GROUND WATER IN NORTH-CENTRAL TENNESSEE
GROUND WATER IN NORTH-CENTRAL TENNESSEE
GROUND WATER IN NORTH-CENTRAL TENNESSEE
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104 <strong>GROUND</strong> <strong>WATER</strong> <strong>IN</strong> <strong>NORTH</strong>-<strong>CENTRAL</strong> <strong>TENNESSEE</strong><br />
(CaCO3) equivalent to all the calcium and magnesium. This quan<br />
tity is the so-called "total hardness" of the water. It is calculated<br />
by the formula<br />
Total hardness = 2.5 Ca +4.1 Mg<br />
in which all quantities are expressed in parts per million. Hardness<br />
is caused both by the bicarbonates and by the sulphates of calcium<br />
and magnesium. The hardness due to sulphates the so-called "non-<br />
carbonate hardness" or "permanent hardness" may be calculated<br />
from the formula<br />
Noncarbonate hardness = 2.5 Ca+4.1 Mg 0.82 HCO3 .<br />
Water with a total hardness less than 50 parts per million is gen<br />
erally considered soft, and under most circumstances its treatment to<br />
remove hardness is not justified on the score of economy. Hardness<br />
between 50 and 150 parts per million does not render the water un<br />
satisfactory for most purposes, but it does increase the consumption<br />
of soap slightly. Hence, it is profitable for laundries and other indus<br />
tries that use large quantities of soap to soften such a water to remove<br />
calcium and magnesium. Hardness exceeding 150 parts per million<br />
is objectionable in common household uses of water, and if the hard<br />
ness is 200 or 300 parts per million it is common practice to soften the<br />
water or to install cisterns and rain catches. When an entire munici<br />
pal supply is softened, the hardness is generally reduced to about 100<br />
parts per million, as the additional improvement from further soften<br />
ing is not deemed an economy. If the hardness is much more than<br />
100 parts per million, the water must generally be treated for. the<br />
prevention of scale formation before it can be used successfully in<br />
steam boilers. The cost and difficulty of adequate softening for this<br />
purpose are likely to be increased materially if the noncarbonate hard<br />
ness is large.<br />
Very few of the ground waters of north-central Tennessee contain<br />
less than 50 parts per million total hardness, and many are so hard<br />
that they should be softened to make them suitable for general use.<br />
In some of the waters the noncarbonate hardness also is relatively<br />
large.<br />
There are two processes for softening water in general use the<br />
lime and soda process and the exchange silicate or so-called "zeolite"<br />
or "permutite" process, which has been developed in recent years.<br />
lii the lime and soda process the noncarbonate hardness, if present<br />
in the raw water, is converted to carbonate hardness by the addition<br />
of enough soda (sodium carbonate) to react with the sulphates of<br />
calcium and magnesium to form the bicarbonates of calcium and mag-