Environ. Sci. Technol. 1998, 32, 2222-2232
Study of the 25-Year-Old Nipisi Oil Spill: Persistence of Oil Residues and Comparisons between Surface and Subsurface Sediments ZHENDI WANG,* MERV FINGAS, SANDRA BLENKINSOPP, GARY SERGY, MICHAEL LANDRIAULT, LISE SIGOUIN, AND PAT LAMBERT Emergencies Science Division, ETC, Environment Canada, 3439 River Road, Ottawa, Ontario, Canada, K1A 0H3
During the years 1970-1972 the Nipisi, Rainbow, and Old Peace River pipeline spills occurred in the Lesser Slave Lake area of northern Alberta. The Nipisi spill was by far the largest of the three spills and is also one of the largest land spills in Canadian history. The most recent field survey was conducted in 1995 in order to determine which cleanup methods were most successful and to provide up-todate information about any changes in residual oil and vegetative recovery 25 years after the spills. The comprehensive chemical analysis data of the Nipisi samples indicate that (a) the Nipisi samples can be categorized into three groups plus the background group, according to the contamination level and degradation degree of the samples; (b) degradation of residual oil of surface samples (0-4 cm) had clearly occurred, and the weathered percentages of this group of samples were estimated to be in the range of 15-43%; (c) subsurface samples (10-40 cm) exhibited great quantities of oil even 25 years after the spills, indicating that the natural recovery rates were slow for this group of samples; (d) the extent of contamination and degree of degradation correlated strongly with sample depth; (e) oil trends were similar across the site, even though different treatments had been used after the spill at the site.
Introduction From 1970 to 1972, spills occurred in the Nipisi area (55°52′ N, 115°07′ W) and two adjacent areas (Rainbow and Old Peace River) in the vicinity of Lesser Slave Lake in northern Alberta, 250 mi northwest of Edmonton. The Nipisi spill was by far the largest of the three spills and is one of the largest land spills in Canadian history (approximately 60 000 barrels over 25 acres). At the Nipisi site, several different methodologies were employed during the cleanup activities including burning, tilling, and fertilizer addition after the spill. The site was then left as an area for future scientific research and evaluation. The surrounding area is predominantly a peat bog/fen habitat. The environment is typical of much of northern Canada and the type of habitat that is crossed by many thousands of miles of crude oil pipelines in Canada (1). The Nipisi site was of interest because of the following: (a) The site had been set aside as a research site, and a variety * Corresponding author telephone: 613-990-1597; fax: 613-9919485; e-mail:
[email protected]. 2222
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of cleanup methods had been tried. It was of interest to determine how successful the cleanup operations were. (b) The spill had occurred in a northern wetland habitat similar to that of the 1994 Russian pipeline spill, a large oil spill in the Kolva basin in western Siberia of the Russian Federation of States (2, 3). In addition, research has traditionally focused on oil spill cleanup techniques for marine environments. Few follow-up studies have been done on freshwater wetland oil spill sites (4-6), and data are badly lacking. Therefore, the study of the Nipisi site enhances our understanding of the long-term fate and effect of oil spills in such northern wetland environments. The Emergencies Science Division (ESD) of Environment Canada has periodically revisited oil spill sites (7-10). During the summer of 1995, ESD staff along with Natural Resources Canada personnel carried out a field survey and sampling program at the Nipisi spill site and two smaller spill sites (Rainbow and Old Peace River). The primary goal of the survey was to provide up-to-date information on oiling conditions and vegetative recovery in the Nipisi area. At selected sites, surface and subsurface samples were collected for chemical analyses of residual oil hydrocarbons. Samples were collected at each of the three spill sites (all within a few kilometers of each other) in each of the identifiable remediation areas. Site histories, details of the 1995 survey, and recoveries of various vegetation types and species at the spill sites have been reported by Blenkinsopp et al. (1). This paper will focus on the analytical chemistry results and findings obtained. Characterization and comparisons of hydrocarbon composition between surface and subsurface sediments at the same location and at different sites help to address spill response questions related to the (a) effectiveness of cleanup techniques, (b) long-term persistence of oil residues, (c) changes in oil character after 25 years exposure to weathering and biodegradation, and (d) relationship of residual oil degradation rate to depth at which samples were collected.
Experimental Section Materials. Distilled chromatographic solvents were used without further purification. Calibration standards used for the determination of individual and total petroleum hydrocarbons include n-alkane standards from C8 to C32 including pristane and phytane, PAH standards (SRM 1491) from the National Institute of Standards and Technology (NIST), and biomarker (hopanes and steranes) standards from Chiron Laboratory in Norway. Sample Collection. A total of 34 samples was collected for oil analysis, 22 at the Nipisi site (see Figure 1 and Table 1 for details), 9 at the Rainbow pipeline spill, and 3 at the Old Peace River site. In most samples, the oil was evident by odor and sight. Most samples were oil in 100% organic materials consisting of sphagnum peat and/or similar decomposed peat. A few of the samples also had a mineral sediment component. Two samples from the Nipisi site were water samples. These samples were obtained from soil cores, the sidewalls of shallow pits, and surface grabs of soil or water. The soil sampling procedures were based on the U.S. EPA Standard Operating Procedure 2012, Soil Sampling (U.S. EPA, 1988). The samples were stored in new, certified clean, glass bottles with Teflon-lined caps (Fisher Scientific, Nepean, ON). Certification was to U.S. EPA protocols for extractable organics including pesticides and PCB. A temperaturecontrolled sample cooler (Canadian Coleman Co., Toronto, ON) was used to store and transport the samples at 4 °C. The S0013-936X(97)01070-5 CCC: $15.00
1998 American Chemical Society Published on Web 06/24/1998
TABLE 1. Description of Nipisi, Rainbow, and Old Peace River Samples (July 19 and 20, 1995) sample location
sample code and depth (cm)a
sample location description
N1
N1-1 (25-30), N1-2 (0-2)
N2
N3
N2-1A (0-2), N2-1B (12-16), N2-1C (30-40), N2-1D (pit water), N2-1E (80-100), N2-2 (0-2), N2-3A (0-2), N2-3B (12) N3-1 (25-28)
N4
N4-1 (15), N4-2 (0-2), N4-3 (15)
N5
N5-1A (0-2), N5-1B (25)
N6
N6-1 (0-2), N6-2 (0-4)
N7
N7-1 (0-2), N7-2 (10-15), N7-3 (pondwater) N8-1 (0-5)
N8 R1/R2 R3
R1-1 (11-18), R2-1A (0-2), R2-1B (40), R2-1C (7-10) R3-1 (water level)
R4 R5 R6
R4-1 (3-6) R5-1 (0-2) R6-1 (17)
R7
R7-1 (40)
P1
P1-1 (5)
P2
P2-1 (12-15)
P3
P3-1 (0-2)
a The sampling depths are shown in parentheses. using Karl-Fischer titration method.
near suspected break point; 20 m north of small collection pond, on right of way east of N1; possible rototill treatment site
70-75
east of N2; outside the drainage ditch; groundwater downstream of oil 500 m southeast of N1; cleanup treatment technique not known; may be reseeded area 300 m southeast of N1; south extension from N2 and similar in appearance 100 m southwest of N1; south adjacent to old wellhead; possible burn site large flattened area, west and southwest of N1
87
13-88
54-83 39-75 29-59 85
outside oil diversion ditch on west side of N7 site; background sample site
87
north end (upstream end) of spill area; 100 m west of wellhead buildings west side of spill site; outside the dyke; background sample about 125 m downstream of R2 150 m downstream of R2; about mid spill area east side of spill area, 100 m south of wellhead buildings south end of spill area
79-89
50 m south of right of way; west side of oiled meadow 50 m south of right of way and 20 m east of P1 on east side of oiled meadow 50 m south of right of way and 100 m west of P1; appears hydrologically isolated from P1 b
H2O content in samples (%)b
93 75 26 78 88 67 77 51
The average values of water content (three determinations) in samples were determined
FIGURE 1. Location of the 1995 Nipisi study site. Pipeline right of ways and the 1995 Nipisi sampling locations (N1-N8) are shown on the map. Inset at the right corner is the map of the surrounding area of the Nipisi spill. The Nipisi spill occurred in 1970-1972 in the Nipisi area and two adjacent areas (Rainbow and Old Peace River), in the vicinity of Lesser Slave Lake in northern Alberta, 250 mi northwest of Edmonton. See Table 1 for details of sample location description. VOL. 32, NO. 15, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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a The values of resolved peaks/TPH for group 3 were not determined due to the presence of vegetation hydrocarbons. b The carbon preference index (CPI) is defined as the ratio of the odd to even carbon-numbered n--alkanes. c The weathering index (WI) is defined as the ratio of the sum of n-C8,10,12,14,16 over the sum of C22,24,26,28,30. d The WI values were unable to be determined for the Group 1 samples due to the nearly complete loss of the low molecular weight n-alkanes.
PL-A:81 PL-B:93 ∼1.0 1.5 1.1 2.4 PL-A:21 PL-B:26 PL-A:55 PL-B:58 reference oils (PL-A & PL-B)
80-100 cm (Nipisi site) 40 cm (Rainbow site)
mixtures of oil and vegetation hydrocarbons
0.8-19
12-673
13-32
not deter- 0.2-1.0 mineda
0.2-0.5 0.4-0.8 1.5-5.4
10-20
0.5-2.3 most samples except 3.6 for N2-1C and 3.4 for N5-1B 16-24 47-55 10 000-165 000 lightly weathered/ degraded ++ group 2
10-40 cm (sub-surface)
190-720
1.0-8.0 33-45 20 000-256 000 200-800 ++ 0-4 cm (surface) group 1
group 3
0-25
0.0 for 0.0-0.2 0.0-0.1 ∼1.0 most samples 1.4-2.5 1.0-2.2 1.0-1.5 ∼1.0
C17/pri C18/phy Alk/Iso TPH (ppm) note
only small amount of vegetation hydrocarbons highly weathered/ degraded no oil
sample depth
various background group
sample grouping
30-74
d
Weathering Indexc (WI) total n-alkanes (mg/g TSEM) CPIb resolved TPH/TSEM peaks/TPH (%) (%) TSEM (mg/g of dried sample) 9
oil contamination
TABLE 2. Sample Grouping Based on Hydrocarbons Analysis Results 2224
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samples were then taken to the ESD lab for detailed oil analysis. Prior to analysis, the samples were tightly sealed and stored in a cold room at 4 °C. Two reference Peace River Pipeline Oil samples, designated as PL-A and PL-B, were supplied from the collection of Drs. Foght and Westlake, University of Alberta. The reference PL-A and PL-B oil samples were subsamples from a full glass mason jar and a half-full glass mason jar, respectively. A date of collection was not recorded, but was assumed to be collected in 1972 based on correspondence records. The reference PL-B oil was labeled “line cut”, a best guess that it was a sample taken directly from the pipeline. Sample Extraction, Cleanup, and Analysis. The water content of the Nipisi samples was determined (three determinations) using the Karl Fischer titration method (Table 1). The sediment samples were placed in 250-mL solventrinsed Erlenmeyer flasks, spiked with the appropriate surrogates (o-terphenyl, mixture of acenaphthene-d10, phenanthrene-d10, benz[a]anthracene-d12, and perylene-d12), and mixed with 100 g of sodium sulfate to dry the samples. If the sample had excessive moisture, additional amounts of sodium sulfate were added, and the samples were then serially extracted three times with 100 mL of dichloromethane (DCM) for 30 min each time using sonication. If there was visible color in the third extraction, an additional extraction was performed. The water samples were successively extracted three times using a liquid-liquid extraction technique in a separatory funnel. The extracts were combined, dried with sodium sulfate, and then filtered through sodium sulfate on a layer of glass wool. The extracts were concentrated to appropriate volumes and solvent-exchanged with hexane by means of rotary evaporation and further concentrated under nitrogen. An aliquot of the concentrated extract was blown down with N2 to a residue and weighed on a microbalance to obtain a total solvent-extractable material weight (TSEM, expressed as mg/g of sample). The reference source oils were directly dissolved in hexane at a concentration of 100 mg/mL and spiked with the appropriate surrogates prior to the column cleanup. A microcolumn packed with 3 g of activated silica gel and topped with 0.5 cm of anhydrous sodium sulfate (11) was employed for sample cleanup and fractionation of the oil. Half of the hexane fraction (F1) was used for analysis of saturates and biomarker compounds; half of 50% benzene fraction (F2) was used for analysis of alkylated PAH homologues and other target PAH. The remaining half of F1 and F2 were combined (F3) and used for determination of total GC-detectable TPH, GC-resolved peaks, and the GCunresolved complex mixture of hydrocarbons (UCM). These three fractions were concentrated to appropriate volumes, spiked with internal standards (5-R-androstane and C30-ββhopane, terphenyl-d14, and 5-R-androstane for F1-F3, respectively), and then adjusted to an accurate preinjection volume for GC/FID and GC/MS analysis. Analyses for n-alkane distribution and TPH were performed on a Hewlett-Packard (HP) 5890 gas chromatograph equipped with a flame-ionization detector (FID) and an HP 7673 autosampler. Analyses of PAH and biomarker compounds were performed on an HP model 5890 GC equipped with a model HP 5972 mass selective detector (MSD). System control and data acquisition were achieved with an HP G1034C MS ChemStation (DOS series). For detailed chromatographic conditions, analysis quality control, and quantification methodology, refer to refs 9-11.
Results and Discussion Determination of Hydrocarbons and Hydrocarbon Groups in Oil Residues. Table 2 presents the hydrocarbon analysis results for the Nipisi samples by gravimetric and GC/FID methods. Examination of the results allows us to classify
FIGURE 2. GC/FID chromatograms for total petroleum hydrocarbon and n-alkane analysis of the reference source oil PL-B and samples N2-1A (0-2 cm), N2-1B (12-16 cm), and R7-1 (∼40 cm), representing group 1-3 samples, respectively. Note that different Y-axis scales are applied from left to right. samples into four groups according to the levels of oil contamination and the extent of weathering and degradation processes. (a) Background group: including samples N3-1, N7-1, N7-3, N8-1, and R3-1. These samples were taken at sites outside the dike. Generally, no petrogenic hydrocarbons, in particular no alkylated PAH homologues and petroleumcharacteristic biomarker compounds such as pentacyclic hopanes and C27-C29 steranes, were detected and can be categorized as background. These samples showed typical biogenic n-alkane distribution in the range of C21-C33 with abundances of odd-carbon-number n-alkanes being much higher than that of even-carbon-number n-alkanes. Three vegetation biomarker compounds with remarkable abundances were detected, and they were identified as 12oleanene (C30H50, MW ) 410.7, RT ) 42.27 min), 12-ursene (C30H50, MW ) 410.7, RT ) 42.74 min), and 3-friedelene (C30H50, MW ) 410.7, RT ) 44.26 min). Formation of a sixmembered ring E from the baccharane precursor leads to the oleanane group. Oleananes and their derivatives form the largest group of triterpenoids and occur in the plant kingdom, specifically from higher plants (12). The friedelenetype triterpenoids arise by increasing degrees of backbone rearrangement of the oleanene skeleton. Methyl migration in ring E of the oleanene precursor leads to the ursene skeleton (12). (b) Group 1: including N1-2, N2-1A, N2-2, N2-3A, N4-2, N5-1A, N6-1, N6-2, R2-1A, and R5-1. These surface samples were taken from the upper 4 cm. They were highly contaminated (TPH levels: 20 000-256 000 ppm), and most of them were highly weathered/degraded. (c) Group 2: including N1-1, N2-1B, N2-1C, N2-3B, N4-1, N4-3, N5-1B, R1-1, R2-1C, R4-1, R6-1, P1-1, P2-1, and P3-1. These subsurface samples were mostly taken from 10 to 40 cm. They were highly contaminated (TPH levels: 10 000
ppm to 165 000 ppm) and lightly to moderately weathered/ degraded. (d) Group 3: including the remaining samples N2-1E, R2-1B, and R7-1. These samples were taken below 80 cm and from a depth of 40 cm at the Nipisi and Rainbow sites, respectively. They were lightly contaminated with oil and vegetation hydrocarbons (TPH < 673 ppm). To compare the compositions of the residual oil in the samples, all quantitative hydrocarbon group data discussed below are expressed relative to the amount of TSEM rather than to the total dry sediment weight. That is, comparison of samples is based on the weight of residual oil. Note that the GC-detectable TPH were only a portion of the TSEM; the remainder is composed of the asphaltenes, polars, and in all likelihood, a very small amount of additional soil components. The ratios of TPH to TSEM, the total resolved peak areas to TPH, and the degrees of depletion of n-alkanes indicate the level of weathering and biodegradation that has occurred in the oil (9, 10, 13). In general, the lower these ratios and the lower the concentrations of n-alkanes are, the greater weathering and degradation in a sample. This relationship was very consistent for group 1 and 2 samples (see Table 2). If, however, the concentrations of hydrocarbons are expressed relative to the total sediment weight, differences in weathering and degradation trends between samples would be not as clear as can be seen in Table 2. It should be noted that the TPH/TSEM ratio is skewed for the group 3 samples because of the high levels of vegetation hydrocarbons and therefore should not be misinterpreted as a highly weathered/degraded sample. Compositional Changes of TPH and n-Alkanes. Assessment of the degradation trends of different Nipisi sample groups can be illustrated by qualitative and quantitative inspection of their GC traces. Figure 2 shows the GC/FID chromatograms for the reference source oil PL-B and samples VOL. 32, NO. 15, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 3. Alkylbenzene and Alkylated PAH Homologue Quantitation Results and Diagnostic Weathering Ratios reference oils group 1 concentrations (µg/g TSEM) BTEX + C3-benzenes total of 5 target alkylated PAHs total of the other 15 EPA priority PAHs sum of C0- to C4-naphthalenes sum of 3 isomers of methyl-DBT ratios naphthalenes/chrysenes phenanthrenes/chrysenes dibenzothiophenes/chrysenes fluorenes/chrysenes 4-:2-/3-:1-methyl-DBTb (3- + 2-m-phen)/(4-/9- + 1-m-phen)c
group 2
group 3
1150-2940 8-22 89-760 2-20
12-225 6000-10990 25-104 2760-7415 30-120
124-1230 0.00 40-434 1.2-10
0.4-2.8 1.8-5.3 0.3-1.2 0.3-1.2 1:0.21:0.261:0.52:0.27a 0.08-0.56
11.7-28.5 5.9-8.6 1.3-2.3 1.6-2.2 1:0.57:0.201:0.48:0.26 0.58-0.78
1.6-11 3.2-7.0 0.6-1.8 0.5-1.8 1:0.49:0.321:0.52:0.31 0.15-0.56
PL-A
PL-B
16 11837 118 7938 152
241 11857 139 8437 132
26.2 7.8 2.0 2.1 1:0.57:0.19
32.7 8.3 2.0 1.9 1:0.58:0.19
0.78
0.79
a
The other 15 EPA priority PAHs include biphenyl, acenaphthane, anthracene, fluoranthene, pyrene, benz[a]anthracene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[e]pyrene, benz[a]pyrene, perylene, indeno[1,2,3-cd]pyrene, dibenz[a,h]anthracene, benzo[ghi]perylene. b MethylDBT, methyl-dibenzothiophene; three isomers of methyl-DBT are 4-, 2-/3-, and 1-methyl-dibenzothiophene; m-phen: methyl phenanthrene. c The ratios of three methyl-DBT were unable to be determined for about half of group 1 samples due to nearly complete degradation of methyl-dibenzothiophenes.
N2-1A, N2-1B, and R7-1, representing group 1-3 samples, respectively. The field survey showed that physicochemical weathering had begun immediately after the Nipisi oil release, largely due to evaporation of low molecular weight saturated and aromatic hydrocarbons (14-16). Since then, the surface and subsurface oil has continually weathered due to physicochemical and biodegradation processes, but at a much slower rate. Figure 2 clearly demonstrates the effects of weathering and degradation on the chemical composition of the oils. The GC chromatogram of hydrocarbons in the reference oil shows a mixture dominated by a homologous series of n-alkanes ranging in carbon number from n-C8 to n-C40 with the maxima around n-C13-n-C15. The lightly degraded samples (N2-1B) still include distinct, well-resolved n-alkanes but with the abundances of the lighter n-alkanes from C8 up to C15 being much reduced. By contrast, the highly weathered sample, N2-1A, shows very different GC trace from the source oil and the lightly weathered oils, in which the n-alkanes including pristane and phytane were almost completely lost, and only a large hump representing the GC unresolved complex mixture of hydrocarbons is seen in the chromatogram. This kind of chromatographic features is typically characteristic of heavily weathered and biodegraded oil (17). GC analysis results clearly illustrate the following: (a) The two reference oil samples had the highest concentrations of n-alkanes (81 and 93 mg/g oil) and the highest ratios of resolved peaks to TPH (21 and 26%), alkanes to isoprenoids (Alk/Iso, defined as the ratio of the sum of n-C14-n-C18 divided by the sum of farnesane, trimethyl-C13, norpristane, pristane, and phytane; 2.4 and 2.4), C17/pristane (2.2), and C18/phytane (1.5). (b) Group 1 samples were highly degraded and have the lowest concentrations of n-alkanes (for most samples, n-alkanes were nearly completely depleted, while only the high molecular weight n-alkanes were detected for the remaining samples), the lowest ratios of resolved peaks to TPH (1.0-8.0%), Alk/Iso (0.0 for most samples), C17/pristane (0.0-0.2), and C18/phytane (0.0-0.1). (c) Group 2 samples were only lightly to moderately degraded. The chromatograms still include well-resolved and highly abundant alkanes from n-C8 to n-C40. The total n-alkanes including pristane and phytane were determined to be in the range of 30-74 mg/g of TSEM. The higher ratios of resolved peaks to TPH, Alk/Iso, C17/pristane, and C18/ phytane were also demonstrated (Table 2). 2226
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(d) For Group 3 samples, it is interesting to note that both petrogenic and biogenic n-alkane distribution were present. The distribution pattern of n-alkanes showed that the abundances of n-alkanes having an odd-number of carbon atoms are obviously higher than n-alkanes having an evennumber of carbon atoms in the range of n-C21-n-C33. The carbon preference index (CPI, defined as the ratio of the odd- to even-carbon-numbered n-alkanes) values were determined to be 1.5-5.4 in the range of n-C8-n-C40 and 1.9-6.9 in the range of n-C21-n-C33, respectively. These are typical characteristics of biogenic samples. The presence of petrogenic hydrocarbons was also obvious, as indicated by the distribution of n-alkanes in a wide range from n-C10 to n-C40 and the notable presence of the chromatographic unresolved complex mixture (UCM). Compositional Changes of Target Alkylated PAH Homologues. Alkylated PAH homologues have been increasingly used as environmental fate indicators and sourcespecific markers of oils in recent years (9, 10, 17-23). Figure 3 shows the GC/MS (in the selected ion monitoring mode) chromatograms of the aromatic fractions for the reference oil PL-B, N2-1A (0-2 cm), P-1 (4-6 cm), N-1B (12-16 cm) and N-1C (30-40 cm), illustrating the effect of field weathering conditions and sample depths on composition changes of alkylbenzenes and alkylated PAH. Table 3 summarizes the quantitation results of alkylbenzenes and alkylated PAH homologues and the diagnostic weathering and degradation ratios. Figure 4 depicts PAH fingerprints of the reference oil PL-B and samples N2-1A, N2-1C, and N2-1E, representing group 1-3 samples, respectively. As well as containing larger quantities of alkyl benzene homologues, the aromatic fraction of the reference oil PL-B contains mainly alkylated naphthalene, phenanthrene, dibenzothiophene, and fluorene homologues (71%, 18%, 4.3%, and 4.2% of the total quantified PAH homologues, respectively) with C1-C4-naphthalenes being dominant (Figure 4). The total of the other 15 EPA priority PAH was determined to be only 139 µg/g oil (Table 3), among which the lower molecular weight biphenyl (95 ppm), acenaphthalene (16 ppm), and acenaphthene (9 ppm) were the most abundant. Only very small amounts (sub-ppm level) of 5- and 6-ring PAH from benzo[a]pyrene to benzo[ghi]perylene (Figure 4) were detected. Even though a few sites were treated by in-situ burning (1) after the spill, no apparent pyrogenic PAH, which are featured by the dominance of the unsubstituted PAH over their alkylated homologues and dominance of 4-6-
FIGURE 3. GC/SIM/MS distribution chromatograms of alkylated benzenes and alkylated PAHs for the reference source oil PL-B and samples N2-1A (0-2 cm), P-1 (5 cm), N2-1B (12-16 cm), and N2-1C (30-40 cm), illustrating the effect of field weathering conditions and sample depths on composition changes of alkylbenzenes and alkylated PAHs. B and N represent benzene and naphthalene, respectively; n, 0, 1, 2, and 3 represent carbon numbers of alkyl groups in alkylbenzenes and alkylated naphthalene homologues. Note that different Y-axis scales are applied from top to bottom. ring PAH over the low molecular weight 2-3-ring PAH (23), were observed. This is probably because the residual oil swamped out these low-level pyrogenic PAH. Compared to the reference oil, the loss of the PAH and their homologues in the Nipisi samples is very apparent. The major compositional changes of PAH are summarized as follows. The totals of alkylated PAH homologous series were determined to be 1150-2940, 6000-10990, and 124-1230 µg/g TSEM for group 1-3 samples, respectively, much lower than the total of alkylated PAH in the reference oils PL-A and PL-B (11837 and 11857 µg/g oil), indicating differences in weathering degree in the different sample groups. In
addition, the other 15 EPA priority PAH were in ranges of 8-22 and 25-104 µg/g TSEM for group 1 and 2 samples, respectively. Almost no other 15 PAH were detected in group 3 samples. The total of BTEX and C3-benzenes were determined to be 16 and 241 µg/g oil for PL-A and PL-B, which indicates that the reference oil PL-A has been weathered since 1972 for whatever reason if compared to the “fresh” line cut oil PL-B. No volatile alkylbenzenes were detected in group 1 and 3 samples. In contrast, however, the group 2 samples still contain relatively large quantities of volatile BTEX and alkylbenzene compounds (12-225 µg/g TSEM). It is surVOL. 32, NO. 15, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. PAH fingerprints of the reference source oil PL-B and samples N2-1A (0-2 cm), N2-1C (30-40 cm), and N2-1E (80-100 cm). N, P, D, F, and C represent naphthalene, phenanthrene, dibenzothiophene, fluorene, and chrysene, respectively; 0-4 represent carbon number of alkyl groups in the alkylated PAH homologous series. The fingerprints of the other 2-6-ring PAHs are shown in the left insets. The abbreviations from Bph to Bp represent the 15 EPA priority unsubstituted PAHs. Refer to Table 3 for the full names of these PAHs. prising to note the existence of relatively large quantities of BTEX and alkylbenzene compounds in sample N2-1C, 25 years after the spill (Figure 3). For highly weathered group 1 samples, striking decreases in the abundance of naphthalenes relative to other PAH were apparent (compared to the reference oil, greater than 90% of naphthalenes were lost). Also, development of a profile 2228
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in each group showing the composition changes of C0 > C1 > C2 > C3 was clear. The chrysene series exhibited the most pronounced relative increase, which was indicated by the increase of their percentages in the total of PAH (for example, increased from 2.2% for the reference oil PL-B up to 10-26% for the group 1 samples). At the same time, decreases in the relative ratios of the sum of naphthalenes, phenanthrenes,
dibenzothiophenes, and fluorenes to the sum of chrysenes were very pronounced as well (as Table 3 shows, the relative ratios decreased from 33, 8, 2, and 2 for PL-B to 0.4-2.8, 1.8-5.3, 0.3-1.2, and 0.3-1.2 for group 1 samples, respectively). The relative enrichment of chrysenes with the degree of PAH weathering was also observed and discussed by Bence et al. (22) for follow-up studies of the Exxon Valdez spill. Compared to the group 1 samples, group 2 samples showed much lighter weathering and degradation, indicated by insignificant development of the relative percentages of C1 < C2 < C3 in each PAH homologous series and by much smaller decreases in the relative ratios of the sum of naphthalenes, phenanthrenes, dibenzothiophenes, and fluorenes to the sum of chrysenes (they were determined to be 12-28, 6-8, 1.3-2.3, and 1.6-2.2, respectively). It has been demonstrated that these ratios can be used, in combination with the parameters describing the degradation of saturates (such as the total n-alkane contents, Alk/Iso, n-C17/pristane, and n-C18/phytane), to illustrate the overall extent and degree of weathering of the Nipisi samples. In general, more weathered oils have higher relative abundances of chrysenes and thus lower ratios of naphthalenes/chrysenes, phenanthrenes/chrysenes, dibenzothiophenes/chrysenes, and fluorenes/chrysenes (Table 3). Another important characteristic of PAH composition changes is the alteration in the relative distributions of some PAH isomeric series, in particular, the methyl-dibenzothiophenes (m-DBT) and methyl-phenanthrenes (mPhen). Previous studies (24, 25) have demonstrated that the distribution fingerprints of the m-DBT isomers vary from oil to oil. Also, 2-/3-m-DBT is found to be the most easily biodegraded of the isomeric m-DBT series (24-26). Furthermore, 1-m-DBT biodegrades slower than 4-m-DBT (2426). This results in a decrease of the ratio of 2-/3- to 4-mDBT and a slight increase of the ratio of 1- to 4-m-DBT. Differential relative biodegradation also can be seen in the decrease of (3- + 2-methyl-phenanthrene) to (4-/9- + 1-methyl-phenanthrene) ratio. Changes in the diagnostic ratios of these source-specific isomeric PAH have been used for source identification (26, 27) and differentiation of physically weathered and biodegraded oils (24, 25). In the Nipisi samples, as weathering proceeded, not only did the concentrations of these isomeric PAH decrease markedly but the differential abundance ratios were also greatly altered, indicating the occurrence of biodegradation. As Table 3 shows, the relative ratios of 4-:2-/3-:1-m-DBT (132 µg/g in total) and (3- + 2-m-Phen)/(4-/9- + 1-m-Phen) were determined to be 1:0.58:0.19 and 0.79 for PL-B. However, these corresponding ratios for group 1 samples were 1:0.21: 0.26-1:0.52:0.27 (2-20 µg/g of TSEM) and 0.08-0.56. Note that the m-DBT ratio could not be determined for about half of group 1 samples due to nearly complete loss of m-DBT. For group 2 samples, the m-DBT and m-Phen ratios were 1:0.57:0.20-1:0.48:0.26 (30-120 µg/g TSEM) and 0.58-0.78, respectively. These two results further clearly demonstrate that group 1 samples were both much more weathered and biodegraded than the group 2 samples. Petrogenic PAH and alkylated PAH homologues were also detected for group 3 samples, which showed similar distribution patterns to group 1 and 2 samples, but with the concentrations being much lower (124-1200 µg/g TSEM). Compositional Changes of Biomarker Compounds and Estimation of Weathered Percentages of Group 1 Samples. Figure 5 shows the GC/MS distribution chromatograms of biomarker triterpanes at m/z 191 of the reference oil PL-B and samples N2-1A (group 1), N2-1C (group 2), and N2-1E (group 3). Figure 5 is characterized by the triterpane distribution in a wide range from C19 to C35 with C29 Rβ- and C30 Rβpentacyclic hopanes being prominent. As Figure 5 shows,
the distribution profiles and patterns of biomarker compounds were similar for all Nipisi samples. The only noticeable difference between samples is the abundance of terpane components relative to the internal standard (IS). For group 1 and 2 samples, the quantitation results illustrate that (a) the concentrations of C29, C30, C23, and C24 in all samples are comparable, and the fresh reference oils have the lowest concentrations of C29, C30, C23, and C24 hopanes. (2) The accumulation of hopanes relative to the reference oils is apparent, especially for the heavily weathered group 1 samples (for example, the concentration of C30 Rβ-hopane in sample N2-1A was approximately 1.8 times that found in PL-B). (c) The ratios of five pairs of target terpane compounds (C23/C24, Ts/Tm, C29/C30, C32 22S/22R, and C33 22S/22R) are almost the same for most samples. Compared to groups 1 and 2 samples, group 3 samples showed much lower concentrations of petrogenic biomarker compounds because of the effect of the vegetation hydrocarbons. However, biomarker compounds in the group 3 samples can be still clearly identified. As an example, Table 4 presents biomarker quantitation results of group 1 samples for the four most abundant hopanes and the relative ratios of C23/C24, Ts/Tm [Ts: 18R(H),21β(H)-22,29,30-trisnorhopane, Tm: 17R(H),21β(H)-22,29,30-trisnorhopane], C29/C30, and 22S/22R of C32 and C33 hopane isomers as well. The ratios of C23/C24, Ts/Tm, and C29 Rβ-/C30 Rβ-hopane have been used in recent years for the purposes of source identification and differentiation of oils, monitoring and evaluation of oil weathering, and degradation under a variety of conditions (9, 10, 28-32). Under most natural weathering conditions, these ratios, in particular the ratio of C29 Rβ-C30 Rβ-hopane, suffer little change and remain characteristic of the source oil. However, it has been demonstrated from Arrow (9) and BIOS (10) oil spill studies that if residual oils in sediments have undergone long-term extensive weathering and biodegradation, C23 and C24 terpane can still be biodegraded. In addition, under very severe degradation conditions, Tm has a relatively faster degradation rate than Ts (33), even though Ts chromatographically elutes out earlier than Tm, resulting in an increase of the Ts/Tm ratio. Table 4 demonstrated that no such changes were observed even for the most heavily weathered and degraded group 1 samples. The steranes in the Nipisi oil samples were much less abundant than hopanes and essentially consist of RRR and Rββ C27 cholestanes, C28 ergostanes, and C29 stigmastanes with the lower molecular weight C21 and C22 steranes being also present and quite prominent. A method to quantify the oil depletion has been developed using the biodegradation-resistant C29 and C30 hopane as an internal conservative reference in comparison with the concentration of C29 and C30 Rβ-hopane in the fresh source oil (28, 29). The estimated weathered percentages of the group 1 samples (using C29 and C30 Rβ-hopanes in the PL-B oil as internal oil references) are also listed in Table 4. It should be understood that the PL-B oil was subsampled from a half-full glass mason jar that had been stored in the lab at the University of Alberta since 1972; therefore, some weathering of this oil might be expected due to 25 years of storage, and it was actually not a true fresh oil (the GC analyses confirmed that this oil has been slightly weathered). Hence, the “true” weathered percentage values of the group 1 samples should be slightly higher than the computed values listed in Table 4. The weathered percentage of PL-A was determined to be ∼6% relative to PL-B. Penetration and Retention of Oil Hydrocarbon Fractions. Interestingly, the three lightly weathered/degraded group 2 samples N1-1, N2-1C, and N5-1B, which were collected at a greater depth (25-40 cm) than the other group VOL. 32, NO. 15, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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2229
FIGURE 5. GC/MS biomarker terpane distribution chromatograms (m/z 191) for the reference source oil PL-B and samples N2-1A (0-2 cm), N2-1C (30-40 cm), and N2-1E (80-100 cm). IS represents the internal standard C30 ββ-hopene; C23, C24, C29, C30, C32, and C33; C23 and C24 terpane, C29 and C30 rβ-hopane, 22S/22R isomer pairs of C32 and C33 hopane, respectively, and Ts and Tm represent 18r(H),21β(H)22,29,30-trisnorhopane and 17r(H),21β(H)-trisnorhopane, respectively.
TABLE 4. Quantitation Results and Diagnostic Ratios of Selected Biomarkers and Estimation of Weathered Percentages of Residue Oils for Group 1 Samples concentrations (µg/g TSEM)
diagnostic ratios
weathered percentages (%)
C23
C24
C29rβ
C30rβ
C23/C24
Ts/Tm
C29/C30
C32(S)/ C32(R)
N1-2 N2-1A N2-2 N2-3A N4-2 N5-1A N6-1 N6-2 R2-1A R5-1
56.7 68.1 83.3 55.1 63.6 72.9 63.7 58.8 57.8 59.7
39.5 43.1 55.1 37.8 40.2 50.3 41.3 39.6 38.7 37.5
163.8 91.7 96.8 63.8 87.0 74.5 90.5 70.8 74.7 66.1
154.4 240.9 242.4 163.8 222.5 197.5 221.0 185.6 189.6 178.5
1.44 1.58 1.50 1.46 1.58 1.45 1.54 1.48 1.50 1.59
1.20 1.19 1.29 1.28 1.15 1.27 1.13 1.15 1.18 1.20
0.39 0.44 0.40 0.39 0.39 0.38 0.41 0.38 0.39 0.37
1.44 1.47 1.54 1.41 1.39 1.46 1.35 1.47 1.40 1.39
1.55 1.43 1.52 1.55 1.46 1.53 1.48 1.49 1.50 1.45
15.2 40.2 43.4 14.1 37.0 26.4 39.4 22.6 26.7 17.1
Ref. PL-A Ref. PL-B
47.0 43.3
29.6 26.9
58.0 54.8
147.1 136.9
1.59 1.61
1.15 1.15
0.40 0.40
1.46 1.43
1.47 1.43
5.5 6.9 (used as reference)
samples
2 samples (10-15 cm), have n-alkane distribution patterns that were remarkably different from the remaining group 2 samples. For example, samples N5-1B and N4-1 have similar concentrations of total n-alkanes, 62-65 mg/g of TSEM, but the n-alkane distribution patterns of these two samples differ greatly. As one would expect of a weathered oil, sample N4-1 has lost most of the low boiling point n-alkanes but 2230
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 15, 1998
C33(S)/ C33(R)
based on C29 hopane
based on C30 hopane
Average
16.4 43.2 43.5 16.4 38.6 30.7 38.1 26.2 27.8 23.3
16 42 43 15 38 29 39 24 27 20 6
retained much of the higher boiling point fraction. For the sample N5-1B, however, the low molecular weight n-alkanes were not depleted while the higher molecular weight n-alkanes were dramatically less than the majority of group 2 samples. It is also noted that the n-alkanes with carbon numbers greater than 34 were entirely “lost”. This distribution of n-alkanes is highly unusual for weathered oil samples.
As a result of this abnormal n-alkane distribution, the samples N2-1C and N5-1B have abnormally high weathering indices (WI, defined as the ratio of the sum of n-C8,10,12,14,16 divided by the sum of n-C22,24,26,28,30) of 3.6 and 3.4, respectively. In contrast, the WI value of the reference oil PL-B was determined to be only 2.6. Normally, the WI of a spilled oil is assumed to decrease with weathering as preferential loss of the low molecular weight n-alkanes occurs by evaporation, biodegradation, and other factors. This is the case for all group 2 samples collected above ∼15 cm, which have the WI values between 0.5 and 2.3 (Table 2). The question then is why are samples N2-1C and N5-1B enriched with low molecular weight n-alkanes when compared with the other lightly weathered group 2 samples? To answer this, we must consider the water washing effect and the interaction between oil hydrocarbon fractions and soil components. The Nipisi site was quite flat, and groundwater movement was relatively slow (1), which means that the oil would not be easily washed away but would largely migrate vertically through the soil column as the groundwater level fluctuated. Recently, a study reported (34) that while oil penetration into and retention by sediment is largely controlled by sediment characteristics such as the content of organic substances, mean grain size, and sorting. Numerous studies have revealed that pesticide retention by soils is greatly affected by the structure and size of the pesticide molecules and the soil organics (35). Similar behavior would be expected for the interactions between the soil organics and the oil hydrocarbons. We suggest that a sort of “chromatographic” process occurred in some of the Nipisi sites. As the water level fell, oil was drawn deeper into the soil where the lighter alkanes were more firmly absorbed to the soil humic substances than the heavier components. Successive water level fluctuations would have the effect of enriching the concentration of the lighter fractions in the deeper samples while depleting the concentration at the surface of the bog, resulting in this abnormal distribution of n-alkanes. The interaction between petroleum fractions and soil components in wetlands is an interesting and neglected area of study. More work is needed to understand this important phenomenon. Relationship of Contamination and Degradation to Sample Depth. Through analyzing the GC data, it is noted that a certain relationship of contamination and degradation to sample depth exist for the Nipisi samples collected at the same location. For example, samples collected from the location N2 demonstrated contamination and degradation trends with sample depth as follows: (a) Contamination level decreases in relation with sampling depth: the TSEM values were determined to be 714, 334, 126, and 19 mg of TSEM/g of dried sample for N2-1A, N2-1B, N2-1C, and N2-1E, which were collected from the depths of 0-2, 12-16, 30-40, and 80-100 cm, respectively. (b) The surface sample N2-1A (0-2 cm), even though it has the highest TSEM value, was the most highly weathered and degraded sample as evidenced by low GC-TPH value, the lowest ratio of resolved peak area to TPH (only 1.0%), very low PAH concentration (
