GROUND WATER IN NORTH-CENTRAL TENNESSEE

142 GROUND WATER IN NORTH-CENTRAL TENNESSEE
is very uncertain. In general, however, wells must be drilled some­
what below the level of the near-by perennial drains in order to assure
an adequate water supply. On the other hand, the likelihood of
entering a water-bearing channeled zone seems to become materially
less as a well is drilled much more than 75 feet below the perennial
surface streams. In these comparatively rugged sections of the
county springs are a relatively important source of water, especially
the perennial tubular springs, which drain large volumes of channeled
limestone. (See pp. 92-95.) The discharge of such a spring may vary
widely during the year, however, so that its reliability can be deter­
mined only by periodic measurements of discharge over a period of
several years.
In the southern and eastern parts of the county several relatively
deep wells have encountered a water-bearing stratum just above the
Chattanooga shale, possibly in the lower part of the Fort Payne for­
mation. Among the wells that have tapped this stratum are Nos. 194,
206, 216, 220, and 226 (pp. 144-146), of which Nos. 206 and 220 over­
flow at the surface by artesian pressure. However, the static level of
the water confined in this bed is about 700 to 725 feet above sea level,
so that the area within which flowing wells may be expected is not
extensive and is limited to parts of the lower reaches of Turnbulland
Jones Creeks. The specific capacity of wells that tap this stratum is
relatively small, that of well 226, for example, being about 0.4 gallon
a minute for each foot of drawdown. This water-bearing stratum
seems to be discontinuous, for it was not found in wells 193, 205, and
222, and a permeable bed is not known farther east where the strata
at this horizon crop out in Cheatham County. Hence, the reliability
of this stratum as a source of water throughout the region is limited.
The chemical analysis of a sample from well 220 (pp. 114-115) shows
that the water from this stratum, although rather hard, is not highly
concentrated and is essentially free from sodium chloride (common salt).
A few wells have been drilled into the beds that underlie the Chat­
tanooga shale in Dickson County, such as Nos. 194, 205, 207, 209,
and 222. Of these, well 222 reaches the lowest stratigraphic horizon,
about 1,260 feet below the Chattanooga shale. All such wells find
these beds to be dry or to contain relatively small amounts of highly
concentrated brine, as in well 194. Hence, deep drilling for water is
not likely to be successful in this area.
The chemical character of the ground waters of Dickson County is
shown by the representative analyses tabulated on pages 112-115. It
is noteworthy that the waters associated with the Fort Payne forma­
tion have a wide range in chemical composition, even in adjacent wells
that reach approximately the same stratigraphic horizon, such as Nos.
190 and 191. Of these two wells, No. 190 yields a moderately hard
calcium bicarbonate water containing 222 parts per million of dissolved