Geology Reference
In-Depth Information
4.5 MARINE CONTAMINATION AND POLLUTION
Both contamination and pollution entail the perturbation of the natural
state of the environment by anthropogenic activity. The two terms are
distinguishable in terms of the severity of the effect, whereby pollution
induces the loss of potential resources.
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Additionally, a clear cause-
effect relationship must be established for a substance to be classified as
a pollutant towards a particular organism. The human-induced distur-
bances take many forms, but the greatest effects tend to be in coastal
environments due to the source strengths and pathways. Waters and
sediments in coastal regions bear the brunt of industrial and sewage
discharges, and are also subject to dredging and spoil dumping. Agri-
cultural runoff may contain pesticide residues and elevated nutrients, the
latter of which may over-stimulate biological activity producing eutro-
phication and anoxic conditions. The deep sea has not escaped contam-
ination. The most obvious manifestations comprise crude oil, petroleum
products and plastic pollutants, but include the long-range transport of
long-lived radionuclides from coastal sources. Additionally, the aeolian
transport of heavy metals has enhanced natural fluxes of some elements,
particularly lead. Three case studies are introduced below to illustrate
diverse aspects of marine contamination and pollution.
4.5.1 Oil Slicks
Major releases of oil have been caused by the grounding of tankers (i.e.,
Torrey Canyon, Southwest England, 1967; Argo Merchant, Nantucket
Shoals, USA, 1976; Amoco Cadiz, Northwest France, 1978; Exxon
Valdiz, Alaska, 1990; Erika, France, 1999; Prestige, Spain, 2002) and
by the accidental discharge from offshore platforms (i.e., Chevron MP-
41C, Mississippi Delta, 1970; Ixtox I, Gulf of Mexico, 1979). Because oil
spills receive considerable public attention and provoke substantial
anxiety, oil pollution must be put into perspective. Crude oil has been
habitually introduced into the marine environment from natural seeps at
a rate of approximately 340
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. Anthropogenic activity has
recently augmented this supply by an order of magnitude; however,
most of this additional oil has originated from relatively diffuse sources
relating to municipal runoff and standard shipping operations. Excep-
tional episodes of pollution occurred in the Persian Gulf in 1991 (910
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) and due to the Ixtox I well in the Gulf of Mexico in 1979 (530
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). In contrast to such mishaps, the Amoco Cadiz discharged
only 250
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of oil in 1978 accounting for the largest spill from a
tanker. The cumulative pollution from tanker accidents on an annual
