In the United States, bottled water is regulated by the FDA as a food product, while tap water is regulated by the Environmental Protection Agency (EPA). Water is generally given a low priority for inspection because of good safety records.
But bottled water is not necessarily safer or more pure than water from the tap. A four-year drinking water study by the National Resources Defense Council (NRDC), released in 1999, found that while most of the 100 brands tested were high quality, some had contamination. At least one sample from one-third of the brands was found to exceed contamination limits allowed by state or bottled water industry standards and guidelines. The contaminants included synthetic organic chemicals, bacteria and arsenic. The FDA exempts bottled water that's packaged and sold in the same state, which accounts for over half of what's out there, so these results aren't even a complete picture.
Bottled water doesn't have to be tested for bacteria and chemical contaminants as often as city water does, and some E. coli or fecal coliform is permitted. Bottled water doesn't have to be disinfected or tested for parasites like giardia, cryptosporidium or viruses.
You might already be drinking it
As for where bottled water comes from, much of it is from the same source as your tap water. If it's not labeled with terms like spring or mineral water, indicating a specific source, bottled water can simply be taken from the municipal supply and filtered.
While the potential for chemicals leaching from plastic containers into water has mainly been linked to single-serving water bottles, the concern could extend to jugs if they're left for longer than their expiry date. The longer the water is in the bottle, the more potential there is for leaching.
One of the biggest potential problems with bottled water coolers is bacteria growth. After the water has been treated and filtered, it's put in jugs to be stored un-refrigerated until use, which could allow bacteria growth inside and out. With the bottle neck exposed and then put into a dispenser without being sanitized, bacteria could be transferred directly to the reservoir. When a bottle is changed, dirty hands can also add bacteria. Bottled water coolers have exposed reservoirs that can also become contaminated if they aren't sanitized routinely.