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The Roles of Photooxidationand Biodegradation in Long-term
Weathering of Crude andHeavy Fuel OilsROGER C. PRINCE*, ROBERT M. GARRETT, RICHARD E. BARE, MATTHEW J. GROSSMAN,
TODD TOWNSEND, JOSEPH M. SUFLITA, KENNETH LEE§, EDWARD H. OWENS,
GARY A. SERGY, JOAN F. BRADDOCK§§, JON E. LINDSTROM & RICHARD R. LESSARD
ExxonMobil Research and Engineering Co., Annandale, NJ 08801, USA
Institute for Energy and the Environment and the Department of Botany and Microbiology, University of Oklahoma,
Norman, OK 73019, USA
§Department of Fisheries and Oceans, Dartmouth, Nova Scotia, Canada B2Y 4T3
Polaris Applied Sciences, Inc., Bainbridge Island, WA 98110, USA
Environment Canada, #200, 4999––98th Ave. Edmonton, AB, Canada T6B 2X3§§Institute of Arctic Biology, University of Alaska Fairbanks, Fairbanks, AK 99775, USA
Shannon & Wilson, Inc., Fairbanks, AK 99709, USA
ExxonMobil Research and Engineering Co., Fairfax, VA 22037, USA
Although spilled oil is subject to a range of natural processes, only combustion, photooxidation and bio-degradation destroy hydrocarbons and remove them from the biosphere. We present laboratory data thatdemonstrate the molecular preferences of these processes, and then examine some oil residues collectedfrom previously documented releases to confirm the important roles that these processes play in removingspilled oil from both marine and terrestrial environments. 2003 Elsevier Science Ltd. All rights reserved.
Introduction
Crude and heavy fuel oils that escape into the en-
vironment, either from natural seeps or from acci-
dental spills, become subject to a variety of physical,
chemical and biological phenomena. Small molecules
evaporate (Fingas, 1999; Sharma et al., 2002), and are
either degraded photochemically (Poisson et al., 2000;
Hurley et al., 2001), or are washed from the atmo-
sphere in rain and then biodegraded (Arzayus et al.,
2001). Under particularly aggressive aeration in water,
as happened in the spill from the OSSA II pipeline into
the flood-stage Rııo Desaguadero on the Bolivian Al-
tiplano in January 2000, this evaporation can extend
into molecules with >30 carbon atoms (Douglas et al.,2002; Prince et al., 2002), but evaporation is more
usually limited to molecules with less than about 15
carbon atoms (Payne et al., 1991; Fingas, 1999). Ter-
restrial spills may soak into the ground, as happened
in the Nipisi, Rainbow and Old Peace River pipeline
spills in the Lesser Slave Lake area of Northern Al-
berta spill (Blenkinsopp et al., 1996). Some molecules,
particularly aromatic hydrocarbons and small polar
molecules such as naphthenic acids, dissolve if suffi-
cient water is present (Lafargue & Le Thiez, 1996;
Burns et al., 2000), and again these are eventually
biodegraded (Herman et al., 1994). Spills at sea or on
lakes and rivers often disperse into the water column,
Spill Science & Technology Bulletin, Vol. 8, No. 2, pp. 145–156, 2003
2003 Elsevier Science Ltd. All rights reserved
Printed in Great Britain
1353-2561/03 $ - see front matter
doi:10.1016/S1353-2561(03)00017-3
145
*Corresponding author.
E-mail address: roger.c.prince@exxonmobil.com (R.C. Prince).
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dramatically increasing the surface area available for
microbial colonization and biodegradation. This oc-
curred during the Braer spill (Thomas, 1993; Thomas& Lunel, 1993), and accelerating the process by the
application of chemical dispersants is an important
option for minimizing the environmental impact of
spills that do not naturally rapidly disperse (Fiocco &
Lewis, 1999; Lessard & DeMarco, 2000; Page et al.,
2000). Dispersants were successfully applied on a large
scale on the spill from the Sea Empress (Lunel et al.,
1997). Other physical processes that occur in water
include the formation of water in oil emulsions,
known as mousses (Fingas et al., 2001), and the in-
teraction of the oil with fine particles of sediment
(Owens, 1999). The oil–fine particle aggregates dis-perse in the water column, often as neutrally buoyant
particles, and the oil is more available for biodegra-
dation (Weise et al., 1999). Concentrations of oil in
local sediments where oil–fine particle interactions
have occurred are typically very low (Boehm et al.,
1987; Sergy et al., 1999). Some spills spontaneously
ignite, as happened during the Haven spill (Martinelli
et al., 1995), and deliberate ignition of spills is an ac-
cepted response option in some situations, such as that
of the New Carissa (Gallagher et al., 2001). The ulti-
mate fate of spilled hydrocarbons that are not col-
lected, burnt or photooxidized is biodegradation, and
stimulating this biodegradation by adding fertilizers
was successful on shorelines oiled following the spill
from the Exxon Valdez (Prince & Bragg, 1997).
Untreated terrestrial spills are not usually subjected
to any dilution, and while biodegradation eventually
removes the majority of the hydrocarbons, it appar-
ently leaves the majority of the resins and polar frac-
tions of the oil. Bioremediation to stimulate the
removal of the hydrocarbons can be an effective
treatment (McMillen et al., 1995; Prince et al., 1997;
Braddock et al., 1997; Radwan et al., 2000).
In contrast, when considering the long-term
weathering of oil spills in the marine environment it isimportant to bear in mind that the vast majority of
most spilled oil is physically dispersed so that it is
impossible to find, and hence to study. As examples,
most of the 2.5 million gallons of Bunker C spilled in
Chadabucto Bay, Nova Scotia, Canada, in February
1970 from the Arrow has ‘‘disappeared’’ (Owens et al.,
1993). So has that from the Baffin Island Oil Spill
experiment conducted on the northern tip of Baffin
Island, Nunavut, Canada in August 1981 (Owens
et al., 1994). In both cases, only remnants are left on
the shorelines. Nevertheless, analysis of these rem-
nants allows us to ascertain how the oils have altered
since the spill, and thus gain some insights into the
likely fate of the oil that has left the beaches.
In this paper we will review what is known about
the photochemistry and biodegradation of crude oils,
principally from work in the laboratory, and then use
this information to implicate these processes in the
transformation of oil spilled in marine and terrestrialenvironments.
Methods
The analyses of this paper rely principally on the
results of gas chromatography coupled with mass
spectrometry. This is a powerful technique that, in
selected ion monitoring mode allows the analysis of a
range of individual hydrocarbons, and in total ion
mode allows an estimation of the total hydrocarbon.
We focus here on normal and iso-alkanes, polycyclicaromatic hydrocarbons and their alkylated forms, and
hopanes and sterane biomarkers (see Douglas et al.,
1992). GC/MS is unable to analyze the majority of the
heteroatom-containing molecules in crude oils and
refined products, often called resins, asphaltenes and/
or polar compounds. These can be analyzed by thin
layer chromatography (Barman, 1996), and we report
some data from laboratory experiments using this
technique. This technique is not able, however, to
distinguish between polar compounds present in crude
oils and non-petrogenic polar compounds in envi-
ronmental samples, so we do not include any data
using this technique on samples collected from spill
sites.
The foundation of our approach is to follow
changes in the chemical composition of the oil, de-
termined with gas chromatography and mass spect-
rometry, using a conserved internal marker in the oil
as a reference compound. Providing we have a sample
of the initial oil, whether in a laboratory experiment or
in examining samples from a historical spill, we can
then determine how much of an individual analyte has
been lost from the experimental or field sample.
Although most hydrocarbons in crude oil are bio-
degradable, some, such as the biomarkers (Peters &Moldowan, 1993) that are molecular fossils of that
biomass that gave rise to the crude oil, are remarkably
resistant to biodegradation (Prince et al., 1994). This is
true for both aerobic (Prince et al., 1994) and anaer-
obic (Caldwell et al., 1998) biodegradation, and they
are also resistant to photooxidation (Garrett et al.,
1998). They can thus serve as conserved internal
markers within the oil, and the loss of other com-
pounds can be assessed with reference to them by
simple proportion.
17a(H)21b(H)hopane is abundant enough in most
crude and heavy fuel oils to be a particularly useful
conserved internal marker. In laboratory experiments
we have shown that it is not generated during oil
biodegradation (Prince et al., 1994), and we have not
seen evidence for its significant biodegradation in
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laboratory or field studies (although we have seen
some evidence for its eventual biodegradation in soil
(Prince et al., 1997)). We can calculate the percentdepletion of other analytes within the oil using the
equation:
%Loss ¼ ½ðð A0= H 0Þ ð As= H sÞÞ=ð A0= H 0Þ 100
Where: As and H s are the concentrations of the target
analyte and hopane in the oil sample, respectively, and
A0 and H 0 are the concentrations in the initially spilled
oil.
Recently Wang et al. (2001) have reported the ap-
parent biodegradation of hopanes in oil remaining
from the 1974 Metula spill, and Bost et al. (2001) have
reported degradation of hopanes and norhopanes bya microbial consortium enriched from a creosote-
contaminated site. Although we have seen no evidence
that biodegradation of 17a(H)21b(H)hopane has oc-
curred in any of the samples discussed here, if it had
occurred our estimates of the extent of biodegradation
of other compounds would be underestimates.
Photooxidation of crude oil
Figure 1 presents total ion gas chromatograms of
an artificially weathered Alaskan North Slope crude
oil (treated to have lost 30% of its initial weight byevaporation) exposed to the atmosphere in a shallow
dish in the dark or exposed to a laboratory UV lamp
(Garrett et al., 1998). There is clearly almost no effect
of the illumination on the total ion GC/MS chroma-
tograms (Fig. 1, left), but the illumination has a sig-
nificant effect on the composition of the oil determined
with thin layer chromatography. The saturates are
unaffected, but the majority of the aromatic hydro-
carbons have been converted to resins or polar mole-
cules (Fig. 1, right). When the aromatic hydrocarbons
are measured with selected ion monitoring GC/MS
(Douglas et al., 1992), a clear pattern emerges; as
shown in Fig. 2, the four-ring chrysene is substantially
more affected than the three-ring phenanthrene and
dibenzothiophene, and in each family the extent of
loss increases with increasing alkylation. Although
smaller aromatics such as naphthalene and benzene
derivatives were not present in the oil used in Figs. 1 &
2, we may surmise that these compounds would be lesssusceptible to photooxidation than phenanthrene and
dibenzothiophene, and recent work bears this out,
even with substantial alkylation (Dutta & Harayama,
2001). Fortunately, although resistant to photooxida-
tion, such molecules are readily biodegraded (Prince,
2002). These patterns of photooxidative loss, with
larger polycyclic aromatic hydrocarbons lost before
smaller ones, and more alkylated compounds lost be-
fore their less alkylated congeners, is quite different
from that seen in biodegradation (Elmendorf et al.,
1994), as we shall see below.
Fig. 1 Photooxidation of an artificially weathered crude oil. On the left are total ion mass chromatograms of an artificially weathered AlaskanNorth Slope crude oil, before and after exposure to a laboratory UV source. On the right is the composition of the oil determined by thin layerchromatography.
Fig. 2 Photooxidation of an artificially weathered crude oil. Thefigure shows the relative losses of alkylated polycyclic aromatic
hydrocarbons caused by exposure to a laboratory UV source. TheC0, C1, C2, C3 nomenclature indicates the number of alkyl carbonson the parent molecule, regardless of position. For example, C2includes dimethyl and ethyl forms.
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Combustion of crude oil
As noted above, burning is sometimes considered asan option for dealing with spilled oil (Buist et al.,
1999; Yoshioka et al., 1999), including in marshes (Lin
et al., 2002). Burning can be an effective way of re-
moving large amounts of oil, but there is always some
residue left when the oil is unable to maintain a high
enough temperature for complete combustion, usually
because the slick becomes so thin that heat loss to the
substrate becomes a dominant process. The residual
oil is typically slightly enriched in pyrogenic hydro-
carbons such as fluoranthene and pyrene, although the
total amount of these compounds in the environment
is reduced by a successful burn (Garrett et al., 2000).None of the samples discussed here has any history,
nor shows any evidence, that combustion was involved
in the weathering processes.
Aerobic biodegradation of crude oil
The aerobic biodegradation of hydrocarbons has
been intensively studied in the last century, and hun-
dreds of cultures of hydrocarbon-degrading aerobic
microorganisms have been studied (Prince, 2002).
These organisms rely upon oxygen as both the initial
oxidant of the hydrocarbon, and the terminal electron
acceptor of respiratory electron flow. Almost all hy-
drocarbons are known to be biodegraded, although
individual strains of organisms typically degrade only
a limited range of substrates. A typical aerobic bio-
degradation pattern for a hydrocarbon fuel is shown
in Fig. 3––on the left, total ion GC/MS chromato-
grams of an intermediate fuel oil that has undergone
aerobic biodegradation by an inoculum collected from
the New Jersey coast, on the right the loss of indi-
vidual compounds identified by selected ion moni-
toring GC/MS. Within one week the n-alkanes,
exemplified here by heptadecane, were essentiallycompletely removed, as were approximately 50% of the
phenanthrene and dibenzothiophene. In contrast, only
some 10% of the branched alkane pristane (2,6,10,14-
tetramethylpentadecane) had been degraded after one
week, but there had been substantial degradation by
three weeks, at which time chrysene biodegradation
had begun. More than 60% of the chrysene was lost at
12 weeks. Figure 4 shows the pattern of degradation
of the alkyl polycyclic aromatics in this experiment; it
is clear that within each family, the unsubstituted
parent compound is degraded most readily, and that
increasing alkylation slowed biodegradation (Elmen-dorf et al., 1994). In summary, under aerobic condi-
tions the n-alkanes are the most readily degraded
hydrocarbons, and the biodegradation of polycyclic
aromatic hydrocarbons decreases with increasing size
and alkylation. These patterns are essentially the op-
posite of those seen for photooxidation.
Perhaps surprisingly, biodegradation in the field
does not usually show very much isomer specificity; all
the isomers of, for example, the methyl dibenzothi-
ophenes or phenanthrenes are lost at more or less the
same rate, although this is not necessarily apparent at
first inspection. For example, the left panel of Fig. 5
shows the methyldibenzothiophenes and methylphe-
nanthrenes of the Arrow cargo oil, and a sample from
Black Duck Cove, Nova Scotia, Canada, a site still
contaminated with oil from the 1970 spill (see below).
While at first glance it seems that there has been
a dramatic preference for the loss of some isomers
(e.g. 4-methyldibenzothiophene) and not others (e.g.
1-methyldibenzothiophene), in fact all the isomers
have been substantially removed when compared to
the residual hopane in the oil (Fig. 5 right panel). We
Fig. 3 Aerobic biodegradation of a heavy fuel oil (IF30). On the leftare total ion mass chromatograms of the oil at various times into theexperiment, and on the right are the relative losses of representativesaturate and aromatic compounds in the oil.
Fig. 4 Aerobic biodegradation of a heavy fuel oil (IF30). The figureshows the relative losses of alkylated polycyclic aromatic hydro-carbons in the samples from Fig. 3, with similar nomenclature toFig. 2.
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have seen no consistent pattern of preferential degra-
dation of individual isomers in the samples we have
collected from the sites discussed here.
As we will discuss below, all the samples we have
collected from the field show changes that we can
readily interpret as the combination of evaporation,
aerobic biodegradation and photochemistry. Never-theless, Fayad and Overton (1995) have reported a
quite different pattern of biodegradation in a labora-
tory experiment with mousse collected during the Gulf
oil spill. In the absence of added nutrients (just indi-
genous nutrients from the Gulf seawater), and at 5
and 20 g oil per liter of seawater, they reported sub-
stantial degradation of aromatic but not aliphatic
hydrocarbons in 144 h. When nutrients were added,
this preference was reversed! There are some puzzling
aspects to this work, including the observation that
much less biodegradation was seen with 10 g oil per
liter of seawater. Unfortunately no data on potential
conserved biomarkers is reported, so it is possible that
the oils in the different tests, albeit from a single
sample of mousse, may have been from a hetero-
geneous mixture of oils. Otherwise it is very hard to
reconcile the data from the different experiments. And
in the absence of data on a conserved internal marker,
their observation of the apparent preferential biode-
gradation of 4-methyldibenzothiophene (their Fig. 3)
may well be akin to that seen in Fig. 5 here, and in fact
reflect very extensive biodegradation of all isomers.
Anaerobic biodegradation of crude oil
The anaerobic degradation of hydrocarbons has
only been clearly demonstrated in the last decade or so
(Heider et al., 1999; Spormann & Widdel, 2000), and
very few defined cultures are in laboratory captivity
(Prince, 2002). In the absence of oxygen, sulfate, which
is reduced to sulfide, and carbon dioxide, which is
reduced to methane, are the most likely oxidants in
most terrestrial and aquatic environments. Although
sulfate reduction and methanogenesis have been well
studied, their involvement in hydrocarbon biode-gradation has not been fully documented, and both
the substrate range and preference of anaerobic hy-
drocarbon-degrading communities are largely un-
known.
We have shown that linear alkanes in crude oil are
readily degraded under sulfate-reducing conditions
by microorganisms from marine sediments (Caldwell
et al., 1998). More recently, we have found that mi-
croorganisms from a terrestrial subsurface environ-
ment catalyze a similar range of crude oil n-alkane
biodegradation under both sulfate-reducing and
methanogenic conditions (Fig. 6). Compared to aero-
bic biodegradation, extensive degradation of branched
alkanes and aromatic hydrocarbons seems to lag far
behind that of the n-alkanes, and under optimal con-
ditions, the anaerobic process, in general, is likely
to be slower than the aerobic process. Nevertheless,
it is apparent that spilled crude oil is subject to bio-
degradation in both aerobic and anaerobic environ-
ments.
The Arrow spill
The wreck of the Arrow in February 1970 released
2.5 million gallons of Bunker C fuel oil into Chedab-
ucto Bay, Nova Scotia, Canada (45 N, 61 W). Only
48 km of an estimated 305 km of oiled shoreline were
cleaned after the spill, and there are still traces of
Fig. 5 Aerobic biodegradation of methyldibenzothiophene and methylphenanthrene isomers. On the left are selected ion chromatograms(m= z ¼ 198 and 192) of cargo from the Arrow and oil from the beach of Black Duck Cove, Nova Scotia, Canada. On the right is the percentageloss of the individual isomers in the field sample.
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residual oil in parts of the affected area that can be
attributed to the spill (Wang et al., 1994; Prince et al.,
1998). Figure 7 shows chromatograms of the original
cargo and of two samples collected in October 1997
from Black Duck Cove, one of the areas where small
amounts of oil can still be found. We note that the
residual surface oil is not very noticeable to the un-informed eye, since the oil is associated with asphalt
pavements in a beautiful day-use park.
One sample, a subsurface sheen, was collected from
the surface of water filling a shallow pit dug in the
intertidal zone of the sheltered beach, while the other
is a sample of exposed asphalt from above the high
tide mark. Both have lost substantial amounts of
their initial hydrocarbon, some 40% and 60% respec-
tively. Note that the subsurface sheen sample still has
molecules with less than 20 carbons, although these
are not resolvable alkanes, while the surface sample
has lost most of these. We attribute this differenceto more extensive evaporation of the surface sam-
ple, coupled with extensive biodegradation in both
samples, and photooxidative loss of alkylated phe-
nanthrenes and chrysenes in the exposed sample
(Fig. 8).
Fig. 7 Samples from Black Duck Cove, Nova Scotia, Canada. On the left are total ion mass chromatograms of the cargo oil and two samplesfrom the cove. On the right are the relative losses of representative saturate and aromatic compounds in the oil.
Fig. 6 Anaerobic biodegradation of an artificially weathered crude oil. On the left are total ion mass chromatograms of an artificially weatheredAlaskan North Slope crude oil, before and after biodegradation under sulfate-reducing and methanogenic conditions. Inocula came from FortLupton, Colorado, and the cultures were incubated for approximately a year. On the right are the relative losses of representative saturated andaromatic compounds in the oil.
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The Baffin Island Oil Spill (BIOS) Project
The Baffin Island Oil Spill (BIOS) Project (Sergy &
Blackall, 1987) released approximately 15 m3 of lightly
weathered (8% weight loss) Lago Medio crude oil onto
the water adjacent to a shoreline on Cape Hatt,
northern Baffin Island, Nunavut, Canada (72310 N,
79500 W) in August 1981. About 45% of the oil
stranded on the previously pristine adjacent beach,
and this subsequently weathered naturally without any
cleanup efforts. By 1989 there had been an approxi-
mately 80% decrease in the total oiled area (Owens
et al., 1994), and in September 2001 we estimated that
coverage had decreased to less than 5% of the initial
area (Owens et al., 2002; Prince et al., 2002).
Wang et al. (1995) examined oil samples collected
in 1993 from this site, and Fig. 9 shows chromato-
grams of the spilled oil and of three samples collectedin September 2001 (Prince et al., 2002). The hydro-
carbon content of these three samples are very dif-
ferent, with the subsurface sample being almost
unchanged in the 20 years since it was spilled with the
exception of the loss of parent and methyl phenanth-
renes and dibenzothiophenes, which we tentatively
attribute to evaporation. In contrast, the oil on some
surface granules was extensively degraded, having lost
approximately 90% of its total hydrocarbons, but it
has still only lost about 20% of its methylchrysenes.
We attribute most of these losses to biodegradation. In
contrast, the oil extracted from a surface pavementhad lost only approximately 50% of its total hydro-
carbon and only 50% of its pristane, but 40% of its
methylchrysenes. We attribute this to rather less ex-
tensive biodegradation coupled with rather more ex-
tensive photooxidation, and the relative losses of the
alkylated forms of chrysene bear this out (Fig. 10).
The Poker-Caribou Creeks Research Watershed experi-
ment
The Poker-Caribou Creeks Research Watershed oil
spill experiment was conducted in 1976 during the
construction of the Trans Alaska Pipeline in order to
examine the potential effects of an oil leak from the
pipeline (Collins et al., 1994). Two 7570 l spills (one in
February and one in July) of hot (57 C) Prudhoe Bay
crude oil were conducted in an open black spruce
(Picea mariana) forest. Samples were collected from
the winter spill site and from an adjacent reference site
in June 2001. The most likely mechanisms for oil loss
at this site are evaporation, biodegradation and
Fig. 9 Samples from the BIOS site, Nunavut, Canada. On the left are total ion mass chromatograms of the initially spilled oil and three samplesfrom the beach. On the right are the relative losses of representative saturate and aromatic compounds in the oil.
Fig. 8 Samples from Black Duck Cove, Nova Scotia, Canada. Thefigure shows the relative losses of alkylated polycyclic aromatichydrocarbons in the samples from Fig. 6.
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photooxidation. Loss by water-washing does not seem
an important fate, since there is no evidence for sig-
nificant water movements at the site and the spill area
did not expand significantly over the first 13 years
(Collins et al., 1994). Figure 11 (left panel) shows gas
chromatograms of the initially spilled oil and of three
samples collected from the site approximately 25 years
later. Perhaps surprisingly, the oil in the mineral soil
horizon (8–18 cm below the surface) is essentially
unchanged despite its 25 years in the environment; less
than 30% of the heptadecane and less than 5% of the
pristane has been lost, and the only significant loss
seems to be of the aromatics, including phenanthreneand dibenzothiophene and their alkyl derivatives (Fig.
11, right panel). Interestingly, this sample still contains
molecules such as dodecane that are thought to be
relatively readily lost by evaporation. In contrast, the
sample from the oiled organic soil horizon (0–8 cm
from the surface) is substantially weathered, having
lost 40% of its total hydrocarbon, and almost 90% of
its heptadecane and methylphenanthrene and methyl-
dibenzothiophene. We attribute most of this loss to
biodegradation. A sample from an oiled surface twig is
even more weathered. Figure 12 shows that neither
chrysene nor its alkyl substituted forms has been lostfrom the soil samples, consistent with quite limited
biodegradation and no significant photooxidation in
these heavily oiled samples (4.5% and 34% oil by
weight, respectively) that were overlain by 5 cm of
moss. Nevertheless, the oil from a surface twig has lost
substantial amounts of these compounds in a manner
consistent with photooxidation rather than biodegra-
dation. Rather similar results, with some samples re-
maining essentially unaltered after 25 years in the
environment, have been obtained by Wang et al.
(1998) in samples from the Nipisi spill near the Lesser
Slave Lakes in northern Alberta. Whether plants can
recolonize the Poker-Caribou site as natural weath-
ering proceeds remains to be seen. Already some
mosses and lichens are beginning to creep across the
surface from unoiled areas adjacent to the site, and it
is possible that the oiled layers may eventually be
Fig. 11 Samples from the Poker-Caribou Flats oil spill site. On the left are total ion mass chromatograms of the initially spilled oil and threesamples from the site. On the right are the relative losses of representative saturate and aromatic compounds in the oil.
Fig. 10 Samples from the BIOS site, Nunavut, Canada. The figureshows the relative losses of alkylated polycyclic aromatic hydro-carbons in the samples from Fig. 8.
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buried by these plants, and then provide a substratum
for succession of other species.
The Erika spill
The Erika, carrying about 30,000 tonnes of heavy
fuel oil as cargo, broke up in a severe storm off the
coast of Brittany on 11 December 1999 (Oudot, 2000).
The left hand panel of Fig. 13 presents a gas chro-
matogram of the cargo oil, and of a sample collected
on 29 March, 2000, at Le Croisic, less than four
(winter) months after the spill. The right hand panel
shows that almost 20% of the hydrocarbon had been
lost, even in this short time. The loss of naphthalenes
may well be due to evaporation, but the loss of hepta-
decane, the phenanthrenes and the dibenzothiophenes
is most likely due to biodegradation. As we saw in
Fig. 3, the fact that no chrysenes had been lost sug-
gests that biodegradation is just beginning, but also
suggests that biodegradation will eventually removemuch of the hydrocarbon content of the spilled oil.
The fate of the resins and asphaltenes, which make up
the majority of the spilled cargo, is less clear, as we will
discuss below.
Discussion
Crude oils and refined products are typically com-
posed of many thousands of individual compounds
(Tissot & Welte, 1984), and we have focussed on only
a few of them in this paper. Nevertheless the com-pounds we have discussed include the most abundant
and most biodegradable, the alkanes, and represen-
tative polycyclic hydrocarbons on the USEPA priority
pollutant list (Keith & Telliard, 1979), which are
usually of the most environmental concern. The ma-
jority of the other hydrocarbons are also biodegrad-
able (see McMillen et al., 1995; Prince, 2002), and it is
reasonable to expect that their biodegradation occurs
concomitantly with the alkanes and polycyclic aro-
matic hydrocarbons. Less is known about the bio-
degradation of many of the polar oil compounds,
including those commonly called resins and asphalt-
enes. Current knowledge suggests that these species
are not very biodegradable, nor are they subject to
significant photooxidative destruction, so they are
likely to remain in the environment for a long time.
Fortunately their inertness seems to be mirrored by
their environmental impact, and indeed in the absence
of hydrocarbons they are difficult to distinguish from
modern soil and sediment components such as humic
and fulvic acids (Burdon, 2001; Rice, 2001). They
completely lack the ‘‘oiliness’’ and ‘‘stickiness’’ asso-
ciated with crude and heavy fuel oils.
Fig. 13 A sample from the Erika spill. On the left are total ion mass chromatograms of the initially spilled oil and a sample from a beach. On theright are the relative losses of representative saturate and aromatic compounds in the oil.
Fig. 12 Samples from the Poker-Caribou Flats oil spill site. Thefigure shows the relative losses of alkylated polycyclic aromatichydrocarbons in the samples from Fig. 10.
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The samples described here show that both biode-
gradation and photooxidation play important roles in
the long-term weathering of crude and heavy fuel oilsin the environment. The most degraded sample de-
scribed from the Arrow Bunker C spill (Fig. 7) has lost
60% of its initial hydrocarbon, essentially all of its
parent, C1, C2 and C3 phenanthrenes, and more than
half of its C1 and C2 chrysenes, the latter by photo-
oxidation. The most degraded sample from the BIOS
crude oil experiment (Fig. 9) had lost 85% of its total
hydrocarbon, and the most degraded sample from the
Poker-Caribou Flats crude oil experiment had lost
70% of its hydrocarbon (Fig. 11). Biodegradation can
be quite rapid, as seen in the sample from the Erika,
which has lost significant amounts of phenanthrene in just a few winter months.
Nevertheless, at both the BIOS and Poker-Caribou
Creeks Research Watershed sites there are also still
some samples that are essentially unchanged from the
date of the spill. What causes this heterogeneity? At
BIOS the least degraded samples have probably never
been inundated by the tide, and so their biodegrada-
tion may be limited by the availability of water. At
Poker-Caribou Creeks Research Watershed the least
degraded samples are from very heavily oiled soil that
has been undisturbed since the spill, and biodegrada-
tion may be limited by inhospitably oily conditions.
But this is not the only heterogeneity that exists at
these sites. At BIOS, for example, the shoreline site
was oiled in 1981. By 1989 only some 20% was still
oiled (Owens et al., 1994), and this had decreased to
only some 5% by 2001. The remaining oil was patchily
distributed, and it is not obvious why some areas re-
mained oiled while adjacent areas were apparently
completely clean. It seems likely that the major loss of
oil was caused by physical factors, but why is the effect
so heterogeneous? The entire beach likely freezes in
the winter, and the areas that have lost oil do not seem
more exposed, or to be more subject to run-off, than
the areas with residual oil. Understanding both thecauses of the physical weathering processes, including
oil–fines interactions (Owens, 1999), and their hetero-
geneity, and integrating this with our understanding
of photooxidation and biodegradation, may allow us
to construct useful models to predict the long-term
weathering of spilled oil, but this remains a challeng-
ing goal.
Acknowledgements— We are grateful to Dr. Jean Oudot for theErika samples.
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