GROUND WATER IN NORTH-CENTRAL TENNESSEE

QUALIFY OF GROUND WATER 103
north-central Tennessee are of this class. Sulphate in a hard water
may increase the cost of softening, and it makes the scale formed in
a steam boiler "hard" and therefore much more troublesome. This
is particularly true if the calcium plus magnesium in a water is much
more than equivalent to the bicarbonate.
Chloride. Chloride (Cl), which is generally not abundant in
moderately concentrated ground waters from shallow sources, may
be derived by solution of rock-forming minerals or by contamination
of the water with sewage. However, the possible sources of chloride
are so many that an abnormally large amount of this constituent in
a natural water is not at all a definite indication of pollution. In
most of the moderately concentrated ground waters of north-central
Tennessee chloride is less than 5 parts per million, although its range
is roughly from 1 to 50 parts per million. The waters associated
with the Bigby limestone contain notably more chloride than most
of the waters associated with the other stratigraphic units of the
region. Some ground waters from deep sources are relatively concen­
trated in chloride, which is derived for the most part from connate
brines associated with marine sediments. Such highly concentrated
waters are common in north-central Tennessee, and the maximum
chloride content shown by the representative analyses is 15,700 parts
per million (No. 352, pp. 116-117).
Chloride has little effect on the suitability of water for domestic
purposes unless it is so concentrated as to impart a saline taste.
Waters that contain several hundred parts per million of chloride
may be corrosive when used in steam boilers, unless this action is
restrained by suitable treatment.
Nitrate. Nitrate (NO3) in natural waters is generally considered
a final oxidation product of nitrogenous organic matter. Hence its
presence in more than nominal quantity in a ground water implies
that the well or spring from which it issues may contain harmful
bacteria derived from cultivated fields or other places where oxidized
nitrogenous matter is common. Most of the ground waters from the
region covered by this report contain less than 1 part per million of
nitrate.
Hardness. Hardness in a natural water is caused almost exclusively
by the salts of calcium and magnesium. It is commonly recognized
by the excessive amount of soap necessary to lather a hard water and
by the curdy precipitate that forms before a permanent lather results.
The constituents that cause hardness are also the active agents in the
formation of the greater part of the scale formed in steam boilers and
in other vessels in which water is heated or evaporated.
In order that hardness may be expressed in a standard unit it is
customarily reported as parts per million of calcium carbonate