GROUND WATER IN NORTH-CENTRAL TENNESSEE

SPEINGS 89
blank sample of water should be taken from each of the observation
wells and springs and preserved as a standard of comparison. Sam­
ples are then taken at regular intervals at each of the observation
points and compared with the corresponding blank sample, and the
collection of samples is continued until the arrival and passing of the
uranin have been noted. From the interval of time between the
dosing of the ground water and the appearance of the dye at a point
a known distance away the rate of movement can be computed. This
is usually determined from the interval between dosage with the dye
and its first appearance at the point of observation, the resultant
figure being probably a minimum value. Though a positive result
from a field test with uranin gives useful information, a negative
result is not conclusive, for the uranin commonly advances as a very
narrow band,63 which may pass between two observation wells and
thereby escape detection.
An instructive example of the use of uranin in tracing the flow of
ground water in limestone has been described recently by Crouch.64
A disappearing stream was traced to springs as much as 5 miles distant,
and the rate of movement was found to be about 160 to 185 feet an
hour.
SPRINGS
GRAVITY SPRINGS
GENERAL FEATURES« ' ' ' '' '
Many localities in north-central Tennessee have no perenjajal
streams of consequence, and hence the springs are an impor^ajpti
present and future source of water supply for municipal and industrial?
uses. The value of any spring for these purposes is determined by
the amount and variability of its discharge, by the temperature of
its water, and by the amount and character of the dissolved and sus­
pended matter in its water.
Most of the springs in this region are gravity springs that is, they
percolate from permeable beds or flow from large openings in the rocks
under the force of gravity, much as a stream flows down its channel.
In such springs water does not issue under artesian pressure. These
gravity springs may be further classified as seepage springs, in which
the water percolates slowly from the many small interstices of a
permeable material; as fracture springs, in which the water flows from
one or more joints or other fractures in the rocks; or as tubular springs,
in which the water issues freely from large tubelike channels that are
> Stiles, O. W., and others, op. cit., p. 73.
« Crouch, A. W., The use of uranin dye in tracing underground waters: Am. Waterworks Assoc. Jour.,
vol. 19, No. 6, pp. 725-728,1928.
68 For a full discussion, see Meinzer, 0. E., An outline of ground-water hydrology, with definitions: U. S.
QeoL Survey Water-Supply Paper 494, pp. 60-53,1923. Bryan, Kirk, Classification of springs: Jour. Geol­
ogy, vol. 27, pp. 522-561, 1919.
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