Occurrence and Transport of Agricultural Chemicals in the Mississippi River Basin, July Through August 1993

Heavy rainfall and severe flooding in the upper Mississippi River Basin from mid-June through early August 1993 flushed extraordinarily large amounts of agricultural chemicals (herbicides and nitrate) into the Mississippi River, many of its tributaries, and, ultimately, the Gulf of Mexico. Even though extremely high streamflows were recorded during the flood in 1993, concentrations of herbicides, such as atrazine, alachlor, cyanazine, and metolachlor, were similar to the maximum concentrations measured during spring and summer 1991 and 1992. It was anticipated that the higher streamflows during the flood would dilute the concentrations of herbicides that are usually flushed into streams in late spring and summer. Instead, concentrations were similar to those measured during much lower flows, but the daily loads of herbicides transported in some reaches of the Mississippi River were higher than those measured in 1991 and 1992.