( ent late in the study. A case study of...)Pesticides in the nation's streams and ground water

In agricultural streams, the pesticides chlorpyrifos, azinphos-methyl, p,p'-DDE, and alachlor were among those most often found at concentrations that may affect aquatic life, with each being most important in areas where its use on crops is or was greatest. According to senior author Robert Gilliom, however, "Pesticide use is constantly changing in response to such factors as regulations and market forces and findings from this decade-long study need to be examined in relation to changes in use during and after the study. For example, levels of the herbicide alachlor declined in streams in the Corn Belt (generally including Illinois, Indiana, Iowa, Nebraska, and Ohio, as well as parts of adjoining states) throughout the study period as its use on corn and soybeans declined, with no levels greater than its aquatic-life benchmark by the end of the study. In contrast, both the use and the levels of atrazine, the most heavily used herbicide in the Corn Belt region, remained relatively high throughout the study period."

In addition, DDT, dieldrin, and chlordane--organochlorine pesticide compounds that were no longer in use when the study began--were frequently detected in bed sediment and fish in urban and agricultural areas. Concentrations of these compounds in fish declined following reductions in their use during the 1960s and elimination of all uses in the 1970s and 1980s, and continue to slowly decline. Just as notable as the declines, however, is the finding that these persistent organochlorine pesticides still occur at levels greater than benchmarks for aquatic life and fish-eating wildlife in many urban and agricultural streams across the Nation.

The USGS study also reported that pesticides seldom occurred alone--but almost always as complex mixtures. Most stream samples and about half of the wel
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Source:United States Geological Survey