Calcium is a major positive ion in natural fresh water. It is widely found in many minerals. Calcium makes up 4.9% of the surface of the earth and 0.07% to 1.7% of the soil. Its concentration in rivers is 15 mg/L and in groundwater it ranges from 1 mg/L to less than 500 mg/L. A common form of calcium is calcite (calcium carbonate). If it coexists with magnesium, they form dolomite. The carbon dioxide from the soil organism decomposition dissolves in water and forms carbonic acid. When the water flows through lime, calcium carbonate is dissolved into calcium bicarbonate (Ca(HCO3)2) which is the main source of calcium ions in water.