Treatment plants, many of them using nearly century-old technology, are simply not up to the task of cleaning up contaminants, said Erik Olson, author of the report by the Natural Resources Defense Council. Pipes carrying water often are old, too, in some cities dating back more than a century, he said.
There was good news as well as bad.
Overall, drinking water purity has improved slightly in most cities in the past 15 years, the study said.
Chicago was singled out for its tap water, earning an "excellent" rating for water quality and compliance with regulations in 2001. Five cities — Denver; New Orleans; Manchester, N.H.; Baltimore and Detroit — were rated good.
Eight got marks of fair: Houston; Los Angeles; Philadelphia; Atlanta; Newark, N.J.; San Diego; Seattle and Washington.
While no city received a failing mark, five were given poor ratings: Albuquerque, N.M.; Boston; Fresno, Calif.; Phoenix and San Francisco.
Fresno apparently violated the standard for nitrates in the water supply, according to the study.
Washington was cited for violating a new national standard for trihalomethanes in 2000, but levels were reduced the following year. Trihalomethanes are a byproduct of the chlorination process for drinking water — and the government says they may increase the risk of cancer.
"People would be surprised to know that their water contains cancer-causing chemicals, toxic chemicals like lead, that it often contains the remnants of pollutants like sewage that slip through some of the treatment plants," said Olson.
An analyst with the National Rural Water Association, which represents 22,000 utilities and communities, took issue with the report.
Mike Keegan said, "There's always been an issue of replacing infrastructure, but now there's more and better infrastructure in the country" than ever before. Compliance with water regulations and standards has been improving, he said.
Environmental Protection Agency spokesman John Millett said that while there are problems in some cities, the vast majority are meeting water quality standards.
Millett said the Bush administration has committed $850 million a year through 2018 for assistance to the nation's drinking water systems.
However, the environmental group said it would take up to $500 billion over the next 20 years to fix the nation's public water system.
The group cited arsenic, chromium and the rocket fuel perchlorate as particularly dangerous. All can end up in the water from industry and manufacturing.
Pesticides, waterborne parasites and lead — which can enter the water supply from old pipes — also are a problem, according to the report.
The study reviewed tap water quality data, EPA compliance records and water suppliers' annual reports to come up with the ratings.
The group also handed out rankings for how a city's reservoirs, rivers or other sources of water are protected, as well as how suppliers notify the public about water quality, in the form of "right to know" reports sent to consumers.
By Jennifer C. Kerr