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CONTENTS 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 .............................................................................................................................................................................. FIGURES 1-2. Maps showing: 1. Locarion of the Upper Colorado River Basin study unit, physiographic provinces, and fixed-station sites for pesticide sampling .......................................................................................................... 2. Location of agricultural synoptic sites in the Grand Valley and Uncompahgre River Valley, Upper Colorado River Basin ............................................................................................................................. 3. Graphs showing streamflow and pesricide sample-collection dates for fixed-station sites, Upper Colorado River Basin, October 1996-May 1998 ...................................................................................................................... 4. (A) Graph showing frequency of pesticide detections and (B) boxplots showing concentrations of detected pesticides for the fixed-station sites sampled in the Upper Colorado River Basin study unit, October 1996-January 1998 ..................................................................................................................................... 5-8. Graphs showing: 5. Number of pesticide detections per sampling date, fixed-station sites, Upper Colorado River Basin, October 1996-January 1998 .............................................................................................................................. 6. Mean number of pesticide detections per sample by month, all fixed-station sites combined, Upper Colorado River Basin, October 1996-January 1998 .............................................................................. 7. Atrazine concentrations at Dry Creek near Begonia Road, Reed Wash near Mack, and Colorado River near State Line, October 1996-January 1998 ................................................................................................... 8. Carbofuran and 2,4-D concentrations at Reed Wash near Mack, October 1996-January 1998 ....................... 9. (A) Graph showing frequency of pesticide detections and (B) boxplots showing concentrations of detected pesticides for the agricultural synoptic sites sampled in the Upper Colorado River Basin, May 1998 .................... 10-11. Graphs showing: 10. Number of pesticide detections per site in the Grand Valley and Uncompahgre River Valley, May 1998....... 11. Frequency of pesticide detections in the Grand Valley and Uncompahgre River Valley, May 1998 ................ III 1 1 3 3 6 8 13 13 21 23 26 26 31 35 40 41 43 44 2 4 22 29 30 31 32 33 36 39 40 CONTENTS V