Geology Reference
In-Depth Information
bacteria. Bioremediation techniques were successfully applied in the
clean up of Prince William Sound and the Gulf of Alaska following the
Exxon Valdez accident. Alternative bioremediation procedures relying
on the addition of exogenous bacteria have still to be proved. Similarly,
successful bioremediation of floating oil spills has yet to be demon-
strated.
Source apportionment of crude oil in seawater and monitoring the
extent of weathering and biodegradation constitute important chal-
lenges in environmental analytical chemistry. As the concentration of
individual compounds varies from one sample of crude oil to another,
the relative amounts define a signature characteristic of the source.
Compounds that degrade at the same rate stay at fixed relative amounts
throughout the lifetime of an oil slick. Hence, a ''source ratio'', which
represents the concentration ratio for a pair of compounds exhibiting
such behaviour remains constant. Conversely, a ''weathering ratio''
reflects the concentration ratio for two compounds that degrade at
different rates and consequently continually changes. Oil spill monitor-
ing programmes conventionally determine four fractions
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:
(i) volatile hydrocarbons;
(ii) alkanes;
(iii) total petroleum hydrocarbons; and
(iv) polycyclic aromatic hydrocarbons (PAHs).
The volatile hydrocarbons, albeit comparatively toxic to marine organ-
isms, evaporate relatively quickly and hence serve little purpose as
diagnostic aids. The alkanes and total petroleum hydrocarbons make
up the bulk of the crude oil. They can be used to some extent for source
identification and monitoring weathering progress. The final fraction,
the PAHs, comprises only about 2% of the total content of crude oil
but includes compounds that are toxic. Moreover, these components
exhibit marked disparities in weathering behaviour due to differences in
water solubility, volatility and susceptibility towards biodegradation. As
demonstrated in Figure 15, both a source ratio (C3-dibenzothiophenes/
C3-phenanthrenes) and a weathering ratio (C3-dibenzothiophenes/
C3-chrysenes) have been defined from among such compounds that
enable the extent of crude oil degradation to be estimated in the marine
environment, as well as for sub-tidal sediments and soils.
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The discussion above focuses on petroleum hydrocarbons from oil
spills, representing just one source of contamination. Marine contami-
nant surveys are conducted not just in response to tanker accidents, but
routinely as a means to assess the quality of the marine environment.
Although analytical difficulties with environmental samples persist and
