Monday, March 6, 2006

The Levels of Pesticides in US Waters Won't Kill People But May Kill Fish

U.S. Geological Survey released a report last week describing the occurrence of pesticides in streams and ground water during 1992-2001. Pesticides were found frequently in streams but infrequently in  ground water.  Pesticides in streams are seldom at concentrations likely to affect humans. However, in streams draining urban and agricultural areas, pesticides were found at concentrations that may affect aquatic life or fish-eating wildlife.

Concentrations of individual pesticides were almost always lower than human health standards and guidelines.  However, pesticides may have substantially greater effects on aquatic ecosystems. More than 80 % of urban streams and more than 50 % of rural agricultural streams had concentrations in water of at least one pesticide—mostly those in use during the study period—that exceeded a water-quality benchmark for aquatic life. Water-quality benchmarks are estimates of concentrations above which pesticides may have adverse effects on human health, aquatic life, or fish-eating wildlife.

Insecticides, particularly diazinon, chlorpyrifos, and malathion frequently exceeded aquatic-life benchmarks in urban streams. Most urban uses of diazinon and chlorpyrifos, such as on lawns and gardens, have been phased out since 2001 because of use restrictions imposed by the EPA. Concentrations of these pesticides may have been declining in some urban streams even before 2001—benchmark exceedences declined late in the study. A case study of diazinon shows declining concentrations in several urban streams in the Northeast during 1998-2004.

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.

DDT, dieldrin, and chlordane—organochlorine pesticide compounds 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. But, 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.

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USGS also reported 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.  Scientists know little about the effects of pesticide mixtures, since most toxicity information and water-quality benchmarks used in this study, has been developed for individual chemicals. USGS indicated "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. 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."

Pesticides in the Nation's Streams and Ground Water 1992-2001 
In-depth information about the pesticide assessment may be found at: http://water.usgs.gov/nawqa/ under "What’s New."

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