Model forecasts of atrazine in Lake Michigan in response to various sensitivity and potential management scenarios

Abstract For more than forty years, the herbicide atrazine has been used on corn crops in the Lake Michigan basin to control weeds. It is usually applied to farm fields in the spring before or after the corn crop emerges. A version of the WASP4 mass balance model, LM2-Atrazine, was used to assess the impact of the historical and future usage of this chemical on lake water concentrations. Long-term model forecasts were performed under various sensitivity and potential management scenarios. The model was calibrated to available lake data and results indicate that atrazine, under average conditions, is decaying very slowly in the lake (0.009/year). This kinetic decay translates into a half-life estimate of 77 years. If the average condition scenario were assumed to remain constant into the future and reflective of conditions in January 1, 2005, it is expected that the lake (excluding Green Bay) would eventually reach a volume-weighted average atrazine concentration of approximately 67 ng/L in the year 2157 (current model prediction is 48 ng/L for year 2011). These forecasted lake-wide concentrations are below known water quality criteria for the protection of aquatic ecosystems.