THE OCCURRENCE AND TRANSPORT OF PESTICIDES IN THE LOWER MISSISSIPPI RIVER DURING THE 2011 FLOOD, APRIL THROUGH JULY

In April through July 2011, the lower Mississippi-Atchafalaya River Basin experienced a flood of historic proportions. This flood marked the first time in history that three of the major flood control structures (Bird’s Point-New Madrid Floodway, Morganza Floodway, and Bonnet Carré Spillway) were operated simultaneously. Because the source water for the flood came from agricultural areas of the Midwest and agricultural land was inundated by opening the Morganza and Bird’s Point-New Madrid Floodways, there was concern that the Mississippi River would export unprecedented amounts of pesticides into the Gulf of Mexico.

In order to characterize the occurrence and transport of pesticides in the lower Mississippi-Atchafalaya River Basin during the flood, the U.S. Geological Survey collected water samples for analysis of pesticides at 11 stations on the Mississippi, Ohio, Yazoo, Arkansas, and Atchafalaya Rivers. Additional water samples were also collected in the two floodways.

Water samples were analyzed for up to 136 pesticides and pesticide degradates. Of the 136 compounds, 118 were not detected above the method reporting level. The remaining 18 compounds fall into several categories: (1) compounds that were frequently detected and showed an increase in concentration during the flood, (2) compounds that were detected almost all of the time at every site, but usually in very low concentrations, (3) and compounds that were infrequently detected. The first group included acetochlor, atrazine and its 3 degradates (deisopropylatrazine, deethylatrazine, and hydroxyatrazine), metolachlor, and simazine. The second group include 2,4-D, 3-4-dichloroanline, alachlor, diuron, metribuzin, prometon, and prometryn. Compounds that were infrequently detected included the fungicides cis-propiconazole, trans-propiconazole, and metalaxyl, and the herbicide, trifluralin.

The fluxes during April through July of 2011 for the most frequently detected pesticides (atrazine, metolachlor, acetochlor, and simazine) were in the mid to low range of historic fluxes. This indicates that the 2011 flood, although of historic proportions concerning flow, did not carry a similarly historic amount of pesticides into the Gulf of Mexico.