Geology Reference
In-Depth Information
Table 5.8 Mean chemical contents of
world river water (Livingstone, 1963)
Substance
Concentration g=g
HCO
3
58.4
Ca
2+
15
SiO
2
13.1
SO
2
4
11.2
Cl
7.8
Na
+
6.3
Mg
2+
4.1
K
+
2.3
NO
3
1
Fe
2+
0.67
Sum
120
Li (1982), as shown in Table 5.9 and Gaillardet et al. (2004) undertook different
compilations of the average concentration of the main trace elements found in rivers.
Lake waters vary greatly in composition, not only from lake to lake but often
within a single lake where marked temperature and compositional stratifications can
occur. Reducing conditions often exist in the lower, more saline levels and these
give rise to relatively high concentrations of nitrite ammonia and Fe
2+
. Reducing
conditions may also lead to the production of hydrogen sulphide gas and the pre-
cipitation of some metal sulphides (including those of iron). Silica and phosphorous
may also be released from sediments. The thermocline zone, which separates the
upper and lower levels of a lake by a notable change in temperature, prevents the
diffusion of atmospheric oxygen into the reduction layer (Henderson, 1982).
5.4.2.2 Groundwater
Nearly all groundwater has its origin in rainfall. It is always slowly flowing to the
ocean, either directly in underground aquifers or via surface flow upon enjoining
with streams.
The hydrogeochemistry of groundwater reflects the water's source, the lithology
of the aquifer and the local chemical conditions as influenced by temperature, pres-
sure and redox potential. It constitutes less than 1% of the total water on Earth
(Table 5.3) with a volume about 35 times greater than that lying in freshwater
surface lakes or flowing in surface streams. White et al. (1963) classified the sources
of groundwater as:
magmatic,
meteoric (e.g. precipitated and surface water),
connate (i.e. water trapped in the pore spaced of a sediment at the time of
deposition),
oceanic.
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