Some of these sources can't be handled by chlorination, which is the method used by the majority of municipal water systems.
"We have a huge number of contamination sources that are poorly regulated if controlled at all," said Erik Olson, water expert at the Natural Resources Defense Council. "The water utilities say: ‘We don't know what to do with them,' and throw up their hands."
Building plants that use carbon filters (like the one in your fish tank), ozone treatment or special membranes has proved too expensive for most cities. So Olson says the key is protecting the original source of a town's drinking water.
Officials in New York City, Seattle, Boston and Manchester, N.H., have aggressively purchased land to keep industrial, mining and farming operations away from their water supplies.
In Cincinnati, city leaders decided instead to build a state-of-the-art treatment system, Olson said.
While the United States has strong environmental regulations, there are some gaps.
It turns out that although the federal Environmental Protection Agency is tasked with keeping drinking water safe, the actual implementation of the law comes down to individual city, county and local municipal authorities.