Water Hardness

Water �hardness’ is defined as the content of metallic ions in waters which react with sodium in soaps. Most water hardness is attributable to calcium (Ca²⁺) and magnesium (Mg²⁺) ions. A general expression used to calculate water hardness is:

hardness = 2.5(Ca²⁺) + 4.1(Mg²⁺)

in which calcium and magnesium are measured in milligrams per litre (mg L^-1) [1]. Water is considered very hard if hardness values exceed 150 mg L^-1 [1]. In comparison, hardness values of < 60 mg L^-1 are indicative of soft waters [1]. Seawater has a hardness level of 500 mg L^-1. Water hardness increases with salinity in a mixing system between seawater and rainwater/river water (Figure 1).

Some important mineral sources of calcium and magnesium ions are calcium carbonate (CaCO₃) and dolomite (CaMg(CO₃)₂) dissolved from limestones.

Figure 1. Changes in water hardness with log salinity in a mixing system between seawater and rainwater.

1. Freeze, R.A. and Cherry, J.A. 1979. Groundwater, Prentice-Hall Inc., Englewood Cliffs, N.J., pp. 604.