In 11 basins in the lower Tennessee River Basin, nonpoint sources of total nitrogen account for at least 74 percent of the annual total nitrogen stream yields; point sources account for less than 26 percent of the annual total nitrogen stream yields (fig. 2). Nonpoint-source contributions to the annual nutrient stream yields were estimated by subtracting point-source inputs from stream yields. These nonpoint-source estimates are conservative because they assume that none of the nutrient inputs from point sources are assimilated or removed in the stream between the point of discharge and the point where the stream nutrient yields are estimated.
Elevated stream yields of total nitrogen occur in the Town Creek [3.5 (tons/mi2)/yr], Elk River (Prospect) [3.0 (tons/mi2)/yr], Shoal Creek [2.9 (tons/mi2)/yr], Clarks River [2.4 (tons/mi2)/yr], and the Flint Creek Basins [2.4 (tons/mi2)/yr]. Few, if any, point sources are present in the Town Creek, Flint Creek, and Elk River (Prospect) Basins, thus nonpoint sources are the primary sources contributing to the elevated annual stream yields of total nitrogen in these basins (fig. 2). Point-source inputs of total nitrogen comprise 26 percent or less of the annual total nitrogen stream yield in the Clarks River Basin and 15 percent or less of the annual total nitrogen stream yield in the Shoal Creek Basin. During periods of low streamflows, a significant correlation exists between median concentrations of total nitrogen and wastewater inputs of total nitrogen among the basins with point sources, which suggests that wastewater discharges can be significant contributors to the amount of nitrogen in transport during periods of low streamflow. In basins without any point sources, nonpoint sources, such as fertilizers applied to crops, lawns, and golf courses, failing septic systems, livestock waste, atmospheric deposition, and natural sources in the rocks and soil, are the primary sources of total nitrogen. Evaluation of nutrient-management strategies within a basin requires quantitative information on nutrient inputs and a thorough understanding of nutrient cycling and transport.
Nonpoint sources of total phosphorus account for 52 to 98 percent of the annual total phosphorus stream yields; point sources account for less than 2 to 48 percent of the annual total phosphorus stream yields in basins unaffected by impoundments. Nonpoint-source inputs of total phosphorus from natural deposits of phosphatic limestone in the lower Duck River (Williamsport Bridge and Hurricane Mills) and the Elk River (Prospect) Basins (fig. 2) contribute to the highest annual stream yields of total phosphorus [greater than 0.88 (tons/mi2)/yr] among the basins in the lower Tennessee River Basin. Stream yields of total phosphorus are less than 0.08 (tons/mi2)/yr in the undeveloped Buffalo River Basin and in the Elk River (Tims Ford Dam) and Duck River (Normandy Dam) Basins, where sampling sites are located directly downstream of reservoirs that act as major sinks for phosphorus. Point-source inputs of total phosphorus account for less than 48 percent of the annual total phosphorus stream yields in the Clarks River and Shoal Creek Basins. Despite the large percentage of inputs from point sources in the Clarks River and Shoal Creek Basins, annual stream yields of phosphorus are low [0.30 (tons/mi2)/yr in Clarks River Basin and 0.15 (tons/mi2)/yr in Shoal Creek Basin] when compared to yields in basins within the lower Tennessee River Basin that contain natural deposits of phosphatic limestone.
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