Calcium (Ca) is an alkaline earth element and is the fifth most abundant element by mass in the Earth's crust. Calcium is chemically reactive. The naturally occurring minerals are the calcium carbonate (CaCO3) and calcium magnesium carbonate (CaMg(CO3)2) with low solubility. Other minerals such as gypsum (CaSO4•2H2O) and anhydrite (CaSO4) are very soluble in water.
Ca2+ is the dominant cation in most river waters as a result of the solubility of carbonate minerals . Thus the high solubility of carbonates and equilibria in the system CaCO3-CO2-H2O will generally dominate the aqueous geochemistry of calcium. However in non-carbonate terrain silicate minerals will be reactive and plagioclase feldspar (especially the anorthite, calcium-rich end member will be an important control in granitic and most igneous rocks. Evaporite rocks with gypsum or anhydrite are also important possible Ca sources in some sedimentary terrain.
Ca2+ can be measured using either ion chromatography (IC) or inductively coupled plasma optical emission spectroscopy (ICP-OES)
Ca2+ concentrations often are used to study chemical weathering. The relatively faster weathering rate of carbonate rocks compared to silicate rocks exercises an important control on water chemistry; in areas where acid rain predominates accelerated rates of dissolution will occur. Dissolution of the mineral (congruent dissolution) takes place until saturation is reached. After rapid mineral dissolution ceases, precipitation can occur. During congruent solution cations will be added in stoichiometric quantities. Therefore, groundwater has similar cation ratios as the parent rocks. For example, Ca+Mg/HCO3 ratio is used to indicate pure carbonate dissolution. to the ration of Ca/HCO3is related to the reaction pathway (stoichiomentry) and also the pCO2 in groundwater. As groundwater proceeds along the flow path in a carbonate aquifer, incongruent dissolution also takes place, whereby the impurities in the solid phase are progressively released and a purer calcite is produced. So Mg/Ca may also be used as a tracer of increasing maturity of groundwater and residence time.
In a karst system, relative saturation indices with respect to calcite and other carbonate minerals are used to evaluate different flow regime. Diffuse flow (water moving slowly through matrix porosity) and conduit flow (fast flowing water through fissures/cracks) have respectively short and long residence time, which will result in significant difference in the saturation indices with respect to calcite and other carbonate minerals.
References and Further Reading
- Edmunds, W.M., Cook, J.M., Darling, W.G., Kinniburgh, D.G., Miles, D.L., Bath, A.H., Morgan-Jones, M. and Andrews, J.N., 1987. Baseline geochemical conditions in the Chalk aquifer, Berkshire, UK: a basis for groundwater quality management, Applied Geochemistry 2, 251–274.
- Langmuir, D., 1971. Geochemistry of Some Carbonate Ground Waters in Central Pennsylvania. Geochimica et Cosmochimica Acta, 35(10): 1023-1045.
- Langmuir, D., 1997. Aqueous Environmental Geochemistry. Prentice-Hall, Englewood Cliffs, New Jersey, 600p.