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The Roles of Photooxidation and Biodegradation in Long-term Weathering of Crude and Heavy Fuel Oils
Spill Science & Technology Bulletin, Vol. 8, No. 2, pp. 145–156, 2003 Ó 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
The Roles of Photooxidation and Biodegradation in Long-term Weathering of Crude and Heavy Fuel Oils ROGER C. PRINCE *, ROBERT M. GARRETT , RICHARD E. BARE , MATTHEW J. GROSSMAN , TODD TOWNSENDà, JOSEPH M. SUFLITAà, KENNETH LEE§, EDWARD H. OWENS , GARY A. SERGYàà, JOAN F. BRADDOCK§§, JON E. LINDSTROM & RICHARD R. LESSARDààà ExxonMobil Research and Engineering Co., Annandale, NJ 08801, USA àInstitute 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 ààEnvironment 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 àààExxonMobil Research and Engineering Co., Fairfax, VA 22037, USA
Although spilled oil is subject to a range of natural processes, only combustion, photooxidation and biodegradation destroy hydrocarbons and remove them from the biosphere. We present laboratory data that demonstrate the molecular preferences of these processes, and then examine some oil residues collected from previously documented releases to confirm the important roles that these processes play in removing spilled oil from both marine and terrestrial environments. Ó 2003 Elsevier Science Ltd. All rights reserved.
Introduction Crude and heavy fuel oils that escape into the environment, either from natural seeps or from accidental 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 atmosphere 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
*Corresponding author. E-mail address: [email protected] (R.C. Prince).
the flood-stage Rıo Desaguadero on the Bolivian Altiplano 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). Terrestrial 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 Alberta spill (Blenkinsopp et al., 1996). Some molecules, particularly aromatic hydrocarbons and small polar molecules such as naphthenic acids, dissolve if sufficient 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, 145
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dramatically increasing the surface area available for microbial colonization and biodegradation. This occurred 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 interaction of the oil with fine particles of sediment (Owens, 1999). The oil–fine particle aggregates disperse in the water column, often as neutrally buoyant particles, and the oil is more available for biodegradation (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 accepted response option in some situations, such as that of the New Carissa (Gallagher et al., 2001). The ultimate fate of spilled hydrocarbons that are not collected, 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 apparently leaves the majority of the resins and polar fractions 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 is important 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 remnants 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, 146
principally from work in the laboratory, and then use this information to implicate these processes in the transformation of oil spilled in marine and terrestrial environments.
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, polycyclic aromatic 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 environmental 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, determined with gas chromatography and mass spectrometry, 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 biodegradable, 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 anaerobic (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 compounds 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 Spill Science & Technology Bulletin 8(2)
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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 percent depletion of other analytes within the oil using the equation: %Loss ¼ ½ððA0 =H0 Þ ðAs =Hs ÞÞ=ðA0 =H0 Þ 100 Where: As and Hs are the concentrations of the target analyte and hopane in the oil sample, respectively, and A0 and H0 are the concentrations in the initially spilled oil. Recently Wang et al. (2001) have reported the apparent biodegradation of hopanes in oil remaining from the 1974 Metula spill, and Bost et al. (2001) have reported degradation of hopanes and norhopanes by a microbial consortium enriched from a creosotecontaminated site. Although we have seen no evidence that biodegradation of 17a(H)21b(H)hopane has occurred 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 by evaporation) 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 chromatograms (Fig. 1, left), but the illumination has a significant effect on the composition of the oil determined with thin layer chromatography. The saturates are unaffected, but the majority of the aromatic hydrocarbons have been converted to resins or polar molecules (Fig. 1, right). When the aromatic hydrocarbons are measured with selected ion monitoring GC/MS
Fig. 2 Photooxidation of an artificially weathered crude oil. The figure shows the relative losses of alkylated polycyclic aromatic hydrocarbons caused by exposure to a laboratory UV source. The C0, C1, C2, C3 nomenclature indicates the number of alkyl carbons on the parent molecule, regardless of position. For example, C2 includes dimethyl and ethyl forms.
(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 less susceptible to photooxidation than phenanthrene and dibenzothiophene, and recent work bears this out, even with substantial alkylation (Dutta & Harayama, 2001). Fortunately, although resistant to photooxidation, 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 before 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 Alaskan North Slope crude oil, before and after exposure to a laboratory UV source. On the right is the composition of the oil determined by thin layer chromatography. Spill Science & Technology Bulletin 8(2)
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Combustion of crude oil As noted above, burning is sometimes considered as an 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 removing 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 hydrocarbons 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 hundreds 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 hydrocarbons are known to be biodegraded, although individual strains of organisms typically degrade only a limited range of substrates. A typical aerobic biodegradation pattern for a hydrocarbon fuel is shown in Fig. 3––on the left, total ion GC/MS chromatograms 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-
Fig. 3 Aerobic biodegradation of a heavy fuel oil (IF30). On the left are total ion mass chromatograms of the oil at various times into the experiment, and on the right are the relative losses of representative saturate and aromatic compounds in the oil. 148
vidual compounds identified by selected ion monitoring GC/MS. Within one week the n-alkanes, exemplified here by heptadecane, were essentially completely removed, as were approximately 50% of the phenanthrene and dibenzothiophene. In contrast, only some 10% of the branched alkane pristane (2,6,10,14tetramethylpentadecane) 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 (Elmendorf et al., 1994). In summary, under aerobic conditions 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 opposite 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 dibenzothiophenes 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 methylphenanthrenes 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. 4 Aerobic biodegradation of a heavy fuel oil (IF30). The figure shows the relative losses of alkylated polycyclic aromatic hydrocarbons in the samples from Fig. 3, with similar nomenclature to Fig. 2. Spill Science & Technology Bulletin 8(2)
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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 percentage loss of the individual isomers in the field sample.
have seen no consistent pattern of preferential degradation 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. Nevertheless, Fayad and Overton (1995) have reported a quite different pattern of biodegradation in a laboratory experiment with mousse collected during the Gulf oil spill. In the absence of added nutrients (just indigenous nutrients from the Gulf seawater), and at 5 and 20 g oil per liter of seawater, they reported substantial 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 heterogeneous 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 biodegradation 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.
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 biodegradation has not been fully documented, and both the substrate range and preference of anaerobic hydrocarbon-degrading communities are largely unknown. 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 microorganisms from a terrestrial subsurface environment catalyze a similar range of crude oil n-alkane biodegradation under both sulfate-reducing and methanogenic conditions (Fig. 6). Compared to aerobic biodegradation, extensive degradation of branched alkanes and aromatic hydrocarbons seems to lag far behind that of the n-alkanes, and under optimal conditions, 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 biodegradation in both aerobic and anaerobic environments.
Anaerobic biodegradation of crude oil
The wreck of the Arrow in February 1970 released 2.5 million gallons of Bunker C fuel oil into Chedabucto 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
The anaerobic degradation of hydrocarbons has only been clearly demonstrated in the last decade or so (Heider et al., 1999; Spormann & Widdel, 2000), and Spill Science & Technology Bulletin 8(2)
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Fig. 6 Anaerobic biodegradation of an artificially weathered crude oil. On the left are total ion mass chromatograms of an artificially weathered Alaskan North Slope crude oil, before and after biodegradation under sulfate-reducing and methanogenic conditions. Inocula came from Fort Lupton, Colorado, and the cultures were incubated for approximately a year. On the right are the relative losses of representative saturated and aromatic compounds in the oil.
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 uninformed 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% respectively. 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 difference to more extensive evaporation of the surface sample, coupled with extensive biodegradation in both samples, and photooxidative loss of alkylated phenanthrenes 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 samples from the cove. On the right are the relative losses of representative saturate and aromatic compounds in the oil. 150
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Fig. 8 Samples from Black Duck Cove, Nova Scotia, Canada. The figure shows the relative losses of alkylated polycyclic aromatic hydrocarbons in the samples from Fig. 6.
Wang et al. (1995) examined oil samples collected in 1993 from this site, and Fig. 9 shows chromatograms of the spilled oil and of three samples collected in September 2001 (Prince et al., 2002). The hydrocarbon content of these three samples are very different, 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 phenanthrenes 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 pavement had lost only approximately 50% of its total hydrocarbon and only 50% of its pristane, but 40% of its methylchrysenes. We attribute this to rather less extensive biodegradation coupled with rather more extensive photooxidation, and the relative losses of the alkylated forms of chrysene bear this out (Fig. 10).
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 (72°310 N, 79°500 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 approximately 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).
The Poker-Caribou Creeks Research Watershed experiment 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 samples from the beach. On the right are the relative losses of representative saturate and aromatic compounds in the oil. Spill Science & Technology Bulletin 8(2)
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Fig. 10 Samples from the BIOS site, Nunavut, Canada. The figure shows the relative losses of alkylated polycyclic aromatic hydrocarbons in the samples from Fig. 8.
photooxidation. Loss by water-washing does not seem an important fate, since there is no evidence for significant 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 phenanthrene and 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 methyldibenzothiophene. 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 lost from 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 biodegradation. Rather similar results, with some samples remaining 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 weathering 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 three samples from the site. On the right are the relative losses of representative saturate and aromatic compounds in the oil. 152
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Fig. 3, the fact that no chrysenes had been lost suggests that biodegradation is just beginning, but also suggests that biodegradation will eventually remove much 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
Fig. 12 Samples from the Poker-Caribou Flats oil spill site. The figure shows the relative losses of alkylated polycyclic aromatic hydrocarbons in the samples from Fig. 10.
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 chromatogram 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 heptadecane, the phenanthrenes and the dibenzothiophenes is most likely due to biodegradation. As we saw in
Crude oils and refined products are typically composed of many thousands of individual compounds (Tissot & Welte, 1984), and we have focussed on only a few of them in this paper. Nevertheless the compounds we have discussed include the most abundant and most biodegradable, the alkanes, and representative polycyclic hydrocarbons on the USEPA priority pollutant list (Keith & Telliard, 1979), which are usually of the most environmental concern. The majority of the other hydrocarbons are also biodegradable (see McMillen et al., 1995; Prince, 2002), and it is reasonable to expect that their biodegradation occurs concomitantly with the alkanes and polycyclic aromatic hydrocarbons. Less is known about the biodegradation of many of the polar oil compounds, including those commonly called resins and asphaltenes. 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’’ associated 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 the right are the relative losses of representative saturate and aromatic compounds in the oil. Spill Science & Technology Bulletin 8(2)
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The samples described here show that both biodegradation and photooxidation play important roles in the long-term weathering of crude and heavy fuel oils in the environment. The most degraded sample described 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 photooxidation. 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 biodegradation 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 biodegradation 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 remained 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 the causes of the physical weathering processes, including oil–fines interactions (Owens, 1999), and their heterogeneity, 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 challenging goal. Acknowledgements—We are grateful to Dr. Jean Oudot for the Erika samples.
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Barman, B.N., 1996. Hydrocarbon-type analysis of base oils and other heavy distillates by thin-layer chromatography with flameionization detection and by the clay–gel method. Journal of Chromatographic Science 34, 219–225. Blenkinsopp, S., Sergy, G., Lambert, P., Wang, Z., Zoltai, S.C., Sitanen, M., 1996. Long-term recovery of peat bogs oiled by pipeline spills in northern Alberta. In: Proceedings of the Nineteenth Arctic and Marine Oil Spill Program Technical Seminar. Environment Canada, Calgary, Alberta, pp. 1335– 1354. Boehm, P.D., Steinhauer, M.S., Green, D.R., Fowler, B., Humphrey, B., Fiest, D.L., Cretney, W.J., 1987. Comparative fate of chemically dispersed and beached crude oil in subtidal sediments of the Arctic nearshore. Arctic 40 (1), 133–148. Bost, F.D., Frontera-Suau, R., McDonald, T.J., Peters, K.E., Morris, P.J., 2001. Aerobic biodegradation of hopanes and norhopanes in Venezuelan crude oils. Organic Geochemistry 32, 105–114. Braddock, J.F., Ruth, M.L., Catterall, P.H., Walworth, J.L., McCarthy, K.A., 1997. Enhancement and inhibition of microbial activity in hydrocarbon-contaminated arctic soils: implications for nutrient-amended bioremediation. Environmental Science & Technology 31, 2078–2084. Buist, I., McCourt, J., Potter, S., Ross, S., Trudel, K., 1999. In situ burning. Pure & Applied Chemistry 71, 43–66. Burdon, J., 2001. Are the traditional concepts of the structures of humic substances realistic? Soil Science 166, 752–769. Burns, K.A., Codi, S., Duke, N.C., 2000. Gladstone, Australia field studies: weathering and degradation of hydrocarbons in oiled mangrove and salt marsh sediments with and without the application of an experimental bioremediation protocol. Marine Pollution Bulletin 41, 392–402. Caldwell, M.E., Garrett, R.M., Prince, R.C., Suflita, J.M., 1998. Anaerobic biodegradation of long-chain n-alkanes under sulfatereducing conditions. Environmental Science & Technology 32, 2191–2195. Collins, C.M., Racine, C.H., Walsh, M.E., 1994. The physical, chemical and biological effects of crude oil spills after 15 years on a black spruce forest, interior Alaska. Arctic 47, 164–175. Douglas, G.S., McCarthy, K.J., Dahlen, D.T., Seavey, J.A., Steinhauer, W.G., Prince, R.C., Elmendorf, D.L., 1992. The use of hydrocarbon analyses for environmental assessment and remediation. Journal of Soil Contamination 1, 197–216. Douglas, G.S., Owens, E.H., Hardenstine, J., Prince, R.C., 2002. The OSSAII pipeline spill: the character and weathering of the spilled oil. Spill Science and Technology Bulletin 7, 135– 148. Dutta, T.K., Harayama, S., 2001. Analysis of long side chain alkylaromatics in crude oil for evaluation of their fate in the environment. Environmental Science & Technology 35, 102– 107. Elmendorf, D.L., Haith, C.E., Douglas, G.S., Prince, R.C., 1994. Relative rates of biodegradation of substituted polycyclic aromatic hydrocarbons. In: Hinchee, R.E., Leeson, A., Semprini, L., Ong, S.K. (Eds.), Bioremediation of Chlorinated and Polycyclic Aromatic Hydrocarbon Compounds. Lewis Publishers, Boca Raton, FL, pp. 188–202. Fayad, N.M., Overton, E., 1995. A unique biodegradation pattern of the oil spilled during the 1991 Gulf War. Marine Pollution Bulletin 30, 239–246. Fingas, M.F., 1999. The evaporation of oil spills: development and implementation of new prediction methodology. In: Proceedings of the 1999 International Oil Spill Conference. American Petroleum Institute, Washington, DC, pp. 185–194. Fingas, M.F., Fieldhouse, B., Lane, J., Mullin, J.V., 2001. What causes the formation of water-in-oil emulsions. In: Proceedings of the 2001 International Oil Spill Conference. American Petroleum Institute, Washington, DC, pp. 109–114. Fiocco, R.J., Lewis, A., 1999. Oil spill dispersants. Pure and Applied Chemistry 71, 27–42. Gallagher, J.J., Hile, H.B., Miller, J.A., 2001. The old New Carissa; a study in patience. In: Proceedings of the 2001 Oil Spill Conference. American Petroleum Institute, Washington, DC, pp. 85–90.
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Garrett, R.M., Pickering, I.J., Haith, C.E., Prince, R.C., 1998. Photooxidation of crude oils. Environmental Science & Technology 32, 3719–3723. Garrett, R.M., Guenette, C.C., Haith, C.E., Prince, R.C., 2000. Pyrogenic polycyclic aromatic hydrocarbons in oil burn residues. Environmental Science & Technology 34, 1934–1937. Heider, J., Spormann, A.M., Beller, H.R., Widdel, F., 1999. Anaerobic bacterial metabolism of hydrocarbons. FEMS Microbiology Review 22, 459–473. Herman, D.C., Fedorak, P.M., MacKinnon, M.D., Costerton, J.W., 1994. Biodegradation of naphthenic acids by microbial populations indigenous to oil sands tailings. Canadian Journal of Microbiology 40, 467–477. Hurley, M.D., Sokolov, O., Wallington, T.J., Takekawa, H., Karasawa, M., Klotz, B., Barnes, I., Becker, K.H., 2001. Organic aerosol formation during the atmospheric degradation of toluene. Environmental Science & Technology 35, 1358– 1366. Keith, L.H., Telliard, W.A., 1979. Priority Pollutants I.––a perspective view. Environmental Science & Technology 13, 416–423. Lafargue, E., Le Thiez, P., 1996. Effect of water washing on light ends compositional heterogeneity. Organic Geochemistry 24, 1141–1150. Lessard, R.R., DeMarco, G., 2000. The significance of oil spill dispersants. Spill Science & Technology Bulletin 6, 59–68. Lin, Q.X., Mendelssohn, I.A., Carney, K., Bryner, N.P., Walton, W.D., 2002. Salt marsh recovery and oil spill remediation after in-situ burning: Effects of water depth and burn duration. Environmental Science & Technology 36, 576–581. Lunel, T., Rusin, J., Halliwell, C., Davies, L., 1997. The net environmental benefit of a successful dispersant operation at the Sea Empress incident. In: Proceedings of the 1997 International Oil Spill Conference. American Petroleum Institute, Washington, DC, pp. 185–194. Martinelli, M., Luise, A., Tromellini, E., Sauer, T.C., Neff, J.M., Douglas, G.S., 1995. The M/C Haven oil spill: Environmental assessment of exposure pathways and resource injury. In: Proceedings of the 1995 Oil Spill Conference. American Petroleum Institute, Washington, DC, pp. 679–685. McMillen, S.J., Requejo, A.G., Young, G.N., Davis, P.S., Cook, P.D., Kerr, J.M., Gray, N.R., 1995. Bioremediation potential of crude oil spilled on soil. In: Hinchee, R.E., Vogel, C.M., Brockman, F.J. (Eds.), Microbial Processes for Bioremediation. Battelle Press, Columbus, OH, pp. 91–99. Oudot, J., 2000. Biodegradability of the Erika fuel oil. Comptes Rendus de LÕAcademie Des Sciences Serie III––Sciences de La Vie––Life Sciences 323, 945–950. Owens, E.H., 1999. The interaction of fine particles with stranded oil. Pure and Applied Chemistry 71, 83–93. Owens, E.H., Sergy, G.A., McGuire, B.E., Humphrey, B., 1993. The 1970 Arrow Spill: what remains on the shoreline 22 years later. In: Proceedings of the Sixteenth Arctic and Marine Oil Spill Program Technical Seminar. Environment Canada, Calgary, Alberta, pp. 1149–1167. Owens, E.H., Humphrey, B., Sergy, G.A., 1994. Natural cleaning of oiled coarse sediment shorelines in Arctic and Atlantic Canada. Spill Science & Technology Bulletin 1, 37–52. Owens, E.H., Sergy, G.A., Prince, R.C., 2002. The fate of stranded oil at the BIOS site twenty years after the experiment. In: Proceedings of the Twentyfifth Arctic and Marine Oil Spill Program Technical Seminar. Environment Canada, Calgary, Alberta, pp. 1–11. Page, C.A., Bonner, J.S., Sumner, P.L., McDonald, T.J., Autenrieth, R.L., Fuller, C.B., 2000. Behavior of a chemically-dispersed oil and a whole oil on a near-shore environment. Water Research 34, 2507–2516. Payne, J.R., Clayton, J.R., McNabb, G.D., Kirstein, B.E., 1991. Exxon Valdez oil weathering and behavior, model predictions and field observations. In: Proceedings of the 1991 International Oil Spill Conference. American Petroleum Institute, Washington, DC, pp. 641–654. Peters, K.E., Moldowan, J.M., 1993. The Biomarker Guide; Interpreting molecular fossils in petroleum and ancient sediments. Prentice-Hall, Englewood Cliffs, NJ.
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Poisson, N., Kanakidou, M., Crutzen, P.J., 2000. Impact of nonmethane hydrocarbons on tropospheric chemistry and the oxidizing power of the global troposphere: 3-dimensional modelling results. Journal of Atmospheric Chemistry 36, 157–230. Prince, R.C., 2002. Petroleum and other hydrocarbons, biodegradation of. In: Bitton, G. (Ed.), Encyclopedia of Environmental Microbiology. John Wiley, New York, pp. 2402–2416. Prince, R.C., Bragg, J.R., 1997. Shoreline bioremediation following the Exxon Valdez oil spill in Alaska. Bioremediation Journal 1, 97–104. Prince, R.C., Elmendorf, D.L., Lute, J.R., Hsu, C.S., Haith, C.E., Senius, J.D., Dechert, G.J., Douglas, G.S., Butler, E.L., 1994. 17a(H),21b(H)-hopane as a conserved internal marker for estimating the biodegradation of crude oil. Environmental Science & Technology 28, 142–145. Prince, R.C., Drake, E.N., Madden, P.C., Douglas, G.S., 1997. Biodegradation of polycyclic aromatic hydrocarbons in a historically contaminated site. In: Alleman, B.C., Leeson, A. (Eds.), In situ and on-site bioremediation 2. Battelle Press, Columbus, OH, pp. 205–210. Prince, R.C., Garrett, R.M., Haith, C.E., Vandermeulen, J.H., Cobanli, S., Mossman, D.C., Lee, K., 1998. The role of biodegradation in the weathering of oil from the 1970 Arrow spill. In: Proceedings of the Twentyfirst Arctic and Marine Oil Spill Program Technical Seminar. Environment Canada, Calgary, Alberta, pp. 717–727. Prince, R.C., Owens, E.H., Sergy, G.A., 2002. Weathering of an Arctic oil spill over 20 years: the BIOS experiment revisited. Marine Pollution Bulletin 44, 1236–1242. Prince, R.C., Stibrany, R.T., Hardenstine, J., Douglas, G.S., Owens, E.H., 2002. Aqueous Vapor Extraction; a previously unrecognized weathering process for oil spills in vigorously aerated water. Environmental Science & Technology 36, 2822–2825. Radwan, S.S., Al Mailem, D., El Nemr, I., Salamah, S., 2000. Enhanced remediation of hydrocarbon contaminated desert soil fertilized with organic carbons. International Biodeterioration & Biodegradation 46, 129–132. Rice, J.A., 2001. Humin. Soil Science 166, 848–857. Sergy, G.A., Blackall, P.J., 1987. Design and conclusion of the Baffin Island Oil Spill project. Arctic 40 (Suppl. 1), 1–9. Sergy, G., Guenette, C.C., Owens, E., Prince, R.C., Lee, K., 1999. Treatment of oiled sediment shorelines by sediment relocation. In: Proceedings of the 1999 International Oil Spill Conference. American Petroleum Institute, Washington, DC, pp. 549–554. Sharma, V.K., Hicks, S.D., Rivera, W., Vazquez, F.G., 2002. Characterization and degradation of petroleum hydrocarbons following an oil spill into a coastal environment of south Texas, USA. Water Air and Soil Pollution 134, 111–127. Spormann, A.M., Widdel, F., 2000. Metabolism of alkylbenzenes, alkanes, and other hydrocarbons in anaerobic bacteria. Biodegradation 11, 85–105. Thomas, D., 1993. The Braer incident at first hand. Petroleum Review (UK) 47, 263–265. Thomas, R., Lunel, T., 1993. The Braer Incident; dispersion in action. In: Proceedings of the Sixteenth Arctic Marine Oilspill Program Technical Seminar, Edmonton, Alberta, pp. 843–859. Tissot, B.P., Welte, D.H., 1984. Petroleum Formation and Occurrence. Springer-Verlag, Berlin. Wang, Z., Fingas, M., Sergy, G., 1994. Study of 22-year-old Arrow oil samples using biomarker compounds by GC/MS. Environmental Science & Technology 28 (9), 1733–1746. Wang, Z., Fingas, M.F., Sergy, G., 1995. Chemical characterization of crude oil residues from an Arctic Beach by GC/MS and GC/ FID. Environmental Science & Technology 29, 2622–2631. Wang, Z., Fingas, M., Blenkinsopp, S., Sergy, G., Landriault, M., Sigouin, L., Lambert, P., 1998. Study of the 25-year-old Nipisi Oil Spill: persistence of oil residues and comparisons between surface and subsurface sediments. Environmental Science & Technology 32, 2222–2232. Wang, Z., Fingas, M.F., Sigouin, L., Owens, E.H., 2001. Fate and persistence of long-term spilled ÔMetulaÕ oil in the marine salt marsh: degradation of biomarkers. In: Proceedings of the 2001 International Oil Spill Conference. American Petroleum Institute, Washington, DC, pp. 115–125.
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doi:10.1016/S1353-2561(03)00017-3
The Roles of Photooxidation and Biodegradation in Long-term Weathering of Crude and Heavy Fuel Oils ROGER C. PRINCE *, ROBERT M. GARRETT , RICHARD E. BARE , MATTHEW J. GROSSMAN , TODD TOWNSENDà, JOSEPH M. SUFLITAà, KENNETH LEE§, EDWARD H. OWENS , GARY A. SERGYàà, JOAN F. BRADDOCK§§, JON E. LINDSTROM & RICHARD R. LESSARDààà ExxonMobil Research and Engineering Co., Annandale, NJ 08801, USA àInstitute 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 ààEnvironment 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 àààExxonMobil Research and Engineering Co., Fairfax, VA 22037, USA
Although spilled oil is subject to a range of natural processes, only combustion, photooxidation and biodegradation destroy hydrocarbons and remove them from the biosphere. We present laboratory data that demonstrate the molecular preferences of these processes, and then examine some oil residues collected from previously documented releases to confirm the important roles that these processes play in removing spilled oil from both marine and terrestrial environments. Ó 2003 Elsevier Science Ltd. All rights reserved.
Introduction Crude and heavy fuel oils that escape into the environment, either from natural seeps or from accidental 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 atmosphere 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
*Corresponding author. E-mail address: [email protected] (R.C. Prince).
the flood-stage Rıo Desaguadero on the Bolivian Altiplano 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). Terrestrial 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 Alberta spill (Blenkinsopp et al., 1996). Some molecules, particularly aromatic hydrocarbons and small polar molecules such as naphthenic acids, dissolve if sufficient 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, 145
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dramatically increasing the surface area available for microbial colonization and biodegradation. This occurred 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 interaction of the oil with fine particles of sediment (Owens, 1999). The oil–fine particle aggregates disperse in the water column, often as neutrally buoyant particles, and the oil is more available for biodegradation (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 accepted response option in some situations, such as that of the New Carissa (Gallagher et al., 2001). The ultimate fate of spilled hydrocarbons that are not collected, 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 apparently leaves the majority of the resins and polar fractions 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 is important 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 remnants 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, 146
principally from work in the laboratory, and then use this information to implicate these processes in the transformation of oil spilled in marine and terrestrial environments.
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, polycyclic aromatic 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 environmental 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, determined with gas chromatography and mass spectrometry, 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 biodegradable, 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 anaerobic (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 compounds 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 Spill Science & Technology Bulletin 8(2)
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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 percent depletion of other analytes within the oil using the equation: %Loss ¼ ½ððA0 =H0 Þ ðAs =Hs ÞÞ=ðA0 =H0 Þ 100 Where: As and Hs are the concentrations of the target analyte and hopane in the oil sample, respectively, and A0 and H0 are the concentrations in the initially spilled oil. Recently Wang et al. (2001) have reported the apparent biodegradation of hopanes in oil remaining from the 1974 Metula spill, and Bost et al. (2001) have reported degradation of hopanes and norhopanes by a microbial consortium enriched from a creosotecontaminated site. Although we have seen no evidence that biodegradation of 17a(H)21b(H)hopane has occurred 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 by evaporation) 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 chromatograms (Fig. 1, left), but the illumination has a significant effect on the composition of the oil determined with thin layer chromatography. The saturates are unaffected, but the majority of the aromatic hydrocarbons have been converted to resins or polar molecules (Fig. 1, right). When the aromatic hydrocarbons are measured with selected ion monitoring GC/MS
Fig. 2 Photooxidation of an artificially weathered crude oil. The figure shows the relative losses of alkylated polycyclic aromatic hydrocarbons caused by exposure to a laboratory UV source. The C0, C1, C2, C3 nomenclature indicates the number of alkyl carbons on the parent molecule, regardless of position. For example, C2 includes dimethyl and ethyl forms.
(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 less susceptible to photooxidation than phenanthrene and dibenzothiophene, and recent work bears this out, even with substantial alkylation (Dutta & Harayama, 2001). Fortunately, although resistant to photooxidation, 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 before 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 Alaskan North Slope crude oil, before and after exposure to a laboratory UV source. On the right is the composition of the oil determined by thin layer chromatography. Spill Science & Technology Bulletin 8(2)
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Combustion of crude oil As noted above, burning is sometimes considered as an 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 removing 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 hydrocarbons 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 hundreds 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 hydrocarbons are known to be biodegraded, although individual strains of organisms typically degrade only a limited range of substrates. A typical aerobic biodegradation pattern for a hydrocarbon fuel is shown in Fig. 3––on the left, total ion GC/MS chromatograms 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-
Fig. 3 Aerobic biodegradation of a heavy fuel oil (IF30). On the left are total ion mass chromatograms of the oil at various times into the experiment, and on the right are the relative losses of representative saturate and aromatic compounds in the oil. 148
vidual compounds identified by selected ion monitoring GC/MS. Within one week the n-alkanes, exemplified here by heptadecane, were essentially completely removed, as were approximately 50% of the phenanthrene and dibenzothiophene. In contrast, only some 10% of the branched alkane pristane (2,6,10,14tetramethylpentadecane) 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 (Elmendorf et al., 1994). In summary, under aerobic conditions 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 opposite 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 dibenzothiophenes 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 methylphenanthrenes 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. 4 Aerobic biodegradation of a heavy fuel oil (IF30). The figure shows the relative losses of alkylated polycyclic aromatic hydrocarbons in the samples from Fig. 3, with similar nomenclature to Fig. 2. Spill Science & Technology Bulletin 8(2)
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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 percentage loss of the individual isomers in the field sample.
have seen no consistent pattern of preferential degradation 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. Nevertheless, Fayad and Overton (1995) have reported a quite different pattern of biodegradation in a laboratory experiment with mousse collected during the Gulf oil spill. In the absence of added nutrients (just indigenous nutrients from the Gulf seawater), and at 5 and 20 g oil per liter of seawater, they reported substantial 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 heterogeneous 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 biodegradation 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.
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 biodegradation has not been fully documented, and both the substrate range and preference of anaerobic hydrocarbon-degrading communities are largely unknown. 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 microorganisms from a terrestrial subsurface environment catalyze a similar range of crude oil n-alkane biodegradation under both sulfate-reducing and methanogenic conditions (Fig. 6). Compared to aerobic biodegradation, extensive degradation of branched alkanes and aromatic hydrocarbons seems to lag far behind that of the n-alkanes, and under optimal conditions, 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 biodegradation in both aerobic and anaerobic environments.
Anaerobic biodegradation of crude oil
The wreck of the Arrow in February 1970 released 2.5 million gallons of Bunker C fuel oil into Chedabucto 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
The anaerobic degradation of hydrocarbons has only been clearly demonstrated in the last decade or so (Heider et al., 1999; Spormann & Widdel, 2000), and Spill Science & Technology Bulletin 8(2)
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Fig. 6 Anaerobic biodegradation of an artificially weathered crude oil. On the left are total ion mass chromatograms of an artificially weathered Alaskan North Slope crude oil, before and after biodegradation under sulfate-reducing and methanogenic conditions. Inocula came from Fort Lupton, Colorado, and the cultures were incubated for approximately a year. On the right are the relative losses of representative saturated and aromatic compounds in the oil.
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 uninformed 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% respectively. 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 difference to more extensive evaporation of the surface sample, coupled with extensive biodegradation in both samples, and photooxidative loss of alkylated phenanthrenes 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 samples from the cove. On the right are the relative losses of representative saturate and aromatic compounds in the oil. 150
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Fig. 8 Samples from Black Duck Cove, Nova Scotia, Canada. The figure shows the relative losses of alkylated polycyclic aromatic hydrocarbons in the samples from Fig. 6.
Wang et al. (1995) examined oil samples collected in 1993 from this site, and Fig. 9 shows chromatograms of the spilled oil and of three samples collected in September 2001 (Prince et al., 2002). The hydrocarbon content of these three samples are very different, 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 phenanthrenes 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 pavement had lost only approximately 50% of its total hydrocarbon and only 50% of its pristane, but 40% of its methylchrysenes. We attribute this to rather less extensive biodegradation coupled with rather more extensive photooxidation, and the relative losses of the alkylated forms of chrysene bear this out (Fig. 10).
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 (72°310 N, 79°500 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 approximately 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).
The Poker-Caribou Creeks Research Watershed experiment 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 samples from the beach. On the right are the relative losses of representative saturate and aromatic compounds in the oil. Spill Science & Technology Bulletin 8(2)
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Fig. 10 Samples from the BIOS site, Nunavut, Canada. The figure shows the relative losses of alkylated polycyclic aromatic hydrocarbons in the samples from Fig. 8.
photooxidation. Loss by water-washing does not seem an important fate, since there is no evidence for significant 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 phenanthrene and 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 methyldibenzothiophene. 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 lost from 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 biodegradation. Rather similar results, with some samples remaining 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 weathering 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 three samples from the site. On the right are the relative losses of representative saturate and aromatic compounds in the oil. 152
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Fig. 3, the fact that no chrysenes had been lost suggests that biodegradation is just beginning, but also suggests that biodegradation will eventually remove much 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
Fig. 12 Samples from the Poker-Caribou Flats oil spill site. The figure shows the relative losses of alkylated polycyclic aromatic hydrocarbons in the samples from Fig. 10.
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 chromatogram 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 heptadecane, the phenanthrenes and the dibenzothiophenes is most likely due to biodegradation. As we saw in
Crude oils and refined products are typically composed of many thousands of individual compounds (Tissot & Welte, 1984), and we have focussed on only a few of them in this paper. Nevertheless the compounds we have discussed include the most abundant and most biodegradable, the alkanes, and representative polycyclic hydrocarbons on the USEPA priority pollutant list (Keith & Telliard, 1979), which are usually of the most environmental concern. The majority of the other hydrocarbons are also biodegradable (see McMillen et al., 1995; Prince, 2002), and it is reasonable to expect that their biodegradation occurs concomitantly with the alkanes and polycyclic aromatic hydrocarbons. Less is known about the biodegradation of many of the polar oil compounds, including those commonly called resins and asphaltenes. 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’’ associated 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 the right are the relative losses of representative saturate and aromatic compounds in the oil. Spill Science & Technology Bulletin 8(2)
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The samples described here show that both biodegradation and photooxidation play important roles in the long-term weathering of crude and heavy fuel oils in the environment. The most degraded sample described 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 photooxidation. 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 biodegradation 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 biodegradation 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 remained 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 the causes of the physical weathering processes, including oil–fines interactions (Owens, 1999), and their heterogeneity, 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 challenging goal. Acknowledgements—We are grateful to Dr. Jean Oudot for the Erika samples.
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