Pesticides (herbicides and insecticides) commonly are used in the study area to control unwanted vegetation and insects and to improve crop production; although useful, pesticides can adversely affect the environment and human health. The greatest potential for adverse effects of pesticides is through contamination of the hydrologic system (Larson and others, 1997). Water is one of the primary pathways by which pesticides are transported from their application areas to other parts of the environment.
Samples collected at the three intensive fixed sites were analyzed for 84 pesticides and pesticide metabolites (table 4). The pesticides detected at the sites and the range in concentrations are listed in table 8. No pesticides were detected in field blanks. The agriculture indicator site had the least number of pesticides detected with 15 in the 32 samples collected. The most commonly detected pesticides at the agriculture site were atrazine, deethylatrazine, and prometon, each detected in at least 20 samples. Diazinon was detected in 12 samples. Concentrations of some pesticides detected (for example, atrazine and diazinon) increased in the spring during peak pesticide application (fig. 9). Concentrations of all pesticides detected at the agriculture site were less than 0.12 microgram per liter (mg/L) (table 8).
The urban indicator site had the most pesticides detected with 25 in the 35 samples collected (table 8). The most commonly detected pesticides were atrazine, deethylatrazine, diazinon, prometon, simazine, and tebuthiuron, each detected in at least 27 samples. Tebuthiuron, bromacil, and diuron had the largest concentrations among the pesticides detected at the urban site, 2.8, 1.6, and 1.1 mg/L, respectively. Concentrations of some of the pesticides detected at the urban site also increased in spring (fig. 10).
Results from the integrator site showed 18 pesticides detected in the 26 samples collected. The most commonly detected pesticides were atrazine, deethylatrazine, diazinon, prometon, and simazine, each detected in at least 20 samples. Carbaryl, chlorpyrifos, lindane, metolachlor, and tebuthiuron were detected in at least 15 samples. Concentrations of all pesticides detected at the integrator site were less than 0.44 mg/L (table 8). As at the agriculture and urban indicator sites, concentrations of some of the pesticides detected at the integrator site showed increases in spring (fig. 11).
Under provisions of the Safe Drinking Water Act, the EPA has established MCLs for concentrations of certain chemicals in drinking water (U.S. Environmental Protection Agency, 1996). These MCLs are health-based standards and are a result of chronic toxicity tests conducted with animals (Larson and others, 1997). Not all pesticides analyzed in the samples collected at the intensive fixed sites have an MCL listed in the drinking water regulations. Concentrations of all pesticides that were detected at the intensive fixed sites and have an MCL were less than the MCL.
Four pesticides detected in a large percentage (38 to 100 percent) of samples collected at the three intensive fixed sites were atrazine, deethylatrazine, diazinon, and prometon. Boxplots of these pesticide concentrations show that median concentrations of atrazine at the three sites were similar (fig. 12). The largest median concentration of deethylatrazine was at the agriculture indicator site. The largest median concentrations of diazinon and prometon were at the urban indicator site. The largest concentrations of three of the four pesticides shown were at the urban indicator site (atrazine, 0.75 mg/L; diazinon, 0.33 mg/L; prometon, 0.24 mg/L).
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