GROUND WATER IN NORTH-CENTRAL TENNESSEE

RUTHERFORD COUNTY 181
and passing through proved wells or springs. Obviously, however,
the uncertainty of finding water increases with the distance from the
proved sites and is very great under even the most favorable
circumstances.
In many if not most parts of the peneplain that covers the central
part of the county springs constitute the most reliable source of water
where perennial streams are absent. The largest of these are tubular
springs whose water is gathered by systems of solution channels in
the limestone (pp. 92-95). Some such springs return to the surface
the water drained from large or small bodies of channeled limestone
and constitute the perennial heads of surface streams. Typical ex­
amples are the city spring at Murfreesboro (No. 439, pi. 4) and Ward
Spring (No. 428). In other springs of this type a subsurface stream
is exposed for a short distance where the roof of its channel has collapsed
or has been pierced by a natural well or subvertical solution channel,
as in the Love Davis cave (No. 413, see also pi. 5, B), the Blue Sink
(No. 450), and the Snail Shell cave (No. 455). Springs 416, 417, and
418, about 5 miles southeast of Smyrna (pi. 4), mark the course of a
rather well-defined subsurface drain. The aggregate discharge from
the tubular springs of this district constitutes the subsurface drainage
that is tributary to the surface streams. Moreover, this ground-
water discharge is subject to seasonal fluctuations comparable in mag­
nitude to the fluctuations of the surface run-off, although the variabil­
ity of all springs is not likely to be the same. Hence, the average or
minimum discharge of a given spring can be determined only by
measuring the flow at regular intervals over a long period.
In the hilly parts of the county ground-water conditions are likely
to differ greatly from place to place, so that drilling is a most uncertain
method of obtaining ground water. In general, few water-bearing beds
probably occur very far below the level of near-by perennial streams.
In many places dug wells derive adequate supplies from the surficial
mantle of rock waste. Both tubular and seepage springs (pp. 90-95) are
common and constitute an adequate source of water for domestic
purposes and for stock in most of these sparsely populated districts.
Usually the ground water from springs and wells of shallow or
moderate depth in Rutherford County is a moderately concentrated
calcium-magnesium bicarbonate water of moderate hardness, such as is
shown by analyses 431, 439, and 466 on pages 118-119. These waters
are suitable for most ordinary uses after softening. Not all the ground
waters, however, are of this type. Drillers in Rutherford County
have noted that water of poor quality is usually encountered in wells
from 60 to 150 feet deep in a zone about 1 mile wide that trends ap~
proximately N. 30° E. through a point about 3 miles west of Mur­
freesboro. This zone also passes through Sulphur Spring (No. 425,
pi. 4), about 4 miles north of Murfreesboro. The water, which is