Seasonal and annual load of herbicides from the Mississippi River basin tothe Gulf of Mexico

Water samples collected from rivers in the Mississippi River Basin were ana lyzed for selected herbicides to evaluate their discharge to the Gulf of Me xico and to identify their predominant source areas within the basin. Sampl es were collected from the Mississippi River at Baton Rouge, LA from 1991 t o 1997 and from sites on the upper Mississippi, the Missouri, and the Ohio Rivers from 1996 to 1997. Atrazine, metolachlor, and alachlor ESA (an alach lor metabolite) were the most frequently detected herbicides in the Mississ ippi River at Baton Rouge, a nd, in general, were present in the largest co ncentrations. The peak annual herbicide load was in 1993 when about 640 met ric tons of atrazine, 320 metric tons of cyanazine, 215 metric tons of meto lachlor, 53 metric tons of simazine, and 50 metric tons of alachlor were di scharged to the Gulf of Mexico. The annual load of atrazine and cyanazine w as generally 1-3% of the amount annually applied in the Mississippi River d rainage 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 M exico.