Page 111 - Soil and water contamination, 2nd edition

98 Soil and Water Contamination

2+
2+
thus precipitate as carbonate ; the permanent hardness is the part of Ca and Mg ions in
-
excess of HCO . To avoid the formation of boiler scale, the hardness of drinking water is
3
-1
usually kept below 150–200 mg l .
Calcium is an important constituent of many igneous rock minerals, especially
plagioclase, pyroxene , and amphibole . The weathering of igneous rocks is slow, so the
Ca concentrations in water that has been in contact with igneous rocks are generally low.
Furthermore, calcium is commonly found in many sedimentary rocks in the form of calcite
and aragonite (both CaCO ), dolomite (Ca, MgCO ), gypsum (CaSO ⋅2H O), or anhydrite
3 3 4 2
(CaSO ). Limestone and chalk consist mostly of calcium carbonate (calcite) and calcium
4
carbonate is also a major cementing agent between the particles of sandstone and other
sedimentary rocks . Calcium is also a component of montmorillonite . Because of the broad
range of Ca-bearing minerals, Ca is plentiful in soil and water, except in pure sands (largely
consisting of quartz minerals) and in acidic leached soils. The Ca content of soils normally
ranges between 0.1 and 1.2 percent, but in CaCO -bearing or CaSO -bearing soils, the Ca
3 4
contents are often considerably larger and in sandy soils and acidic leached soils (laterite
soils) often smaller (Scheffer and Schachtschabel, 1989). In these latter soils, Ca deficiency
may occur for plant growth, although Ca deficiency is rare. Therefore, Ca fertilisers are
seldom applied. Nevertheless, a major anthropogenic Ca source is lime (calcium carbonate),
a fertiliser applied to increase the pH and/or to eliminate toxic aluminium and manganese in
agricultural soils, forest soils, or lake waters. In this case, the carbonate is the effective agent.
Two major mechanisms control the Ca concentrations in water: calcite precipitation
and dissolution , and cation exchange at the negatively charged surfaces of the soil particles.
Furthermore, Ca ions form complexes with some organic anions , but these complexes barely
affect the Ca concentrations in natural water (Hem, 1989). Because the Ca concentrations
are for the greater part controlled by calcite precipitation and dissolution, Ca concentrations
are often correlated with the most abundant carbonate species in natural water: bicarbonate
-
(HCO ). The solubility of calcite is regulated not only by the Ca concentration but also by
3
the carbonate equilibria, which, in turn, are controlled by the CO partial pressure (pCO )
2 2
-
and pH . Section 5.10 gives more detail about the relation between Ca and HCO and also
3
the solubility of calcite. Since Ca ions are divalent, they adhere more strongly to surface
charge sites than monovalent ions. Because Ca is generally the dominant divalent ion in
2+
surface water and groundwater, Ca ions occupy most surface sites. Cation exchange may
cause changes in the ratio between calcium and other dissolved cations . However, in systems
in which water is in contact with an extensive area of solid surface, the Ca:Na ratio tends to
remain relatively constant (Hem, 1989).
5.5 MAGNESIUM
Magnesium (Mg) is an alkaline-earth metal and has only one oxidation state of significance:
2+
Mg . It is an essential constituent of plant and animal nutrition. About one-fifth of the
Mg taken up by plants is used for the production of chlorophyll . A deficiency of Mg leads
to interveinal chlorosis , i.e. yellowish coloration of leave tissue between the darker veins,
especially of older leaves, because Mg is mobilised from older leaves and transported to
younger tissues. Furthermore, Mg is an activator for many critical enzymes in both plants
and animals. In animals and humans, it is also used for building bones and tendons, and
together with calcium it plays a role in the regulation of heart activity. Magnesium also
contributes to the total water hardness and is thus undesirable in large concentrations in
drinking water (see above).
Magnesium occurs typically in dark-coloured minerals present in igneous rocks such
as plagioclase, pyroxenes, amphiboles, and the dark-coloured micas . In metamorphous

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