SCSB# 395
MLRA 123: Nashville Basin
J.T. Ammons1, R.E. Yoder1, and R.J. Luxmoore2
1University of Tennessee and 2Oak Ridge National Laboratory

Chapter Content

Location, Extent, and Landuse
The Nashville Basin extends from southern Kentucky through central Tennessee, and into northern Alabama. This major land resource area (MLRA 123) occupies a total land area of 1,458,200 ha (Fig. 1). A variety of cropping systems are present and grazing is prevalent. Tobacco and corn are grown on the alluvial soils and the residual limestone soils are used primarily for hay land and pasture. There is significant residential and urban development in the Nashville-Davidson County Metro area near the center of this MLRA.

According to USDA (1981), the average annual precipitation is 1,275 mm, the average annual temperature ranges between 14 to 16°C, and the average freeze-free period is 192 days.

Physiography and Geology
In the Central Basin, Paleozoic Era rocks are from the Ordovician period. Shale and limestone outliers of Devonian-Mississippian periods cap the many small hills around the edge of this MLRA. Phosphates are the major resource of the Central Basin, but other products include crushed limestone and Zinc (Miller, 1974).

A summary of the soil series in this MLRA are given in Table 1. STATSGO soils are depicted in Fig. 1. Pedons from Jackson and Lincoln counties are representative of the Central Basin MLRA 123. Soil morphology, classification, and parent materials are described for both pedons in this MLRA (Tables 2 and 3). Total elemental concentrations are given in Tables 4 and 5.

MLRA 123 lies in the Interior Low Plateaus subregion of the Appalachian Plateaus and Valley and Ridge ground water region of North America. The topography is generally mature and well-rounded, with several major areas of karst. Altitudes within the subregion range from 100 to 300 m above mean sea level. There are three conceptual models required to cover the diversity of ground water occurrence with this area (Back et al., 1988).

In areas of Mississippian rocks, the regolith varies from a thickness of 10 m to more than 35 m; in these areas the ground water occurrence varies from diffuse to conduit and the secondary permeability and storage vary from very poorly to very well developed. Ground water flow in these systems is generally limited to secondary permeability in rocks within 100 m of the ground surface.

The Ordovician rocks found in this MLRA dip gently or lie flat. Ground water in these rocks flows through shallow vertical joints and openings in horizontal bedding planes. This flow is usually concentrated in valleys and within 100 m of the surface.

The Knox aquifer which is part of a regional freshwater flow system occurs in buried paleokarst. The depth of the aquifer below the land surface varies from 100 to 460 m. The transmissivity of the aquifer is homogeneous but quite low, with flow to wells of about 0.3 L s -1 . It is thought that the permeability of the rocks beneath the Knox aquifer is extremely low.

The water of the shallow carbonate rock aquifers in the MLRA is a calcium magnesium bicarbonate type that is typically fresh (<1000 mg L -1 ) for depths less than 100 m (Brahana and Bradley, 1986a, 1986b). This suggests an interaction between the aquifers and water from outside the aquifers. Salinity increases with depth, suggesting less interaction with outside water sources as aquifer depths increase. Near the Nashville Dome, saline water may be encountered at relatively shallow depths, probably from isolated, stagnant flow cells that are in contact with evaporites. Salinity within the Knox aquifer does not increase with depth suggesting it is part of a deep regional flow system.

Water and Solute Transport
The prevalence of karst topography in this MLRA makes water and solute transport quite variable.

Literature Cited
Back, W., J.S. Rosenshein, and P.R. Seaber, editors. 1988. Hydrogeology: The geology of North America, Volume O-2. The Geological Society of America, Boulder, Colorado.

Brahana, J.V. and M.W. Bradley. 1986a. Delineation and description of the regional aquifers of Tennessee; The Central Basin aquifer system. p. 35. U.S. Geological Survey Water-Resources Investigations Report 82-4001.

Brahana, J.V. and M.W. Bradley. 1986b. Delineation and description of the regional aquifers of Tennessee; Highland Rim aquifer system. p. 38. U.S. Geological Survey Water-Resources Investigations Report 82-4045.

Miller, R.A. 1974. The geological history of Tennessee, Bulletin 74. Tennessee Department of Conservation, Division of Geology, Nashville, Tennessee.

USDA-SCS. 1981. Land Resource Regions and Major Land Resource Areas of the United States. Agriculture Handbook 296. Washington, DC.

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