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The occurrence and transport of pesticides were investigated at nine stream sites in the lower Tennessee (LTEN) River Basin as part of the U.S. Geological Survey's National Water-Quality Assessment Program. Sites were selected to represent the three dominant crops in the LTEN River Basin: corn, soy beans, and cotton, and to represent varying amounts of agricultural land use. Atrazine, metolachlor, and 2,4-D are the most extensively used pesticides in the LTEN River Basin; however, glyphosate, fluometuron, methyl parathion, and aldicarb are the most heavily applied chemicals in areas of intensive cotton cultivation. The long growing season for cotton (from April to October) makes this crop especially susceptible to yield loss because of pests; consequently, cotton requires more intense chemical management than other crops require. From January to September 1999, a total of 60 samples were collected from all 9 sites and analyzed for more than 100 pesticides and 15 metabolites (breakdown products). Minimum reporting levels ranged from 0.002 to 0.08 micrograms per liter (µg/L). Two sites on streams draining intensive cotton cultivated areas in the Flint River Basin, Alabama, were sampled weekly or biweekly and during storms in order to quantify annual rates of constituent transport. This intensive sampling frequency is required to evaluate occurrence and transport of the more soluble and mobile chemical constituents that move offsite rapidly with surface runoff. Seven other sites in the Flint River Basin and on the Duck and Elk Rivers were sampled intermittently to assess occurrence of pesticides across varying land uses and in source water for public water supplies.
Three of the most commonly used pesticides were also the most frequently detected pesticides at the nine sites: atrazine and its metabolites, metolachlor, and fluometuron and its metabolites were detected more frequently (more than 80 percent of the samples), and occurred at higher concentrations, than other compounds. Some discrepancies exist between detection frequency and usage rates. These discrepancies can be partly explained by differences between the pesticides' physical properties, such as water solubility and soil sorption.
Pre-plant and pre-emergent herbicides were detected most frequently and at highest concentrations in storm runoff in April and May (at least 10 times higher than base flow concentrations); for many of these herbicides, the only occurrences of concentrations above reporting levels were in spring storm runoff. Concentrations of atrazine ranged from 0.024 µg/L (in March base flow) to 24 µg/L (in April storm runoff); concentrations of fluometuron ranged from less than 0.05 µg/L (in base flow throughout the year) to 3.3 µg/L (in May storm runoff).
Concentrations of atrazine exceeded 2 µg/L (the Canadian water quality guideline for protection of freshwater aquatic life; no guideline has been established in the United States) in 15 percent of the samples from all sites. No other chemical exceeded established guidelines for protection of aquatic life. Because the effects of mixtures of pesticides and their metabolites are not known, the risk to human health and aquatic life from pesticide exposure may not be adequately characterized by comparisons with existing guidelines. In many samples, concentrations of metabolites (deethylatrazine, demethylfluometuron, and demethylnorflurazon) equalled or exceeded the corresponding parent compound (atrazine, fluometuron, and norflurazon).
U.S. Department of the Interior, U.S. Geological Survey
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