DAVID GREENE, HOST:
Earlier this year, a chemical spill in West Virginia forced officials to put a ban on drinking water that affected some 300,000 people. This also highlighted an unsettling truth: While officials test our drinking supply, they're only targeting a few chemicals. Many contaminants go undetected.
Here's NPR's Elizabeth Shogren.
ELIZABETH SHOGREN, BYLINE: Toxic chemicals can make it into tap water for years without experts knowing it. That's because of a basic fact about how treatment plants test their water.
MIKE WEHNER: You really only find the things you look for.
SHOGREN: Mike Wehner is a manager with the Orange County Water District in California. The troubling consequences of this truth were brought home to him when California added 1,4-dioxane to the list of chemicals it might someday regulate, and Wehner's utility decided to test for it.
WEHNER: We detected 1,4-dioxane in our treatment plant, and not really being very effectively removed by the treatment processes that were employed at the time.
SHOGREN: Wehner says it was a complete surprise. Industries use 1,4-dioxane to dissolve oily or greasy substances. The federal government says it probably causes cancer. Fortunately, Orange County's problem was caused by only one company. Officials quickly tracked it down and got it to stop releasing the chemical. Still, agency officials decided to install expensive new water treatment equipment.
Wehner says that's because they learned something important: If they missed this toxic chemical, they could be missing others.
WEHNER: If 1,4-dioxane could get through our system, we should be confident that we have barriers that will remove not just that contaminant, but other contaminants, as well.
SHOGREN: That all happened a dozen years ago, a time when most tap water suppliers never bothered to test for 1,4-dioxane. Now the federal Environmental Protection Agency is requiring 6,000 of the biggest drinking water suppliers to test for the chemical. Early results show 1,4-dioxane has made it into tap water elsewhere.
In North Carolina, utilities that get water from the Cape Fear River measured levels as high as nine parts per billion. That caught the attention of Detlef Knappe. He's an engineering professor at North Carolina State University.
DETLEF KNAPPE: That would translate into cancer risks in the range of 30 in a million. Those are levels where we become concerned from a public health perspective.
SHOGREN: Knappe and his students started testing the water on the Cape Fear and its tributaries. The further upstream they went, the higher levels they found, including in tap water in a small water treatment plant. Nobody knows where it's coming from. That's because the state doesn't make companies test for it in their wastewater.
Susan Massengale represents the state's Department of Environment and Natural Resources.
SUSAN MASSENGALE: We don't really have the resources to go looking for a lot of things that don't already fit into our regulatory scheme.
SHOGREN: She says generally, if the EPA doesn't require it, they won't spend the money to do it. Still, she says her agency will look at Knappe's results and EPA's data and will consider a response.
OK, that's just two states. But the more places that look for 1,4-dioxane, the more find it. Two hundred and eighty drinking water suppliers have found it mostly in low concentrations that probably aren't dangerous. But these early results are causing a stir among drinking water experts.
ANDY EATON: Maybe we're looking at a very widespread industrial pollutant here.
SHOGREN: Andy Eaton is technical director of a water testing laboratory in California. He tests water from around the country. So far, the EPA is not talking about the results.
There is no federal standard for how much 1,4-dioxane is safe to drink. That's true for thousands of other toxic chemicals, just like the one that tainted drinking water in West Virginia.
Elizabeth Shogren. NPR News. Transcript provided by NPR, Copyright NPR.