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Spills of Emulsified Fuels: Risks and Response
2
Behavior and Fates: Summary and Evaluation of Available Information
BEHAVIOR AND FATE
Physical Characterization
Much of the scientific research on the fate of emulsified fuels has been focused on a particular commercial product called Orimulsion manufactured in Venezuela. The following properties specifically refer to Orimulsion-400, which replaced an earlier version of the product called Orimulsion-100. Orimulsion is a mix of approximately 70 percent natural bitumen (pitch) and 30 percent fresh water, with additives to maintain the emulsion. The additives are 1,100 parts per million (0.11 percent) MEA, an emulsion stabilizer, and 1,350 ppm (0.135 percent) AE, a water-soluble nonionic surfactant (Golder and Associates, 2001) that retards the coalescence of bitumen droplets. The designations 100 and 400 refer to the type of nonionic surfactant used in the formulation. (Much of the existing literature discusses studies of Orimulsion-100 thus this report makes the distinction as needed.)
With a specific gravity greater than 1.0, Orimulsion is classified for regulatory purposes as a Group V oil (Stout, 1999). The density of the product is intermediate between fresh and salt water (Figure 2.1). It is comparable in pour point1 and viscosity to an industrial fuel oil. The dynamic viscosity of Orimulsion varies with temperature but is in general lower than that of many Group V oils.
1
The lowest temperature at which a substance, such as oil, will flow under specified conditions.
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Spills of Emulsified Fuels: Risks and Response
FIGURE 2.1 Density-temperature relationship for Orimulsion-400, bitumen, and different salinity waters. SOURCE: Reprinted from PDVSA–Intevep,1998. Permission granted by Bitor America Corporation.
The flash point is > 95°C, and thus Orimulsion is classified as nonflammable by the National Fire Protection Association (NFPA).
Pure bitumen has a high viscosity, on the order of a million centipoise at typical ambient water temperatures. The density and the pour point are higher than those of the emulsified Orimulsion product (Figure 2.1). Jokuty et al. (1995) list a pour point of 38oC for bitumen versus a pour point of 3oC for fresh Orimulsion. In completely quiescent conditions, dispersed bitumen droplets would be expected to sink or to be neutrally buoyant in water at 15°C and salinity less than 10-15 parts per thousand (ppt)2 (Crosbie and Lewis, 1998a), but they would be expected to float in more saline water.
The average bitumen droplet size in fresh Orimulsion is around 20 µm (Ostazeski et al., 1998a, 1998b). Although the droplet size distribution (Figure 2.2 ) is somewhat bimodal (Stout, 1999), almost all of the droplets range in size from 1 to 80 µm. Exposure to even low-salinity water (>5-7 psi (Practical Salinity Units) collapses the surfactant in Orimulsion (Brown et al., 1995; Crosbie and Lewis, 1998b); releasing the bitumen from the emulsion where it can form
2
The documents examined report concentrations in metric units such as mg/L or parts per million. This difference reflects differences in common use between scientific practice within the community. This report may use either or both, depending on use in the literature reviewed. For the concentrations and conditions discussed in this report, the conversion between systems is unwarranted.
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Spills of Emulsified Fuels: Risks and Response
FIGURE 2.2 Histogram showing average bitumen droplet size in fresh Orimulsion-400. SOURCE: Stout, 1999.
droplets. If the droplets collide, the bitumen may coalesce and form larger droplets, which (based on Stokes equation, Figure 2.3), increases the likelihood that the droplet will either surface or sink, depending on its relative buoyancy compared to the surrounding water.
Because Orimulsion is not a homogeneous fluid, its properties may change significantly during the course of an accidental release. According to Febres et al. (1995), at a concentration greater than 20,000 ppm the product retains its emulsion properties, while at a concentration less than 10,000 ppm the material is expected to behave like dispersed bitumen droplets. Such high concentrations would exist only for an open-water spill within the immediate vicinity of the spill source and for a very short time. The exception would be a spill scenario in which mixing with the ambient water and subsequent dilution of the product were restricted. For the most part, predicting Orimulsion spill behavior becomes a matter of predicting the behavior of the dispersed bitumen cloud.
Chemical Characterization
The Cerro Negro bitumen used to produce Orimulsion comes from the Orinoco Belt in the Eastern Venezuelan Basin (Bitumenes Orinoco). This bitumen is highly weathered (degraded) in nature and consists primarily of highmolecular-weight, multi-ring aromatic hydrocarbons and resins that account for
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Spills of Emulsified Fuels: Risks and Response
FIGURE 2.3 Re-float velocity of bitumen droplets (assumed to be spherical) by Stokes equation in seawater (35 ppt) at 15oC.
63 to 69 percent of the fuel (Ostazeski et al., 1998a, 1998b; Brown et al., 1995; Jokuty et al., 1999; Stout, 1999). Figure 2.4 presents flame ionization detector gas chromatographic profiles. These profiles show neat Orimulsion (sample 3), an oil-in-water dispersion of 18,250 mg/L Orimulsion (dissolved phase) filtered through a 1-µm (micron) membrane (sample 4), and an oil-in-water dispersion of 5,475 mg/L Orimulsion that has not been filtered (sample 8), (Wang and Fingas, 1996). The neat Orimulsion is characterized by a bimodal unresolved complex mixture (Fig. 2.4A). Very few of the individual constituents present in the Orimulsion are partitioned into the dissolved phase (Fig. 2.4B). The unfiltered sample shows the total petroleum hydrocarbons clearly associated with the bitumen droplets as evidenced by the pattern which is almost identical to the whole Orimulsion pattern (Fig. 2.4C).
Orimulsion has very low concentrations of BTEX that are at least an order of magnitude lower than in the typical No. 6 fuel oil it is likely to replace (Table 2.1). Individual and total polynuclear aromatic hydrocarbon (PAH) concentrations in Orimulsion are up to one order of magnitude below those typically found in crude oils and refined products (Table 2.2). Orimulsion is relatively high in sulfur, nickel, and vanadium (Table 2.1), with the latter two constituents tied up as metalloporphyrins, which make them biologically unavailable.
The additives used in the production of Orimulsion-400 are a water-soluble nonionic surfactant (a narrow distillate cut of widely used AE) and an emulsion
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Spills of Emulsified Fuels: Risks and Response
FIGURE 2.4 Flame ionization detector gas chromatographic profiles for neat Orimulsion (sample 3), an oil-in-water dispersion filtered through a 1-µm membrane (sample 4), and an oil-in-water dispersion that has not been filtered (sample 8). SUR and IS represent surrogate and internal standards, respectively. SOURCE: Wang and Fingas, 1996. Copyright 1996 American Chemical Society.
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Spills of Emulsified Fuels: Risks and Response
TABLE 2.1 Physical Properties and Chemical Composition of Orimulsion-400 and No. 6 Fuel Oil
Parameter
Orimulsion-400
No. 6 fuel oil
Density (g/ml at 15oC)
1.01-1.02
0.94-1.02
Pour point (oC)
0-3
-17-36
Viscosity (cP)
200-350 (at 30oC)
325-47,000 (at 15oC)
Mean particle size (µm)
14-20
Not applicable
Sulfur content (wt%)
2.85
0.7-3.0
Hydrocarbon groups (wt%)
Saturated
14
11
Aromatic
47
55
Resins
22
20
Asphaltenes
17
14
Total BTEX (ppm)
36
464
Benzene
0
17
Toluene
4
100
Ethylbenzene
19
47
Xylene
13
300
Total PAH (µg/g bitumen)
3,040
317,627
Naphthalenes (C0-C4)
474
46,600-106,000
Phenanthrenes (C0-C4)
854
50,200-113,000
Dibenzothiophenes (C0-C4)
1,330
18,713
Fluorenes (C0-C4)
348
12,800-30,700
Chrysenes (C0-C4)
168
21,600-48,700
Metals (ppm)
Nickel
55
37
Vanadium
310
32
Zinc
19
45
SOURCE: Bitumenes Orinoco, S.A., b; National Oceanic and Atmospheric Administration ADIOS Model Database.
stabilizer, MEA. The purpose of these components is to maintain the stability of the bitumen droplets in the emulsion by preventing particle-particle agglomeration and coalescence.
The AE that make up the surfactant are composed of a long-chain fatty (alkyl) alcohol (hydrophobic) and an ethylene oxide (EO) chain (hydrophilic), connected by an ether linkage. The nomenclature of AE is determined by the average number of carbons in the alkyl chain of the alcohol and the number of EO groups in the hydrophilic moiety (e.g., C10-12EO8 represents an alcohol with 10 to 12 carbons attached to polyethylene oxide with 8 EO units).
The alcohol ethoxylate used in Orimulsion-400 is known as GENAPOL X 159 and is complexly branched. It contains a mixture of highly branched C12 (22-30 percent) and C13 (70-78 percent) fatty alcohols with anywhere from 9 to 22 (EO9 to EO22) EO groups (Bjornestad et al., 1998; Bowadt et al., 1998). The
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Spills of Emulsified Fuels: Risks and Response
TABLE 2.2 PAH Concentrations in Orimulsion-400, Crude Oils, and Petroleum Products
Sample Type PAHs
Average Light Crude (average of 19 oils)
(µg/g oil)
Prudhoe Bay crude
(ug/g oil)
Average Heavy Crude (average of 6 oils)
(µg/g oil)
Arab. Med. Crude
(µg/g oil)
Diesel No. 2
(µg/g oil)
Average Bunker C (average of 4)
(µg/g oil)
Orimulsion
(µg/g oil)
Cold Lake Bitumen
(µg/g oil)
Naphthalene
C0-N
45.6
46
21.0
21.0
1404.6
28.2
9.3
47.0
C1-N
540.4
540
148.9
148.9
5174.0
87.6
47.2
128.0
C2-N
1696.9
1697
427.0
427.0
7031.6
327.8
165.7
490.0
C3-N
2082.9
2083
602.4
602.4
5591.4
541.0
243.0
839.0
C4-N
990.6
991
313.2
313.2
2963.6
507.0
309
705.0
Sum
5356
5356
1513
1513
22165
1492
774
2209
Phenanthrene
C0-P
41.4
41
80.4
80.4
455.8
68.4
45.7
91.0
C1-P
348.3
348
324.2
324.2
657.5
210.2
133.4
287.0
C2-P
462.3
462
405.5
405.5
223.5
406.3
306.5
506.0
C3-P
380.3
380
356.4
356.4
33.0
472.2
425.4
519.0
C4-P
262.2
262
225.8
225.8
6.3
202.3
228.6
176.0
Sum
1494
1494
1392
1392
1376
1359
1140
1579
Dibenzothiophene
C0-D
185.8
186
37.4
37.4
367.4
34.9
16.7
53.0
C1-D
623.5
624
124.9
124.9
414.9
136.8
67.6
206.0
C2-D
1092.9
1093
212.3
212.3
160.5
337.5
222.9
452.0
C3-D
967.5
968
192.5
192.5
30.6
405.0
364.0
446.0
Sum
2870
2870
567
567
973
914
671
1157
Fluorene
C0-F
43.1
43
18.6
18.6
179.4
21.1
10.1
32.0
C1-F
120.3
120
60.6
60.6
404.5
54.8
38.6
71.0
C2-F
240.6
241
98.0
98.0
375.0
135.8
126.7
145.0
C3-F
225.8
226
109.3
109.3
221.3
152.2
134.4
170.0
Sum
630
630
286
286
1180
364
310
418
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Spills of Emulsified Fuels: Risks and Response
Chrysene
C0-C
17.1
17
32.0
32.0
0.5
21.4
14.9
28.0
C1-C
26.3
26
82.9
82.9
0.0
39.8
29.5
50.0
C2-C
37.7
38
139.8
139.8
0.0
62.0
53.9
70.0
C3-C
34.5
35
72.5
72.5
0.0
44.6
46.3
43.0
Sum
116
116
327
327
1
168
145
191
TOTAL PAH
10466
10466
4086
4086
25696
4297
3040
5554
Other PAHs
Biphenyl
26.9
26.90
2.7
2.7
363.4
11.2
5.4
Acenaphthalene
10.2
10.22
0.7
0.7
31.7
2.7
1.6
Acenaphthene
7.5
7.50
1.0
1.0
24.5
3.2
11.2
Anthracene
3.1
3.06
0.8
0.8
59.5
6.5
2.7
Fluoranthene
3.2
3.19
0.7
0.7
0.1
3.4
3.2
Pyrene
4.3
4.26
4.8
4.8
0.3
1.8
6.5
Benz(a)anthracene
0.9
0.86
5.2
5.2
0.0
0.3
3.4
Benzo(b)fluoranthene
1.3
1.26
3.2
3.2
0.0
2.2
1.8
Benzo(k)fluoranthene
0.4
0.43
0.7
0.7
2.1
0.3
Benzo(e)pyrene
3.1
3.13
12.2
12.2
0.0
6.7
2.2
Benzo(a)pyrene
0.5
0.46
2.4
2.4
0.1
0.3
2.1
Perylene
0.1
0.09
0.8
0.8
0.0
0.2
6.7
Indeno(1,2,3cd)pyrene
0.1
0.06
0.2
0.2
0.1
1.9
0.3
Dibenz(a,h)anthracene
0.2
0.21
1.4
1.4
0.0
49.5
0.2
Benzo(ghi)perylene
0.6
0.58
7.0
7.0
0.0
48.6
1.9
TOTAL
62
62
44
43.8
480
141
49
Source: Environment Canada, 2001.
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Spills of Emulsified Fuels: Risks and Response
selected group of C12 and C13 branched AE was used in Orimulsion-400 rather than nonylphenol ethoxylates used in Orimulsion-100, because of the latter’s potential for endocrine disruption and the fact that the degradation metabolites were more toxic and persistent than the parent compounds (Harwell and Johnson, 2000).
Because of their widespread use in household products, much of the research on microbial degradation of AE has been based on their removal efficiencies in wastewater treatment facilities. For various types of treatments, 86-99 percent of the AE in wastewater influents are degraded to some intermediate form (Talmage, 1994). It has long been held that the major, if not only, route for linear AE biodegradation is that of cleavage at the ether bridge between the alkyl chain and the EO moiety (Swisher, 1987). After that, biodegradations of fatty alcohols and polyethylene glycols (PEG) were believed to proceed independently and more slowly.
Branching of the alkyl chain in the vicinity of the central ether bridge appears to inhibit central ether cleavage (Di Corcia et al., 1998). In addition, the same study (Di Corcia et al., 1998) reported that bacterial attack on the ethoxy chain produced metabolites with the EO either shortened or, to a lesser extent, oxidized to a terminal carboxylic acid group. They also reported end-of-chain oxidation of both alkyl side chains in branched AE to form very polar di- and tricarboxylic acids. Marcomini et al. (2000a,b) reported fast biomediated etherlinkage cleavage of linear and short-chain (methyl or ethyl) C2-monobranched AE with slower biodegradation of the released PEG by both hydrolytic shortening and oxidative hydrolysis to form shorter PEG oligomers and carboxylated PEG. Taken together, these studies suggest that the AE mixtures used in Orimulsion-400 formulations are capable of slow aerobic biodegradation. In fact, one study on the rate of biodegradation of GENAPOL X 159 stated that “the toxicity of the AE was still 100 percent even after 56 days of biodegradation.” This is in accordance with the results of Bjornestad et al. (1998) who found biodegradation of only 35 percent of the AE. This rate of biodegradation is at variance however with oxygen consumption studies on the specific AE mixture used for Orimulsion-400 that have shown a biodegradability (compared to complete chemical oxygen demand with potassium dichromate) of 79 percent in seawater over a 28-day period (VKI, 1997a).
MEA, the substance used to stabilize Orimulsion, is widely used in healthcare products and the surfactant industry. It is not considered mutagenic or carcinogenic (Johnson, 1998) and is utilized and metabolized by plant, animal, and microbial cells. As such, it is quickly transformed in the environment with a halflife of days to weeks (Johnson, 1998). Oxygen consumption studies on MEA have shown a biodegradability (compared to complete chemical oxygen demand with potassium dichromate) of 74 percent in seawater over a 28-day period (VKI, 1997b). Because of its high water solubility, MEA has a low potential for bioaccumulation. This is discussed in greater detail in Chapter 3.
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Spills of Emulsified Fuels: Risks and Response
TABLE 2.3 Selected Dissolved Aromatic Hydrocarbon Concentrations (Mean and Standard Deviation, µg/L) After Five-Day, Zero-Headspace Equilibrium Exposure Studies
Fuel
Benzene
Toluene
Xylene
Naphthalene
No. 6 fuel oil
73.0 ± 15.8
197.3 ± 29.2
126.5 ± 17.2
142.5 ± 23.6
Orimulsion
5.9 ± 4.2
57.8 ± 5.1
3.8 ± 1.6
4.9 ± 2.5
SOURCE: Brown et al., 1995.
A detailed chemical characterization of the water-soluble components contained in the 30 percent water added to make Orimulsion and the water-soluble fractions (WSF) generated by dispersions of Orimulsion into fresh and salt water was completed by Potter et al. (1997). In addition, Brown et al. (1995) examined the dissolution behavior of Orimulsion and No. 6 fuel oil spilled in water of varying salinities. These studies concluded that the majority of the surfactants are contained in the aqueous phase and that they would be diluted by the receiving water during an Orimulsion spill. BTEX constituents were not observed in 1:9 (volume:volume) dispersions of Orimulsion in water at a detection limit of 0.2 µg/L (Potter et al., 1997). Table 2.3 presents benzene, toluene, xylene and naphthalene concentrations from zero-headspace, five-day equilibrium exposure studies comparing Orimulsion and No. 6 fuel oil (Brown et al., 1995). In general, levels of volatile and water-soluble components are higher by up to one order of magnitude for the No. 6 fuel oil tested in their studies compared to Orimulsion.
To date, no detailed chemical analyses of the dissolved-phase PAH concentrations in the 30 percent aqueous phase of Orimulsion have been reported. Using equilibrium partition theory and the initial concentrations of PAH in neat Orimulsion, Stout (1999) and French (2000) calculated the initial concentrations of PAH expected in the aqueous phase of Orimulsion; these values are presented in Table 2.4. The highest concentrations of any PAH are for the naphthalenes, and they are only around 2 µg/L—in general agreement with the values measured by Brown et al. (1995), with initial concentrations for other PAH rapidly decreasing to values in the range of 0.1-0.8 µg/L. The sum of the PAH, or the total PAH (TPAH) with log Kow values <5.6 is only 14.9 µg/L. Brown et al. (1995) examined the time-series kinetics of dissolution behavior from Orimulsion and No. 6 fuel oil, and concluded that little or no additional environmentally significant PAH dissolution occurs upon release of Orimulsion to the environment. In their dilution studies, however, they did not dilute the Orimulsion fuel enough to allow for continued dissolution of PAH from the bitumen phase. Instead, they concluded that the dissolved-phase components measured in their experiments were simply from the dilution of the already near-equilibrium concentrations of PAH in the initial aqueous phase. A similar conclusion was reached by Stout (1999) who examined the dissolution behavior of the high-molecular-weight PAH frac-
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Spills of Emulsified Fuels: Risks and Response
TABLE 2.4 Dissolved PAH Concentrations Estimated in the Aqueous Phase of Fresh Orimulsion
PAH
Molecular Weight (g/mol)
Log Kow
Concentration in Bitumen (mg/kg)
Dissolved Concentration (µg/L)
Naphthalene
128
3.37
15.4
2.363
C1 naphthalenes
142
3.87
43.03
2.263
C2 naphthalenes
156
4.37
136.7
2.464
C3 naphthalenes
170
5
189.37
0.886
C4 naphthalenes
185
5.55
267.97
0.386
Biphenyls
154
3.9
5
0.247
Acenaphthylene
152
4.07
0
0
Acenaphthene
154
3.92
10.66
0.504
Dibenzofuran
168
4.31
5.42
0.111
Fluorene
166
4.18
13.52
0.366
C1 fluorenes
181
4.97
57.39
0.286
C2 fluorenes
196
5.2
184.39
0.562
C3 fluorenes
211
5.5
272.13
0.436
Dibenzothiophene
184
4.49
28.19
0.393
C1 dibenzothiophene
199
4.86
133.96
0.846
C2 dibenzothiophene
214
5.5
345.13
0.553
C3 dibenzothiophene
228
5.73
692.83
0.679
Phenanthrene
178
4.57
067.78
0.796
Anthracene
178
4.54
0
0
C1 phenanthrenes-anthracenes
192
5.14
143.84
0.499
C2 phenanthrenes-anthracenes
207
5.25
366.41
1.003
C3 phenanthrenes-anthracenes
222
6
459.43
0.252
C4 phenanthrenes-anthracenes
237
6.51
241.73
0.0446
Fluoranthene
202.3
5.22
0
0
Pyrene
202.3
5.18
0
0
Sum (Kow<5.6)
2,286
14.964
SOURCE: Stout, 1999.
NOTE: Concentrations are calculated from the PAH composition of whole Orimulsion data from Stout (1999) and equilibrium partitioning (data provided by D. French McCay, A.S.A. Narragansett, RI) to estimate dissolved PAH composition (PAH concentrations listed as zero if log Kow >5.6).
tion of Orimulsion-400 after 24 hours’ exposure in benchtop 4-liter stirred-beaker experiments. Figure 2.5 presents a histogram plot showing the relative abundance of high-molecular-weight PAH in fresh Orimulsion and the dissolved constituents in the water generated by gently stirring a 1,700-mg/L dispersion of Orimulsion in fresh water for 24 hours. The PAH dissolved in the water are enriched in the more soluble low-molecular-weight two-ring PAH (naphthalene and its alkyl-substituted homologues), which were the predominant dissolvedphase components in the near-equilibrium 30 percent aqueous phase in the original Orimulsion fuel mixture (e.g., see Table 2.4). As discussed further in the section titled “Water Column Processes - Dissolution,” the high-molecular-weight
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Spills of Emulsified Fuels: Risks and Response
(Bitumenes Orinoco, S,A., a; Brown et al., 1995; Johnson et al., 1998; Stout, 1999). Brown et al. (1995) examined the interaction of Orimulsion bitumen with fine and coarse sediment in saline water and reported that adhesion equilibrium reached up to 1 and 2.5 mg of bitumen per gram of coarse sediment at low and high sediment loads, respectively. Adhesion approached maximum values of 2,000 and 6,000 mg of Orimulsion bitumen per gram of fine sediment in batch equilibrium experiments. Orimulsion interactions with SPM are physical adhesion processes that seem to be favored by high salinity, sediment surface area, and/or high organic carbon content (Brown et al., 1995). In comparing the behavior of Fuel Oil No. 6 and Orimulsion, Brown et al. (1995) reported that the adhesion of No. 6 fuel oil to Tampa Bay sediments was negligible compared to Orimulsion. For 15-minute contact experiments, the fine sediment fraction showed approximately 1,400-mg/g loadings for Orimulsion and only 6 mg/g for No. 6 fuel oil. The coarse fraction loadings were 3.3 mg/g and 0.03 mg/g for Orimulsion and No. 6 fuel oil, respectively. Under brackish or full-strength seawater salinities, dispersed bitumen-SPM interactions can lead to formation of bitumen-SPM agglomerates that can be transported to the bottom (Brown et al., 1995; Stout, 1999). In flume studies conducted under extremely high-energy and full-strength seawater salinity conditions, up to 11 percent of the total bitumen was observed to be transferred eventually to the bottom in water containing high (45 mg/L) suspended loads of kaolinite (Stout, 1999). Similar experiments conducted in fresh water showed no transport of bitumen-kaolinite agglomerates to the bottom, with the majority of the dispersed bitumen remaining suspended in the water column, presumably because of the effectiveness of the surfactant in fresh water.
Long-Term Fate and Microbial Degradation of Sedimented Bitumen Droplets
It appears that Orimulsion is capable of being biodegraded although the rates are believed to be extremely slow. Brown et al. (1995) observed overgrowth of microbial life in Orimulsion weathering reactors containing Tampa Bay seawater, and Lapham et al. (1999) confirmed the slow aerobic biodegradation of Orimulsion bitumen, with 1-3 percent degraded over a 21- to 60-day period. Compared to degradation rates for crude oil, where 10 percent of the alkanes and 50 percent of the PAH were degraded over similar time scales, it is clear that the rates for bitumen degradation are extremely low. However, the authors concluded that it was remarkable that any microbial respiration of the highly degraded bitumen droplets occurred at all. They also observed that bitumen degradation was greater in the presence of the AE surfactant, which was believed either to act as a co-metabolite providing a readily available carbon and energy source to the bacteria or merely to increase bitumen reactivity by promoting its dissolution in water. In a subsequent study, Proctor et al. (2001) found that the addition of seagrass and pinfish to sediment microcosms stimulated the in situ degradation
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Spills of Emulsified Fuels: Risks and Response
of bitumen by as much as two- to ninefold. This suggests that bioremediation augmented by the addition of natural marine carbon substrates may be a viable option for responding to spilled Orimulsion in the marine environment. Their studies also demonstrated that the bacteria were not growing on Orimulsion but were simply respiring it as a co-metabolite.
SHORELINE PROCESSES
Orimulsion Spills on Land and Shorelines
Fresh Orimulsion spilled directly onshore initially behaves similarly to heavy oils of like viscosity. It will tend to penetrate deeper into beaches that are wet and have gravel substrates. However, the mobility of Orimulsion decreases rapidly as it weathers and comes in contact with dry substrates, which could reduce the contamination threat to groundwater (AEA Technology, 1996). When spilled onto sand, the fresh emulsion is filtered, breaking the emulsion. The particles can penetrate approximately 4-5 cm before they occlude the available pore space, resulting in a maximum loading of approximately 55-60 percent bitumen in the surface layer on the contaminated sand (Harper and Kory, 1997). The water phase is available for percolation into the substrate, and the potential for groundwater contamination will be a function of local geology and spill size.
When floating bitumen strands on shorelines and dries, it becomes stickier and will not resuspend with the tide. Weathered bitumen, which is significantly stickier than fresh Orimulsion, will not penetrate sand as readily and is expected to initially penetrate only the coarsest beach sediments (cobble-boulder). With surface warming of the bitumen-coated substrates by solar radiation, however, the weathered bitumen may become fluid enough to percolate deeper into sediments (Harper and Kory, 1997).
Stranding of Weathered Bitumen
As long as bitumen that is dispersed in the water remains wet within the sediments, it will penetrate freely into pebble beaches but will remain on or near the surface of sand beaches (Harper and Kory, 1997). Unlike weathered bitumen patties, such dispersed bitumen is more mobile within the coarse-grained beach substrate than a typical heavy fuel oil. Dispersed bitumen can be flushed from sediments, but normal tidal flushing may not provide sufficient energy to remove it completely. Once the bitumen droplets have been exposed to air, they become much stickier and can form a tenacious coating on the surface of the beach sediments. This coating is difficult to remobilize, even if there is subsequent rewetting of the bitumen.
Bitumen that has already formed weathered patties or ropes on the water surface before reaching the shoreline is highly viscous and sticky, with many of
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the same characteristics as weathered fuel oils. It is unlikely to penetrate into sediments finer than pebbles (Harper and Kory, 1997). Sticky bitumen patties can mix with sand suspended by waves. Mixing with more than 1 percent suspended sand can cause the droplets to become heavier than typical seawater and sink.
USE OF MODELING AND SCENARIOS TO UNDERSTAND THE BEHAVIOR OF ORIMULSION SPILLS
A widely used oil spill model has been modified to simulate possible Orimulsion behavior (French and Mendelsohn, 1995). This model has been used to evaluate the effects of hypothetical Orimulsion spills in Tampa Bay and Delaware Bay and River (French McCay and Galagan, 2001) and as an analysis tool in a series of Orimulsion test spills in the Caribbean (French et al., 1997). Although the model incorporates standard spill model approximations and assumptions, certain parameters specific to Orimulsion were estimated from laboratory or small field experimental data. One such key parameter is the rate of bitumen coalescence, which was estimated by doing an empirical curve fit to bitumen concentration (French, 2000). Since this curve fit uses confined sample data, it is most likely too large. Although the algorithms of the model have been published (French and Mendelsohn, 1995; French et al., 1996), the committee was unable to verify that the numerical code in the model accurately represents these algorithms.
Other spill models have likewise been modified to simulate Orimulsion spill behavior using somewhat different approximations (VKI, 1999) and would presumably provide different predictions, although no direct comparison of the different models has been done. Barring an actual spill event, such model comparisons provide guidance on the sensitivity of selected environmental and computational parameters in model forecasting and may be an area of future research. Because each spill is unique, even an actual spill would only validate a model involving those particular circumstances. Nevertheless, modeling an emulsified bitumen fuel spill in different scenarios can be useful in sensitivity analysis and in determining where additional data are required. French and Mendelsohn (1995) used their model to assess the sensitivity of the model predictions to key factors such as bitumen coalescence. They also performed comparative studies of hypothetical Orimulsion and Fuel Oil No. 6 spills. They concluded that a large variety of scenarios are necessary to span the range of possible effects of emulsified bitumen spills, both in an absolute sense and in a relative sense for comparison with spills of other oil products. The committee selected six distinct spill scenarios to assess the expected behavior and fate of an Orimulsion spill.
The scenarios developed by the committee were (1) marine—open water, (2) marine—nearshore, (3) estuarine (brackish water), (4) nontidal river, (5) fresh water—quiescent, and (6) on land near water. The emphasis of these scenarios
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is on the behavior and fate of the bitumen component because of its unique properties and behavior once spilled into water. The dissolved constituents are also important, but the processes of mixing and dilution of water-soluble compounds is well understood.
Marine—Open Water
Figure 2.8 presents the schematic fate diagram for open-water marine spills of Orimulsion. Away from the immediate vicinity of an open-ocean spill, the surfactant will be diluted or degraded rapidly so that the emulsified fuel will become an independent cloud of bitumen droplets. The dissolved components in the water phase will mix quickly into the surrounding water. In open-water settings, concentrations should rapidly decrease due to mixing and turbulence. Some of the bitumen will coalesce and make its way to the surface as either tarballs or tar patties. The remaining bitumen will generally disperse due to water turbulence. The surface tarballs or patties will be transported by winds and surface currents, in a fashion similar to tarballs from any weathered fuel oil slick. Being only slightly buoyant, they will be subject to overwash, making their observation by spill response personnel difficult. Tarball fields can be spread over long distances and subsequently become reconstructed in convergence zones. Thus, a line of stranded tarballs from the spill could appear on beaches far away from the spill site. Upon exposure to the air, the surfaced bitumen will greatly increase its adhesion properties and could attach itself to any floating material it encounters. Mesocosm roof-top experiments have shown that prolonged exposure to sunlight can eventually lead to sinking of surface bitumen patties or
FIGURE 2.8 Schematic representation of the distribution and fate of Orimulsion-400 spilled into an idealized marine open water environment.
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FIGURE 2.9 Schematic representation of the distribution and fate of Orimulsion-400 spilled into an idealized marine nearshore environment.
tarballs (Brown et al., 1995). Suspended bitumen and tar particles may also be subject to zooplankton grazing and, ultimately, sedimentation in fecal material.
Bitumen droplets are of a size range similar to single-celled algae (i.e., phytoplankton) and natural silt particles, which are fed on by filter feeders. However, most filter feeders studied to date have proven to be quite selective in their choice of food, and they can reject mineral and other particles. Copepods have been known to ingest droplets of Bunker C fuel oil following an oil spill (Conover, 1971), and up to 10 percent of the hydrocarbons in the water column were associated with the plankton and their feces. Laboratory studies with crustacean zooplankton, primarily copepods, demonstrated uptake of a variety of aromatic and paraffinic hydrocarbons from oil-contaminated food or water (Corner et al., 1976a, b; Lee, 1975); however, there are no data to show if similar behavior might be expected from ingested bitumen droplets.
Marine—Nearshore
Figure 2.9 presents the schematic fate diagram for nearshore coastal marine spills of Orimulsion, which are expected to behave differently than open-water spills. Because of the land and bottom boundary conditions, vertical diffusive mixing could be reduced and pockets of higher near-surface concentrations of bitumen and dissolved components could persist longer than would occur in open water. This would encourage a greater rate of coalescence and the formation of surface slicks. If the bitumen does resurface in quiescent water, it will tend to form a thin film of less than 0.1 mm (Sommerville, 1999) and can easily be resuspended if sufficient energy becomes available. Surface slicks of weathered
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bitumen can also become stranded at the high-water mark on the shoreline during a falling tide and onshore winds. Once deposited, they would be expected to weather as described in the previous section on shoreline interactions. If large patties or ropes of the weathered bitumen mix with sand suspended in the surf zone, they may form large “sand rollers,” which are mixtures of tar and sand that can roll down and populate the nearshore bottom. Once deposited, they may be subject to burial in offshore bars. They could also become buried in the intertidal zone during depositional phases on beaches.
Suspended bitumen droplets would probably not strand on the beach but would be transported along the shoreline by the alongshore current. They could, however, be scavenged by suspended particulate matter as described earlier. In any segment of coastline that is of lower energy (or in calmer offshore waters), these bitumen/ suspended sediment combinations could sink. Suspended or dispersed bitumen droplets that are carried offshore would be subject to the same long-term fate as bitumen droplets produced from an open-ocean Orimulsion spill.
Estuarine (Brackish Water)
Figure 2.10 presents the schematic fate diagram for an Orimulsion spill in estuarine or brackish water environments. Although the surfactant in Orimulsion will degrade in water with salinity greater than 5-7 ppt, the dispersed bitumen droplets will typically not become buoyant until the salinity of the surrounding water is 15 ppt or greater (Deis et al., 1997). Thus, it is possible for the bitumen droplets to begin to coalesce and then either float to the surface, sink to the bottom, or remain neutrally buoyant, depending on the encompassing water den-
FIGURE 2.10 Schematic representation of the distribution and fate of Orimulsion-400 spilled into an idealized estuarine (brackish water) environment.
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sity. If the water is stratified and quiescent, it is possible that increased concentrations of bitumen could form at the boundary between the less dense and more dense water. A likely scenario for an Orimulsion spill in a brackish, low-energy estuary of varying salinity would be that a small amount of the bitumen would coalesce and float to the surface, some would be scavenged by suspended particulates, and most would either sink very slowly to the bottom or remain in the water column, gradually being flushed by the normal water exchange in the estuary. Dissolved components (PAH, surfactants) are expected to decrease at rates determined by the degree of tidal mixing and flushing.
If the water is sufficiently brackish to cause some of the bitumen to float, this weathered floating bitumen could adhere to estuarine vegetation and impact shorelines. Bitumen/particle agglomerations could settle into the bottom sediment in quiescent areas where fine sediment accumulation occurs. If there is insufficient energy to cause coalescence, any buoyant bitumen may float to the surface but be subject to resuspension if disturbed. Suspended and dispersed bitumen droplets that are carried out of the estuary to coastal or offshore waters would be subject to the same long-term fate as bitumen derived from an open ocean Orimulsion spill.
Nontidal River
Figure 2.11 presents the schematic fate diagram for an Orimulsion spill in a nontidal energetic riverine environment. A river spill of Orimulsion would typically result in rapid mixing of the bitumen and dissolved components throughout the water column with subsequent rapid dilution. The surfactant would usually remain effective for a sufficient period of time to allow the bitumen droplets to
FIGURE 2.11 Schematic representation of the distribution and fate of Orimulsion-400 spilled into an idealized nontidal river environment.
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become effectively dispersed to low enough particle densities to discourage coalescence. If there is a heavy sediment load in the river, there could be a minor amount of scavenging of the bitumen, although laboratory flume experiments have indicated that bitumen-SPM interactions are insignificant in fresh water conditions. There would most likely be little sinking of the pure bitumen outside of any previously formed scour pits or other naturally quiescent pools, where the droplets might temporarily collect but could easily be resuspended. Water intakes would have to be closed until the cloud of droplets passed. Flood conditions, a not uncommon situation in spill accidents, could strand bitumen-laden water on inundated floodplains. As the water drained off, the bitumen could remain on the top sediment layer. The long-term fate under normal conditions may be deposition in calm deltaic areas where other fine sedimentary material accumulates, although there are no data to support this hypothesis.
Fresh Water—Quiescent
Figure 2.12 presents the schematic fate diagram for an Orimulsion spill into a freshwater region with low or near-zero currents. A spill of Orimulsion into a quiescent freshwater pool would be similar to a spill in a river with less mixing and essentially no bitumen-SPM interaction. As in the river spill, the surfactant would probably remain effective long enough to allow the bitumen droplets to diffuse to a low enough concentration to inhibit coalescence, as long as the receiving water was of sufficient volume to allow breakdown of the emulsion. However, the low mixing energy would delay this process. The PAH fraction in the carrier aqueous phase would be subject to limited dilution if the volume
FIGURE 2.12 Schematic representation of the distribution and fate of Orimulsion-400 spilled into a quiescent fresh water environment.
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FIGURE 2.13 Schematic representation of the distribution and fate of Orimulsion-400 spilled into an idealized land-to-water (through wetlands) environment.
fraction of the Orimulsion were significant compared to the freshwater pool volume. Given a breakdown of the emulsion, some of the bitumen could settle temporarily to the bottom, but it could also be resuspended easily. If there were any turbulent energy, the bitumen droplet concentration would be nonzero throughout the water column, with the settling motion of the bitumen offset by random particle motion. For a spill on the surface of a pond, the steady-state vertical concentration of droplets could be an exponentially decaying function upward from the bottom of the pond (Hemond and Fechner, 1994).
On Land Near Water
Figure 2.13 presents the schematic fate diagram for an Orimulsion spill on land with subsequent migration through a wetland into a riverine environment. If Orimulsion is released on land, it would be expected to behave initially like a fuel oil with a similar viscosity. Penetration of fresh Orimulsion into soil and sandy sediments would be limited to the upper few centimeters, as described in the discussion of Orimulsion spills on land and shorelines. The suspended bitumen droplets are filtered out of the spilled material by the sand or soil, whereas the dissolved-phase surfactants and low-molecular-weight PAH constituents may percolate deeper into the soil and could eventually interact with groundwater. With prolonged exposure to air, the bitumen droplets remaining in the upper soil layers would dry, increasing their stickiness and viscosity, and any pooled Orimulsion would eventually turn into tar mats. If sufficient Orimulsion were spilled to eventually reach a freshwater body, some of the bitumen would be subject to dispersion in the receiving water body. However, under calm condi-
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tions, it would most likely behave like a highly viscous, high-density oil and settle toward the bottom in tar patties, tarballs, or tar mats, depending on the spill and environmental circumstances. The dissolved PAH fraction in the aqueous phase would be subject to limited dilution, and it could remain in the receiving waters at higher concentrations than in any of the other scenarios considered.
SUMMARY OF THE BEHAVIOR AND FATE OF SPILLED ORIMULSION
All of our understanding of the behavior of spilled Orimulsion is based on small-scale laboratory studies, flume tests, small (2-10 barrels) experimental spills in harbors and open water, and computer models developed from these data. Therefore, the ability to predict what happens in a real, large spill event remains limited, and responders should be prepared for a wide range of possibilities for response and cleanup. To provide better prediction tools during spills, the different Orimulsion models should be compared for the same spill scenarios and the strengths and weaknesses of each model should be evaluated.
Orimulsion behavior varies significantly when spilled into fresh water or into salt water due to the denaturing of the surfactant when the salinity of the receiving water is greater than 5-7 ppt. As a result, there is a greater tendency for dispersed bitumen droplets to coalesce and surface in brackish or salt water. The competing processes of coalescence (with possible surfacing or sinking) and dispersion into the water column dictate the behavior of bitumen droplets from an Orimulsion spill in marine or brackish water. Therefore, further research must be done to quantify the processes of coalescence and dispersion of the bitumen droplets. In particular, the role of turbulent energy (magnitude and structure), salinity, spill volume, and spill rate should be evaluated.
For spills into open water, Orimulsion would quickly behave as a cloud of dispersed bitumen droplets that are separated from the dissolved PAH and surfactants in the 30 percent water phase, which quickly mixes into the receiving water body. The bitumen droplets chemically resemble the residue that would be found in a heavy fuel oil spill at the end of short-term weathering processes. The concentrations of dispersed bitumen droplets and water-soluble constituents decrease significantly due to dispersion into the surrounding water column. Predictions of the dispersion are dependent on knowledge of the vertical and horizontal diffusion parameters that are key factors used in computer models. To improve the accuracy of model predictions in support of local spill response plans, site-specific studies are needed to define diffusion coefficients (energy dissipation rates) for areas where Orimulsion shipment and/or loading and offloading operations are planned.
The PAH concentration and chemical composition of bitumen are similar to the end product or heavily weathered residues from most crude oil spills. Significant quantities of the water-soluble PAH constituents have already leached from
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the bitumen droplets. Long-term leaching of PAH can occur at appreciable rates given the high surface-area-to-volume ratios of these 1- to 80-µm-diameter bitumen droplets. However, given the relatively low concentration of the highmolecular-weight PAH in the bitumen itself, this continued leaching is not expected to be environmentally significant. Limited data suggest that bitumen droplets may interact with SPM to a greater extent than No. 6 fuel oil (which does not break up into small droplets and remains suspended in the water column). Because the bitumen droplets are recalcitrant and remain in the environment for a long time, their interactions with SPM and retention in nearshore, estuarine, and riverine sediments may be important. Additional dispersed bitumen-SPM studies using a wider variety of suspended particulate material types (including organic-rich substrates) at different salinities are recommended.
The ultimate fate of spilled bitumen droplets is sedimentation, where continued biodegradation of the bitumen droplets is expected to be extremely slow. Based on the highly weathered nature of bitumen, the bioavailability of PAH is likely to be low. However, given the high surface-area-to-volume ratio of the very fine bitumen droplets, the potential for PAH uptake exists and thus should be evaluated.
For Orimulsion spills on land, the water phase can separate from the bitumen and infiltrate the underlying substrate. Because low- and intermediate-molecular-weight PAH and surfactants reside in the aqueous phase, they may reach groundwater. The potential for groundwater contamination is very site specific and outside the scope of this study.
Recent literature shows that the surfactant AE are subject to a variety of microbial breakdown pathways to highly oxygenated water-soluble intermediates. Identified breakdown products include fatty acids, alcohols, carboxylated polyethylene glycols, and (for branched AE) intermediate compounds with carboxylic acid groups on one or more alkyl groups (side chains) as well as the terminal end of the polyethylene oxide moiety. Branched AE such as those found in Orimulsion degrade slowly, and intermediate products can still be isolated (at low concentrations) from laboratory experiments after weeks to months. Therefore, AE and their intermediate degradation compounds can persist for weeks to months after a release. Federal and state agencies should consider developing information on the ambient concentration of these compounds and their degradation products in the environment.
Representative terms from entire chapter:
fuel oil
