The answer, said some House lawmakers at a hearing on Capitol Hill today, is that we often don't know for sure.
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Holding up a bottle of Dasani, Rep. Bart Stupak, D-Mich., scoured the label for information on the source of the water.
"It doesn't say anything about sources or anything, does it?" he asked the panel of government and beverage industry experts.
"So we don't know where this water really came from -- Nevada, Connecticut, New York or Pennsylvania?" he asked. The panel agreed.
With more Americans turning to bottled water believing it safer and healthier than tap water, there are growing concerns over how water is marketed, sourced and tested for safety and purity.
Consumption of bottled water in the U.S. has doubled over the past 10 years, according to the Beverage Marketing Corporation.
Many of the most popular brands of bottled water claim a high level of purity, but often consumers and government regulators can't easily verify those claims.
Volvic claims its water is "extremely pure and distinctly different," Ice Mountain Natural Spring Waters says it's "pure as the driven snow," and one brand -- Aquamantra -- claims its product "resonates with the energy and frequency of well-being."
In a study of 188 bottled water brands by the Environmental Working Group -- a nonprofit public health advocacy group -- only two brands listed specific water sources and treatment methods on their labels and offered a recent water quality test report on their Web sites.
"Consumers spend 1,900 times more for bottled water than for tap water, yet they rarely know basic information about exactly what's in their water bottle," said Jane Houlihan, an Environmental Working Group researcher.
Houlihan says 25 percent of bottled waters are simply "tap water in a bottle."
But industry representatives say it's normal for water to come from municipal sources. In most cases, the water gets specifically purified before minerals are added for taste.
Those mineral additions are supposed to be noted on the label by government regulation.
Still, there is no broad requirement of manufacturers to print the water's source, how or whether it's treated, and what chemicals it contains on the container label.
Bottled water -- a multibillion-dollar industry -- is considered a "food" by the Food and Drug Administration and is regulated for safety and accuracy in labeling.
The agency targets bottled water labels which promise medical benefits, such as curing cancer or prolonging life, and tests samples of bottled water for microbial, radiological or chemical contamination. But it can only do so much.
"We can't say there's zero risk here," Stupak said. "There have been 23 recalls from 2001 to 2008 of bottled water ... But because it's bottled water, we assume it's safe."
Municipal tap water suppliers are required by law to produce an annual water quality report that is available to consumers. Bottled water does not face the same requirement.
Still, not all lawmakers agree that bottled water needs tougher regulation.