Occurrence and transport of agricultural chemicals in the Mississippi River basin, July through August 1993

Heavy rainfall and severe flooding in the upper Mississippi River Basin from mid-June through early August 1993 flushed extraordinarily large amounts of agricultural chemicals (herbicides and nitrate) into the Mississippi River, many of its tributaries, and, ultimately, the Gulf of Mexico. Even though extremely high streamflows were recorded during the flood in 1993, concentrations of herbicides, such as atrazine, alachlor, cyanazine, and metolachlor, were similar to the maximum concentrations measured during spring and summer 1991 and 1992. It was anticipated that the higher streamflows during the flood would dilute the concentrations of herbicides that are usually flushed into streams in late spring and summer. Instead, concentrations were similar to those measured during much lower flows, but the daily loads of herbicides transported in some reaches of the Mississippi River were higher than those measured in 1991 and 1992. The total atrazine load transported to the Gulf of Mexico from April through August 1993 (539,000 kilograms) was about 80 percent higher than that for the same period in 1991 and 235 percent higher than for the same period in 1992. The concentrations of atrazine and cyanazine in a few individual samples exceeded health-based limits for drinking water. However, because drinking-water regulations are based on the average of at least four quarterly samples, the annual average concentrations in the Mississippi River probably will not exceed these limits for 1993. Nitrate concentrations were similar to those measured during spring and summer 1991 and 1992. The loads of nitrate-nitrogen transported into the Gulf of Mexico during July and August 1993 were as much as 5,734 metric tons per day. These loads generally are similar to those measured in spring 1991 and 1992 but larger than those measured in summer 1991 and 1992. The total nitrate-nitrogen load transported to the Gulf of Mexico from April through August 1993 (827,000 metric tons) was about 37 percent larger than that for this same period in 1991 and 112 percent larger than that for the same period in 1992. The transport of extraordinarily high loads of nitrate and large amounts of fresh-water into the Gulf of Mexico during midsummer when primary production is highest could increase phytoplankton biomass and affect the gulf ecosystem along the Louisiana coast.