The broad flat Mississippi River Alluvial Plain, located on both sides of the Mississippi River (predominately in Arkansas, Louisiana, Mississippi, and Missouri) is one of the most agriculturally productive areas in the United States. The major crops grown in this area include corn, cotton, rice, and soybeans. Although pesticides are used in row crop production throughout the United States to improve yields and protect crops, pesticide use in the Mississippi River Alluvial Plain (the Delta) exceeds that of most other parts of the United States. The greater use of pesticides is largely because of the intense weed and insect pressure in the humid southeastern United States and the types of crops, such as cotton, that are slow growing and very susceptible to yield loss. There is concern that this heavy use of pesticides may contaminate surface water. In addition to the parent compounds, metabolites of the pesticides may also contaminate surface water. The toxicological significance of the presence of these metabolites in surface water is not usually known because most studies focus on the toxicity of the parent compounds. In 1996 the U.S. Geological Survey’s National Water-Quality Assessment Program began an assessment of the surface-water quality of the Mississippi Embayment study area. Surface-water samples were collected from streams and analyzed for selected herbicides and metabolites. These included atrazine, cyanazine, fluometuron, norflurazon, and propanil, some of the most heavily used herbicides in the Delta. In addition, the surface-water samples were analyzed for at least one metabolite for each herbicide. The results indicate that for some herbicides, such as propanil, the parent compound is almost never detected above the reporting level in surface water; however, 3,4-dichloroaniline, a metabolite of propanil, was detected frequently throughout the growing season. Norflurazon and its metabolite are detected in about equal concentrations. Other herbicides such as atrazine and fluometuron were detected frequently with a range in concentrations from 1 to 10 μg/L, and their metabolites were also detected frequently, but at lower concentrations ranging from 0.05 to 1 μg/L. Metabolites represent about 10 to 30 percent of the total concentration in samples collected at the beginning of the growing season, but become proportionally more significant later in the season as the parent compound dissipates and degrades. Metabolites represented more than 50 percent of the total concentration at the end of the growing season. 

Atrazine, a symmetrical triazine used for broadleaf control on corn, forms its major metabolites by dealkylation of the ethyl or isopropyl side chains. Surface water concentrations are highest in the early spring following application and usually dissipate to background levels by fall. Metabolite concentrations follow the same pattern of occurrence as atrazine, but the magnitude of the concentrations are less. Other triazines, such as simazine and propazine can form the same metabolites as atrazine. The agricultural uses for these two compounds are limited in the Delta, and it can be assumed that most of the deisopropylatrazine and deethylatrazine is from atrazine. However, Fletcher Creek drains a small urban basin in Memphis, Tennessee, and the high deisopropylatrazine concentrations are not from atrazine use, but from urban lawn care use of simazine.

Cyanazine, a symmetrical triazine used as a postemergent herbicide on cotton in the Delta, forms its major metabolite by nucleophilic addition of a hydroxyl ion. It can then be further degraded to deisopropylatrazine. Because cyanazine is used as a postemergent herbicide, its occurance in surface water of the Delta is considerably delayed compared to atrazine. Cyanazine was detected in low concentration throughout the year, and the highest concentration occurs after application during June and July. Cyanazine-amide was not detected until cyanazine concentrations begin to rise following application. Cyanazine-amide concentrations followed the same pattern as cyanazine, but were lower until early fall when the cyanazine and cyanazine-amide concentrations were about equal.

Fluometuron, a substituted urea, is used in cotton for the control of many broadleaf and grass species. The degradation pathway for fluometuron involves stepwise demethylation followed by hydrolysis to the aniline derivative. Concentrations of fluometuron were highest after cotton planting and decline after July, but remained at detectable concentrations throughout the study. Demethylfluometuron was the most frequently detected metabolite, but concentrations were less than fluometuron until early fall when fluometuron and demethylfluometuron concentrations were about 0.1 μg/L. The intermediate metabolite trifluoromethyl-phenyl urea (TFMPU) was not usually detected, and trifluoromethyl aniline ( TFMA) was infrequently detected.

Norflurazon is used in soybean and cotton for the control of many grasses and sedges. One of its metabolites, demethylnorflurazon, is formed by demethylation. Norflurazon and demethylnorflurazon were detected in low concentrations throughout the study. The maximum concentrations for norflurazon and demethylnorflurazon were 0.89 and 0.77 μg/L, respectively. The concentrations of norflurazon and its metabolites were fairly equal throughout the study with slightly more norflurazon detected in April - June and slightly more demethylnorflurazon detected in August - December.

Propanil, a substituted amide, is used in rice to control certain annual broad leaf and grass weeds. It is the most heavily used herbicide in the study area, based on mass of active ingredient, and yet is infrequently detected above the method reporting level of 0.05 μg/L in the surface water of the Delta. This indicates a very rapid dissipation of propanil or that it is tightly absorbed to soil particles. Propanil is metabolized to propionic acid and 3,4-dichloroaniline. The metabolite of propanil, 3,4-dichloroaniline, is found frequently in the surface water of the Delta, with the highest concentrations occurring immediately after application.

Metabolites, as a category, represented about 25 percent of the total herbicide concentration (parent compound + all metabolites) in the Bogue Phalia at the beginning of the growing season (April) and increased to more than 50 percent by December. The absolute concentration was about 10 μg/L in April and about 1 μg/L in December. This indicates the importance of analyzing for metabolites as well as the parent compounds when evaluating the effects of pesticides on water quality.