Consumers are able to compare brands of bottled water by the source and the purity of the water. Sources of bottled water are generally listed on the bottle, and studies by federal, state and private agencies and organizations assess water purity.
Bottled water labeled purified, distilled or P.W.S., meaning public water source, usually comes from tap water. From 25 to 47 percent of bottled water in the United States is tap water, as of 2015. Artesian water is drawn from a well tapping an underground aquifer, and spring water flows up naturally from an underground aquifer. Mineral water must contain 250 parts per million or more of minerals and other elements. Sparkling water contains carbon dioxide, but when natural carbon dioxide is lost in the treatment process, more is added artificially. The terms mountain water or glacier water on bottles are advertising terms that have no standard definitions.
The U.S. Environmental Protection Agency regulates tap water purity, while the U.S. Food and Drug Administration oversees the purity of bottled water sold inter-state, and local state agencies regulate the purity of bottled water brands sold only within single states. The Natural Resources Defense Council reports that federal and state oversight of bottled water purity is inadequate and that a significant amount of bottled water contains chemical or bacterial contaminants. Consumers can access studies such as those by the Environmental Working Group to assess the purity of specific bottled water brands.