Geology Reference
In-Depth Information
matter and surficial rock composition
18
for the major elements. Ele-
ments resistant to chemical weathering or biological activity (e.g. alu-
minium, titanium, iron, phosphorus) show some enrichment in the
riverine solids, while more soluble elements are subject to weathering
and are depleted in the solids, being transported largely in solution
(sodium, calcium). Some pollutant elements such as the metals lead,
cadmium, and zinc tend to be highly enriched in the solids relative to
surficial rocks or soils due to inputs from human activities.
The dissolved components of river water typically exhibit significantly
higher concentrations than in rainwater
19
(Table 4), due to leaching from
rocks and soils. Some insight into the processes governing river water
composition may be gained from Figure 7. Starting from the point of
lowest dissolved salts concentrations, the ratio of Na/(Na
þ
Ca) appro-
aches one. This is similar to rainwater, and is termed the precipitation
dominance regime. It is typified by rivers in humid tropical areas of the
world with very high rainwater inputs and little evaporation. As the
dissolved solids concentration increases the ratio Na/(Na
þ
Ca) declines,
indicating an increasing importance for calcium in the rock dominance
regime. Here, increased weathering of rock provides the major source of
dissolved solids. As dissolved solids increase further, the abundance of
calcium decreases relative to sodium as the water becomes saturated with
respect to CaCO
3
, and this compound precipitates. Waters in the evap-
oration/precipitation regime are typified by rivers in very arid parts of
the world (e.g. River Jordan) and the major seas and oceans of the
world.
20,21
In total, the rivers of the world carry around 4.2
10
12
kg per
year of dissolved solids to the oceans and 18.3
10
12
kg per year of
suspended solids.
Table 4 Average concentrations of the major constituents dissolved in rain and
river water
Concentrations (mg L
1
)
Rain water
Constituent
River water
Na
1
1.98
6.3
K
1
0.30
2.3
Mg
21
0.27
4.1
Ca
21
0.09
15
Fe
0.67
Al
0.01
Cl
3.79
7.8
SO
4
2
0.58
11.2
HCO
3
0.12
58.4
SiO
2
13.1
pH
5.7
