GROUND WATER IN NORTH-CENTRAL TENNESSEE

100 GROUND WATER IN NORTH-CENTRAL TENNESSEE
and can be determined for a given source only by sampling and ana­
lyzing the water periodically for several years.
Silica. A small amount of silica (SiO2) is carried in solution by
most ground waters, even by those that issue from relatively pure
limestone. The dissolved silica in a water may be deposited wholly
or in part as a constituent of the hard scale formed in boilers, but
otherwise it has no effect on the use of the water for domestic or most
industrial uses. In the representative ground waters from north-
central Tennessee the dissolved silica ranges between 2.2 and 45 parts
per million; it is more than 15 parts per million in about two-thirds of
the samples analyzed.
Iron. The iron (Fe) contained by ground waters is derived from
pyrite and other iron-bearing minerals. As these minerals are usually
distributed very unevenly through the water-bearing rocks, the iron
content of ground waters, even those from the same geologic formation,
may differ materially from place to place. Although a relatively
large amount of iron may be taken into solution by ground water, if
much more than 0.1 part per million is present the excess is likely to
separate out as a reddish-brown gelatinous sediment after the water
has been pumped and allowed to stand in contact with the air. If this
sediment be formed in large volume it may obstruct pipes. If the iron
content be several parts per million, the water is likely to produce
stains on linen during laundering and on enamel-ware and porcelain-
ware utensils and plumbing fixtures. Even a very small quantity of
dissolved iron renders a water unfit for some industrial chemical uses.
In waters that contain hydrogen sulphide (H2S), such as many of the
ground waters of north-central Tennessee, an excess of dissolved iron
may separate out as a suspension of the black ferrous sulphide (FeS).
Such are the so-called black sulphur waters of this region. The excess
iron may be removed from most waters by aeration as by pumping
through spray nozzles and filtration, although some waters require
the addition of lime or other precipitating agent.
The iron content of the representative ground waters from north-
central Tennessee ranges from 0.02 to 29 parts per million but is less
than 0.5 part per million in half the waters and less than 1.0 part per
million in three-fourths of them. In most of the waters iron is present
in sufficient quantity to be troublesome for some purposes.
Calcium. Calcium (Ca) may be taken into solution as the bicar­
bonate by the reaction of natural waters containing carbon dioxide
or natural acids with calcium carbonate, which is the essential con­
stituent of limestone and a minor constituent of many other rocks.
Calcium is the most abundant of the bases in nearly all the slightly
and moderately concentrated ground waters of north-central Tennes­
see. In those waters which contain less than 500 parts per million