GROUND WATER IN NORTH-CENTRAL TENNESSEE

120 GROUND WATER IN NORTH-CENTRAL TENNESSEE
REI^ATTON TO STRATIGRAPHY
As is brought out in the following paragraph, the analyses of repre­
sentative ground waters tabulated above do not disclose any out­
standing relation between the chemical character of the ground waters
and the stratigraphic horizon of the water-bearing beds. In general
most of the waters fall into one of three classes first, slightly arid
moderately concentrated calcium bicarbonate waters having moder­
ate carbonate hardness; second, highly concentrated calcium or
calcium-magnesium sulphate waters with considerable noncarbonate
hardness; third, highly concentrated brines or sodium chloride-sul­
phate waters. All members of the geologic column contain waters
that fall into these three classes and have about the same range in
concentration. The waters of the first class are those that circulate
freely through permeable beds and channeled limestone at shallow
or moderate depth and issue from most of the springs and from wells
less than 200 feet deep. Most of them contain from 150 to 400 parts
per million of dissolved mineral matter and from 125 to 325 parts
per million of carbonate hardness. Usually they are/essentially free
from iron and hydrogen sulphide. Hence they are suitable for all
ordinary purposes if softened. The waters of the second and third
classes, which can not be sharply discriminated, occur at greater
depth in permeable beds or at moderate depth in beds of low per­
meability. The brines are probably connate or modified connate
waters originating in an epoch of marine sedimentation. The cal­
cium sulphate waters owe their high concentration to slow percola­
tion or stagnation in strata containing soluble sulphates or possibly
to reaction on limestone of sulphuric acid formed by oxidation- and
hydrolysis of pyrite. These highly concentrated waters issue from
a few springs and from some wells less than 50 feet deep, though
generally they occur in strata more than 100 feet below the surface.
So far as is known, the water in all strata more than 350 feet below
the surface is highly concentrated and is not entirely satisfactory for
most ordinary uses, especially for domestic consumption.
The general character of the ground water associated with different
stratigraphic units in north-central Tennessee is shown by the ac­
companying table of average, minimum, and maximum quantities of
mineral constituents, which is based only on the 101 representative
analyses. The accuracy of the relations suggested by this table is
limited by several factors. It is impossible to tell the precise strati-
graphic horizon of the water-bearing bed of some wells, because
complete records of the strata penetrated are not available. This
table shows the absolute minimum and maximum of each constituent
"without reference to any other constituent. All the minima are not
necessarily derived from the same analysis; neither are all the max­
ima necessarily derived from a single analysis. Some wells penetrate