Atrazine, ametryn, bromacil, simazine and norflurazon were the most frequently detected pesticides in surface water samples and DDE, DDD and ametryn were the most frequently detected pesticides in sediment samples collected over the period November 1991 to June 1995 in a monitoring network that includes 27 stations in south Florida canals. The 744 pesticide detections during this time period represent about 2% of the total number of analytical determinations. Many of the most frequently detected compounds were used in large amounts in the monitoring area based on pesticide usage estimates included in this study. Spatial trends in pesticide detections followed use patterns. The maximum atrazine detections occurred in winter to late spring and were associated with usage on turfgrass and agricultural products. Endosulfan residues above the Florida water quality criterion were occasionally observed in surface water in the Homestead area and most of the exceedences occurred in confined waters. Methods with lower MDLs have recently been developed and should increase the number of detections in future sampling. Pesticides which bind strongly to soil, pesticides that are highly persistent and those used in large amounts were some of the more frequently found pesticides in sediments.

The South Florida Water Management District is a state agency that manages surface and ground water quantity and quality in south Florida. Since 1984 surface water and sediment have been sampled for pesticides at various frequencies and locations in the District's 1400-mile system of canals. Based on monitoring data from 1992 to 2001 the most common pesticides detected in surface water samples were herbicide compounds, especially ametryn and atrazine, while DDE and DDD were the most frequently detected in sediment samples. Exceedances of state surface water quality standards occurred in certain basins for several insecticides including endosulfan. In addition, the concentrations of several ubiquitous organochlorine compounds in sediment were similar to or exceeded threshold effect levels based on a comparison to the NOAA screening quick reference tables (or SQuiRTs) for sediment.

The South Florida Water Management District has collected surface water and sediment samples for analysis of over 80 pesticides since 1992. Residues of atrazine and DDE-p,p(') are nearly ubiquitous throughout the sampling area. Surface water concentrations of atrazine at nine of 14 selected monitoring locations exhibited a downward trend and none of the detected concentrations exceeded established levels of concern. Seventy-five percent of the sampled locations had average sediment residues of DDE-p,p(') at levels that may impact sediment-dwelling organisms. Monitoring locations with the highest average DDE-p,p(') concentrations (based on detected values) and the highest percentage of pesticide detections did not exhibit consistent trends. Some locations exhibited increasing trends while trends at other locations showed a decrease in concentrations. Additionally, specific location concentrations for ethion, diazinon, and endosulfan documented periods of surface water quality exceedances. However, product label changes and grower education have substantially contributed to the reduced number of exceedances over time.

6-month incubation (Table 111). In whole grain of both treatments, malathion monocarboxylic acid was the most prevalent degradation product, followed by an unknown compound. The dockage fraction, however, contained larger quantities of the unknown compound than malathion monocarboxylic acid. Both treatments contained smaller amounts of malathion and larger amounts of the degradation products after 6 months than after 2 months. Malaoxon was not found in the grain. The unknown compound did not correspond to malathion, malaoxon, malathion mono-or dicarboxylic acid, malaoxon mono-or dicarboxylic acid, demethyl malathion, diethyl mercaptosuccinate, thiomalic acid, or the alkyl phosphate metabolites. Malathion dicarboxylic acid was not detectable in this study because it did not contain the radiocarbon label. ACKNOWLEDGMENTWe are grateful to R. Beeman, who developed and assisted in the HPLC analysis, and to T. Hopkins for helpful suggestions concerning sample oxidation.Registry No. Malathion, 121-75-5; malathion monocarboxylic acid, 35884-76-5.

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