Pesticides are present throughout the year in most streams in urban and agricultural areas of the United States, according to a new report released by the U.S. Geological Survey on Friday. The report is based on data from 51 major river basins and aquifer systems from Florida to the Pacific Northwest, Hawaii and Alaska, plus a regional study in the High Plains aquifer system.
The pesticides are seldom found at concentrations likely to affect humans, but they may affect aquatic life or fish-eating wildlife in many streams, particularly those draining urban and agricultural areas, the USGS researchers learned. The toxic chemicals are less common in ground water, they found.
The USGS study, which covers the years 1992-2001, found that pesticides seldom occurred alone but almost always as complex mixtures. Most stream samples and about half of the well samples contained two or more pesticides, and frequently more.
The common occurrence of pesticide mixtures in streams, means that the total combined toxicity of pesticides in water, sediment, and fish may be greater than that of any single pesticide in a stream.
Senior author Robert Gilliom puts a high priority on understanding more about what happens when pesticides mix together in streams. "The potential effects of contaminant mixtures on people, aquatic life, and fish-eating wildlife are still poorly understood and most toxicity information, as well as the water-quality benchmarks used in this study, has been developed for individual chemicals."
"Studies of the effects of mixtures are still in the early stages," he said, "and it may take years for researchers to attain major advances in understanding the actual potential for effects."
The report, "Pesticides in the Nation's Streams and Ground Water, 1992-2001," Circular 1291 is available at http://water.usgs.gov/pubs/circ./circ1291. In-depth information about the pesticide assessment may be found at: http://water.usgs.gov/nawqa/ under "What's New."