Starting Wednesday, the vast majority of Americans can learn about every potentially harmful chemical in their drinking water and what scientists say are the safe levels of those contaminants. The Environmental Working Group's (EWG) new national Tap Water Database is the most complete source available on the quality of U.S. drinking water, aggregating and analyzing data from almost 50,000 public water systems in all 50 states and the District of Columbia.

The organization has earned a reputation for ambitious data-mining research projects that shake up policy debates and consumer markets. EWG's online Farm Subsidy Database, listing millions of subsidy recipients, and its Skin Deep guide to more than 70,000 personal care products, draw tens of millions of visitors every year.