GOOD.is/Transparency has been always providing beautiful infographics. This is a look at the five most and least polluted water systems in America (in cities with more than 100,000 population), based on the total number pollutants found in the water, and how often those pollutants were found during continued testing.
The infographics shows that even though water comes out of the faucet looking clean and clear, there is a good chance it has some pretty nasty stuff in it.

http://www.good.is/post/Transparency-How-Clean-Is-Your-Tap-Water