An Introduction to Environmental Chemistry

tions stood at 47 g l-^1. Rivers draining into the Salton Sea have a Na+/Ca^2 +
ratio of about 5 : 1 (on an atomic basis) but as a result of evaporation and
CaCO 3 precipitation, this ratio increases to 27 : 1 in the Salton Sea itself.
Other examples of rivers in which evaporation plays an important role include
the Jordan and Rio Grande. In all these arid areas, dissolution of evaporite
minerals in the catchments may also contribute to the increasing dominance of
NaCl.
The classification of riverwater composition in Fig. 5.3 is simplified and does
not always work. For example, weathering of feldspars (see Section 4.4.4) can
produce solutions of low ionic strength, but rich in sodium and silica, which plot
in the bottom right of Fig. 5.3. This effect probably influences the classification
of the Rio Negro. Weathering of evaporite minerals will also affect the compo-
sition of rivers. For example, in the Amazon catchment there are a small number
of tributaries draining areas of predominantly evaporite rock. These have very
high total cation concentrations and are characterized by high sodium, chloride,
calcium and sulphate concentrations from the weathering of the evaporite min-
erals, halite and gypsum. Despite these complications, Fig. 5.3 remains a useful
way to compare factors controlling riverwater chemistry. Indeed, it is remarkable
that most of the world’s major rivers can be rationalized in this straightforward
way.

5.3.1 Alkalinity, dissolved inorganic carbon and pH buffering

In Section 4.4 we saw that most soilwaters that feed rivers and groundwater have
near-neutral pH, with HCO 3 - as the major anion. This results from the dissolu-
tion of CO 2 in water (see eqn. 4.7) and from the acid hydrolysis of silicates and
carbonates. The total concentration of weak acid anions like HCO 3 - in water is
referred to as alkalinity. These anions are available to neutralize acidity (H+)
in natural waters, consequently it is important to understand their chemical
behaviour.
In continental waters, bicarbonate (HCO 3 - ) and carbonate (CO 32 - ) ions are the
most important components of alkalinity, although in seawater other ions also
contribute to alkalinity. The relative importance of HCO 3 - and CO 32 - depends on
the pH of the solution and can be calculated from the known dissociation con-
stants (see Box 4.5) of these ions and the solution pH.
The first dissociation of dissolved carbon dioxide (expressed here as carbonic
acid),

eqn. 5.6

has a dissociation constant,

eqn. 5.7

The ‘a’ denotes activity, the formal thermodynamic representation of concentra-
tion (see Section 2.6).
Similarly, for the second dissociation of carbonic acid,

K

aa

(^1) a
3
23

= ◊ = 1064

+-

H HCO -

HCO

.

H CO 23 ()aq ªH()+aq+HCO 3 - ()aq

The Chemistry of Continental Waters 151