Temporal trends of atrazine, simazine, and metolachlor in a U.S. Midwest agricultural watershed: A 10-year study

Authors: Gonzalez, Javier; Smiley, Peter

Submitted to: Environmental Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/24/2025
Publication Date: 3/13/2025
Citation: Gonzalez, J.M., Smiley, P.C. 2025. Temporal trends of atrazine, simazine, and metolachlor in a U.S. Midwest agricultural watershed: A 10-year study. Environmental Processes. https://doi.org/10.1007/s40710-025-00753-7.
DOI: https://doi.org/10.1007/s40710-025-00753-7

Interpretive Summary: Most agriculture systems rely on pesticides to minimize pest damage and maximize yield. Herbicides are the most common pesticides used in agriculture. However, herbicide losses often occur from agricultural fields to adjacent streams, rivers, and lakes and degrade water quality. Communities throughout the Midwest United States, e.g., the city of Fort Wayne, Indiana, rely on Great Lake tributaries as their drinking water sources. There is a lack of information on long-term trends in herbicide concentrations in agricultural streams and rivers, which is needed to develop effective watershed management plans. For ten years, we measured the concentrations of three herbicides commonly used to control weeds in corn (atrazine, metolachlor, simazine) in the Cedar Creek watershed, a tributary of the St. Joseph River, Indiana, USA. Our objective was to determine if these herbicide concentrations increased or decreased over time and if time explained the long-term trends in herbicide concentrations better than other selected environmental factors, including climate, hydrology, land use, and adoption of conservation practices. Our results indicated that there were no annual or seasonal trends for atrazine and metolachlor and that their concentrations were better predicted by climatic, hydrologic, and land use factors. During the time of application (May and June), atrazine concentrations typically exceeded the drinking water standards, which indicates a need to implement conservation practices to reduce atrazine concentrations during this period. Simazine concentrations decreased over a decade and the decrease was attributed to the declining use of this herbicide. Our findings provide valuable information to researchers, policymakers, producers, and local and state governments to guide and monitor watershed management at the large agricultural watershed scale.

Technical Abstract: Most crop production systems rely on pesticide usage to minimize pest damage to crops and maximize yield. Herbicides are the most common pesticides used in agriculture and their off-site transport to water bodies occurs frequently and subsequently impacts water quality, which is a worldwide concern. Long-term trends in herbicide concentrations in agricultural streams and rivers and the environmental factors associated with these trends are limited and needed to develop watershed conservation strategies. To address this research gap, in a 10-year study, instream atrazine, metolachlor, and simazine concentration were measured in the Cedar Creek watershed, a tributary of the St. Joseph River, Indiana, USA, to determine if trends occur in atrazine, metolachlor, and simazine concentrations in Cedar Creek and if these herbicide concentrations are better predicted by time than other environmental factors. Our decade-long watershed study indicated that no annual or seasonal trends occurred in atrazine and metolachlor concentrations and their concentrations were better predicted by climatic, hydrologic, and land use variables. Atrazine concentrations during the spring regularly exceeded the drinking water standards, which indicates a need for targeted conservation efforts to reduce atrazine concentrations after application. Annual and spring downward trends for mean simazine concentrations were observed and attributed to declining simazine usage. These findings provide information that will help guide watershed management and monitoring efforts within large agricultural watersheds.