72Occurrence of Hard Water
Hard water is caused by soluble, divalent, metallic cations, (positive ions having valence of 2). The
principal chemicals that cause water hardness are calcium (Ca) and magnesium (Mg). Strontium,
aluminum, barium, and iron are usually present in large enough concentrations to contribute
significantly to the total hardness.
Water hardness varies considerably in different geographic areas of the contiguous 48 states. This
is due to different geologic formations, and is also a function of the contact time between water and
limestone deposits. Magnesium is dissolved as water passes over and through dolomite and other
magnesium-bearing minerals. Because groundwater is in contact with these formations for a longer
period of time than surface water, groundwater is normally harder than surface water.
Expressing Water Hardness Concentration
Water hardness is generally expressed as a concentration of calcium carbonate, in terms of
milligrams per liter as CaCO 3. The degree of hardness that consumers consider objectionable will
vary, depending on other qualities of the water and on the hardness to which they have become
accustomed. We will show two different classifications of the relative hardness of water:
Comparative classifications of water for softness and hardness
Classification mg/L as CaCO 3 * mg/L as CaCO 3 +
Soft 0 – 75 0 – 60
Moderately hard 75 – 150 61 – 120
Hard 150 – 300 121 – 180
Very hard Over 300 Over 180
Source: Adapted from sawyer 1960 and Briggs and Ficke 1977.
- Per Sawyer (1960)
- Per Briggs and Ficke (1977)
Types of Water Hardness
Hardness can be categorized by either of two methods: calcium versus magnesium hardness and
carbonate versus non-carbonate hardness. The calcium-magnesium distinction is based on the
minerals involved. Hardness caused by calcium is called calcium hardness, regardless of the salts
associated with it, which include calcium sulfate (CaSO 4 ), calcium chloride (CaCl 2 ), and others.
Likewise, hardness caused by magnesium is called magnesium hardness. Calcium and
magnesium are normally the only significant minerals that cause harness, so it is generally
assumed that
Total harness = calcium hardness + magnesium hardnessThe carbonate-noncarbonate distinction, however, is based on hardness from either the
bicarbonate salts of calcium or the normal salts of calcium and magnesium involved in causing
water hardness. Carbonate hardness is caused primarily by the bicarbonate salts of calcium and
magnesium, which are calcium bicarbonate, Ca(HCO 3 ) 2 , and magnesium bicarbonate Mg(HCO 3 ) 2.
Calcium and magnesium combined with carbonate (CO 3 ) also contribute to carbonate hardness.