GROUND WATER IN NORTH-CENTRAL TENNESSEE
GROUND WATER IN NORTH-CENTRAL TENNESSEE
GROUND WATER IN NORTH-CENTRAL TENNESSEE
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74 <strong>GROUND</strong> <strong>WATER</strong> <strong>IN</strong> <strong>NORTH</strong>-<strong>CENTRAL</strong> <strong>TENNESSEE</strong><br />
circulates slowly and before it moves a great distance becomes satu<br />
rated in calcium bicarbonate and thereby depleted in solvent power, as<br />
its natural acid is neutralized by reaction with the limestone. Conse<br />
quently, it is commonly held, as by Swinnerton, 46 that below the water<br />
table the limestone does not dissolve readily and continuous systems<br />
of large solution passages do not form. This hypothesis seems to be<br />
compatible with the relation between the principal systems of solution<br />
channels and the surface streams (pp. 23-24) in north-central<br />
Tennessee. On the other hand, Davis 46 contends that limestone<br />
caverns are formed in part by solution and in part by corrasion and<br />
that they may be formed at any depth below the water table.<br />
Where a stratum of limestone has primary permeability or is<br />
thoroughly jointed and is overlain and underlain by impermeable<br />
rocks, ground water may circulate along that stratum under hydro<br />
static pressure, as through a conduit. Under such conditions, if the<br />
geologic structure is favorable, circulation may take place to con<br />
siderable depths below the water table. If the circulation is relatively<br />
rapid, so that ground water passes entirely through the limestone<br />
conduit before it becomes saturated, then the original openings in the<br />
limestone may be enlarged by solution from one end of the conduit to<br />
the other, regardless of its depth below the water table. Such con<br />
ditions are not known to have existed in north-central Tennessee.<br />
Systems of solution passages may be formed in limestone during<br />
successive erosion epochs and then buried beneath younger sediments<br />
after submergence.<br />
In some regions the limestone formations can be discriminated more<br />
or less sharply by the abundance and size of the solution passages and<br />
other water-bearing openings. This is true to some extent in parts of<br />
north-central Tennessee, but generally the effect of differential sol<br />
ubility is a secondary factor in determining the water-bearing proper<br />
ties of the limestone of this region.<br />
SOURCES AND CmCTJLATION OF <strong>GROUND</strong> <strong>WATER</strong><br />
In general the ground water that occurs in the limestone of north-<br />
central Tennessee is derived from two sources meteoric water, or<br />
that which falls as rain and percolates to the water table, and connate<br />
or fossil water, or that which was trapped in the sediments by the<br />
deposition of overlying beds and has not since been flushed from the<br />
rocks. The waters of meteoric origin are those which circulate freely<br />
through joints and solution openings and are discharged from most<br />
springs and wells. They generally contain only small or moderate<br />
amounts of dissolved mineral matter and are suitable for most ordi-<br />
48 Swinnerton, A. C., Changes in base-level indicated by caves in Kentucky and Bermuda [abstract]:<br />
Geol. Soc. America Bull., vol. 40, p. 194,1929.<br />
« Davis, W. M., Origin of limestone caverns: Geol. Soc. America Bull., vol. 41, No. 3, pp. 475-628,<br />
1930; The origin of limestone caverns: Science, new ser., vol. 73, No. 1891, pp. 329-330,1931.