Scientists studied eleven herbicides and insecticides frequently detected in the Corn Belt region, which generally includes Illinois, Indiana, Iowa, Nebraska and Ohio, as well as parts of adjoining states. This area has among the highest pesticide use in the nation, mostly herbicides used for weed control in corn and soybeans. As a result, these pesticides are widespread in the region's streams and rivers, largely resulting from runoff from cropland and urban areas.
Elevated concentrations 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. Four of the eleven pesticides evaluated for trends were among those most often found in previous USGS studies to occur at levels of potential concern for healthy aquatic life. Atrazine, the most frequently detected, is also regulated in drinking water.
'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 increased rapidly to a constant level by about 1996 and then declined. Cyanazine use also decreased rapidly from 1992 to 2000, as it was phased out because of environmental concerns. Metolachlor use did not markedly decrease until about 1998 when S-metolachlor, a more effective version that requires lower application rates, was introduced. 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. According to Skip Vecchia, senior author of the report on this analysis, '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.'
Additional information, including data, reports and maps on pesticide status, trends and use may be found at the USGS Pesticide National Synthesis Project website.