Tom, the "ring" is from dissolved minerals coming back out of the water. As water evaporates, the salts become more concentrated until they finally start to precipitate out. Chlorine is a gas. It is dissolved in water, but it will evaporate as well--faster than the water will--thus the concentration of chlorine will go down in water that sits.
Even rain has lots of stuff dissolved in it. Think about acid rain (elemental sulfur going through several stages of oxidation and ending up as sulfuric acid). Think about rain in the city where there is heavy automobile traffic and the resulting exhaust.
I water my seedlings when they're in the basement-under-lights stage with water I've saved from the dehumidifier. But that's mostly because we don't have a water faucet in the basement and I'm too lazy to traipse up and down the stairs with jugs of water from the kitchen sink tap.
