Consider page 14 from the District of Columbia Water and Sewer Authority's "Drinking Water Quality Report 2005." (Download the report.) The Lead Results graph shows lead concentrations in the district's water during 2004 and 2005. In 2004, D.C.'s residents were getting water that was in gross violation of the Environmental Protection Agency's guideline for lead, which fixes the 90th-percentile concentration at 15 parts per billion (ppb) or less. In 2004, the 90th percentile in the nation's capital was 50 to 60 ppb.

While the lead levels did improve the next year, another table in the district's report shows that during the July to December 2005 monitoring period, 10 of 102 samples exceeded 15 ppb. Such results mean that almost 10 percent of the faucets might have been dispensing water above the EPA's action level. "Although the district did not technically violate the EPA rule," says Deborah Wallace, a senior project leader in the Consumer Reports Technical Division who conducted our recent testing of water filters, "the ratio of samples with elevated lead concentrations to total samples was so close to violation that compliance is really a matter of luck or of a sampling program very carefully designed to produce the appearance of compliance."

One aspect of CCRs could be confusing to consumers. Some contaminants, especially arsenic, lead, nitrate, nitrite, and radon, that are present in concentrations above federal or state limits but appear in only a small proportion of the samples require only a health warning in the report. They do not need to be highlighted in the tables. What's more, those warnings are often written so that they seem more like an explanation of a substance's potential health effects rather than an alert to its possible presence in your water. And you might have to scour the report for them. Fortunately, CCRs include a contact at the relevant water system to answer your questions and clear up any murky areas in the report, so call the company if you have any questions.

And remember, even the most thorough CCRs describe only the quality of the system's water as a whole and cannot provide specifics about what's flowing out of your tap. To find out whether the water at your home has picked up contaminants such as copper, lead, bacteria, and cysts, have it tested. Call the EPA's Safe Drinking Water Hotline (800-426-4791) to see whether your municipality provides free or inexpensive testing or to find a certified testing lab in your area. For more information from the EPA, visit the Ground Water & Drinking Water site.