GROUND WATER IN NORTH-CENTRAL TENNESSEE

92 GROUND WATER IN NORTH-CENTRAL TENNESSEE
material and those that have a large discharge generally yield water
that contains little dissolved matter, but those that issue by slow
percolation from partly leached rock yield water that is relatively
highly concentrated. These relations are brought out by Plate 8 and
by the corresponding chemical analyses, which are tabulated on
pages 110-119. The concentration and chemical character of the
water may vary somewhat from season to season, although the change
is not likely to be either large or rapid. Moreover, the water usually
contains very little suspended matter as it issues from the water­
bearing bed. Hence, in case it is desirable to soften the water for use,
it will probably not be necessary to modify the treatment process from
season to season in order to produce satisfactory results.
FRACTURE SPRINGS
A great many springs issue from fractured rocks in north-central
Tennessee and probably owe their origin to the circulaton of ground
water along crevices, although most of the rocks are appreciably
soluble, so that the crevices have been more or less enlarged by solu­
tion. Hence the springs that issue from these rocks are tubular
springs rather than fracture springs. However, the Lower Ordo-
vician rocks that crop out in the Wells Creek Basin of southeastern
Stewart County (see pp. 190-193) are very closely jointed and are
much less soluble than the younger rocks, at least in part. In this
locality, therefore, numerous fracture springs occur, which for the
most part yield less than 5 gallons a minute and are subject to wide
seasonal fluctuations.
TUBULAR SPRINGS
The largest and most reliable of the perennial springs of north-
central Tennessee are those that issue from tubelike solution channels
in the limestone. A tubular spring usually falls into one of three
classes. First, it may be the outlet of a subsurface drainage channel
into the surface stream to which it is adjusted, such as No. 416, in
Rutherford County. (See pi. 9, A.) Second, it may be the outlet
of a portion of a subsurface drainage system that has been captured
by a surface stream during a period of downcutting. The subsurface
drainage in the limestones of north-central Tennessee has cut down
to an equilibrium surface of solution over extensive areas during
the later stages of each cycle of peneplanation. (See pp. 18-23
and 78-82.) Hence, when such an underground. drainage sys­
tem is breached during a later cycle of surface erosion, the tubular
springs produced tend, within a given district, to occur at approxi­
mately the same distance below the related peneplain. Such a rela­
tion seems to exist in the dissected portion of the Highland Rim
plateau in the western part of the region. Finally, a tubular spring
may be a portion of an underground stream that is exposed when the