Calcium is an essential constituent in many igneous rock minerals, especially the chain silicates and the feldspars. Calcium also occurs in other silicate minerals that were produced in metamorphism. Therefore, some calcium is to be expected in water that has been in contact with igneous and metamorphic rock. The calcium concentration of this water generally is low, mainly because the rate of decomposition of most igneous rock minerals is very slow.
The most common forms of calcium in sedimentary rock are the carbonates. The two crystalline forms, calcite and aragonite, both have the formula CaC03. The mineral dolomite can be represented as CaMg(CO3)2. Limestone consists mostly of calcite with mixtures of magnesium and other impurities. Other calcium minerals common in sediments include sulfates such as gypsum and anhydrite.
In sandstone and other detrital rock, calcium carbonate commonly is present as a cement between particles or a partial filling of the inter spaces. Calcium is also present in the form of absorbed ions on negatively charged mineral surfaces in soils and rocks.
The chemical controls of calcium concentration involve a complex equilibrium. The equilibrium involves the carbonates, which are the major factor in limiting the solubility of calcium in most natural waters. The pH also controls the solubility of calcium carbonate in the water.
Figure __ (from Hem 1975) illustrates the equilibrium concentration of calcium in mg/L as it relates to carbonate and pH.
However, in environments where hydrogen ions are supplied for rock weathering by processes other than disassociation of dissolved carbon dioxide species, such as oxidation of sulfur or sulfides, calcium may be brought into solution in amounts greater than the stoichiometric equivalent of the bicarbonate concentration. In such a system, the maximum calcium concentration that could be reached would generally be determined by an equilibrium in which gypsum is the stable solid.
When water moves through soil, calcium may be exchanged for other cations (primarily sodium). The reverse reaction may also occur under some conditions. The presence of exchangeable calcium in soil tends to hold clay particles in a granular structure and thereby increases infiltration and percolation.
Commonly, streams which drain humid areas contains more calcium than any other cation. Streams in more arid regions, and especially where some of the more soluble rock types are exposed, tend to have much higher dissolved calcium concentrations.
When stream water is impounded in a storage reservoir or lake, changes may occur in calcium concentration as a result of calcium carbonate precipitation. The increased pH near the water surface caused by algae and plankton may bring about supersaturation and precipitation can occur around the edges of the reservoir or lake. A conspicuous white deposit called marl, which is formed in these areas, is partly made up of calcium carbonate.
Ground water associated with limestone would commonly be expected to be near saturation with respect to calcite. However, most available analytical ground water data do not include pH determinations that are satisfactory for testing calcium carbonate equilibrium concentrations.
The wide variation in calcium concentration and the corresponding correlations shown with hardness, stratification and biological productivity have stimulated many attempts at lake classification based on the calcium ion. The distribution of calcium in stratified lakes generally follows a more or less characteristic pattern.
In soft water lakes, the calcium content generally ranges from 0.7 to 2.3 mg/L, and during summer lake stagnation, little stratification of calcium exists.
Lakes of medium hardness and medium calcium content typically contain a moderately increased calcium concentration in the hypolimneon during stratification.
The hypolimneon of hard water lakes characteristically contains a greatly increased load of calcium. In some of these lakes, the curve of calcium concentration shows a steady increase of the ion from the surface to the upper region of the hypolimneon.