U.S.A.Population: 300 million Problem: Emerging contaminants Up to his knees: Schoenfuss captures fish for study in the Grindstone River near Hinckley, Minn., looking for chemicals that mimic hormones

The common white sucker is nobody's favorite fish. It's a bottom feeder that trout fishermen in Colorado happily toss back into the water. But it's also what scientists call a sentinel - a species whose health (or lack thereof) can warn us about problems in the environment. So imagine the reaction of environmental endocrinologist David O. Norris of the University of Colorado when he discovered some alarming changes in the sucker population of Boulder Creek. Upstream, where the water flows pure and clear out of the Rocky Mountains, the ratio of males to females is 50-50, just as nature intended. Downstream, below the wastewater-treatment plant in Boulder, the females outnumber the males by 5 to 1. Even more worrisome, Norris found that about 10 percent of the fish were neither clearly male nor female, but had sexual characteristics of both. "On the one hand, we were excited [to make such a dramatic finding]," says Norris. "At the same time, we were appalled."There's something fishy in the nation's water supply. True, its quality has improved dramatically since passage of the Clean Water Act in the 1970s. Toxic substances and pollutants are now routinely filtered out. But across the nation, something's causing disturbing effects on aquatic wildlife. In a search for culprits, scientists are zeroing in on a group of compounds they call "emerging contaminants," including pharmaceuticals, cosmetics and antibacterial soaps. Although we like to think that these compounds disappear when we wash them down the drain or flush them down the toilet, a lot of them are clearly ending up in water. Could they possibly affect human health? At this point, no one knows for sure. "We have lots of questions, but very few answers," says environmental chemist Christian Daughton at the Environmental Protection Agency.

Scientists aren't worried about any one of these chemicals in isolation. Most are found in minute doses, if they're found at all. Toxicologist Amy Perbeck at the Michigan Department of Environmental Quality calculated that the levels of ibuprofen in Michigan drinking water were so low that a person would have to consume 17,000 gallons to get the amount in one pill. But new technology is allowing scientists to screen for mere traces of compounds, down to levels that were previously undetectable - and they find just about everything they look for. A 2002 study by the U.S. Geological Survey detected such compounds in 80 percent of the 139 streams it examined, many of which were downstream from urban areas. None of the chemicals on its own appears to be toxic at minuscule doses. "But what happens when a person is exposed to a whole cocktail of them?" asks Perbeck.

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