Water Quality Assessment of the South Platte River Basin, Colorado , Nebraska, and Wyoming

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FIGURES 1 -5 6. 7 -10. 11-43. 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, 197540, in the South Platte River Basin ....................................... ............................... 7 5. Mean annual runoff (1951 -80) in the South Platte River Basin ................................. ............................... 8 Graph showing annual mean streamflow and mean monthly streamflow at Bijou Creek at Wiggins, Colorado, and Big Thompson River at Estes Park, Colorado ...................................................... ............................... 9 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 SouthPlatte River Basin ............................................................................................ ............................... 12 9. Locations of selected surface - water - quality sites in the South Platte River Basin, wateryears 1980 -92 .................................................................................................. ............................... 20 10. Locations of selected ground- water - quality sites in the South Platte River Basin, wateryears 1980- 92 .................................................................................................... ............................... 21 Graphs showing: 11. Description of components of a boxplot and related statistical tests .......................... ............................... 33 12. Distribution of nutrient sites and samples by site characteristics ............................... ............................... 39 13. Relation of nitrite plus nitrate samples and streamflow distribution ........................... ............................... 41 14. Dates of nutrient sample collection for surface -water sites in the South Platte River Basin ..................... 42 15. Number of selected ground -water samples, by year and month, in the South Platte River Basin, wateryears 1980- 92 .................................................................................................... ............................... 47 16. Nutrient concentrations in surface water in the South Platte River Basin, water years 1980 -92 .............. 48 17. Nutrient concentrations at selected sites along the South Platte River, water years 1980 -92 ................... 54 18. Relations of land use to concentrations of selected nutrients in the South Platte River Basin, wateryears 1980- 92 .................................................................................................... ............................... 56 19. Statistical relations of wastewater - treatment -plant discharges to concentrations of selected nutrients in the South Platte River Basin, water years 1980- 92 ................................................ ............................... 58 20. Relations of land use and concentrations of total organic nitrogen plus ammonia, as nitrogen, by season, in the South Platte River Basin, water years 1980 -92 ............................................... ............................... 59 21. Relations of selected nutrient concentrations and time at the South Platte River at Julesburg, Colorado, water years 1980- 92 .................................................................. ............................... 61 22. Relations between nutrient concentrations and percentiles of flow at selected surface -water sites in the South Platte River Basin, water years 1980- 92 ................................................ ............................... 65 23. Relations between total phosphorus concentrations, percentile of flow, and suspended - sediment concentrations for selected surface -water sites in the South Platte River Basin, water years 1980- 92.... 66 24. Instantaneous nitrite plus nitrate loads on the Big Thompson River, Colorado, June 19-20, 1991........... 72 25. Concentrations of dissolved nitrite plus nitrate, as nitrogen, in ground water from the South Platte River Basin, grouped by aquifer and well type, water years 1980 -92 ....................... ............................... 73 26. Concentrations of dissolved nitrite plus nitrate, as nitrogen, in ground water from wells completed in the alluvial aquifer from the South Platte River Basin, grouped by land use, water years 1980 -92 ............. 74 27. Relations of concentrations of dissolved nitrite plus nitrate, as nitrogen, and total nitrate, as nitrogen, to well depth for selected wells completed in the alluvial aquifer, South Platte River Basin, wateryears 1980 -92 ................................................................................................... ............................... 75 28. Concentrations of dissolved organic nitrogen plus ammonia, as nitrogen, in ground water from the South Platte River Basin, grouped by aquifer, well type, year of sample collection, and season of sample collection, water years 1980 -92 ................................................................................ ............................... 76 29. Concentrations of dissolved organic nitrogen plus ammonia, as nitrogen, in ground water from wells completed in the alluvial aquifer, South Platte River Basin, grouped by land use, well type, year of sample collection, and season of sample collection, water years 1980- 92 ............ ............................... 77 IV CONTENTS