Farmers, exterminators, homeowners and others in the U.S. use roughly one billion pounds of pesticides every year to eliminate weeds, insects and other nuisances. But many of those chemicals end up in the nation's waterways. In fact, a 10-year survey reveals that even in remote areas--far from pavement runoff or agricultural by-products--65 percent of streams contain traces of one or more pesticides.

Hydrologist Robert Gilliom of the U.S. Geological Survey (USGS) and his team tested waters in 51 areas across the country for 75 pesticides between 1992 and 2001, including water samples from 186 streams and more than 5,000 wells along with sediment from more than 1,000 waterways. Overall, pesticides were less common in so-called groundwater, such as that in wells, although 33 percent of deep wells that tap aquifers contained one or more of the chemicals.

Only 10 percent of the agricultural streams surveyed exceeded human health standards for any of these chemicals, most located in the so-called Midwestern corn belt, where herbicides like atrazine are regularly employed. But fish and other aquatic life face potentially harmful concentrations in 57 percent of streams in agricultural areas and in 83 percent of city streams. And these fish are still carrying the toxic legacy of DDT and its derivatives 20 years after its ban, according to the USGS. Around 70 percent of city stream beds turned up the organochlorine pesticide as well.

Concentrations of the pesticides varied from region to region. Atrazine predominated in the Midwest, whereas the highest concentrations of DDT were found in the Southeast and Pacific Northwest, where it was used on local crops in the past. The USGS survey also revealed that pesticides were rarely the only chemicals in the water: more than 90 percent of contaminated streams had at least two detectable chemicals and 20 percent carried 10 or more.

"The common occurrence of pesticide mixtures, particularly in streams, means that the total combined toxicity of pesticides in water, sediment and fish may be greater than that of any single pesticide compound that is present," Gilliom explains. "Studies of the effects of mixtures are still in the early stages, and it may take years for researchers to attain major advances in understanding the actual potential for effects. Our results indicate, however, that studies of mixtures should be a high priority."