eqn. 5.3
eqn. 5.4
eqn. 5.5
At pH 8 the HPO 42 - species predominates. Understanding dissociation behaviour
is very important in calculating the solubility of an ion. To do this it is important
to know which species is the dominant one. A water chemist, for example, may
wish to know if iron phosphate (FePO 4 ) can form in a river, as this species helps
regulate phosphorus concentrations in freshwaters. To know this it is necessary
to calculate the PO 43 - concentration from the total dissolved inorganic phospho-
rus concentration that is measured analytically and contains all the species in
equations 5.3–5.5. This can be done knowing the pH and dissociation constant
(see Box 4.5) for equations 5.3–5.5. A worked example for the carbonate system
is given in Section 5.3.1
Silicon is mobilized by the weathering of silicate minerals, mainly feldspars
(see eqn. 4.14), and is transported in natural waters at near-neutral pH as undis-
sociated silicic acid (H 4 SiO 4 ), an oxyanion (Fig. 5.2). Silicate minerals weather
slowly, such that rates of input—and concentrations—of silicon in most fresh-
waters are quite low. Despite this, where silicates are the main component of
bedrock or soil, H 4 SiO 4 can be a significant dissolved component of freshwater.
5.3 Water chemistry and weathering regimes
Comparison of dissolved major ion compositions in four large rivers draining very
different crustal areas (Table 5.2) shows the dominance of calcium (Ca), magne-
sium (Mg), sodium (Na) and potassium (K). Overall, however, the chemistry of
each river is different and weathering regimes control most of these variations.
The dissolved ion composition of freshwater depends upon:
1 the varying composition of rainfall and atmospheric dry deposition;
2 the modification of atmospheric inputs by evapotranspiration;
HPO^24 - ()aq++ 23 H()+aqªPO^34 ()-aq H()+aq
H PO 24 - ()aq ++H()+aqªHPO^24 - ()aq 2 H()+aq
H PO 34 ()aq ªH PO 24 ()aq+H()+aq
The Chemistry of Continental Waters 145
Table 5.2Dissolved major ion composition (mmol l-^1 ) of some major rivers. Data from
Meybeck (1979); except Rio Grande from Livingston (1963).
Mackenzie (1) Orinoco (2) Ganges (3) Rio Grande (4)
Ca^2 + 0.82 0.08 0.61 2.72
Mg^2 + 0.43 0.04 0.20 0.99
Na+ 0.30 0.06 0.21 5.10
K+ 0.02 0.02 0.08 0.17
Cl- 0.25 0.08 0.09 4.82
SO 42 - 0.38 0.03 0.09 2.48
HCO 3 - 1.82 0.18 1.72 3.00
SiO 2 0.05 0.19 0.21 0.50
Drainage basin characteristics: (1) northern arctic Canada; (2) tropical northern South America; (3)
southern Himalayas; (4) arid southwestern North America.
