If memory serves me correct, there are around 600+ chemicals found in drinking water. Some are added by the water companies such as Chlorine and Flouride. Some chemicals are also absorbed from old pipework. On a side note, I was quite alarmed to discover on my travels that the same Flouride compound used in our drinking water is the same stuff used by the Nazis on the Jews during WW2 as a mind suppressant
I've watered my maples with tapwater, with no ill effects. Since I got a water butt last year (and no that doesn't mean I've got cellulite on my backside :p) I prefer to water my trees with rainwater as much as possible. The only noticeable benefits seem to be that my pots don't get anywhere the amount of limescale/salt deposits as they did before. Whether there are unseen benefits, I couldn't really say. I would hazard a guess that the Chlorine in the water doesn't go down to well with natural bacteria found in the soil. I haven't noticed a huge improvement in growth, but I do prefer to use water from the sky than the hose.
All the best,
Aaron