Environmental Setting and Water-Quality Issues in the Lower Tennessee River Basin

SUMMARY

The lower Tennessee River Basin is one of 59 National Water-Quality Assessment study units in which water-quality assessments have been or are being conducted by the U.S. Geological Survey. The lower Tennessee River Basin study unit encompasses about 19,500 mi² and extends from Chattanooga, Tennessee, to Paducah, Kentucky. Geology and physiography were used to subdivide the study unit into nine subunits that represent areas of relative geologic and physiographic homogeneity. This subdivision provides framework in which natural variability in water quality can be quantified and the effects of human-related factors on water quality can be assessed. The boundaries for these subunits are similar to the ecoregion boundaries. The nine subunits delineated in order of size are the Western Highland Rim, Eastern Highland Rim, Coastal Plain, Outer Nashville Basin, Plateau Escarpment and Valleys, Cumberland Plateau, Transition, Inner Nashville Basin, and Valley and Ridge.

Much of the lower Tennessee River Basin is underlain by carbonate rocks, with smaller areas of unconsolidated sediments and siliciclastic rocks present in the western and eastern parts of the study unit, respectively. Most of the rock units in the study unit are undeformed and relatively flat-lying to gently dipping. Karst landforms, such as sinkholes, caves, and disappearing streams, are present in parts of the study unit underlain by carbonate rocks. Generally, the soils overlying bedrock in the study unit have moderate to slow infiltration rates.

The lower Tennessee River Basin lies within parts of the Coastal Plain, Interior Low Plateaus, and Appalachian Plateaus Physiographic Provinces. Relief is low to moderate in much of the study unit; high relief occurs between the Highland Rim and Cumberland Plateau Physiographic Sections of the Interior Low Plateaus and Appalachian Plateaus, respectively. Land-surface altitudes generally increase from about 500 ft above sea level in the west to more than 2,000 ft above sea level in the east.

The main stem of the Tennessee River is highly regulated with few free-flowing stream reaches. Six reservoirs are located on the main stem and many additional reservoirs are located on tributaries to the main stem. Mean-annual streamflow in the Tennessee River increases from about 35,900 to about 65,600 ft³/s from Chattanooga, Tennessee, to Paducah, Kentucky. The Elk and Duck Rivers are the two largest tributaries and contribute about 26 percent of the streamflow gained in the study unit.

The Cretaceous sand, Pennsylvanian sandstone, Mississippian carbonate, and Ordovician carbonate aquifers account for most of the shallow ground-water flow in the study unit. Ground-water flow paths typically are short in these aquifers, and much of the recharge to the aquifers is discharged within the study unit. The shallow ground-water system generally is within 300 ft of land surface and flow is through unconsolidated sediments, solution openings in bedding planes and joints in bedrock, and in fractured bedrock. The Mississippian carbonate aquifer is the most productive in the study unit and is characterized by flow in solution openings. Estimates of the contribution of ground-water discharge to streamflow from the principal aquifers indicate that at least 50 percent of streamflow is contributed by ground-water discharge to streams.

Surface water is the principal source of water for both industrial use and public supply, and accounts for about 70 percent of the water used for drinking water. Much of surface water used is withdrawn from the main stem of the Tennessee River. Ground-water use is highest in the Eastern Highland Rim, where Huntsville, Alabama, the largest city in the study unit, uses ground water for about 40 percent of the water supply. Point-source discharges were estimated for 1995. The estimated total nitrogen input from wastewater discharges in the study unit was about 13,430 tons and the estimated total phosphorus load was 770 tons.

Land use in the study unit largely reflects the geomorphology of the basin and the distribution of people. Large forested areas are present in the Plateau Escarpment and Valleys, Transition, and Western Highland Rim where the topography has moderate to high relief. Forest land is the largest land use in the Coastal Plain, Cumberland Plateau, and Valley and Ridge as well. Pasture land is the dominant land use in the Inner and Outer Nashville Basin and the Eastern Highland Rim. Cultivated land constitutes 6 percent or less of the land use in all subunits except the Eastern Highland Rim, where about 16 percent of the land is cultivated. Urban and developed land is 1 percent or less of the land use in most subunits.

Cultural factors most likely to have a widespread effect on water quality in the lower Tennessee River Basin are related to animal and rowcrop agriculture. Corn, soybeans, cotton, and wheat were the primary crops grown in the study unit in 1992. The Eastern Highland Rim and Coastal Plain were the most intensively farmed subunits. Cotton acreage in the Eastern Highland Rim was the largest crop by subunit. About 42 million chickens, 0.9 million cattle, and 0.3 million hogs were produced in 1992. The number of chickens per square mile was highest in the Cumberland Plateau. Cattle production was most intense in the Inner and Outer Nashville Basin, and hogs were raised primarily in the Coastal Plain.

Nutrient inputs from agricultural activities and atmospheric deposition (nonpoint sources) were estimated for 1992. The estimated total nitrogen input from these sources was about 195,000 tons, with about 74 percent of the total contributed by fertilizer and livestock waste. On a per unit area basis, nitrogen inputs were highest in the Eastern Highland Rim. Nitrogen inputs from livestock waste were highest in the Cumberland Plateau, Inner and Outer Nashville Basin, and Valley and Ridge. Nitrogen inputs from fertilizer application were highest in the Eastern Highland Rim, Coastal Plain, and Inner Nashville Basin. About 14,500 tons of phosphorus were contributed to the study unit as fertilizer and 23,500 tons were produced by livestock in the study unit in 1992. On a per unit area basis, livestock waste was the larger source of phosphorus in most subunits. In the Eastern Highland Rim and Coastal Plain, fertilizer input was about the same or slightly more than inputs from livestock waste.

About 3.7 million pounds of pesticides (active ingredient) were applied to agricultural land in the study unit in 1992. The herbicides atrazine, monosodium methanearsonate (MSMA), 2,4-D, and metalochlor were used in the largest amount (more than 200,000 lb of each). Insecticides were used to a lesser extent and were applied primarily on cotton grown in the Eastern Highland Rim. About half of the pesticides used were applied to crops in the Eastern Highland Rim where about 538 lb of pesticides per square mile were applied.

Nutrient and fecal coliform data collected between 1980 and 1996 indicate that natural setting likely has as important an effect as cultural factors on surface- and ground-water quality in the lower Tennessee River Basin. These constituents represent water-quality issues for both surface and ground water in the lower Tennessee River Basin. Median nitrate concentrations were less than 1 mg/L for surface and ground water in all subunits except for the Eastern Highland Rim and Cumberland Plateau, where the nitrate concentration at the ninetieth percentile exceeded the maximum contaminant level for drinking water for nitrate (10 mg/L). In general, median concentrations of nitrogen species were highest in subunits where percentages of agricultural land use were highest. Median phosphorus concentrations in surface water were less than 1 mg/L in all subunits. The Inner and Outer Nashville Basin had the highest concentrations, probably a result of naturally occurring sources of phosphorus in phosphatic limestones in the Ordovician carbonate rocks. Available data for phosphorus concentrations in ground water were limited, but concentrations were generally less than 1 mg/L. Median counts of fecal coliform were higher in surface water than ground water. The highest median counts in surface water were in the Valley and Ridge (7,500 col./100 mL) and the Outer Nashville Basin (5,000 col./100 mL). Highest median counts in ground water were in the Inner and Outer Nashville Basin.