Pesticide Levels Lower in Corn Belt Rivers

Scientists studied 11 herbicides and insecticides frequently detected in the Corn Belt region, which stretches across Illinois, Indiana, Iowa, Nebraska and Ohio, as well as parts of adjoining states.

This area has the highest pesticide use in the nation, mostly herbicides used for weed control in corn and soybeans. Borne by runoff from cropland and urban areas, these pesticides are widespread in the region’s streams and rivers.

Elevated concentrations of these chemicals can affect aquatic organisms in streams as well as the quality of drinking water in some high-use areas where surface water is used for municipal supply.

Spraying pesticide on a corn crop in Illinois (Photo by farmerdoodah)

The USGS study is based on analysis of 11 pesticides for 31 stream sites in the Corn Belt for two partially overlapping time periods - 1996 to 2002 and 2000 to 2006.

Pesticides included in the trend analyses were the herbicides atrazine, acetochlor, metolachlor, alachlor, cyanazine, EPTC, simazine, metribuzin and prometon, and the insecticides chlorpyrifos and diazinon.

Additional detailed analyses of relations between concentrations and use focused on four herbicides mainly used for weed control in corn - atrazine, acetochlor, metolachlor and alachlor - at a subset of 11 sites on the main rivers and selected large tributaries in the Ohio, Upper Mississippi and Missouri River basins.

Atrazine, the most frequently detected chemical, is regulated in drinking water.

Four of the pesticides evaluated were among those most often found in previous USGS studies to occur at levels of potential concern for healthy aquatic life.

The declines documented in pesticide concentrations closely followed declines in their annual applications, the authors said, indicating that reducing pesticide use is an effective and reliable strategy for reducing pesticide contamination in streams.

Declines in concentrations of the agricultural herbicides cyanazine, alachlor and metolachlor show the effectiveness of U.S. Environmental Protection Agency regulatory actions as well as the influence of new pesticide products, the USGS reports.

Declines in concentrations of the insecticide diazinon from 2000 to 2006 correspond to the EPA’s national phase-out of nonagricultural uses of this chemical, the USGS report finds.

"Pesticide use is constantly changing in response to such factors as regulations, market forces, and advances in science," said Dan Sullivan, lead scientist for the study.

"For example, acetochlor was registered by the EPA in 1994 with a goal of reducing use of alachlor and other major corn herbicides - acetochlor use rapidly increased to a constant level by about 1996, and alachlor use declined," he said.

"Cyanazine use also decreased rapidly from 1992 to 2000, as it was phased out because of environmental concerns," Sullivan said.

"Metolachlor use did not markedly decrease until about 1998, when S-metolachlor, a more effective version that requires lower application rates, was introduced," he said. "Each of these declines in use was accompanied by similar declines in concentrations."

Although trends in concentration and use almost always closely corresponded, concentrations of atrazine and metolachlor each declined in one stream more rapidly than their estimated use.

Skip Vecchia, senior author of the report on this analysis, said, "The steeper decline in these instances may be caused by agricultural management practices that have reduced pesticide transport, but data on management practices are not adequate to definitively answer the question. Overall, use is the most dominant factor driving changes in concentrations."

Only one pesticide - simazine, which is used for both agricultural and urban weed control - increased from 1996 to 2006.

Concentrations of simazine in some streams increased more sharply than its trend in agricultural use, suggesting that non-agricultural uses of this herbicide, such as for controlling weeds in residential areas and along roadsides, increased during the study period, the authors said.

Glyphosate, an herbicide which has had rapidly increasing use on new genetically modified varieties of soybeans and corn, and which now is the most heavily used herbicide in the nation, was not measured until late in the study and thus had insufficient data for analysis of trends.

Find the new report and maps on pesticide status, trends and use at the USGS Pesticide National Synthesis Project website.

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