Environ. Sci. Technol. 1996, 30, 2332-2339
Environmental Stability of Selected Petroleum Hydrocarbon Source and Weathering Ratios G R E G O R Y S . D O U G L A S , * ,† A. EDWARD BENCE,‡ ROGER C. PRINCE,§ SARA J. MCMILLEN,‡ AND ERIC L. BUTLER| Arthur D. Little, Inc., Acorn Park, Cambridge, Massachusetts 02140, Exxon Production Research, Houston, Texas 77252, Exxon Research & Engineering, Annandale, New Jersey 08801, and ENSR Consulting & Engineering, Acton, Massachusetts 01720
Weathering and biodegradation alter the composition of spilled oil, making it difficult to identify the source of the release and to monitor its fate in the environment. Using intertidal sediment and terrestrial soil data that cover a wide range of oil weathering states, we show that ratios of alkylated dibenzothiophenes and phenanthrenes are useful for source identification even up to 98% depletion of total polycyclic aromatic hydrocarbons (PAHs). Furthermore, we find that some ratios of alkylated naphthalenes, phenanthrenes, and chrysenes can qualitatively assess the extent of weathering an oil has undergone since a spill. These source and weathering ratios appear to successfully describe oil depletion and to identify sources in subtidal sediment data from the M/C Haven spill in Italy, the Exxon Valdez spill in Alaska, and a North Sea oil spill.
Introduction Petroleum released into the environment is subject to a range of chemical, physical, and biological processes, together known as weathering, that change its composition (1-4). Light molecules evaporate, and some molecules are washed out by dissolution. Photochemistry can also alter the composition of a spilled oil, but biodegradation is the major pathway of degradation (1-4). These changes make the unambiguous identification of the source of an oil something of a challenge. Conventional oil spill monitoring programs typically measure four fractions of oil; the volatile aromatic hydrocarbons, the alkanes, the total petroleum hydrocarbons, and the polycyclic aromatic hydrocarbons. While the volatile aromatics (including benzene, toluene, ethylben* Corresponding author telephone: 617-498-5384; fax: 617-4987296. † Arthur C. Little Inc. ‡ Exxon Production Research. § Exxon Research & Engineering. | ENSR Consulting & Engineering.
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zenes, and xylenes) can be toxic to organisms, their residence times in the environment after a surface spill are short, and they are of little use in source identification. The alkanes and total petroleum hydrocarbons, typically analyzed by gas chromatography with flame ionization detection (GC/FID), constitute the bulk of an oil. They can be used to assess total oil concentrations, the extent of degradation, and to a limited degree, the source. The polycyclic aromatic hydrocarbons, particularly the two- to four-ring compounds (with which we include the threering heterocyclic dibenzothiophenes), can be used to identify both source and extent of degradation. Although they commonly represent less than 2% of the bulk composition of an oil, these compounds include toxic compounds that can be of long-term concern. The polycyclic aromatic compounds degrade at different rates, depending upon their relative water solubility, volatility, and biodegradability, and the extent of weathering is affected by the degree of exposure and environmental conditions. Consequently, the relative distributions of these compounds in an oil can vary widely, both spatially and with time. Ratios of compounds that degrade at the same rates retain the initial oil signature until they can no longer be detected (5, 6). These ratios are termed “source” ratios and can be contrasted to ratios that change substantially with weathering and biodegradation, which are termed “weathering” ratios. Three-ring alkylated polycyclic aromatic compounds are particularly useful for source identification because they are present in petroleum and many of its refined products at suitable concentrations, their relative concentrations vary among different oils making them source specific, and they can be quantitatively measured using routine analytical methods (7-11). Furthermore, it has been shown in studies of the Exxon Valdez oil spill that some polycyclic aromatic compound ratios remain relatively constant over wide concentration ranges and weathering states (9). This feature, combined with the source-specific nature of some ratios in the spilled oil, facilitated identification and quantification of multiple sources of hydrocarbons, including a substantial natural petroleum hydrocarbon background, in the subtidal sediments of Prince William Sound, AK. Age-dated cores demonstrated that the diagnostic polycyclic aromatic hydrocarbon ratios in the natural background did not change with time even in samples that had been deposited up to 130 years ago (9). Source ratios are thus likely to play an increasingly important role in defining the analytical and interpretive strategy used for characterizing a petroleum-contaminated site. Such studies would identify and quantify pre-spill hydrocarbon concentrations, identify and quantify spilled hydrocarbons, and quantify the degree of weathering and biodegradation of the spilled oil. This information would help document the exposure of natural resources to spilled oil, identify the principal responsible parties, allocate responsibility for multiple-source spills, and predict and monitor the effectiveness of remediation activities. In this paper, we examine polycyclic aromatic compound ratios to document the stability of selected ratios that have been used to identify and allocate sources in marine oil
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1996 American Chemical Society
TABLE 1
Analytes Measured in This Work Polycyclic Aromatic Hydrocarbons naphthalenea (N) C1-naphthalenes (N1) C2-naphthalenes (N2) C3-naphthalenes (N3) C4-naphthalenes (N4) acenaphthylenea (AC) acenaphthenea (AE) fluorenea (F) C1-fluorenes (F1) C2-fluorenes (F2) C3-fluorenes (F3) anthracenea (A) phenanthrenea (P) C1-phenanthrenes/anthracenes (P1) C2-phenanthrenes/anthracenes (P2) C3-phenanthrenes/anthracenes (P3) C4-phenanthrenes/anthracenes (P4) dibenzothiophene (D)
C1-dibenzothiophenes (D1) C2-dibenzothiophenes (D2) C3-dibenzothiophenes (D3) fluoranthenea (FL) pyrenea (PY) C1-fluoranthene/pyrenes (FP1) benz[a]anthracenea (BA) chrysenea (C) C1-chrysenes (C1) C2-chrysenes (C2) C3-chrysenes (C3) C4-chrysenes (C4) benzo[b]fluoranthenea (BB) benzo[k]fluoranthenea (BK) benzo[a]pyrenea (BAP) indeno[1,2,3-c,d]pyrenea (IP) dibenz[a,h]anthracenea (DA) benzo[g,h,i]perylenea (BP)
Saturated Hydrocarbons pristaneb (PR) phytaneb (PH)
C30-17R(H)21β(H)-hopane n-C7-n-C36b a
Analyte is a U.S. EPA priority pollutant PAH.
b
Analyte concentration determined by GC/FID analysis.
spills, evaluate a quantitative approach for measuring oil weathering and biodegradation, and broaden the approach to other spill environments including oil-contaminated soils. The analytical data used to evaluate the stability of the ratios were obtained during a 1990 bioremediation study of oiled intertidal sediment from Prince William Sound, AK, following the Exxon Valdez oil spill (12-16). Quantification of oil depletion using the saturated pentacyclic triterpane biomarker compound C30-17R(H),21β(H)-hopane, hereafter referred to as hopane, as an internal reference marker was pioneered in these studies. The chemistry data provide a database (149 samples) of oiled beach sediment samples that reflect a wide range of weathering and biodegradation states of a single oil. Early results of the study demonstrated the efficacy of two fertilizers, Inipol EAP22 and Customblen, to stimulate biodegradation (13). Analysis of the polycyclic aromatic hydrocarbons showed that the two- and three-ring compounds were readily degraded, while the four-ring chrysenes were more resistant (10). The most stable of the measured compounds was hopane (15). Our approach was to first identify diagnostic analyte ratios by calculating relative percent depletion for each analyte and the relative standard deviation (standard deviation/mean) of all two-analyte ratios in the 1990 samples. The source and weathering ratio approach was then tested using chemistry data from the M/C Haven oil spill in the Mediterranean (17), benthic sediments from the Exxon Valdez oil spill in Prince William Sound (9), and a North Sea oil spill. The field observations were then confirmed in a controlled laboratory oiled soil biodegradation study (18).
Methods Analytical Methods. The compounds that were measured in common for these studies include the 16 U.S. EPA priority pollutant polycyclic aromatic hydrocarbons, their associated alkylated homologues, n-alkanes, the isoalkanes
pristane and phytane, and hopane (Table 1). The samples were extracted and analyzed by capillary gas chromatography with flame ionization detection (GC/FID) and capillary gas chromatography with mass spectrometry (GC/MS) operated in the selected ion mode (10). Approximate GC/ FID method detection limits for sediment samples (50 g dry weight) are 2 mg/kg for total resolved and unresolved GC/FID detectable hydrocarbons and 0.001 mg/kg for individual alkanes. Approximate method detection limits for polycyclic aromatic hydrocarbons, dibenzothiophenes, and hopane are 1 µg/kg (dry weight) in sediments and 5 mg/kg in oils (9). Method detection limit studies were based on the EPA protocol entitled Definition and Procedure for the Determination of the Method Detection Limit, Code of Federal Regulations 40 CFR Part 136. For quality control of the GC/FID and GC/MS methods, a procedural blank, an Alaskan North Slope control oil, a matrix-spike sample, and a National Institute of Standards and Technology standard reference material were analyzed with each batch of 15 field samples. Quality control criteria are described elsewhere (9, 10). Note that the polycyclic aromatic hydrocarbons and the saturated triterpanes of each sample were measured, with appropriate quantitation standards, in the same chromatographic separation. Interpretive Methods. An ideal source ratio would be unique to that particular source, and the two analytes would degrade at similar rates. The tremendous number of oils in commerce and oils from natural seeps makes it unlikely that a truly unique fingerprint of polycyclic aromatic hydrocarbons can be found, but one group of analytes that varies widely in different oils is the dibenzothiophene family; their concentrations reflect the sulfur content of the oil, and this varies widely in different oils (19). The determination of degradation rates requires a quantitative means of monitoring degradation, and this requires a conserved internal marker within the oil to act as a standard. The triterpane hopane has been identified as the most resistant analyte of those listed in Table 1 (15), and calculations of total oil and individual analyte depletion are based upon
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the assumption that, in the time frame involved (3-4 years), hopane undergoes no degradation. As biodegradation proceeds, the hopane concentration in the remaining oil increases due to the removal of other components. The concentration of hopane in the weathered oil (H1), measured on an oil weight basis, relative to its concentration in the initial source oil (H0) is a function of the amount of oil degraded (8, 10).
% oil depletion ) [1 - (H0/H1)] × 100
(1)
Individual analyte depletion, corrected for oil loss, is then determined by
% analyte depletion ) [1 - ((C1/C0)(H0/H1))] × 100 (2) where C1 is the analyte concentration in the degraded oil, and C0 is the analyte concentration in the source oil. Because hopane does degrade very slowly under some environmental conditions (19), the calculations of total oil and individual analyte depletion are conservative and provide a minimum estimate of oil disappearance. Soil Study Methods. A laboratory study (18) was designed to evaluate the end point and rates of biodegradation and the stability of the source ratios and utility of weathering ratios in a well-constrained system. Clean loam (37% sand, 53% silt, 10% clay) with a total organic carbon content of 1.24% was sieved (2 mm) and 3.4% crude oil was added on a dry weight basis. Nutrients were added to the soil as ammonium nitrate and potassium phosphate to achieve a C:N:P ratio of 100:2:1. The samples were split, and the study was performed in duplicate at 22 °C. Moisture in each system was maintained at 14% (70% water holding capacity), the system was mixed at regular intervals to introduce oxygen, and the pH was maintained between 6 and 8. Indigenous bacteria present in the soil degraded the added oil; no microbial amendments were added. Soil samples were collected at the initiation of the experiment and 1, 3, and 6 months later.
Results Applying eq 2 to the hydrocarbon analyses of the 149 shoreline samples from the 1990 Prince William Sound bioremediation studies reveals that the oil in the different samples had lost 30-70% of its initial content (13, 16). Figure 1 shows the percent depletion of individual alkanes in a moderately degraded sample from that suite, collected in July 1990 (16 months after the spill). Total petroleum hydrocarbon depletion based on GC/FID detectable hydrocarbons is 60% for this sample, relative to fresh Exxon Valdez cargo oil. As expected, shorter alkanes (n-C10-nC28) are more depleted than the longer chain alkanes (nC28+), and the isoalkanes pristane and phytane are less depleted than the adjacent n-alkanes (n-C17 and n-C18). The differential depletion of n-alkanes to branched alkanes has been used as an indicator of biodegradation [ratios of n-C17/pristane and n-C18/phytane (20)], but as can be seen from Figure 1, the branched alkanes are themselves depleted, so such ratios substantially underestimate overall biodegradation in moderately degraded samples. Pristane and phytane are about equally depleted, so they are possible candidates for source ratios in weathered oil (21), but again their ready degradation makes them unsuitable in mod-
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FIGURE 1. Percent alkane and total petroleum hydrocarbon [F1 + F2] depletion, based on the conservation of hopane (14), for a degraded Alaska North Slope crude oil extracted from a shoreline sediment sample collected in July 1990 in Prince William Sound, 16 months after the Exxon Valdez oil spill. C10-C34 are linear alkanes with that number of carbons; PR and PH are pristane and phytane. TPH represent the depletion of the total petroleum hydrocarbons in the sample.
FIGURE 2. Percent polycyclic aromatic hydrocarbon depletion, based on the conservation of hopane (14), for the sample of Figure 1. The abbreviations are listed in Table 1. Compounds with no data were below our detection limit in this crude oil.
erately to heavily degraded samples. In addition, natural sources of pristane from copepods may alter the pristane/ phytane ratios in sediments, confounding its application as a source ratio. Nevertheless alkane profiles can provide data to separate refined petroleum products [e.g., diesel fuel (4)] from crude oils. The depletion of the individual polycyclic aromatic hydrocarbons in the same sample is shown in Figure 2. As noted by other investigators, the rate of polycyclic aromatic hydrocarbon degradation in the environment decreases with ring size and, within a homologous series, decreases with increasing alkylation (10, 22, 23). Figure 2 shows that some analytes have similar percent depletions, indicating that their ratios remain relatively constant as weathering proceeds. For example, the C2-phenanthrenes (P2), the sum of all dimethyl and ethyl isomers, are 78% depleted. The C2-dibenzothiophenes (D2) are 71% depleted in the same sample, and C3-phenanthrenes (P3) and C3-dibenzothiophenes (D3) are 54% and 50% depleted, respectively. As noted above, since the dibenzothiophenes vary widely in different oils, the relative amounts of phenanthrenes and dibenzothiophenes are good candidates as source indicators. We shall return to this below. The D2/P2 and
FIGURE 3. Percent relative standard deviation of selected target analyte ratios for 149 ANS crude oiled beach sediment samples collected in Prince William Sound in 1990 as part of the 1990 Bioremediation Monitoring Program (15) approximately 15-16 months after the Exxon Valdez oil spill. The number of samples that were used to determine the percent relative standard deviation (ratio greater than 0) are listed above each bar.
D3/P3 ratios were the most stable for Exxon Valdez cargo crude under the conditions in Prince William Sound. The relative standard deviation for each analyte ratio (630 possibilities) in all the samples (149) was calculated and ranked from highest (very sensitive weathering ratio) to lowest (most stable ratio). Values ranged from 480% for the C0-fluorene/C1-phenanthrenes (F/P1) to 7% for the C3dibenzothiophenes/ C3-phenanthrenes (D3/P3) and 17% for the C2-dibenzothiophenes/C2-phenanthrenes (D2/P2) ratios (Figure 3). Of course, compounds that weather to below their respective detection limits during the early stages of oil degradation cannot provide reliable source or weathering ratios. For example, for the F/P1 ratio, only 7 out of 149 samples had both compounds present above their detection limits. For ratios involving the lower molecular weight analytes, a large relative standard deviation is due primarily to their high solubilities and volatilities, whereas for heavier compounds the large values are probably the result of varying degrees of biodegradation. These relationships have been used to develop weathering indicators of varying sensitivity for different stages of the weathering process (4). For light product degradation (e.g., diesel fuel) and for crudes in the early stages of weathering, the C3-naphthalenes/C2-phenanthrenes (N3/ P2) ratio may be preferred, and for the later stages of crude oil degradation, a less sensitive weathering ratio such as C3-dibenzothiophenes/C3-chrysenes (D3/C3) may be used (10). These ratio approaches provide a more reliable measurement of total oil weathering and biodegradation than conventional mass balance methods in heterogeneous systems (13, 16). When calibrated to the source oil, these ratios can be used to define the weathering state of a sample collected from the environment. Thus, using hopane as a conserved internal marker (15), the Exxon Valdez oil sample in Figure 1 has lost approximately 83% of its initial total polycyclic aromatic hydrocarbons and 60% of the total hydrocarbons, relative to the fresh crude. This difference reflects the facts that the polycyclic aromatic hydrocarbons are only 1-2% of the total oil (10) and that they are more degradable, on a percentage basis, than the total oil. In fact approximately 50% of the polycyclic aromatic hydrocarbons in the spilled oil were naphthalenes, which are
FIGURE 4. Plot of C3-dibenzothiophenes/C3-chrysenes (weathering ratio) versus C3-dibenzothiophenes/C3-phenanthrenes (source ratio) for fresh and degraded crude oil samples collected from three separate oil spills; ([) a North Sea crude oil, (9) Alaska North Slope crude oil, and (b) Iranian crude.
both quite volatile and among the most degradable compounds in crude oil (22; Figure 2). Distinguishing among Weathered Crude Oils. Plots of a source ratio versus a weathering ratio provide a means of resolving multiple sources as well as differences in the extent of degradation for samples from a single source. Figure 4 plots the weathering ratio D3/C3 versus the source ratio D3/P3 for three different oil spills; the Exxon Valdez (Alaskan North Slope crude oil), M/C Haven (heavy Iranian crude oil), and a North Sea production leak (North Sea crude oil). The plot demonstrates the stability and usefulness of the source ratio over a wide range of weathering and biodegradation for the three oils. Distinguishing Spilled Oil from Other Sources. M/C Haven. On April 11, 1991, an explosion on the M/C Haven resulted in a fire and the release of approximately 145 000 t of heavy Iranian crude oil near Genoa, Italy. The spill was unusual because the tanker burned for 3 days, and approximately 70% of the cargo was consumed. Approximately 7.8 million gal of oil was released into the sea (17); much of this was confined in the vessel during the fire, and the intense heat modified the physical and chemical properties of the cargo and influenced the transport and ultimate fate of the oil in the marine environment. Figure 5 is a plot of the D3/C3 ratio (weathering) versus D3/P3 (source) ratio for sediment, water, oil, and tar samples collected in the spill zone of the M/C Haven oil spill within the first 3 months of the spill. The decrease in the D3/C3 ratio from 10-12 to
