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Table 3. Source or cause, and significance of dissolved-mineral constituents and physical properties of water
(modified from Popkin, 1973, p. 85)

[µS/cm, microsiemens per centimeter at 25 degrees Celsius; mg/L, milligrams per liter; µg/L, micrograms per liter]

Constituent or property

Source or cause


conductance (µS/cm)

Mineral content of the water.

Indicates degree of mineralization. Specific conductance is a measure of the capacity of the water to conduct an electric current. Varies with temperature, concentration, and degree of ionization of the constituents.


Acids, acid-generating salts, and free carbon dioxide lower the pH. Carbonates, bicarbonates, hydroxides, phosphates, silicates, and borates raise the pH.

pH is a measure of the activity of hydrogen ions. A pH of 7 indicates neutrality of a solution. Values higher than 7 denote increasing alkalinity; values lower than 7 indicate increasing acidity. Corrosiveness of water generally increases with decreasing pH. However, excessively alkaline water also may be corrosive.

Hardness as calcium carbonate (CaCO3)

In most water, nearly all the hardness is due to calcium and magnesium. All metallic cations other than the alkali metals also cause hardness.

Consumes soap before a lather will form and deposits soap curd on bathtubs. Hard water forms scale in boilers, water heaters, and pipes. Hardness equivalent to or less than the bicarbonate and carbonate concentration is called carbonate hardness. Any hardness in excess of this is called noncarbonate hardness. Water with hardness of 60 mg/L or less is considered soft; 61 to 120 mg/L, moderately hard; 121 to 180 mg/L, hard; more than 180 mg/L, very hard.

Calcium (Ca) and magnesium (Mg)

Dissolved from many rocks and soil, but especially from limestone, dolomite, and gypsum. Calcium and magnesium are detected in large quantities in some brines. Magnesium is present in large quantities in seawater.

Causes most of the hardness and scale-forming properties of water; soap consuming (see hardness).

Sodium (Na) and potassium (K)

Dissolved from many rocks and soil; also in ancient brines, seawater, industrial brines, and sewage.

Large concentrations, in combination with chloride, give a salty taste. Moderate concentrations have little effect on the usefulness of water for most purposes. Sodium salts may cause foaming in steam boilers. A large sodium concentration may limit the use of water for irrigation.

Bicarbonate (HCO3) and carbonate (CO3)

Action of carbon dioxide in water on carbonate rocks such as limestone and dolomite.

Bicarbonate and carbonate produce alkalinity. Bicarbonates of calcium and magnesium decompose in steam boilers and hot-water facilities to form scale and release corrosive carbon dioxide gas. In combination with calcium and magnesium, causes carbonate hardness.

Sulfate (SO4)

Dissolved from rocks and soil containing gypsum, iron sulfides, and other sulfur compounds. Commonly present in mine water and in some industrial wastes.

Sulfate in water containing calcium forms hard scale in steam boilers. In large concentrations, sulfate in combination with other ions gives bitter taste to water, and may have a laxative effect on some people. Some calcium sulfate is considered beneficial in the brewing process.

Chloride (Cl)

Dissolved from rocks and soil. Present in sewage and found in large concentrations in ancient brines, seawater, and industrial brines.

In large concentrations in combination with sodium, gives salty taste to drinking water. In large concentrations increases the corrosiveness of water towards some metals.

Fluoride (F)

Dissolved in minute to small concentrations from most rocks and soil. Added to most water by fluoridation of municipal supplies.

Fluoride in drinking water reduces the incidence of tooth decay when the water is consumed during the period of enamel calcification. However, it may cause mottling of the teeth and renal dysfunction, depending on the concentration of fluoride, the age of the child, quantity of drinking water consumed, and susceptibility of the individual.

Silica (SiO2)

Dissolved from many rocks and soil, commonly less than
30 mg/L. Large concentrations, as much as 250 mg/L, generally occur in alkaline water.

Forms hard scale in pipes and boilers. Transported in steam of high-pressure boilers to form deposits on blades of turbines. Inhibits deterioration of zeolite-type water softeners.

Dissolved solids

Primarily mineral constituents dissolved from rocks and soil.

Water containing more than 1,000 mg/L dissolved solids is unsuitable for many purposes.

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