5.2 Element chemistry
The 20 largest rivers on Earth carry about 40% of the total continental runoff,
with the Amazon alone accounting for about 15% of the total. These rivers give
the best indication of global average riverwater chemical composition, which can
be compared with average continental crust composition (Table 5.1). Three fea-
tures stand out from this comparison:
1 Four metals dominate the dissolved chemistry of freshwater, all present as
simple cations (Ca^2 +, Na+, K+and Mg^2 +).
2 The low concentration of ions in freshwater.
3 The dissolved ionic composition of freshwater is radically different from
continental crust, despite the fact that all of the cations in riverwater, with the
exception of some of the sodium and chloride (see Section 5.3), are derived from
weathering processes.
Although it is not meaningful to derive a single global average composition for
groundwater—because of the marked differences in aquifer rocks—it is nonethe-
less true that most groundwaters share the three features listed above for
riverwater (see Table 5.3).
The difference between crustal and dissolved riverwater composition is par-
ticularly marked for aluminium and iron relative to other metals (Table 5.1). This
difference results from the way specific metal ions react with water.
Ionic compounds dissolve readily in polar solvents like water (see Box 4.1).
Once in solution, however, different ions react with water in different ways (Fig.
5.1). Low-charge ions (1+, 2+, 1-, 2-) usually dissolve as simple cations or anions.
These ions have little interaction with the water itself, except that each ion is
surrounded by water molecules (Fig. 5.1a). In general, for elements with similar
atomic number, the smaller the ionic radius the higher the charge on the
ion. Small high-charge ions react with water, abstracting OH-to form uncharged
and insoluble hydroxides, liberating hydrogen ions in the process (Fig. 5.1b), for
example:
Fe^3 ()+aq+Æ + 33 H O 21 () FeOH 3 ()s H+()aq eqn. 5.1142 Chapter Five
Table 5.1Comparison of the major cation composition of average upper continental crust
(from Wedepohl 1995) and average riverwater (from Berner & Berner 1987); except
aluminium and iron from Broecker and Peng (1982).
Upper continental Riverwater
crust (mg kg-^1 ) (mg kg-^1 )
Al 77.4 0.05
Fe 30.9 0.04
Ca 29.4 13.4
Na 25.7 5.2
K 28.6 1.3
Mg 13.5 3.4