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The term "hardness" refers to the ability of water to inhibit the formation of soap lather and to promote the formation of a white scale resulting from the heating of water. White scale often forms on plumbing fixtures and in shower stalls. Hardness is attributed mainly to the amount of dissolved calcium and magnesium ions in the water and is reported in several different units, including terms of the equivalent concentration of calcium carbonate. Hardness is usually reported in milligrams per liter (mg/L) of calcium carbonate (equivalent to parts per million, or ppm); it is also sometimes reported in grains per gallon (gpg), where 1 gpg equals 17.118 mg/L. The range of hardness can be classified as soft (0-60 milligrams per liter (mg/L)), moderately hard (61-120 mg/L), hard (121-180 mg/L), and very hard (greater than 180 mg/L).

The primary source of hardness- producing calcium and magnesium is carbonate materials, including weathered shell composed of calcium carbonate (present as both calcite and aragonite), magnesium calcite, and secondary precipitated calcite. These are common minerals in Coastal Plain aquifers of marine origin. Hard water does not pose a health hazard. Hardness can be reduced through the use of "water-softeners" which use ion-exchange media to replace the calcium and magnesium ions with sodium. Alternatively, polyphosphate compounds can be used to precipitate the calcium carbonate which can then be removed by filtration.

[Source of data in table: "A User's Guide for the Artesian Aquifers of the Maryland Coastal Plain--Part Two:Aquifer Characteristics" (H. Hansen, 1972; Maryland Geological Survey)]