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Colorado Water Science Center

PESTICIDE OCCURRENCE IN A SMALL URBAN AND A SMALL AGRICULTURAL BASIN, SOUTH PLATTE RIVER BASIN, APRIL THROUGH DECEMBER 1993

National Water-Quality Assessment (NAWQA) Program
South Platte River Basin Study

by Robert A. Kimbrough and David W. Litke 1

ABSTRACT

A study of pesticides in surface water in a small urban and a small agricultural basin was completed by the U.S. Geological Survey's National Water-Quality Assessment Program, South Platte River Basin Study Unit. This study defined and contrasted the occurrence of pesticides in the two land-use settings. Although Pesticide concentrations in surface water have been documented separately for urban and agricultural areas, little work has focused on comparing pesticides in the two environments using a common sampling and laboratory protocol.

Water samples were collected at the mouths of two tributary streams of the South Platte River in Colorado at least monthly during April-December 1993. The urban basin, located in the Denver metropolitan area, had a contributing drainage area of 25 square miles with predominantly residential and commercial land use. The agricultural basin, located in Weld County, had a contributing drainage area of 78 square miles with predominantly irrigated land. Corn, alfalfa, pinto beans, sugar beets, and onions were the major crops grown in the agricultural basin. Twenty-one water samples collected from each basin were analyzed for 47 pesticides. In the urban basin, 24 pesticides were detected in one or more samples, and in the agricultural basin, 30 pesticides were detected in one or more samples.

The herbicide atrazine was the most frequently detected pesticide in each basin. Atrazine was detected in 20 of the urban samples and all of the agricultural samples. The concentrations of atrazine were greater in the agricultural basin where the median value was 0.12 microgram per liter (µg/L) and the maximum value was 2.7 µg/L. The median concentration of atrazine in the urban basin was 0.018 µg/L and the maximum concentration was 1.1 µg/L. The herbicide prometon was the second most frequently detected pesticide in each basin. Prometon was detected in 19 of the urban samples and all of the agricultural samples. Median concentrations of prometon were similar for each basin and equaled 0.063 µg/L for the urban basin and 0.066 µg/L for the agricultural basin.

In addition to atrazine and prometon, two herbicides in the urban basin and eight in the agricultural basin were detected in more than 50 percent of the samples. The compound DCPA had the highest concentration of any herbicide in each basin even though other herbicides, such as atrazine and prometon, were detected more frequently. Commonly used for weed control in urban lawns and in agricultural crops such as onions, DCPA had a maximum concentration of 3.2 µg/L in the urban basin and 40 µg/L in the agricultural basin.

Four insecticides in the urban basin and five in the agricultural basin were detected in more than 25 percent of the samples. Carbaryl, detected in 14 of the urban samples and 6 of the agricultural samples, had the highest concentration of any insecticide in each basin. Available to the public for use on trees and turf grass, and commonly applied to corn, pinto beans, and sugar beets, carbaryl had a maximum concentration of 2.5 µg/L in the urban basin and 1.5 µg/L in the agricultural basin.

Several pesticides were detected only during the growing season whereas other pesticides persisted through the fall after the growing season ended. Consistently present in water samples collected in October through December 1993 were atrazine and prometon (in both basins); simazine (in the urban basin); and cyanazine, DCPA, EPTC, and metolachlor (in the agricultural basin). Regardless of when pesticide detections occurred, the concentration of pesticides in both basins generally was higher in storm-water runoff compared to baseflow conditions.

1 U.S. Geological Survey, Box 25046, MS 415, Denver Federal Center, Lakewood, CO 80225

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