GROUND WATER IN NORTH-CENTRAL TENNESSEE

OCCURRENCE OF GROUND WATER IN LIMESTONE 87
The courses followed by some of the larger underground channels
may sometimes be traced approximately on the surface where sink
holes or natural wells occur in line, the altitude and relative size of the
sinks and the altitude of the associated ground water being used to
discriminate channels that are related to different levels of planation
by solution. Furthermore, some solution channels follow persistent
joints and may be approximately straight for considerable distances.
Hence, where the limestones are broken by one or more well-defined
sets of joints the most probable courses of a water-bearing channel
can be projected approximately by drawing lines through the proved
wells or springs and parallel to the dominant sets of joints. Obviously
the uncertainty of these projected courses increases with the distance
from the proved sites. Solution channels may also be formed at
several altitudes along a single joint, so that the course of a shallow
channel which can be traced by springs, dry caverns, or small sinks
may coincide with that of another water-bearing channel at greater
depth. However, solution channels and joints usually depart some­
what from a true plane, so that wells located at sites chosen by one
of the methods of projection outlined above may not be successful
in finding a large yield of water. Moreover, the traces of many large
solution channels that are proved by existing wells and of most small
channels and joints are not indicated at the surface. Hence the
development of ground-water supplies in limestone by drilling wells
is likely to involve failures even under the most favorable circum­
stances.
In north-central Tennessee the limestones fortunately are somewhat
jointed at most places, and small openings along the joints and along
bedding planes are generally water bearing 50 feet or more above and
below the level of the perennial surface drains. Hence, sufficient
water for household use and for watering stock can be obtained by
wells of moderate depth in most parts of the region. However, the
water found in the deeper crevices in many places holds considerable
dissolved mineral matter and may be unsatisfactory in chemical
character for some uses. Comparatively few wells yield as much as
100 gallons a minute. The water-bearing properties of the limestones
in different parts of the area are discussed in the county descriptions
(pp. 124-233).
In studying the sanitary condition of a water supply from the lime­
stone or the possibility of interference between two or more sources,
it is frequently desirable to trace the direction of flow of the ground
water. For this purpose the use of a water-soluble dye, such as the
sodium salt of fluorescein, is usually effective.69 The principle of its
use is to dose the circulating ground water with a solution of the dye
' Dole, R. B., Use of fluorescein in the study of underground waters: U. S. Geol. Survey Water-Supply
Paper 160, pp. 73-85,1906. Stabler, Herman, Fluorescein, an aid to tracing waters underground: Recla­
mation Record, vol. 12, pp. 122-123,1921.