24 REPORT OF INVESTIGATIONS NO. 50
and rocks, any bacteria or color present will usually be removed. The fluctuations in the quality and temperature of ground water are smaller than in most surface-water bodies because of the long period of time required for ground water to percolate downward to the aquifer and then laterally through the aquifer to the point of discharge.
Dissolved mineral constituents in water are usually reported in parts per million (ppm) (one unit weight of a constituent in a million unit weights of water). Hardness of water is caused by the presence of alkaline earth metals, mostly calcium and magnesium, and is expressed as an equivalent quantity of calcium carbonate. Specific conductance is a measure of the ability of water to conduct an electric current and may be used in estimating the dissolved mineral content. The most important dissolved constituents can usually be related to specific conductance (fig. 25). Color is expressed in units of the platinum-cobalt scale. The symbol pH is a measure of the acidity or alkalinity of a solution, and is expressed as the negative logarithm of the hydrogen-ion concentration.
RELATION OF QUALITY OF WATER TO USE
The amount and type of dissolved and suspended materials in water determines its value for a particular use. Water suitable for one use may be entirely unsatisfactory for another. For example, sea water is used for cooling purposes, whereas it is unsatisfactory for most other industrial use. Water used in the manufacturing of products such as high-grade paper and textiles must be very low in dissolved solids.
Table 4 shows the more common characteristics of water quality. Some constituents in water can be removed inexpensively whereas other constituents can be removed only by expensive distillation. Hardness of water may be removed by the relatively simple and inexpensive ion-exchange method which replaces the calcium and magnesium with sodium from common table salt. Iron, color, and turbidity may be economically removed by flocculation, settling, and filtration. The removal of sodium, chloride, and sulfate is difficult and expensive.