For all the pathogens and chemicals monitored by the federal government to protect drinking water, a far broader universe of “emerging contaminants” is going unregulated.

The Environmental Protection Agency keeps tabs on scores of substances that have surfaced in water systems around the country, with the aim of restricting those that endanger public health. But partly because the rules that the agency must follow are complicated and contentious, officials have failed to successfully regulate any new contaminant in two decades.

Only once since the 1990s has the EPA come close to imposing a new standard — for perchlorate, a chemical found in explosives, road flares, rocket fuel and, it turns out, the drinking water of over 16 million people.

The years of inaction, critics say, have left many Americans at potential risk from substances that few even realize might be in their water in the first place.

“We live in a country where we’ve made a fundamental decision that chemicals are safe unless they’re proven to be bad,” said Jeffrey Griffiths, a public health professor at Tufts University School of Medicine who studies waterborne diseases. “We have this system which is biased toward the presumption of innocence.”

Here in North Carolina, one of the contaminants on the government’s watch list has been found in rivers and streams on which more than a million people depend.

Since 2013, Detlef Knappe and a team of researchers at N.C. State University have logged hundreds of miles as they gather samples along the Cape Fear River basin. From Greensboro in the heart of the state to the coastal city of Wilmington, they have identified troubling levels of 1,4-dioxane, a byproduct of plastics manufacturing that can be found in paint strippers, varnishes, detergents, shampoos and cosmetics. The EPA has deemed it a “likely human carcinogen,” although limited data exist on the cancer risks it poses for people.