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Scientists are concerned by the increasing variety of contaminants in the environment. Traces of pesticides and caffeine were found in the Zumbro River, above, in Minnesota.Credit Brent Frazee/Kansas City Star, via MCT, via Getty Image

Deborah Swackhamer, a professor of environmental health sciences at the University of Minnesota, decided last year to investigate the chemistry of the nearby Zumbro River. She and her colleagues were not surprised to find traces of pesticides in the water.

Neither were they shocked to find prescription drugs ranging from antibiotics to the anti-convulsive carbamazepine. Researchers realized more than 15 years ago that pharmaceuticals – excreted by users, dumped down drains – were slipping through wastewater treatment systems.

But though she is a leading expert in so-called emerging contaminants, Dr. Swackhamer was both surprised and dismayed by the sheer range and variety of what she found. Caffeine drifted through the river water, testament to local consumption of everything from coffee to energy drinks. There were relatively high levels of acetaminophen, the over-the-counter painkiller. Acetaminophen causes liver damage in humans at high doses; no one knows what it does to fish.

“We don’t know what these background levels mean in terms of environmental or public health,” she said. “It’s definitely another thing that we’re going to be looking at.”

Or, she might have said, one of many, many other things.

The number of chemicals contaminating our environment is growing at exponential rate, scientists say. A team of researchers at the U.S. Geological Survey tracks them in American waterways, sediments, landfills and municipal sewage sludge, which is often converted into agricultural fertilizer. They’ve found steroid hormones and the antibacterial agent triclosan in sewage; the antidepressant fluoxetine (Prozac) in fish; and compounds from both birth control pills and detergents in the thin, slimy layer that forms over stones in streams.

“We’re looking at an increasingly diverse array of organic and inorganic chemicals that may have ecosystem health effects,” said Edward Furlong, a research chemist with the U.S.G.S. office in Denver and one of the first scientists to track the spread of pharmaceutical compounds in the nation’s waterways. “Many of them are understudied and unrecognized.”