VALLEY AND RIDGE AQUIFERS
INTRODUCTION
The Valley and Ridge Physiographic Province is characterized by a sequence of folded and faulted, northeast-trending Paleozoic sedimentary rocks that form a series of alternating valleys and ridges that extend from Alabama and Georgia to New York. The province is more areally extensive in Segment 11 than in Segment 10; therefore, the aquifers in the province are discussed in greater detail in that Atlas Chapter. The Valley and Ridge Province in the eastern part of Tennessee in Segment 10 (fig. 115) is underlain by rocks that are primarily Cambrian and Ordovician in age. Minor Silurian, Devonian, and Mississippian rocks also are present in the province. Soluble carbonate rocks and some easily eroded shales underlie the valleys in the province, and more erosion-resistant siltstone, sandstone, and some cherty dolomite underlie ridges.
The arrangement of the northeast-trending valleys and ridges and the broad expanse of the Cambrian and the Ordovician rocks in eastern Tennessee are the result of a combination of folding, thrust faulting, and erosion. Compressive forces from the southeast have caused these rocks to yield, first by folding and subsequently by repeatedly breaking along a series of thrust faults as shown in figure 116. The result of the faulting is that geologic formations can be repeated several times across the faults; for example, the carbonate-rock aquifers in the Chickamauga, the Knox, and the Conasauga Groups are repeated across the thrust faults shown in figure 116. In eastern Tennessee, the thrust faults are closely spaced and are more responsible than the folds for the present distribution of the rocks. Following the folding and thrusting, erosion produced the sequence of ridges and valleys on the present land surface.
The general hydrogeologic characteristics of the entire Valley and Ridge Province are fairly consistent. However, unique characteristics can be attributed to local differences in rock type and geologic structure.
HYDROGEOLOGIC UNITS
The principal aquifers in the Valley and Ridge Province of Segment 10 consist of carbonate rocks that are Cambrian, Ordovician, and Mississippian in age (fig. 117). These aquifers, which are typically present in valleys and rarely present on broad, dissected ridges, underlie more than one-half of the Valley and Ridge Province in Tennessee (fig. 115). Most of the carbonate-rock aquifers are directly connected to sources of recharge, such as rivers or lakes, and solution activity has enlarged the original openings in the carbonate rocks. Other types of rocks in the province can yield large quantities of water to wells where they are fractured or contain solution openings or are directly hydraulically connected to sources of recharge.
GROUND-WATER MOVEMENT
Ground water in the Valley and Ridge aquifers primarily is stored in and moves through fractures, bedding planes, and solution openings in the rocks. These types of openings are secondary features that developed after the rocks were deposited and lithified. Little primary porosity and permeability remain in these rocks after the process of lithification. Some ground water moves through primary pore spaces between the particles that constitute the alluvium along streams and the residuum of weathered material that overlies most of the rocks in the area.
In the carbonate rocks, the fractures and bedding planes have been enlarged by dissolution of part of the rocks. Slightly acidic water, especially that circulating in the upper 200 to 300 feet of the zone of saturation, dissolves some of the calcite and dolomite that compose the principal aquifers. Most of this dissolution takes place along fractures and bedding planes where the largest volumes of acidic ground water flow.
Ground-water movement in the Valley and Ridge Province in eastern Tennessee is localized, in part, by the repeating lithology created by thrust faulting and, in part, by streams. Major streams are parallel to the northeast-trending valleys and ridges, and tributary streams are perpendicular to the valleys and ridges. Older rocks (primarily the Conasauga Group and the Rome Formation) have been displaced upward over the top of younger rocks (the Chickamauga and the Knox Groups) along thrust fault planes (fig. 118) thus forming a repeating sequence of permeable and less permeable hydrogeologic units. The repeating sequence, coupled with the stream network, divides the area into a series of adjacent, isolated, shallow ground-water flow systems. Within these local flow systems, most of the ground-water movement takes place within 300 feet of land surface. In recharge areas, most of the ground water flows across the strike of the rocks. The water moves from the ridges where the water levels are high toward lower water levels adjacent to major streams that flow parallel to the long axes of the valleys (fig. 118). Most of the ground water is discharged directly to local springs or streams, but some of it moves along the strike of the rocks, following highly permeable fractures, bedding planes, and solution zones to finally discharge at more distant springs or streams. Although fracture zones locally are present in the clastic rocks, the highly permeable zones, which are primarily present in the carbonate rocks, act as collectors and conduits for the water.
WELL YIELDS AND SPRING DISCHARGE
Yields of wells completed in the principal Valley and Ridge aquifers range from about 1 to 2,500 gallons per minute (table 7). The largest yields (2,500 gallons per minute) are reported for wells completed in the Honaker Dolomite of the Conasauga Group. Large yields also are reported for wells completed in limestone or dolomite of the middle and lower parts of the Chickamauga Group, the Knox Group, and the Shady Dolomite (all about 500 gallons per minute). The median yields of wells completed in the principal aquifers range from about 11 to 350 gallons per minute; the largest median yields are for wells in the Shady Dolomite (350 gallons per minute), the middle part of the Conasauga Group (100 gallons per minute), and the Newman Limestone (55 gallons per minute).
The discharges of springs that issue from the principal Valley and Ridge aquifers in eastern Tennessee vary greatly; measured discharges range from about 1 to 5,000 gallons per minute (table 7). The largest springs issue from the Newman Limestone and the Lenoir Limestone of the Chickamauga Group. Springs that issue from the Knox Group discharge as much as 4,000 gallons per minute. The median discharges of springs that issue from the principal aquifers range from 20 to 175 gallons per minute. The largest median discharges are from springs that issue from the Shady Dolomite (175 gallons per minute), the Knox Group (50 gallons per minute), and the upper part of the Conasauga Group (40 gallons per minute). Many springs discharge as much as 10 times more water during periods of abundant rainfall than during extended periods of little or no rainfall.
GROUND-WATER QUALITY
The chemical quality of water in the freshwater parts of the Valley and Ridge aquifers is similar for shallow wells and springs (fig. 119). The water is hard, is a calcium magnesium bicarbonate type, and typically has a dissolved-solids concentration of 170 milligrams per liter or less. The ranges of concentrations are thought to be indicators of the depth and rate at which ground water flows through the carbonate-rock aquifers. In general, the smaller values for a constituent represent water that is moving rapidly along shallow, short flow paths from recharge areas to points of discharge. This water has been in the aquifers for a short time and has accordingly dissolved only small quantities of aquifer material. Conversely, the larger values represent water that is moving more slowly along deep, long flow paths. Such water has been in contact with aquifer minerals for a longer time and thus has had greater opportunity to dissolve the minerals. Also, water that moves into deeper parts of the aquifers can mix with saltwater that might be present at depth.
In places where the residuum that overlies the carbonate rocks is thin, the Valley and Ridge aquifers are susceptible to contamination by human activities. The complex network of fractures, bedding planes, and solution openings developed in the carbonate rocks allows rapid local ground-water movement. The natural ground-water quality is subject to degradation in places where landfills and other waste-disposal sites, underground storage tanks, and septic tank systems are located.
FRESH GROUND-WATER WITHDRAWALS
Fresh ground-water withdrawals from the aquifers in the Valley and Ridge Province in eastern Tennessee were about 82 million gallons per day during 1985 (fig. 120). This amount constitutes about 16 percent of the ground water used in the State. About 31 million gallons per day was withdrawn for public supply, and about 20 million gallons per day was withdrawn for industrial, mining, and thermoelectric power purposes. About 19 million gallons per day was withdrawn for domestic and commercial supplies, and about 12 million gallons per day was withdrawn for agricultural use.
