Geoscience Reference
In-Depth Information
When still resident in the water body, or even
after deposition, these contaminants can have a
significant ecological impact, and thus be regarded
as pollutants (see Chapter 1 for definitions).
For example, toxicity may cause fatalities and
endanger parts of the food chain or, if at sub-
lethal levels, may have an impact on the ability
to reproduce. Such issues may occur whether a
pollutant is in solution within the water body,
or within the sediment where it remains bio-
available. Furthermore, some species can toler-
ate pollutants and store them within their body
tissues, a process known as bioaccumulation or
biomagnification (Clark 1992).
Although the environmental levels of contam-
inants/pollutants have declined since the mid-
twentieth century (see Fig. 7.9), developments in
chemical engineering have resulted in the environ-
mental presence of a new suite of compounds
known as organohalogens. These are generally
used as pesticides or in electrical equipment but,
importantly, are unable to be broken down and,
unlike metals, can be toxic at very low concen-
trations. The organohalogens include such groups
of compounds as the PCBs (polychlorinated
biphenyls), HCB (hexachlorobenzine), HCH
(hexachlorocyclohexane, lindane), and DDT
(dichloro-diphenyl-trichloroethane) (McLusky
1989). These compounds show the classic trends
of bioaccumulation and biomagnification, with
the greatest impact on the higher predators.
More significant for estuarine systems is that
much of the world's waste and surplus applica-
tion of these substances has drained into them
and, because of the accumulatory nature and
storage potential associated with clays, have
become stored.
Another significant pollutant in deltas and
estuaries, particularly in terms of its visibility,
is oil. By their very nature, estuaries and deltas
lend themselves to the establishment of ports,
harbours and oil refineries. Indeed, some of the
larger deltas (e.g. Mississippi) also contain oil
fields of their own. Oil spills are a very emotive
issue and whereas the media may not appear
too concerned to discuss metal pollution in
salt marsh sediments, they will readily report
on a large oil tanker spill. Although an oil
tanker accident may appear more newsworthy,
a factory releasing small quantities of oil on
a daily basis is likely to be more significant in
the long term than one major release. Nelson-
Smith (1972) cites one example. A typical
refinery effluent may contain small traces of oil
(
c
. 10-20 ppm) that are not readily detectable
with the naked eye. If, however, this is linked to
a discharge of 455,000 L min
−1
, then on a daily
basis this equates to 6825 L of oil. Ironically,
the major accident which receives the greatest
publicity is somewhat of a rarity. Farmer (1997)
reports data from the International Tanker
Owners Pollution Federation (ITOPF) which
shows that only 12% of marine (i.e. not neces-
sarily delta or estuarine) oil is derived from
tanker accidents. The greatest source (37%) is
from industrial sources and urban runoff, such
as that reported by Nelson-Smith (1972), and
directly affects estuaries and deltas.
Despite the continued inputs of oil, perhaps
the most remarkable aspect of estuarine and
deltaic environments is their resilience. Although
the oiled parts of plants may die, they gener-
ally grow again once the oil has broken down.
DeLaune et al. (1994) studied the impacts of oil
on salt marshes by artificially oiling areas of
Spartina
marsh and trying different methods of
cleaning, notably leaving the oiled marsh to be
cleaned by flushing with sea water, applying a
dispersant, and the cutting and removal of the
oiled growth. They found that after 95 days of
monitoring, there was no major difference in
the plots and hence concluded that the best
course of action is to leave marshes to recover
and regrow naturally. Such conclusions have also
been supported by Gilfillan et al. (1995), who
showed that 15 years after the 1978 Amoco
Cadiz spill on the coast of Brittany, France, the
areas of marsh where recovery was most suc-
cessful were those that had been left alone. Teal
et al. (1992) also showed that 20 years after a
large spill in Buzzard's Bay, Massachusetts,
marsh growth was as good as areas unaffected
by the spill. However, in both cases, regeneration
took up to 15-20 years to achieve, suggesting
that regrowth may initially be at a slower rate
than pre-spill.
