Wondering what’s coming out of the tap? The Environmental Working Group can tell you, thanks to analysis of 20 million tap water quality tests performed between 2004 and 2009. There’s a database available of the results at http://www.ewg.org/tap-water/whats-in-yourwater.

To find your water source, enter your zip code in the widget on the right side of the screen. I entered 90210 and got a list of dozens of utilities that supply water in that area. I chose the City of Burbank and got a screen that looked like this:

At the top of the page you get a summary about how a particular water supplier compares to the rest of the country. As you move down the page, you get information on what contaminants in the water are over the legal/health limit (if any) and what contaminants are actually found in the water (if any — but all the places I looked at found some contaminants.) At the bottom of the page you’ll get a pollution summary with an overview of what kinds of pollutants are found, and a summary of the EPA violations that water provider has had.

The testing history shows how the testing for particular contaminants go over time. For some locations you might see that the water was really bad in 2004 and 2005, for example, but then the tests get cleaner over time, presumably as changes are made in the water filtration.

You might want to know more than the site tells you about the contiminants, though. The only contaminant I knew much about was selenium. I have heard of Chloroform and Trichlorofluoromethane, but I don’t know why they in particular are considered toxic/dangerous. If you hold your mouse over the name of the contaminant you’ll get more details, but I found there was not enough data. How about a link to PubMed or Wikipedia or something?

You can click on individual contimnants for more data. I wanted to get more data on Ethylbenzene so I went to its individual page. I got details on the number of test days and the number of tests conducted, and then a very useful table of what the legal limits are for the contaminant.

But I still didn’t get a full understanding of why Ethylbenzene is dangerous. I had to get that from ATSDR, at http://www.atsdr.cdc.gov/tfacts110.html. Maybe that registry can be connected with this drinking water database.

In short, there’s a lot of information aggregated here, and the level of analysis is extensive, but you might have to go outside the site to get enough data for context.