Page 122 - Soil and water contamination, 2nd edition

Major dissolved phase constituents 109
[Ca ][HCO 3 ] 0 . 2
K s 10 (5.12)
[H ]

Now the electroneutrality principle imposes that

2 C + C = C + 2 C + C (5.13)
Ca 2 + H + HCO 3 CO 3 2 OH

Together with the above equations, these additional equations allow us to calculate the
concentrations of the carbon dioxide species in a solution in equilibrium with calcite . The
+
H needed for the dissolution of calcite is almost entirely derived from carbonic acid , i.e.
dissolved carbon dioxide . The overall reaction equation is then:
CaCO 3 (s ) + CO 2 + H 2 O Ca 2 + + 2HCO 3 (5.14)

In water bodies that are in contact with solid calcite and the free atmosphere, the partial
-3.5
CO pressure will remain constant at 10 atmosphere and the corresponding HCO -
2 3
-1
-1
concentration at pH = 8.3 will be 1 mmol l (= 61 mg l ). Note, however, that equilibration
between the gas and liquid phase is a relatively slow process, so water bodies exposed to

the free atmosphere may not always be in equilibrium with the atmospheric partial CO
2
pressure; this is especially likely if the CO in the water is produced or consumed chronically
2
in substantial amounts. In surface waters with abundant aquatic vegetation or algae , the
photosynthesis process consumes lots of dissolved CO , so the equilibrium in Equation
2
(5.14) shifts to the left. The dissolved CO is then partly replenished from the atmosphere.
2
The CO consumption in the water may lead to supersaturation, followed by precipitation
2
of calcite. This process occurs in particular in isolated lakes and leads to the formation of
calcareous deposits (gyttja) on the lake bed. In the unsaturated zone or in volcanic areas,
where the pCO is higher, the equilibrium in Equation (5.14) shifts to the right and more
2
calcite dissolves. If this water comes into contact with the free atmosphere, the reduction
of the pCO pushes the equilibrium in Equation (5.14) to the left again and calcite is
2
precipitated. Accordingly, the degassing of CO from the exfiltrating water brings about the
2
formation of stalactites and stalagmites in caves and massive travertine (calcite) formations
near springs.
In the case of a water body isolated from the free atmosphere, for instance in deep
groundwater, the pCO drops as calcite dissolves, since the CO is not replenished. The
2 2
+
dissolution of calcite consumes H ions (see Equation 5.11); as a result, the pH increases
and the carbonate equilibrium in Equation (5.11) shifts to the right. The total TIC
concentration in such isolated groundwater bodies is given by the concentration of TIC in

the zone where the water became isolated from the gas phase plus the amount of TIC derived
from additional dissolution of calcite. In the case of deep groundwater, the zone where the
water becomes isolated from the atmosphere is usually the root zone. Because the calcite
2+
-
dissolution results in an increase of the Ca ions that equals the increase of the HCO ions,
3
the TIC concentration can be calculated from the following mass balance :
TIC = TIC root + Ca 2 (5.15)
where TIC = the concentration of total inorganic carbon in the isolated groundwater body
2+
-1
-1
(mol l ), TIC = the TIC concentration in the root zone (mol l ), and ΔCa = the increase
root
2+
-1
of the Ca concentration resulting from calcite dissolution (mol l ). If there is no supply of
CO from deeper layers or from the decomposition of organic matter, as is usually the case in
2
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