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Water Hardness


The Floridan aquifer, which underlies all of Florida. yields water with a hardness of less than 180 parts per million (ppm) in the northwestern part and throughout much of the center of the State. In most of the rest of the State, the aquifer yields water having a hardness greater than 180 ppm and as much as 1,900 ppm.

Hardness of ground water is caused by dissolved salts from the geologic formations through which the water passes or by direct pollution. Sometimes pollution may indirectly increase the hardness of ground water by increasing the acid content and thus increasing the solution of calcium and magnesium salts.

Water from deep zones of the aquifer tend to be harder than water from more shallow zones. Only the hardness of water from the upper part of the aquifer is shown.

The term "hardness" is applied to the soap neutralizing power of water. Soap will neither cleanse, nor lather, until all of the hardness of the water is precipitated. Any substance, such as calcium, magnesium, iron, manganese, copper, barium, lead, and zinc, that will form an insoluble curd with soap causes hardness. Hardness is usually attributed to calcium and magnesium because other elements are seldom present in significant concentration.

Hardness is usually reported in terms of an equivalent concentration of calcium carbonate which is the amount of calcium carbonate required to cause a hardness equal to that caused by all the substances producing hardness in the water.

The terms "temporary hardness" or "carbonate hardness" and "permanent hardness" or "non-carbonate hardness," have been used extensively. When carbonates or bicarbonates are present in the water in concentrations equivalent to, or greater than, the calcium and magnesium, the scale that forms upon evaporation will consist primarily of calcium carbonate and magnesium hydroxide. Such hardness is termed "carbonate" or, because the scale can be removed with acid, "temporary." When the alkalinity is low, sulfates and chlorides may form scales that cannot readily be dissolved by acid, and such hardness is called "non-carbonate" or "permanent" (McKee and Wolf. 1963, p. 196).

In Florida the hardness is mostly carbonate hardness. In general, the non-carbonate hardness will be less than half of the carbonate hardness; however, in local areas the non-carbonate hardness may exceed the carbonate hardness, The small inset map shows the non-carbonate hardness in the Floridan aquifer in Florida. The carbonate hardness may be approximated by subtracting the value on the non-carbonate hardness map from the value of the total hardness map. In general, only the highly mineralized water in the southern part of Florida contains much non-carbonate hardness.

Detrimental effects of hardness include excessive soap consumption in homes and laundries, formation of scums and curds in homes, laundries. and textile mills, yellowing of fabrics, toughening of vegetables cooked in hard water, and the formation of scale in boilers, hot water heaters, pipes and cooking utensils.

There have been many attempts to classify water as soft, moderately hard, or hard. Such adjectives are subject to almost as many interpretations as there are readers. The classification described below is that used by the U. S. Geological Survey. Water with a hardness of less than 60 ppm is generally rated as soft, and its treatment for the removal of hardness is rarely justified. Hardness between 60 and 120 ppm is called moderately hard and does not seriously interfere with the use of water for most purposes, but it does slightly increase the consumption of soap, and its removal by a softening process is profitable for laundries and allied industries. Hardness between 120 and 180 ppm is called hard and is unsuitable for many industrial processes and requires treatment for the prevention of scale in boilers. Hardness greater than 180 ppm is called very hard and is objectionable for most industrial and domestic uses.

Hard waters have no harmful effects upon the health of consumers (McKee and Wolf, 1963) and are commonly used. Many people in Florida use water having a hardness of from 200 to 400 ppm. Some municipal supplies in Florida are softened and the hardness generally reduced to about 85 ppm.

William J. Shampine, "Hardness of Water from the Upper Part of the Floridan Aquifer in Florida" Prepared by United States Geological Survey in cooperation with the Bureau of Geology, Florida Department of Natural Resources Tallahassee, Florida, 1965, revised 1975.


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