Municipal water systems in the United States and Puerto Rico supply water for many commercial and industrial uses as well as for domestic wells. It is generally known that our water resources are unequally distributed throughout the country, but it is not quite so well understood that the quality of our water resources is also variable. This hydrologic investigations atlas shows, State by State, some of the chemical quality aspects of our public water supplies. This information can be used to evaluate the suitability of the public supplies for many uses – among them, manufacturing processes, food processing, cooling water, and domestic use.