A Screening-Level Model Evaluation of Atrazine in the Lake Michigan Basin

Atrazine, a widely used herbicide in the agricultural regions of the Lake Michigan basin, was selected as a priority toxic chemical for study in the United States Environmental Protection Agency (U.S. EPA)-sponsored Lake Michigan Mass Balance Project. A surface-water, screening-level model for atrazine in the Lake Michigan basin was developed to obtain an initial insight into its transport behavior and ultimate fate. Estimates of tributary loadings and atmospheric loadings for model computations were made for the period-of-usage of the chemical beginning in 1964. Most of these loading estimates were based on total annual usage rates in the United States. Data from the literature were used to estimate these historical loadings. Approximately 30% of the total load of atrazine entering the lake is associated with precipitation, and the remainder is from tributary loads. An unsteady-state, Water Quality Simulation Program (WASP) model based on the principle of conservation of mass, was used to predict concentrations of atrazine in Lake Michigan and Green Bay, Wisconsin. Model predictions agree with recent field-measured atrazine concentrations in the lake when atrazine is assumed to be conservative in the lake. The persistence of atrazine predicted for Lake Michigan contrasts sharply with the relatively short half-lives of the chemical measured on agricultural fields as reported in the literature. It was estimated that if loadings of atrazine were to continue into the future at a rate equivalent to that of 1993, the lake would reach a steady-state concentration of 160 ng/L in approximately 300 years.