I assume there might be something in Milwaukee Metropolitan Sewage District website, but it's got information overload. Not to mention it could be buried in the reports that are over 400+ pages.
So, they key is mostly the mash temp (I know H2O plays a part, but I'm not there yet). Mixing with distilled is just playing with your water chemistry if you say, have hard water?
I've always used straight tap H2O for all my beers and never noticed anything "off". Who said, if your water tastes fine, so will your beer?