In 1997, the U.S. Geological Survey (USGS) began an assessment of the lower Tennessee (LTEN) River Basin as part of the National Water-Quality Assessment (NAWQA) Program. The lower Tennessee River Basin corresponds to the lower half of the Tennessee River Valley (fig. 1). Surface-water-quality data collected in the LTEN River Basin from 1980 to 1996 have been compiled, screened, and evaluated to provide a general description of water-quality conditions, to identify trends in selected water-quality constituents, and to assist the design of NAWQA data-collection activities within the LTEN River Basin. Assessment efforts have been focused on nitrogen, phosphorus, and suspended sediment because these water-quality constituents have been identified by Federal, State, and local resource-management and regulatory agencies in the basin as the issues of greatest concern to the quality of surface-water resources.
State water-quality regulatory agencies within the LTEN River Basin have documented that although the quality of surface water in the basin is generally good, poor water quality impairs beneficial uses locally in 109 stream segments and 3 lakes (Alabama Department of Environmental Management, 1996; Georgia Department of Natural Resources, 1996; Kentucky Natural Resources and Environmental Protection Cabinet, 1996; Mississippi Department of Environmental Quality, 1996; Tennessee Department of Environment and Conservation, 1996). Nutrient overenrichment is listed as causing impairment in 37 stream segments and 2 lakes. Although nitrogen and phosphorus are essential nutrients for plant and animal growth, nutrient overenrichment of streams and lakes can promote excessive growth of algae and other aquatic plants. The subsequent decay of this growth in organic matter may deplete dissolved oxygen and adversely affect fish and other aquatic life. Excessive growth of algae and other aquatic plants also is accompanied by increased levels of dissolved organic matter that may cause taste and odor problems. These problems increase water-treatment costs and thus impair the use of the water resource as a drinking-water supply.
Although nutrient overenrichment is a major cause of impairment, siltation and suspended sediment are the dominant causes of impairment in streams throughout the LTEN River Basin. Siltation and suspended sediment are listed by State water-quality regulatory agencies as causing impairment in 63 of the 109 impaired stream segments in the basin. The increased turbidity associated with elevated suspended-sediment concentrations reduces light penetration and primary productivity of the water bodies. In addition, excessive sediment deposition (siltation) on streambeds degrades habitat for benthic organisms and reduces spawning grounds for fish. Excessive sediment deposition in reservoirs interferes with recreational use (in the tributary embayments), obstructs commercial navigation channels, and reduces storage capacity (D. Meinert, Tennessee Valley Authority, written commun., 1998).
The purposes of this report are to (1) describe and quantify the major sources of nitrogen and phosphorus to surface waters in the LTEN River Basin; (2) describe the seasonal and spatial patterns and temporal trends of concentrations and loads of nitrogen, phosphorus, and sediment; and (3) relate spatial and temporal patterns of nutrient concentrations and loads to spatial and temporal variation in sources and other environmental factors. The analyses presented in this report are based on historical water-quality data collected at 49 monitoring sites during the period October 1979-September 1996 (water years 1980-96), and on information on nutrient sources for several years during this period. Comparison of nutrient sources with loads is based on data from 1992. The water-quality data were obtained from various monitoring programs and special studies conducted by the Alabama Department of Environmental Management (ADEM), Flint Creek Watershed Project (FCWP), Geological Survey of Alabama (GSA), Kentucky Department for Environmental Protection (KDEP), Ohio River Valley Sanitation Commission (ORSANCO), Tennessee Department of Environment and Conservation (TDEC), Tennessee Valley Authority (TVA), and USGS. Water-quality constituents used in the analyses included total nitrogen, total ammonia plus organic nitrogen, total nitrite plus nitrate nitrogen, total ammonia nitrogen, total phosphorus, dissolved orthophosphorus, and suspended sediment. The data analyses included graphic summaries of nutrient concentrations, regression analysis of nutrient and sediment concentrations to estimate instream loads and trends, and correlation analysis of instream nutrient loads with watershed inputs.
The LTEN River Basin NAWQA study unit covers a 19,500-square-mile (mi2) area in the lower half of the Tennessee River Valley (fig. 1). The study unit upstream boundary, which coincides with the downstream boundary of the upper Tennessee River Basin study unit, is located at river mile 465 on the main stem of the Tennessee River at Chattanooga, Tennessee. The study unit encompasses parts of Tennessee, Georgia, Alabama, Mississippi, and Kentucky that drain to the Tennessee River and its tributaries between river mile 465 and the confluence with the Ohio River at Paducah, Kentucky (Woodside and Mitchell, 1998).
The LTEN River Basin is subdivided into 14 major hydrologic units (fig. 2). Seven of these units, representing 37 percent of the basin area, make up the drainage areas of five major tributaries to the Tennessee River: the Elk (two units), Duck (two units), Sequatchie, and Buffalo Rivers, and Bear Creek. The remaining units, representing 63 percent of the basin, correspond to direct drainage to the main stem of the Tennessee River, or to groupings of minor tributaries to the main stem (no individual tributary draining more than 600 mi2, or 3 percent of the basin).
The spatial variation in concentrations and loads of nitrogen, phosphorus, and sediment within hydrologic units is affected by both natural and anthropogenic factors. Natural factors that affect water quality include geology, physiography, soils, land cover, climate, and hydrology. Kingsbury and others (1999) used geologic and physiographic boundaries to divide the LTEN River Basin into subunits in which natural factors affecting water quality are relatively homogeneous (fig. 2).
Anthropogenic factors that affect water quality include reservoirs, land use, population distribution, and urban, industrial, and agricultural activities. The distribution of urban and agricultural land use in the LTEN River Basin (fig. 3) corresponds to the distribution and amount of nonpoint sources (terms in bold can be found in the Glossary) of nitrogen, phosphorus, and sediment in the basin. The influence of certain urban and industrial activities is represented by the distribution of point-source discharges of wastewater (fig. 4). A more thorough discussion of the environmental setting of the LTEN River Basin is given in Kingsbury and others (1999).
Although many investigators have reported nutrient and sediment concentration data from ambient monitoring sites in the LTEN River Basin (Tennessee Valley Authority, 1972; Carriker and others, 1981; Meinert, 1991; Parr, 1991; Meinert and Fehring, 1992), few reports have presented estimates of instream loads, yields, or trends. Parr (1991) reported streamflow-concentration relations and temporal trends for selected water-quality constituents, including nutrients, in data collected during 1986-89 at sites in the TVA fixed-station tributary monitoring network. Trimble and Carey (1984) combined reservoir sediment-accumulation data with suspended-sediment load data from samples from the 1930's, 1960's, and from 1975 to 1982 to estimate instream sediment loads for several subbasins in Tennessee, including part of the LTEN River Basin. Estimates of sediment yields for central and eastern Tennessee basins (including those in the LTEN River Basin) were about 800 tons per square mile per year [(tons/mi2)/yr], whereas sediment yields for western Tennessee basins, which are intensively agricultural and channelized, ranged from 700 to 1,000 (tons/mi2)/yr. The similarities in these values did not match the expected result that sediment yields for western Tennessee basins would far exceed sediment yields for central and eastern Tennessee basins because of the greater availability of loose sediment in western Tennessee watersheds compared with other parts of the State. Trimble and Carey (1984) explained these results by citing the regional differences in land-water and instream delivery processes.
The authors gratefully acknowledge the following Federal and State agencies for their cooperation in providing data and interpretations used in this report: Tennessee Valley Authority (TVA), Tennessee Department of Environment and Conservation (TDEC), Alabama Department of Environmental Management (ADEM), Kentucky Department for Environmental Protection (KDEP), Ohio River Valley Sanitation Commission (ORSANCO), Geological Survey of Alabama (GSA), Mississippi Department of Environmental Quality (MDEQ), and Georgia Department of Natural Resources (GDNR). Sherry Wang of TDEC, Frank Sagona and Dennis Meinert of TVA, and M.W. Treece and Douglas Harned of USGS provided excellent technical guidance and review throughout this effort. The authors also thank several individuals who gave special assistance in compiling the data: E.A. Thornton and Glenda Gose of TVA; Steve Fishel and Jack Hughes of TDEC; Hubert Savoy of the University of Tennessee, Agricultural Extension Service; Steve Foster, Tom Cleveland, and Mike Reif of ADEM; Sydney DeJarnette of GSA; Glen Odom of MDEQ; and Vicky Prather of KDEP.
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