This is a beautiful video capturing what happens to a single drop of Water.

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How safe is the water that flows out of your tap? The answer very much depends on where you live.

It’s now easier than ever for consumers to find out what’s in their tap water. The Environmental Working Group (EWG) today released the results of a three-year investigation of municipal water supplies across the U.S.

The research and advocacy group looked at water quality tests performed by water utilities since 2004 and created an extensive database that contains info on the contaminants found in 48,000 communities in 45 states.

EWG also rated 100 big city (population over 250,000) water utilities. Below are the top and bottom results.

Cities with the best water:

1. Arlington, TX
2. Providence, RI
3. Fort Worth, TX
4. Charleston, SC
5. Boston, MA
6. Honolulu, HI
7. Austin, TX
8. Fairfax County, VA
9. St. Louis, MO
10. Minneapolis, MN

Cities with the worst water:

1. Pensacola, FL
2. Riverside, CA
3. Las Vegas, NV
4. Riverside County, CA
5. Reno, NV
6. Houston, TX
7. Omaha, NE
8. North Las Vegas, NV
9. San Diego, CA
10. Jacksonville, FL

Your city not on the list? Here is the full 100-city listing or you can search for your town by ZIP code.

If you live in one of the few areas that weren’t investigated, you can get an annual report of what’s in your public drinking water. If your water comes from a well, then see the EPA’s guidelines for those who use private wells.

The results of the investigation raise some concerns about municipal water supplies in the U.S. EWG says 316 different contaminants were found in the nation’s tap water. The group also points out that more than half of those contaminants aren’t regulated by the Environmental Protection Agency.

Establishing more effective source water protection programs and developing enforceable government standards for contaminants would go a long way toward improving the nation’s water supply, according to the EWG.

In the meantime, no one is suggesting that you go out and start drinking bottled water (although, of course, in emergency situations it can be necessary). Experts still agree  that drinking tap is preferable to drinking bottled water. Bottled water is much more expensive than tap, it takes a huge toll on the planet, and it’s not necessarily any safer than tap. “Bottled water is not regulated in the same way as tap water,” says Olga Naidenko, a senior scientist at EWG. “With bottled water, consumers often do not know what they are getting, and 25 to 40 percent of bottled water on the market is simply tap poured into a bottle.”

Knowing what’s in your water is the key. Once you know which contaminants are present, you can find the best filter to get rid of them. EWG put together a thorough buying guide to help you find an affordable water filter that works for you. You can read the guidelines or search for products based on contaminants you want to get rid of.

Environmental journalist Lori Bongiorno shares green-living tips and product reviews with Yahoo! Green’s users. Send Lori a question or suggestion for potential use in a future column. Her book, Green Greener Greenest: A Practical Guide to Making Eco-smart Choices a Part of Your Life is available on Yahoo! Shopping and Amazon.com.