Environmental Engineering Reference
In-Depth Information
5.3.1
Transport of matter
(Table 5.1), but it is this part that most affects land-
scape structure and functioning. Net exchange (pre-
cipitation minus evaporation) between atmosphere
and continents amounts to 37 × 1 0
9
m
3
y r
− 1
. It is dis-
tributed very unevenly over the Earth but in most areas
this water fl ows to seas and oceans, either through
rivers and lakes as surface water or through the soil as
groundwater. Eventually, also groundwater fl ows into
the surface-water system but upwelling groundwater
may in the meantime sustain
wetlands
that are
outside the reach of surface water. Spring mires or
groundwater-fed fen meadows high above the surface
water are typical examples. The size of groundwater
systems differs greatly from catchment to catchment
and depends on both the topography and the permea-
bility of geological strata, but most systems are at least
several square kilometres in size. Toth (1963) devel-
oped the concept of nested systems (Figure 5.3) where
small fl ow systems are embraced by larger systems.
These systems may differ considerably, not only in fl ow
rates but also - depending on the chemical composi-
tion of the soil strata - in water chemistry (see also
Chapter 16 ).
Rivers transport large amounts of solutes, sediments
and sometimes biomass. The latter two are deposited
on fl oodplains during fl ooding periods and this causes
natural fertilization in otherwise nutrient-poor land-
scapes. Five millennia ago, the agriculture in Egypt was
based on regular fl ooding by the river Nile, and this
could sustain a large human population in an arid
environment that was otherwise very hostile.
Both surface and groundwater also transport dis-
solved substances such as nutrients (soluble com-
pounds of N, P and K) and other ions. The most common
ones are called major ions (Hem 1959) and they
By w ind
Large quantities of material are transported by air
fl ows, especially along the edges of continents where
winds are generally stronger than in more interior
areas. Under natural conditions this is a major mecha-
nism for the exchange of chloride between sea and
inland areas. Rainwater analyses from different sites
showed a decrease in Cl
−
content from over 100 to less
than 1 mg l
− 1
between coast and inland (Figure 5.2). In
more intensively human-affected landscapes air trans-
port of pollutants has become an important issue.
Examples include the transport of sulphur dioxide and
mineral nitrogen from industrial and agricultural
sources, and the transport of radioactive gases. Dis-
tances covered by wind transport can be huge. Holland
et al
. (2005) estimated that part of the atmospheric
N-deposition in western Europe may originate from
sources in the United States, a distance of over 5000 km!
Also the amount of material can be large. The deposi-
tion of air-borne nitrogen has at least been doubled or
tripled in large parts of Europe from natural back-
ground values of circa 350 eq. N ha
− 1
yr
− 1
and may now
make up more than 25% of the available nitrogen in
the system (Verhagen & van Diggelen 2006 ).
By w ater
Only a very small portion of all the water on the planet
is found in the atmosphere and on the continents
100
United Kingdom
The Netherlands
Sweden
Australia
Table 5.1
Freshwater reservoirs on the land.
10
Reservoir
Mass
(10
15
kg)
Proportion
(%)
Ice sheets
26 200
80.0
1.0
Groundwater
6200
19.0
Lakes, fresh water
126
0.4
Lake Baikal
22
Great Lakes, North America
32
East African lakes
36
0.1
0
50
100
150
200
250
300
350
Lakes, brackish water
105
0.4
Distance from the sea (km)
Caspian Sea
80
Soil, unsaturated zone
150
0.5
Figure 5.2
Chloride concentration in precipitation in
relation to distance from the coast. (FromVoigt 1980.)
Rivers
2
