Chemistry and the Environment - Green Chemistry and Technology - page 52

4.3 Pollution of the oceans and biomagnify due to their lipid solubility, mov-

ing up the trophic levels entering the human food
chain through finfish and shellfish. The PAHs com-
prise about 2% of crude oil but include some of the
slowest components to degrade. The PAH compo-
nents thus may be used to identify pollution sources
and monitor the weathering process.
Hydrocarbons also adsorb readily onto particulates
and accumulate in sediments where they pose a
chronic threat to the bottom-dwelling (benthic) flora
and fauna. Soluble hydrocarbons such as the mono-
cyclic aromatics kill plankton exposed to them at
levels above 5 mg l -1 . Anthropogenic sources of
marine oil pollution exceed natural sources (exclud-
ing biosynthesis) by a factor of between 3 and 10.
Removal processes include evaporation, dissolu-
tion, dispersion, emulsification, photochemical oxi-
dation, adsorption and biodegradation.

'Pollution is nothing but the resources we are not
harvesting. We allow them to disperse because
we've been ignorant of their value.'
R. Buckminster Fuller (1895-1983)

Pathways by which anthropogenic pollutants are
delivered to the oceans include river run-off, wet
and dry atmospheric deposition, marine dumping
(e.g. of sewage sludge, radioactive waste and dredge
spoil), discharges from ships and coastal pipeline
discharges.
Pollutants that enter the oceans may be diluted
and dispersed by circulation mechanisms, concen-
trated by ingestion and retention by marine biota,
degraded by chemical and biological processes or
deposited in the sediments. The pollutants thus af-
fect all compartments of the ocean and have their
greatest impact in coastal and enclosed seas close
to the sources. The most significant pollutants in-
clude hydrocarbons, sewage, persistent organic com-
pounds, toxic metals and artificial radionuclides.

Sewage

Sewage may reach the oceans in untreated or par-
tially treated forms through rivers or coastal dis-
charge or as dumped sewage sludge. It poses
aesthetic and health risks and carries with it other
pollutants such as metals, nutrients and organics. At
the beginning of the 1990s some 5 million tonnes of
wet sewage sludge were dumped by the UK in the
North Sea every year but this practice has now
ceased.

Hydrocarbons

The main sources of in situ marine hydrocarbon pol-
lution are tanker operations and accidents, maritime
bilge and fuel oils, coastal refineries, marine termi-
nals and offshore production. The main sources from
land are municipal wastes, including motor vehicle
fuels and lubricants, industrial wastes, urban run-off
and atmospheric fall-out. There are no consistent
estimates of marine oil release but a figure of be-
tween one and three million tonnes per year may be
realistic and this figure may be declining.
Crude oil is the main in situ component and is
a complex mixture of aliphatic and aromatic hydro-
carbons with some heterocyclic compounds. Mo-
lecular weights range from 40 to over 200, with con-
sequent ranges of vapour pressure, oil-water parti-
tion coefficient and water solubility. The effects can
depend upon the composition of the oil as
well as the prevailing weather and ecology of the
area.
Oil pollution adversely affects marine communi-
ties and has damaging impacts on marine and coastal
habitats. The longer chain aliphatic hydrocarbons
and PAHs deplete marine populations over extensive
areas because of their toxicity, rapid uptake by biota
and environmental persistence. They bioaccumulate

Persistent organic pollutants

Anthropogenic organic compounds exist in a
large variety and all may potentially find their
way to the oceans. Halogenated compounds such
as organochlorine pesticides, PCBs, dioxins, diben-
zofurans and chlorinated solvents pose a major
threat. As an example, over 60% (230 000 t) of
all environmental PCBs are estimated to reside in
the water of the open oceans, 35% (130 000 t) in
the coastal sediments and relatively tiny amounts in
soils (2400 t), terrestrial waters (3500 t) and the air
(550 t).
Other persistent organics that are causes for con-
cern in the marine environment are organophos-
phorus pesticides, solvents, endocrine disruptors and
PAHs. The PAHs are now believed to be 1000 times
more toxic to marine organisms after exposure to
sunlight than in their unphotolysed form [24].