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) . Water is considered very hard if hardness values exceed 150 mg L-1 . In comparison, hardness values of < 60 mg L-1 are indicative of soft waters . 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.
- Freeze, R.A. and Cherry, J.A. 1979. Groundwater, Prentice-Hall Inc., Englewood Cliffs, N.J., pp. 604.