I've placed small open containers of NYC tap and Poland Spring water in my frost-free freezer and allowed them to evaporate, and I've found lots of ugly brown residue in the former, but none in the latter.
People in my neighborhood seem to have abandoned tap water in favor of Poland Spring, but that deprives their kids of the fluorine.
Personally, I'd risk the contaminants, which don't appear to be anywhere near critical, rather than confer lifelong dental problems on my kids.
