NONPOINT-SOURCE CONTAMINATION OF THE MISSISSIPPI RIVER AND ITS TRIBUTARIES BY HERBICIDES

A study of the Mississippi River and its tributaries during July-Augus t 1991, October-November 1991, and April-May 1992 has indicated that t he entire navigable reach of the river is contaminated with a complex mixture of agrochemicals and their transformation products derived fro m nonpoint sources. Twenty-three compounds were identified, including triazine, chloroacetanilide, thiocarbamate, phenylurea, pyridazine, an d organophosphorus pesticides. The upper and middle Mississippi River Basin farm lands are major sources of herbicides applied to corn, soyb eans, and sorghum. Farm lands in the lower Mississippi River Basin are a major source of rice and cotton herbicides. Inputs of the five majo r herbicides atrazine, cyanazine, metolachlor, alachlor, and simazine to the Mississippi River are mainly from the Minnesota, Des Moines, Mi ssouri, and Ohio Rivers. Ratios of desethylatrazine/atrizine atrazine potentially are useful indicators of groundwater and surface water int eractions in the Mississippi River. These ratios suggested that during base-flow conditions, there is a significant groundwater contribution to the river. The Mississippi River thus serves as a drainage channel for pesticide-contaminated surface and groundwater from the midwester n United States. Conservative estimates of annual mass transport indic ated that about 160 t of atrazine, 71 t of cyanazine, 56 t of metolach lor, and 18 t of alachlor were discharged into the Gulf of Mexico in 1 991.