The ratio of Na+: (Na++Ca^2 +) can therefore be used to discriminate between
rainwater and weathering sources in freshwaters. When sodium is the dominant
cation (sea-salt contribution important), Na+: (Na++Ca^2 +) values approach 1.
When calcium is the dominant ion (weathering contribution important), Na+:
(Na++Ca^2 +) values approach 0.
The composition of dissolved ions in riverwater can be classified by compar-
ing Na+: (Na++Ca^2 +) values with the total number of ions present in solution
(Fig. 5.3). Note that the total dissolved ions or salts can also be expressed as the
ionic strength of the water (Box 5.1). Data which plot in the bottom right of Fig.
5.3 represent rivers with low ion concentrations and sodium as the dominant
cation. These rivers flow over crystalline bedrock (low weathering rates) or over
extensively weathered, kaolinitic, tropical soils (low weathering potential, chem-
ical index of alteration (CIA) c.100 (see Table 4.9)). The Rio Negro, a tributary
of the Amazon (Fig. 5.4), draining the highly weathered tropical soils of the
central Amazonian region, has low ionic strength (Box 5.1) with weathering-
derived sodium as the major cation. The Onyx River in the dry valleys of Antarc-
tica is a better example of a low-ionic-strength, sea-salt-sodium-dominated river.
This river has its source as glacial melt water and has a starting chemistry almost
totally dominated by marine ions. As it flows over the igneous and metamorphic
rocks of the valley floor, its composition evolves to higher ionic strength with an
increasing proportion of calcium (Fig. 5.3).
Major river systems flow over a wide range of rock types, acquiring the dis-
solved products of weathering reactions. Freshwaters originating in areas with
active weathering processes will have higher ion concentrations and an increas-
ing predominance of calcium over sodium. These rivers plot along a trend from
A to A¢on Fig. 5.3. The Mackenzie and Ganges (Table 5.2) fall within this group,
despite very different geomorphological settings.
The Amazon and its tributaries are a good example of a river system where the
chemistry of the lower reaches integrates the products of differing soil and bedrock
weathering regimes (Fig. 5.4). Rivers draining the intensely weathered soils and
sediments of the central Amazonian region, such as the Rio Negro, have low total
cation concentrations of less than 200meq l-^1 (i.e. sum of all major cations concen-
trations ¥charge; see also footnote to Table 4.10). The Rio Negro has water rel-
atively enriched in sodium, silica, iron, aluminium and hydrogen ions, because of
the limited supply of other cations from weathering reactions. By contrast, rivers
draining easily erodible sedimentary rocks (including carbonates) of the Peruvian
Andes are characterized by high total cation concentrations of 450–3000meq l-^1 ,
including abundant calcium, magnesium, alkalinity (see below and Box 5.2) and
sulphate. Between these two extremes in water composition are rivers with quite
low total cation concentrations, with sodium enriched relative to calcium and mag-
nesium, but also with high concentrations of silica, consistent with the weathering
of feldspars (e.g. albite (see eqn. 4.14)). These rivers drain areas without large
amounts of easily weatherable rock, but drain soils not so completely degraded as
the lowest concentration group characterized by the Rio Negro.
In arid areas, evaporation may influence the major dissolved ion chemistry
of rivers. Evaporation concentrates the total amount of ions in riverwater.
The Chemistry of Continental Waters 147