, then sometimes go from cleaning seat pockets with chewing gum and full vomit bags to the next plane where we handled the drinking water nozzle with uncleaned hands.
The potable water is slightly safer when used for coffe as it has at least been boiled, but frequently when I worked as an airport caterer, we got complaints of metallic tasting water. My guess is all sorts of stuff gets into systems.
