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Results of two studies are presented in this report. The first part of the report presents results of pesticide data collected at surface-water sites in the agricultural areas along the South Platte River from Henderson, Colorado, to North Platte, Nebraska, during the 1994 growing season. Samples were collected at 16 to 20 sites on three occasions from May through August 1994 (synoptic samples) and at 2 sites approximately biweekly during the same period (anchor-site samples). The second part of the report presents results of pesticide data collected at two surfacewater sites in the Denver metropolitan area. One site was located on the main-stem South Platte River in Denver, whereas the second was located on a tributary (Cherry Creek) at its confluence with the South Platte River, 0.4 mile upstream from the main-stem site. Eighteen samples were collected at each site from December 1993 through November 1994. Thirty-nine pesticides were detected at least once during the agricultural study. Ten pesticides accounted for about 80 percent of the synoptic sample detections. Of the 10 pesticides, 6 commonly are used for irrigated agriculture in the study area (atrazine, metolachlor, DCPA, cyanazine, EPTC, and carbofuran); one (desethylatrazine) is a metabolite of atrazine; two (prometon, simazine) primarily are used for long-term weed control in noncropped areas; and one (diazinon) is an insecticide used around households and commercially on some vegetable crops. Of the 39 pesticides detected during the agricultural study, only 8 had median concentrations greater than their respective method detection limits (the 8 median concentrations ranged from 0.004 to 0.15 microgram per liter). Collectively, there were 22 detections of 10 pesticides that were measured at concentrations greater than 1 microgram per liter. The herbicide DCPA was measured at the highest concentration of 30 micrograms per liter. Pesticide concentrations generally were less than standards or guidelines for drinking water and aquatic life. Atrazine, desethylatrazine, and prometon were detected throughout the agricultural study area, and each persisted throughout the growing season at consistent concentrations. Ground-water discharge may transport atrazine, desethylatrazine, and prometon to surface waters in the study area because these three compounds also have been detected at equivalent concentrations in alluvial ground water in the study area. Other commonly detected pesticides occurred less frequently and had geographic or temporal fluctuations in their occurrence, indicating that pesticide use varies throughout the study area. For example, occurrences of metolachlor, which commonly is applied to corn, primarily were limited to sites located in corn production areas. Occurrences of diazinon, which is used in urban areas and on vegetables, primarily were limited to

Pesticides in surface water in agricultural and urban areas of the South Platte River basin, from Denver, Colorado, to North Platte, Nebraska, 1993-94