Geoscience Reference
In-Depth Information
sediments. Figure 7.9 shows a typical example
from a salt marsh in the Severn estuary. What
is noticeable here is that the concentration of
individual metals varies with depth. This trend
is a historical one, with the levels of metals
deposited in each layer of sediment reflecting that
in the estuary at the time of deposition. Hence,
sediment deposited before 1850 contains rela-
tively little contamination, whereas that deposited
in the 1950s has significantly higher levels. Such
a trend is typical of many industrial estuaries
and can be divided into three zones. The lower
zone (I) represents background contaminant
levels, when the estuary or delta sediments
were being deposited in times of no industrial-
ization, and reflects levels from natural erosion.
The middle zone (II) marks a period of rapidly
increasing contaminant levels, and represents a
time of rapid industrialization and declining
environmental quality. In the UK, the start of
this zone can be linked to the onset of the indus-
trial revolution in the mid-nineteenth century.
The uppermost zone (III) shows declining levels
of contamination, and links to the cleaning up
of the environment, increased environmental
awareness and legislation, and a general decline
in heavy industries.
Zn
2+
Cu
+
Cla
y
K
+
Al
3+
Zn
2+
Zn
2+
Cu
+
Mg
2+
Cu
+
Mg
2+
Al
3+
Al
3+
K
+
Pt
2+
Pt
2+
Zn
2+
Fig. 7.8
Hypothetical representation of the adsorption of
metals to clay particles. The positively charged metals are
attracted to the negatively charged clays. As the clays flocculate,
so the metals are laid down with the sediments.
involve chemical combination of metals with
sulphides and sulphates, or within mineral lattices
(see Chapter 1).
The main issue with metal contaminants is that
they cannot be readily observed, and therefore
attract little attention. The fact remains though,
that many muddy estuaries and deltas contain
significant quantities of metals stored within their
Cu (ppm)
Pb (ppm)
Zn (ppm)
Age
1989
1960
1936
1900
III
1958
10
20
30
40
50
60
70
80
90
100
110
120
130
140
II
1850
I
Fig. 7.9
Metal depth profiles from a salt marsh deposit in the Severn estuary, south-west UK. Note the variation with depth, which
reflects the changing level of contaminants within the estuary over time. I, II and III represent 'chemozones'. These are distinct zones
that can be identified in industrialized estuarine sediments. Chemozone I represents static, background levels of the uncontaminated,
pre-industrialized estuary; II represents increasing contaminant levels during industrial growth; III represents declining contaminant
levels representing industrial decline and increased emission legislation (see text). (Modified from French 1996.)
