Stick with filtered tap water, keep reusable bottles handy, and learn to decipher bottle labels.

This may be where your bottled water comes from...if you're lucky.

RODALE NEWS, EMMAUS, PA—Think that premium bottled water you’re swigging was taken from some crisp glacial aquifer? Do a little homework, and you may find that’s only, at best, a “possible source,” according to a new report released by the Environmental Working Group (EWG). In an attempt to get bottled water companies to be more forthright about where their water is coming from and what they’re doing to it, the environmental nonprofit analyzed the labels of some 200 bottles. They found that for all the purity and clarity they tout, many companies are obscuring the origins of their water.

THE DETAILS: In 2008, EWG posted requests on its website and through emails asking people to send labels from water bottles to their office, and then graded those labels based on how much they revealed about specific water sources, how the water was filtered or treated, and if it contained any contaminants. The group received labels from 137 brands purchased in 30 states. It repeated the analysis again in May and June 2009, after a law went into affect in California requiring bottled water companies to post information about specific water sources and treatment methods on their websites.

On all the labels the group analyzed in both 2008 and 2009, EWG found that only two brands, Ozarka Drinking Water and Penta Ultra-Purified Water, listed the specific water sources and treatment methods on their labels and offered water-quality reports on their websites. Most of the large national brands, they found, were vague about where they got the water, listing springs and rivers as “possible sources,” and noting that it “originates from public water sources.” The vagaries extended to how, or if, the water underwent additional filtration or treatments methods. EWG found that 44 percent of the labels they analyzed provided no information about treatment.