By Nancy J. Bauch and Norman E. Spahr

Available from the U.S. Geological Survey, Branch of Information Services, Box 25286, Denver Federal Center, Denver, CO 80225, USGS Water-Resources Investigations Report 00–4005, 46 p., 11 figs.

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Forty-four river, stream, and drain sites in the Upper Colorado River Basin in Colorado were sampled during 1996­98 to determine the occurrence and distribution of pesticides in the basin. In a fixed-station study, 57 surface-water samples were collected from October 1996 through January 1998 at four sites. Each site was sampled approximately monthly for up to a year, with more frequent sampling during the spring and summer growing season. In a synoptic study, surface-water samples were collected at 43 sites in the agricultural areas of the Grand Valley and the Uncompahgre River Valley in May 1998. Each site was sampled once.

Pesticide concentrations generally were low and varied seasonally and across land uses. Thirty-five pesticides were detected at least once in the two studies, and 11 pesticides accounted for almost 82 percent of the detections. Herbicides were more commonly detected than insecticides, and only the herbicides alachlor and atrazine were detected in more than 50 percent of the samples. Carbofuran was the most commonly detected insecticide and was detected in 19 percent of the samples. Pesticide detections increased and were measured at higher concentrations in the summer months and at the agriculture sites. All pesticide concentrations were less than drinking-water standards, and most complied with human-health advisories and criteria for the protection of freshwater aquatic life. Pesticides in the Upper Colorado River Basin in Colorado were detected slightly less frequently and generally at lower concentrations than in 20 National Water-Quality Assessment Program study-unit basins that collected water-quality data from 1992 through 1996. Results from surface-water sampling conducted during 1996­98 in the Upper Colorado River Basin in Colorado indicate that beneficial uses of water were not being impaired by the presence of pesticides in surface waters in the basin.

Foreword

Abstract

Introduction

Purpose and Scope

Description of the Upper Colorado River Basin Study Unit

Study Design

Sampling and Laboratory Methods

Acknowledgments

Quality-Control Methods and Results

Hydrologic Conditions During Sampling

Occurrence and Distribution of Pesticides

Pesticides at Fixed-Station Network

Temporal Distribution

Spatial Distribution

Pesticides at Synoptic Network

Comparison of Pesticides at Fixed-Station Network and Synoptic Network

Comparison with Results of Previous Studies

Summary and Conclusions

References