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.