The website 24/7 Wall St. examined the quality of water supplies in most major America cities, using data collected from multiple sources for five years (ending in 2009) by Environmental Working Group (EWG), based in Washington, D.C. The fact that the data covered a half-decade is important because it shows that the presence of certain chemicals is persistent.
Based on the EWG's methodology, 24/7 Wall St. came up with its 10 worst cities list. These cities' water quality rank is based on three metrics, in order of increasing importance:
* The percentage of chemicals found based on the number that were tested for
* The total number of contaminants found
* The most dangerous average level of a single pollutant.
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