How can I tell if the water I purchase started out as tap water?

Roughly 45% of the water sold in single-serve bottles comes from a municipal water source.

By law, bottled water that comes from a municipal water supply has to disclose this on its label unless the bottler takes steps to further purify the water, which most do. In this case, the label will say "purified water" or "purified drinking water," but the original source is probably tap water.

Water labeled "spring water" comes from an underground water spring, but it may be piped to the bottling plant.

"Mineral water" comes from an underground source and must contain no less than 250 parts per million total dissolved solids, such as salts, sulfur compounds, and gasses. No minerals may be added to the water by the bottler.

"Artesian water" or "artesian well water" must come from a well that taps a confined aquifer.

How can I tell if there are contaminants in the bottled water I purchase?

You probably can't. Tap water is regulated by the Environmental Protection Agency (EPA), which requires yearly public reports identifying the contaminants found in local water sources. But bottled water is regulated by the FDA, which has no such requirement.