Water samples collected from rivers in the Mississippi River Basin were analyzed for selected herbicides to evaluate their discharge to the Gulf of Mexico and to identify their predominant source areas within the basin. Samples were collected from the Mississippi River at Baton Rouge, LA from 1991 to 1997 and from sites on the upper Mississippi, the Missouri, and the Ohio Rivers from 1996 to 1997. Atrazine, metolachlor, and alachlor ESA (an alachlor metabolite) were the most frequently detected herbicides in the Mississippi River at Baton Rouge, and, in general, were present in the largest concentrations. The peak annual herbicide load was in 1993 when about 640 metric tons of atrazine, 320 metric tons of cyanazine, 215 metric tons of metolachlor, 53 metric tons of simazine, and 50 metric tons of alachlor were discharged to the Gulf of Mexico. The annual load of atrazine and cyanazine was generally 1−3% of the amount annually applied in the Mississippi River drainage basin; the annual load of acetochlor, alachlor, and metolachlor was generally less than 1%. During 1996−1997 the Ohio River contributed about 50% of the discharge and 50% or more of the herbicide load to the Gulf of Mexico.
