The relative magnitude of point- and nonpoint-source loads of nutrients in streams is important to those attempting to manage these loads. Point-source loads are primarily contributed by discharges from wastewater treatment plants and industries, and were estimated for the South Platte River Basin at National Pollutant Discharge Elimination System permit sites using site-specific data where available and estimated nutrient concentrations else where. Nonpoint-source loads to the land surface were estimated using county data on fertilizer and manure applications and using precipitation chemistry data. The difference between calculated stream loads and known upstream point-source loads often is assumed to be equivalent to the nonpoint-source load.
Nonpoint-source nitrogen loads to the land surface of the South Platte River Basin was estimated to be 292,000 tons, of which 132,000 tons was from fertilizer, 94,000 tons was from manure, and 66,000 tons was from wet and dry atmospheric deposition. Total point-source nitrogen loads are estimated to be less than 10,000 tons. The total nitrogen load transported in the South Platte River at Roscoe, Nebraska, during a median-flow year was estimated to be 6,300 tons. Therefore, most of the nitrogen load to the land surface of the Basin, therefore, was not transported in streams. A large part of the nitrogen load probably does not enter groundwater or surface water because it is taken up by plants or remains immobile in the unsaturated zone; that part of the load that does enter groundwater or surface water is decreased by processes such as denitrification in groundwater and biologic uptake in surface water. Instream loads also are decreased by removal of water at ditch diversion points.
1 U.S. Geological Survey, Box 25046, MS 415, Denver Federal Center, Lakewood, CO 80225