BLUE RIDGE AQUIFERS, OZARK PLATEAUS AND SOUTHEASTERN COASTAL PLAIN AQUIFER SYSTEMS
BLUE RIDGE AQUIFERS
The Blue Ridge Physiographic Province in Segment 10 is in easternmost Tennessee (fig. 121). Rocks that underlie the province range in age from Precambrian to Ordovician. Precambrian rocks include sandstone and metasedimentary, igneous, and metamorphic rocks. Cambrian rocks primarily are sandstone with some dolomite, and Ordovician rocks are primarily limestone and dolomite. Blue Ridge aquifers are discussed in more detail in the Atlas Chapter that describes Segment 11, where these aquifers are areally extensive; the aquifers are discussed briefly in the Chapters that describe Segments 6 and 12, where these aquifers locally are present.
Ground water in the Blue Ridge Physiographic Province generally is present in fractured bedrock. The bedrock, which consists of sedimentary, metasedimentary, and crystalline igneous and metamorphic rocks, is progressively more deformed and metamorphosed toward the southeast. Sedimentary rocks in the Blue Ridge Province primarily are well-cemented sandstone, limestone, and dolomite with minor shale.
Locally, regolith and stream-valley alluvium also can provide ground water. The bedrock is overlain by regolith that ranges from 1 to 150 feet thick. Alluvium that consists of boulders, gravel, silt, sand, and clay locally covers the floor of major stream valleys and can be several tens of feet thick.
Ground water from the Blue Ridge aquifers is used primarily for domestic supplies. Well yields from these aquifers are adequate for domestic, livestock, and small public supplies. The specific capacities of 13 wells finished in sandstone and phyllite in Great Smoky Mountains National Park, Tenn., range from 0.04 to 13 gallons per minute per foot of drawdown with a median of 0.57 gallon per minute per foot of drawdown. The yield of these wells ranges from less than 1 to about 125 gallons per minute with a median of 6 gallons per minute. Wells completed in Cambrian and Precambrian sandstone, metamorphic rocks, and crystalline rocks rarely have large yields, unless a well is open to major fracture zones.
Ground-water occurrence in the Blue Ridge aquifers is determined by the number, size, and degree of interconnection of fractures. Rocks in the Blue Ridge Province generally are massive and have little or no primary porosity. The rocks generally are nonporous and impermeable except within a few hundred feet of land surface where fractures (present in all rock types) provide secondary permeability (fig. 122). Fractures are less common at depth and, therefore, regional ground-water flow is not significant. Most of the water available from these fractures is within about 300 feet of land surface. However, sparse data indicate that fresh ground water can be obtained locally from fractures as deep as 1,500 feet below land surface.
The saturated regolith that overlies the bedrock and the alluvium in major stream valleys store ground water and release it slowly into the bedrock fractures. The regolith and alluvium, which locally are aquifers, supply sufficient water for domestic wells. However, wells completed in regolith might go dry during late summer and early autumn when water levels usually decline because of a decrease in precipitation or increased withdrawals or both.
Ground-water circulation in the Blue Ridge aquifers is localized. Most of the ground water moves along short, shallow flow paths. Precipitation recharges the regolith and alluvium and then percolates downward into the bedrock aquifers. Discharge is to seeps and springs, as base flow to streams and rivers, and as withdrawals from wells. The amount of ground-water discharge to streams and rivers ranges between 400,000 and 800,000 gallons per day per square mile of area and averages about 600,000 gallons per day per square mile of area throughout the Blue Ridge. This large rate of discharge is controlled primarily by large quantities of precipitation and large infiltration rates.
The chemical quality of the water in the Blue Ridge aquifers generally is suitable for most uses. Water from wells completed in sandstone aquifers (fig. 123) typically is a calcium magnesium bicarbonate type. Dissolved-solids concentrations are less than 300 milligrams per liter. Water from wells completed in crystalline rocks of Precambrian age has a smaller dissolved-solids concentration and is softer than water from wells completed in sandstone aquifers.
OZARK PLATEAUS AQUIFER SYSTEM
Because only a small part of the Ozark Plateaus aquifer system is within Segment 10 (fig. 124), the aquifer system is only briefly summarized here. A complete description of the geology, hydrology, and water-quality of the aquifer system is presented in the Chapter of this Atlas that describes Segment 3.
The principal aquifers in consolidated rocks of the Ozark Plateaus aquifer system consist of limestone and minor dolomite of Mississippian through Ordovician age. Unconsolidated sand and gravel aquifers in Quaternary deposits overlie the aquifers in consolidated rocks along the Mississippi River and its tributaries.
Water in the limestone and dolomite aquifers of the Ozark Plateaus aquifer system primarily is stored in and moves through fractures and bedding planes because of the low primary porosity and permeability of the rocks. Dissolution of the carbonate rocks creates enlarged openings along the fractures and bedding planes; these openings allow water to move rapidly through the aquifers. The aquifers are recharged by downward leakage through overlying sand and gravel deposits and directly through fractures, sinkholes, and swallow holes where the aquifers crop out. Springs are common points of discharge for the limestone and dolomite aquifers.
Yields of wells completed in the Ozark Plateaus aquifer system in Illinois generally are less than 25 gallons per minute but might be several hundred gallons per minute where well withdrawals induce additional recharge from nearby springs or streams. Yields of wells completed in the consolidated rocks that contain some sand and shale commonly are less than those of wells completed in limestone and dolomite.
Water from wells completed in the Ozark Plateaus aquifer system is hard and is a calcium magnesium bicarbonate type. Dissolved-solids concentrations generally range from 350 to 1,000 milligrams per liter and increase toward the northeast as the aquifers dip into the Illinois Basin. Hardness (as calcium carbonate) ranges from 200 to 400 milligrams per liter; sulfate concentrations generally range from 25 to 125 milligrams per liter; nitrate typically is less than 5 milligrams per liter. Chloride concentrations range from less than 50 milligrams per liter near the Mississippi River to more than 1,000 milligrams per liter toward the northeast. Iron concentrations generally range from 0.3 to more than 5 milligrams per liter.
SOUTHEASTERN COASTAL PLAIN AQUIFER SYSTEM
The part of the Southeastern Coastal Plain aquifer system within Segment 10 is restricted to small areas in portions of six counties in western Tennessee (fig. 125) The aquifer system extends into Segments 5 and 6, and is discussed in detail in the chapter of this Atlas that describes the aquifers in Segment 6, where the aquifer system is most extensive.
The Southeastern Coastal Plain aquifer system is divided into four regional aquifers, which consist mostly of semicon-solidated sand, separated by three regional confining units of clay, mudstone, and chalk. Only the lowermost regional aquifer, called the Black Warrior River aquifer, and its overlying confining unit are present in Tennessee. The Black Warrior River aquifer consists of Late Cretaceous sands of fluvial and deltaic origin, interbedded with clay and minor gravel. The geologic units that compose the aquifer are primarily the Tus-caloosa and the Eutaw Formations and the Coffee Sand.
Water enters the Black Warrior River aquifer in upland re-charge areas and moves westward and southwestward, down the dip of the sand beds, to discharge to streams. A small amount of the water moves into deep, confined parts of the aquifer. The water is stored in and moves through intergranular pore spaces. Water generally is present under unconfined conditions in and near aquifer recharge areas except where lenses of clay form local confining beds.
Although the Black Warrior River aquifer is moderately permeable, the aquifer is thin, and its transmissivity is accordingly moderate. Estimated transmissivity values for the part of the aquifer in Tennessee are 5,000 feet squared per day or less. Yields of wells completed in the aquifer generally are less than 50 gallons per minute, but, locally, yields of as much as 300 gallons per minute have been reported.
Water from the Black Warrior River aquifer is hard to moderately hard and is a calcium bicarbonate type. Dissolved-solids concentrations in water from the aquifer are small because the silica minerals that compose the aquifer do not readily dissolve. Locally, objectionable concentrations of iron have been reported.