The best way to learn about your local drinking water quality is to read the annual drinking water quality report/consumer confidence report that water suppliers now send out by July 1 of each year. The reports often are sent out with water bills, but they may be sent separately. The reports tell where drinking water comes from, what contaminants are in it, and at what levels.
The U.S. Environmental Protection Agency offers information about local drinking water systems through their Local Drinking Water Information Web page. Many water suppliers are now posting their water-quality monitoring results on the Web, and these results can be accessed from this site.
Another source of information is the EPA site Surf Your Watershed. This site answers questions such as, "Where does my drinking water come from?"