GROUND WATER IN NORTH-CENTRAL TENNESSEE

106 GROUND WATER IN NORTH-CENTRAL TENNESSEE
takes from the silicate an equivalent amount of sodium. In this way
the filtered water is completely softened, although the total quantity
of dissolved mineral matter is not decreased, and the quantity of
sodium may be so increased as to induce foaming to a troublesome
degree. When all its exchangeable sodium has been replaced the
silicate becomes inert. If, however, a concentrated solution of com­
mon salt is passed through the inert silicate, the exchange reaction is
reversed, sodium from the salt displaces the calcium and magnesium,
and the silicate is reactivated. After it has been flushed with soft
water the exchange silicate can again be used as a softening agent.
The exchange silicate process softens water more completely than
the lime and soda process, although it does not reduce the total
quantity of dissolved solids. It is likely to be somewhat more costly
than the lime and soda process if the water to be treated has only
carbonate hardness. On the other hand, it is generally less costly if
the water contains much noncarbonate hardness and if salt for reacti­
vating the silicate is not unduly expensive. The principal advantages
of the exchange silicate process are that the water can be completely
softened if desired and that close technical control is not essential.
During recent years this process has been successfully adapted to
treating as much as 5,000,000 gallons of water daily, as at McKees
Rocks, Pa.; it has also been adapted to the needs of the single house­
hold in units of small capacity that can be operated with nominal
attention.
Hydrogen sulphide. Hydrogen sulphide (H2S) is a gas that gives the
characteristic odor to sulphur waters, the same odor that is associated
with the decomposition of eggs and other organic substances that
contain considerable sulphur. It is easily detected by its character­
istic odor in concentrations as slight as 1 part per million or less,
although it is difficult to determine quantitatively if the concentratio
is much less than 5 parts per million. Hydrogen sulphide is quickly
dissipated or is oxidized to sulphate when a sulphur water is allowed
to stand in contact with air, so that it must be precipitated as an
insoluble sulphide, usually cadmium sulphide, when a sample for
quantitative analysis is taken. It is generally held that hydrogen
sulphide in ground waters of meteoric origin is formed by the reduction
of sulphates, as is brought out by a review of the literature cited by
Renick.69
Hydrogen sulphide, if present in large quantities, imparts to a
water a decidedly disagreeable odor and taste that makes the water
unfit for domestic uses. In the presence of air it combines with the
dissolved iron to form the black ferrous sulphide, which generally
Renick, B.C., Some geochemical relations of ground water and associated natural gas in the Lance
formation, Montana: Jour. Geology, vol. 32, pp. 668-684,1924.