in tap water you're biggest concern is chlorides which will evaporate if left out in the open for a hour or two... I would not recommend well water. You take a much bigger risk of exposing your plants to things more dangerous than chlorides- like VOC's(volatile organic compounds), choloform (bad bacteria), as well as unacceptable levels of secondary contaiminats such as iron, manganese, magnesisum, sodium, copper, nitrate, nitrite, TDS (total dissolved solids)etc... I don't know where you live but in the U.S. there is no federal guidlines for well-water. It is up to the homeowner to test it themselves and fix any problems. And unfortunately, to test for many of the things listed above (and there are thousands more) costs several hundred dollars