We. Pereira et Fd. Hostettler, NONPOINT-SOURCE CONTAMINATION OF THE MISSISSIPPI RIVER AND ITS TRIBUTARIES BY HERBICIDES, Environmental science & technology, 27(8), 1993, pp. 1542-1552
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.