Annual loads of herbicides in Lake Erie tributaries of Michigan and Ohio

Loading rates of the herbicides atrazine, alachlor, cyanazine, metolachlor, and metribuzin to western and central Lake Erie for the period 1983-1993 were determined for the Raisin, Maumee, Sandusky, Cuyahoga, and Grand River tributaries. Knowledge of these loads is important as input: (1) to the development and refinement of the Lakewide Management Plans (LAMPs); (2) to understand the dynamics of these herbicides in the Great Lakes for the prediction of their effects on human and ecosystem health; (3) for the comparative assessment of agricultural pollution and its management on a regional and national scale. Loads were calculated using a new implementation of the Beale Ration Estimator. Variability in annual loads is substantial, and is linked to year-to-year differences in rainfall and river discharge, particularly during the several months following herbicide application. The highest loads were observed in 1990, and the lowest loads in drought year of 1988; the highest annual loads exceeded the lowest by as much as 60-fold. The Sandusky and Maumee rivers have substantially higher unit area loads than the other tributaries, reflecting the extent of row crop agriculture in these basins and their propensity toward surface runoff of precipitation because of fine-grained soils. For the Maumee and Sandusky rivers, atrazine and metolachlor loads are typically in the range 1-2 g/ha/yr and have not exceeded 4 g/ha/yr., with atrizine the highest, typically about 1 g/ha/yr and rarely reaching 3 h/ha/yr. Herbicides are infrequently detected in the Grand River, and loads are low and estimated with considerable uncertainty. Total loads for the five stations combined range from 500-20,000 kg/yr and 600-14,000 kg/yr for the atrazine and metolachlor to 100-6,000 kg/yr for alachlor, 70-3,300 kg/yr for cyanazine, and 60-2,800 kg/yr for metribuzin.