Water-Quality Assessment of the South Platte River Basin, Colorado, Nebraska, and Wyoming-Analysis of Available Nutrient, Suspended-Sediment, and Pesticide Data, Water Years 1980-92 by Kevin F. Dennehy, David W. Litke, Peter B. McMahon, Janet S. Heiny, and Cathy M. Tate U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 94-4095 Denver, Colorado 1995 U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary U.S. GEOLOGICAL SURVEY Gordon P. Eaton, Director The use of trade, product, industry, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. For additional information write to: Copies of this report can be purchased from: District Chief U.S. Geological Survey U.S. Geological Survey Earth Science Information Center Box 25046, MS 415 Open-File Reports Section Denver Federal Center Box 25286, MS 517 Denver, CO 80225 Denver Federal Center Denver, CO 80225 CONTENTS Abstract...................................................................._ 1 Introduction .......................................................................................................................................................................... 2 Purpose and scope ...................................................................................................................................................... 2 Acknowledgments ...................................................................................................................................................... 2 Description of the South Platte River Basin......................................................................................................................... 2 Physical setting .......................................................................................................................................................... 3 Hydrologic setting ...................................................................................................................................................... 3 Water use .................................................................................................................................................................^ 11 Urban practices .......................................................................................................................................................... 15 Agricultural practices ................................................................................................................................................. 15 Water-quality issues in the basin ................................................................................................................................ 16 Sources and characteristics of available water-quality data ................................................................................................. 18 Data-selection criteria and screening procedures ....................................................................................................... 18 Quality assurance of data ........................................................................................................................................... 19 Site characterization.............................................^ 32 Data-analysis methods.......................................................................................................................................................... 32 Nutrients.......................................................................................................^ 34 Description of surface-water data............................................................................................................................... 34 Description of ground-water data ............................................................................................................................... 40 Concentrations in surface water ................................................................................................................................. 47 Trends in surface water .............................................................................................................................................. 60 Relation of concentration to streamflow .................................................................................................................... 60 Loads in surface water ............................................................................................................................................... 67 Concentrations in ground water ................................................................................................................................. 71 Suspended sediment...................................................................^ 78 Description of data...................................................................................................................................................... 81 Concentrations............................................................................................................................................................ 84 Loads........................................................................................................................................................................... 88 Pesticides ...................................................................................................................._^ 88 Description of surface-water data............................................................................................................................... 88 Description of ground-water data ............................................................................................................................... 91 Concentrations in surface water ................................................................................................................................. 97 Concentrations in ground water.................................................................................................................................. 101 Summary .............................................................................................................................................................................. 101 South Platte River Basin ............................................................................................................................................ 101 Sources and characteristics of available water-quality data ....................................................................................... 106 Nutrients ......................................................................... 106 Suspended sediment.................................................................................................................................................... 107 Pesticides ............................................................................ 107 References cited ................................................................................................................................................................... 108 Supplemental data .........................................................................................................................................................^ Ill CONTENTS III FIGURES 1 5. Maps showing: 1. Location of the study unit and selected cities and streams in the South Platte River Basin...................... 4 2. Average annual precipitation (1951-80) and location of National Atmospheric Deposition Program (NADP) sites in the South Platte River Basin .......................................................................................... 5 3. Population density in the South Platte River Basin ................................................................................... 6 4. Major land uses, 1975-80, in the South Platte River Basin ...................................................................... 7 5. Mean annual runoff (1951-80) in the South Platte River Basin ................................................................ 8 6. Graph showing annual mean streamflow and mean monthly streamflow at Bijou Creek at Wiggins, Colorado, and Big Thompson River at Estes Park, Colorado..................................................................................... 9 7 10. Maps showing: 7. Major aquifers of the South Platte River Basin ......................................................................................... 10 8. Major interbasin water transfers, water diversions, reservoirs, and municipal discharges in the South Platte River Basin ........................................................................................................................... 12 9. Locations of selected surface-water-quality sites in the South Platte River Basin, water years 1980-92 ................................................................................................................................. 20 10. Locations of selected ground-water-quality sites in the South Platte River Basin, water years 1980-92................................................................................................................................... 21 11-43. Graphs showing: 11. Description of components of a boxplot and related statistical tests ......................................................... 33 12. Distribution of nutrient sites and samples by site characteristics .............................................................