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

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45-46. 30. Relation of concentrations of dissolved organic nitrogen plus ammonia, as nitrogen, to well depth 13 2. for selected wells completed in the alluvial aquifer, South Platte River Basin, water years 1980- 92...... 78 31. Concentrations of dissolved phosphorus, as phosphorus in ground water from the South Platte 17 4. River Basin, grouped by aquifer, well type, year of sample collection, and season of sample 18 5. collection, water years 1980 -92 ................................................................................. ............................... 79 32. Concentrations of dissolved phosphorus, as phosphorus, in ground water from selected wells completed 22 6. 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 ................. 80 33. Relation of concentrations of dissolved phosphorus, as phosphorus, to well depth for selected wells completed in the alluvial aquifer, South Platte River Basin, water years 1980 -92 ... ............................... 81 34. Number of suspended- sediment samples per month for surface -water sites that have sediment data in the South Platte River Basin, water years 1980 -92 ............................................... ............................... 83 35. Number of suspended- sediment samples per decile of flow for surface -water sites that have sediment data in the South Platte River Basin, water years 1980 -92 ............................................... ............................... 83 36. Number of suspended- sediment samples by land use for surface -water sites that have sediment data in the South Platte River Basin, water years 1980 -92 ................................................ ............................... 84 37. Suspended- sediment concentrations for surface -water sites that have sediment data in the South Platte River Basin, water years 1980- 92 .......................................................... ............................... 86 38. Particle size (percent finer than sand) for suspended- sediment samples for surface -water sites that have sediment data in the South Platte River Basin, water years 1980 -92 ............... ............................... 87 39. Monthly suspended- sediment concentrations for surface -water sites that have sediment data in the South Platte River Basin, water years 1980 -92 ............................................... ............................... 89 40. Relation of suspended- sediment concentration to instantaneous streamflow for surface -water sites that have sediment data in the South Platte River Basin, water years 1980 -92 ........ ............................... 89 41. Suspended- sediment load for surface -water sites that have sediment data in the South Platte RiverBasin, water years 1980 -92 ............................................................................. ............................... 90 42. Relation of suspended- sediment load to instantaneous streamflow for surface -water sites that have sediment data in the South Platte River Basin, water years 1980 -92 ....................... ............................... 91 43. Percentage of detections of selected pesticides, by land use, in surface water in the South Platte River Basin, water years 1980 -92 .................................................... ............................... 98 44. Map showing areal distribution of atrazine and 2,4 -D analyses at surface -water sites in the South Platte River Basin, water years 1980 -92 .......................................................... ............................... 99 Graphs showing: 45. Relations of 2,4 -D concentrations to streamflow at selected surface -water sites in the South Platte RiverBasin, water years 1980 -92 ............................................................................. ............................... 102 46. Relations of 2,4 -D concentrations to time at selected surface -water sites in the South Platte River Basin, water years 1980 -92 ............................................................................. ............................... 103 TABLES Table 1. Major interbasin water transfers, water diversions, reservoirs, and municipal discharges in the South Platte CONTENTS V RiverBasin ................................................................................................................................ ............................... 13 2. Estimated monthly net irrigation requirements for selected crops in the Greeley, Colorado area, 1988 ................. 17 3. Most common pesticides applied annually in the South Platte River Basin ............................. ............................... 17 4. Organizations contacted as potential sources of water - quality data in the South Platte River Basin ...................... 18 5. Characteristics of selected surface- water - quality sites in the South Platte River Basin, wateryears 1980 -92 ................................................................................................................ ............................... 22 6. Characteristics of selected ground- water - quality sites in the South Platte River Basin, wateryears 1980 -92 ................................................................................................................ ............................... 27 7. Summary of selected nutrient data for surface -water sites in the South Platte River Basin, wateryears 1980 -92 ................................................................................................................ ............................... 35 8. Summary of statistics for selected dissolved nutrients in ground -water samples, by aquifer, well type, and land use in the alluvial aquifer in the South Platte River Basin, water years 1980- 92...., % .............................. 44 CONTENTS V