years in the Egyptian North Western Desert (Barakat et al., 2001). These oil residues were released from the maintenance drainage of floating storage tanks and.
Environmental Forensics (2002) 3, 219±225 doi:10.1006/enfo.2002.0095, available online at http://www.idealibrary.com on
Compositional Changes of Aromatic Steroid Hydrocarbons in Naturally Weathered Oil Residues in the Egyptian Western Desert A. O. Barakat* Department of Environmental Sciences, Faculty of Science, Alexandria University, Alexandria, Egypt
Y. Qian, M. Kim and M. C. Kennicutt II Geochemical and Environmental Research Group, Texas A&M University College Station, TX 77845, U.S.A. (Received May 2002, Revised manuscript accepted July 2002) Naturally weathered oil residues from an arid dumpsite in Al-Alamein, Egypt were analyzed for monoaromatic and triaromatic steranes to demonstrate the utility of biomarker compounds in assessing the chemical composition changes during the degradation of the released oil residues in a terrestrial environment. The characterizations of individual aromatic compounds were based on gas chromatography/mass spectrometry (GC/MS) analyses. The results showed that triaromatic sterane distributions were similar in the oil residues of varying weathering degradation extents and correlated with a fresh crude oil sample of the Western Desert-sourced oil. Molecular ratios of triaromatic sterane compounds (ratios of C28 20R/C28 20S, C27 20R/C28 20R, and C28 20S/[C26 20R C27 20S]) were proved to be suitable for source identi®cation. Major changes in chemical compositions during weathering of the oil residues were the depletion of short chain mono- and tri-aromatic steranes in samples that had undergone extensive degradation. The results of triaromatic sterane distribution are in good agreement with weathering classi®cation based on the analyses of saturate and aromatic hydrocarbons and the ratios of n-alkanes, PAHs and saturate biomarker compounds. # 2002 AEHS. Published by Elsevier Science Ltd. All rights reserved. Keywords: oil spill; biomarkers; chemical ®ngerprinting; source parameters; weathering parameters.
Introduction
the biosphere is aromatic steranes. The distribution of these geochemical markers has been increasingly used as a ®ngerprint for the study of the maturation of the organic matter of sediments (Mackenzie et al., 1981) as well as for the correlation of crude oils and source rocks (Seifert and Moldowan, 1978). In a previous article, we studied the eect of weathering on oil residues spilled over a period of 15 years in the Egyptian North Western Desert (Barakat et al., 2001). These oil residues were released from the maintenance drainage of ¯oating storage tanks and from ballast waters of tankers operating between EI-Hamra terminal in AI-Alamein and Alexandria Harbor. The accumulated residue oil at a 20 acre dump site was collected from ®ve locations from dierent depths. The previous study focused on the quantitative determination of n-alkanes, isoprenoid alkanes, polycyclic hydrocarbons, and sterane and terpane biomarkers. The data were discussed in terms of the source speci®city and weathering trends in molecular ratios of individual aliphatic and aromatic compounds. This study extends the earlier investigation by studying the stability of mono- and tri-aromatic sterane biomarkers during weathering. The main objectives of this work were to determine the compositional changes of aromatic steranes as a result of physical, chemical and biological processes in the terrestrial
In recent years, there have been many studies on the source and fate of hydrocarbon contamination in the environment (e.g. Bence et al., 1996; Boehm et al., 1997; Hostettler and Kvenvolden, 1994; Hostettler et al., 1999; Le DreÂau et al., 1997; Sauer et al., 1993, 1998; Wang et al., 1994, 1995). Source identi®cation of spilled oils becomes increasingly dicult with the lapse of time due to changes in oil compositions caused by exposure to light, air, water and microorganisms. The most common approach to the characterization of a spilled oil and identifcation of its potential source relies on analysis with gas chromatography (GC) and gas chromatography/mass spectrometry (GC/MS) of aliphatic and/or aromatic hydrocarbons. Correlations are made on the basis of the molecular distribution of aliphatic and aromatic compounds and aromatic compounds and ratios of source speci®c compounds, known as `biomarkers', which have distinctive chemical structures that are closely related to the organic compounds produced by plants, bacteria and algae. One important class of biomarkers that were formed during diagenesis and maturation of sterols in *Author for correspondence. Tel.: 203-54632500; E-mail: abarakat@ dataxprs.com.eg
219 1527-5922/02/030219+07 $35.00/00
# 2002 AEHS. Published by Elsevier Science Ltd. All rights reserved.
220 A. O. Barakat et al.
environment by quantitatively determining the concentrations of major compounds. Con®rming the stability of these compounds under severe natural environmental conditions would be valuable for their use as internal tracking standards in determining the source and types of degraded petroleum hydrocarbons (Prince et al., 1994).
Experimental Sample Collection Oil residue samples from ®ve sites at the spill location were collected and stored frozen until analysis. A total of 14 surface and subsurface samples were collected from dierent depths up to 30 cm (total accumulation depth over the discharge history). A sample of fresh Western Desert-sourced oil, provided by the Western Desert Petroleum Company (WEPCO), was also analyzed with the sample set. A brief description of the samples and results for bulk hydrocarbon composition and selected compound ratios of saturate and aromatic compounds are given in Table 1. Extraction and Separation Oil residue/soil mixture samples (5±100 mg) were extracted with high purity dichloromethane for 15 min by sonication. The extracts were concentrated and the total extractable organic matter (EOM) was determined gravimetrically. An aliquot of the concentrated extract was treated with n-pentane to precipitate asphaltene from the extract. About 5±50 mg of the extract was forti®ed with hexane solutions of surrogate standards containing d10-phenanthrene and d10-deuterated chrysene. The forti®ed extract was puri®ed by alumina column chromatography. The eluent was evaporated to a ®nal volume of about 0.5 ml under puri®ed nitrogen and separated into aliphatic and aromatic fractions using a high performance liquid chromatography (HPLC) method described by McDonald and Kennicutt (1992). The separation was performed on a 25 cm 9.4 mm (i.d.) cyano/ amino propylsilane bonded preparative column (Whatman). A Hewlett±Packard (Palo Alto, California) 1050
HPLC system with an autosampler and quaternary pump interfaced to a Perkin±Elmer (Norwalk, Connecticut) Model 25 Refractive Index Detector were used. A linear solvent gradient from 100% hexane with a 2 min hold time, to 100% dichloromethane with an 8 min hold time, followed by an ethyl acetate cleanup step was employed. An internal standard was added (d10-¯uorene) to each sample prior to instrumental analyses to monitor recovery of surrogates. Gas Chromatography±Mass Spectrometry (GC/MS) The GC/MS analysis of the aromatic biomarker fractions were performed with a Hewlett±Packard 5890-GC interfaced to a Hewlett±Packard 5970 mass spectrometry detector (MSD) operated in the selected ion monitoring mode (SIM). Separation was accomplished on a 30 m 0.32 mm, 0.25 mm ®lm thickness DB-5 fused silica capillary column (J&W Scienti®c Inc). The samples were injected in the splitless mode and GC temperature was raised from 40 to 2808C at 108C/min with a ®nal holding time of 16 min. The procedure used for quanti®cation of individual biomarker compounds has been described elsewhere (McDonald and Kennicutt, 1992). The GC/MS was calibrated with solutions of standard compounds at ®ve concentrations prior to sample analysis. A response factor of one of more analytes of similar structure and molecular weight was used for biological markers when no authentic standard was available. A standard reference oil sample was analyzed as part of the internal laboratory QA/QC procedures. Compound identi®cation was based on comparison of the retention times of the target compounds with standard compounds and with a reference oil.
Results and Discussion The results of bulk hydrocarbon composition analyses (Table 1) indicated that the samples exhibited a considerable amount of compositional heterogeneity and were at various stages of alterations. Detailed discussions of the eect of weathering on the composition and distribution of alkanes, polynclear aromatic
Table 1. Sample description and bulk properties of the oil residue samples (Barakat et al., 2001) Sample (depth, cm)
Description
S-1 (0±1) S-1 (2±3) S-1 (13±15) S-1 (27±30) S-2 (0±1) S-2 (6±8) S-2 (13±15) S-3 (0±1) S-3 (5±7) S-3 (12±15) S-4 (surface) S-5 (1±7) S-5 (7±15) S-5 (20±25) Source oil
Oily black clay Oily black clay Oily black clay Oily black clay Yellowish ®ne sediment with coarse sand Oily black shales Grey oily ®ne sediment Oily black clay Oily black clay Yellowish/grey ®ne sediment Very hard black asphaltic material Black/dark brown sticky tar Dark gray sticky tar Yellowish sticky ®ne sediment
n-Alkanes mg/g EOM 45.0 60.0 76.6 70.2 42.2 73.7 34.6 48.7 63.3 50.7 1.7 50.6 4.8 1.4 138
Total PAHs mg/g EOM 3559 20491 8566 6048 1209 4279 2502 2311 3939 2398 97 4018 1994 247 60136
TRP mg/g EOM 65.3 101.2 117.7 104.2 56.3 101.2 54.8 66.4 95.1 78.2 3.8 73.2 21.1 9.4 220.0
Compositional Changes of Aromatic Steroid Hydrocarbons in Naturally Weathered Oil Residues 221
hydrocarbons (PAHs), and sterane and terpane biomarkers were provided in a previous paper (Barakat et al., 2001). Brie¯y, the analyses of the weathered oil residues indicated that, despite the long-term extensive weathering, it was still possible to correlated the oil residue samples based on the PAH composition and the selective use of biomarker parameters. The ratios of C2-DBTs/C2-PHENs (C2-dibenzothiophenes/C2-phenanthrenes), C3-DBTs/C3-PHENs (C3-dibenzothiophenes/C3-phenanthrenes), C2-CHRYs/C1-CHRYs (C2-chrysenes/C1-chrysenes) and C2-DBTs/C1-DBTs (C2-dibenzothiophenes/C1-dibenzothiophenes) were stable in this arid terrestrial environmental up to the depletion of approximately 99% of total n-alkane or aromatic hydrocarbon and up to the depletion of 98% of total resolved peaks. Biomarker ratios of hopanes and steranes were also useful for source identi®cation even for severely weathered oil residues, although some exceptions were encountered for an extensively weathered surface sample (S-4). The results of biomarker compositions were in good agreement with weathering classi®cation based on alkanes and aromatic hydrocarbons. Samples with lowest pregnane indices (PI, de®ned as the percentage of the concentrations of C21 and C22 steranes to the total concentration of steranes) and tricyclic terpane indices (TriTI, de®ned as the percentage of the concentrations of C19 to C30 tricyclic terpanes to the total concentrations of triterpanes) showed the lowest concentrations of n-alkanes and
alkylated PAH homologues, the highest weathering ratios [WR S (n-C23 to n-C34)/(n-C11 to n-C22)], the lowest concentrations of total resolved peak (TRP), and the highest ratios of C2 and C3-alkylated CHRYs to their counterparts in the PHEN and DBT series (Figure 1). Aromatic steranes are another groups of biomarker compounds that are resistant to degradation and can be used track the sources of biodegraded oils. Mass chromatograms of the monoaromatic (m/z 253) and triaromatic steranes (m/z 231) in the aromatic hydrocarbon fractions of representative samples are shown in Figures 2 and 3, respectively. Peak identi®cation is summarized in Table 2. The m/z 253 chromatogram of the fresh oil sample is characterized by series of 20R and 20S C27 ±C29 18-nor5b (H), 17b(Me)- and 18-nor-5a (H), 17b(Me)-chloesta8, 11, 13-trienes ( peaks 3±11 in Figure 2a). In addition, C21 and C22 ring-C monoaromatic sterane hydrocarbons, have been identi®ed ( peaks 1 and 2 in Figure 2a). The distribution showed higher relative abundance of the C29 monoaromatic steranes which was also a characteristic feature of the Western Desert oils from the AI-Alamein Basin (Barakat, 1994). The eects of evaporation and degradation on the monoaromatic sterane compositions of the oil residues are illustrated in Figure 2 (b±f). With the exception of the most weathered oil sample (S-4), the loss of C21, C22, C27 and C28 monoaromatic steranes ( peaks
Figure 1. Plots of TRI versus selected weathering ratios reported in Barakat et al. (2001). W oil residues, q source oil. WR S (n-C23 to n-C34)/ (n-C11 to n-C22); PI sum of concentrations of C21 and C22 steranes ( pregnanes) over total concentration of steranes 100; TrTI is the sum of concentrations of C19 to C30 tricyclic terpanes/total concentration of triterpanes 100; TRI is the sum of concentrations of C20 to C21 triaromatic steranes/total concentration of triaromatic steranes 100.
222 A. O. Barakat et al. 10
7+8 10 a) Source oil
5+6
5+6 1
3
3 2
Relative percentage (%)
7+8
b) S-1 (27-30 cm)
1
2
10
10 c) S-2 (6-8 cm)
7+8
d) S-3 (0-1 cm)
7+8
5+6
5+6 3
3 1 2
1
2
10 5+6 7+8 10
7+8 5+6
f) S-5 (20-25 cm)
e) S-4 (surface)
3
3
4
9
11
1 2
Retention Time Figure 2. Mass chromatograms (m/z 253) showing the distribution of monoaromatic steranes in representative weathered oil residue samples from Al-Alamein and a fresh Western Desert-sourced oil.
1, 2, 7 and 8 in Figure 2) in the weathered samples is apparent. The C27, C28, and C29 monoaromatic steranes have roughly the same abundances in the weathered samples except for the extensively weathered sample S-4 (surface). Another signi®cant dierence between the m/z 253 fragmentogram of the most weathered sample (S-4) and other samples is that the C21 and C22 short chain steranes were greatly decreased in their abundances relative to C27 ±C29 series. The loss of the lower molecular weight C21 and C22 monoaromatic steranes indicated that volatilization was one of the major processes for the weathering of this sample. The extensive volatilization, coupled with biodegradation and other physical and chemical reactions, could have caused the changes in the compositions of most hydrocarbons in S-4 (surface), even for the relatively stable biomarkers.
On the other hand, the triaromatic sterane distributions shown in Figure 3 (m/z 231) are remarkably similar among the samples in spite of the varying degrees of degradation with some exceptions for the extensively weathered sample S-4 (surface). A good correlation existed between the oil residues and the fresh oil. The triaromatic sterane distribution was also similar to the Western Desert oils from the AIAlamein Basin as reported earlier by Barakat (1994). In addition, the molecular ratios of triaromatic sterane compounds (ratios of C28 20R/C28 20S, C27 20R/C28 20R, and C28 20S/(C26 20R C27 20S) in Table 3) were almost identical for most of the weathered samples and were similar to those of the fresh oil. The similarities in triaromatic sterane compositions suggested that the spilled oil residues and the Western Desert oil came from a common origin.
Compositional Changes of Aromatic Steroid Hydrocarbons in Naturally Weathered Oil Residues 223 D
G
D
E
a) Source oil
b) S-1 (27-30 cm)
F A
F
B A
B C
Relative percentage (%)
C
D
D E
c) S-2 (6-8 cm)
G
d) S-3 (0-1 cm)
G
E F
F
A
G
E
B
A
C
B
C
D
D G
E
f) S-5 (20-25 cm)
e) S-4 (surface) F
F G E C
B A
C
0.00
A
B
42.00
44.00
46.00
48.00
50.00
52.00
54.00
60.00 0.00
42.00
44.00
46.00
48.00
50.00
52.00
54.00
60.00
Retention Time Figure 3. Mass chromatograms (m/z 231) showing the distribution of triaromatic steranes in representative weathered oil residue samples from Al-Alamein and a fresh Western Desert-sourced oil.
The quantitation results (Table 3) illustrate a number of chemical changes that occur during weathering of the oil residues. Although, the concentrations of C27 (20R) and C28 (20R) triaromatic steranes are comparable, the fresh oil has the lowest concentrations, while the highly weathered samples from S-4 (surface), S-5 (20±25 cm) and S-5 (0±1 cm) have the highest concentrations. Figure 3 showed that the peak intensities of C20 and C21 triaromatic steranes in the source oil were greater than the weathered samples, indicating that, relative to the C26 ±C28 homologues, short chain C20 and C21 triaromatic steranes were lost during weathering processes. The dierential
weathering of C20 and C21 vs C26 ±C28 triaromatic steranes was more pronounced in the highly weathered samples obtained from S-4, and S-5 (20±25 cm) as indicated by the lower values of the ratios C20/ (C20 C28) and the triaromatic sterane index (TRI) (Table 3).
Acknowledgements We are grateful to the Western Desert Petroleum Company (WEPCO) for providing the samples used in this study. Technical support by J. Alcala-Herrera is
224 A. O. Barakat et al. Table 2. GC-MS peak identi®cation for Figures 2 and 3 Peak label
Compound
Monoaromatic steranes (m/z 253) 1 2 3 4 5 6 7 8 9 10 11 Triaromatic steranes (m/z 231) A B C D E F G
C21-monoaromatic sterane C22-monoaromatic sterane C27-5b(H)-monoaromatic sterane C27-5b(H)-monoaromatic sterane C27-5a(H)-monoaromatic sterane C28-5b(H)-monoaromatic sterane C27-5a(H)-monoaromatic sterane C28-5a(H)-monoaromatic sterane C29-5a(H)-monoaromatic sterane C28-5a(H)-monoaromatic sterane C29-5a(H)-monoaromatic sterane
(20S) (20R) (20S) (20S) (20R) C28-5a(H)-monoaromatic sterane (20S) (20R) C29-5b(H)-monoaromatic sterane (20S) (20S) (20R) C29-5b(H)-monoaromatic sterane (20R) (20R)
C20-triaromatic sterane C21-triaromatic sterane C26-triaromatic sterane (20S) C26 (20R) C27 (20S)-triaromatic sterane C28-triaromatic sterane (20S) C27-triaromatic sterane (20R) C28-triaromatic sterane (20R)
Table 3. Triaromatic steranes biomarker parameters in AI-Alamein oil residues Sample (cm)
C27 20R, mg/g EOM
C28 20R, mg/g EOM
S-1 (0±1) S-1 (2±3) S-1 (13±15) S-1 (27±30) S-2 (0±1) S-2 (6±8) S-2 (13±15) S-3 (0±1) S-3 (5±7) S-3 (12±15) S-4 (surface) S-5 (1±7) S-5 (7±15) S-5 (20±25) Source oil
2.72 5.85 5.84 8.4 3.52 6.61 5.88 4.02 4.59 3.89 16.2 14.1 6.50 16.5 1.14
3.88 7.87 7.34 12.2 5.13 9.91 8.96 5.83 6.79 5.42 12.6 13.3 9.00 20.9 1.52
(C2820R/ C2820S)* 1.00 0.94 0.89 0.93 0.94 0.95 0.96 0.96 0.94 0.94 1.35 1.04 0.95 0.86 0.92
(C2720R/ C2820R){ 0.70 0.74 0.80 0.69 0.69 0.67 0.66 0.69 0.68 0.72 1.29 1.06 0.72 0.79 0.75
(C2820S/ [C2620R C2720S]){ 1.01 0.99 1.01 1.03 1.03 1.04 0.99 1.05 1.04 1.07 0.48 0.75 0.97 1.07 1.06
% (C21/ [C21 C28])}
TRIk
22.1 30.5 35.0 29.1 27.6 26.1 25.7 24.4 27.2 25.5 8.3 18.1 22.4 16.0 54.4
14.1 19.5 20.1 17.7 15.3 15.5 16.2 14.5 16.5 15.6 4.1 11.9 12.9 10.8 35.9
*G/E in Figure 3. { F/G in Figure 3. { E/D in Figure 3. }[A/(A G)] 100 in Figure 3. k[(A B)/(SA-G)] 100 in Figure 3.
appreciated. A.O.B. is grateful to the Fulbright Foundation for a research fellowship.
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