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HYDC03 12/5/05 5:37 PM Page 107

Chemical hydrogeology 107

Table 3.8 Selected hydrochemical data for the Lincolnshire Limestone aquifer to illustrate cation exchange. Data from Edmunds and
Walton (1983).
−
+
2+
+
−1
−1
−1
Na (mg L ) Ca (mg L ) Cl (mg L ) Na /Cl −
Freshwater (Sample 0, Ropsley) 14 135 42 0.51
Mixed water (Sample 14, Pepper Hill) 280 17 114 3.74
Saline water (Sample 19, Deeping St Nicholas) 920 9.5 1100 1.27
c = (114/1100)920 + (1 − 114/1100)14 landfill leachate plumes and the remediation of sites
Na,mix
contaminated by organic pollutants. The major redox
= 107.9 mg L −1
sensitive components of groundwaters and aquifers
− + 2− −
are O , NO /N /NH , SO /HS , Mn(II)/Mn(IV)
Similarly, equation 3.24 can be re-written to calculate 2 3 2 4 4
+ and Fe(II)/Fe(III). Redox sensitive trace elements
the amount of Na released in the cation exchange
include As, Se, U and Cr in addition to Fe and Mn.
reaction:
The toxic effects of these elements differ greatly for
various redox species, for example Cr(III)/Cr(VI),
c = c − c eq. 3.26
Na,react Na,sample Na,mix and so it is important that the behaviour of these ele-
ments can be predicted on the basis of the groundwa-
Now, using the result for c found above:
Na,mix ter redox conditions.
During redox reactions, electrons are transferred
c = 280 − 107.9 = 172.1 mg L −1
Na,react between dissolved, gaseous or solid constituents and
−1
(or 7.5 meq L )
result in changes in the oxidation states of the reac-
2+ tants and products. The oxidation state (or oxidation
A similar calculation for Ca removed from the
number) represents the hypothetical charge that an
mixed groundwater sample results in:
atom would have if the ion or molecule were to dis-
sociate. The oxidation states that can be achieved by
c = (114/1100)9.5 + (1 − 114/1100)135
Ca,mix the most important multi-oxidation state elements
= 122.0 mg L −1 that occur in groundwater are listed in Table 3.9. By
definition, oxidation is the loss of electrons and re-
and duction is the gain of electrons. Every oxidation is
accompanied by a reduction and vice versa, so that
c = 17 − 122.0 =−105.0 mg L −1 an electron balance is always maintained (Freeze &
Ca,react
−1
(or 5.3 meq L ) Cherry 1979).
For every redox half-reaction, the following form
of an equation can be written:
3.9 Redox chemistry
−
oxidized state + ne = reduced state eq. 3.27
Reactions involving a change in oxidation state are
referred to as oxidation-reduction or redox reactions. As an example, the redox reaction for the oxidation of
Redox reactions have a controlling influence on the Fe can be expressed by two half-reactions:
solubility and transport of some minor elements in
−
+
groundwater such as Fe and Mn and also on redox 1 /2O + 2H + 2e = H O (reduction) eq. 3.28
2
2
− 2−
sensitive species such as NO and SO . The extent
3 4
3+
2+
to which redox reactions occur in groundwater 2Fe = 2Fe + 2e − (oxidation) eq. 3.29
systems is therefore significant with respect to many
practical problems, for example issues of ground- The complete redox reaction for the oxidation of Fe
water quality for drinking water, the attenuation of is found from the addition of equations 3.28 and 3.29