A large range in annual flow-weighted concentrations of total nitrogen in the lower Tennessee River Basin suggests that human activities have caused concentrations of total nitrogen to increase by as much as fivefold. Flow-weighted concentrations of total nitrogen range from about 0.5 milligrams per liter (mg/L) in a stream draining an undeveloped basin to 2.8 mg/L in a stream draining a mixture of land uses, including urban development, pasture land, and cropland. Flow-weighted concentrations of total nitrogen are low (0.55 mg/L) in the undeveloped Buffalo River Basin and similar to concentrations of total nitrogen for undeveloped basins throughout the Southeast (Clark and others, 2000). Flow-weighted concentrations also are low (0.53 mg/L) in the Duck River Basin (Normandy Dam). Denitrification within Normandy Lake during the summer when the reservoir is stratified results in lower concentrations of total nitrogen in the water discharged from the reservoir. Flow-weighted concentrations of total nitrogen are highest in the Clarks River (2.8 mg/L) and Town Creek (2.3 mg/L) Basins. The Clarks River Basin receives inputs of total nitrogen from a variety of point and nonpoint sources; whereas, the primary source of total nitrogen in the Town Creek Basin is from nonpoint sources, such as animal feeding operations, cropland, pasture, and failing septic systems (not estimated in this report). Flow-weighted concentrations of total nitrogen in Clarks River and Town Creek also are elevated (greater than the 70th percentile) in comparison to 372 streams and rivers across the Nation (D.K. Mueller, U.S. Geological Survey, oral commun., 2000).
Although national water-quality criteria limiting nutrient fluxes have not been established, the National Technical Advisory Committee (1968) recommended phosphorus concentrations not exceed 0.05 mg/L for waters entering impoundments. Among the eight streams and rivers investigated in the lower Tennessee River Basin, flow-weighted concentrations of total phosphorus range from 0.02 mg/L in the undeveloped Buffalo River Basin to 0.73 mg/L in the Duck River Basin (Williamsport Bridge). Phosphorus levels in the undeveloped Buffalo River Basin are similar to the median flow-weighted concentration of total phosphorus (0.022 mg/L) for undeveloped basins throughout the Nation (Clark and others, 2000). Annual flow-weighted concentrations of total phosphorus in the lower Duck River (Williamsport Bridge and Hurricane Mills) and the Elk River (Prospect) Basins exceed 0.52 mg/L, which is above the 90th percentile of flow-weighted concentrations of total phosphorus among 372 streams and rivers across the Nation (D.K. Mueller, U.S. Geological Suvey, oral commun., 2000). Natural deposits of phosphatic limestones in the Duck River and Elk River Basins (fig. 2) contribute to the large variation, nearly fortyfold, in concentrations of total phosphorus throughout streams in the lower Tennessee River Basin. The presence of phosphatic limestone is an important factor to consider as regional boundaries are established to attain region-specific water-quality criteria for total phosphorus.
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