Oil Spill Dispersants Efficacy and Effects (2005) / Chapter Skim
Currently Skimming:
4 Transport and Fate
4 Transport and Fate
Pages 135-192
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From page 135... ...
The latter portion of the chapter and Appendix E discuss how the mechanisms are integrated into computer oil spill models that simulate the fate of spilled oil, and how such models are used (or might be used) for purposes of pre-planning, emergency response, and natural resource damage assessment.
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From page 136... ...
Surface Transport Oil spilled directly on a calm water surface spreads radially by gravity and is resisted by inertia, viscosity, and surface tension until the slick reaches a thickness of ~0.1 mm.
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From page 137... ...
Slick thicknesses were estimated during several well-documented oil spills, usually indirectly by dividing volume/area (Mackay and Chau, 1986; Lunel and Lewis, 1993a,b; Lewis et al., 1995a,b; Walker et al., 1995; Brandvik et al., 1996; Brown et al., 2000)
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From page 138... ...
. Because of the presence of water, the resulting oil slick will be significantly thinner than those produced by oil spilled directly on the surface.
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From page 139... ...
. Vertical Transport Dispersion of a surface slick, whether caused naturally or through application of chemical dispersants, results in the formation of droplets that are entrained into the water column and transported with the subsurface currents.
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From page 140... ...
Some models assume that the depth of this floor is simply proportional to wave height. Unless there is significant interaction with suspended particulates, most oil droplets will be positively buoyant and will rise toward the surface.
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From page 141... ...
Horizontal Subsurface Transport Subsurface advection of dispersed and dissolved phase oil by a uniform current affects the location of the oil, but does not, in itself, cause
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From page 142... ...
Without horizontal mixing, and under sufficiently calm weather conditions, vertically dispersed oil droplets could all ultimately resurface given enough time. Horizontal mixing consists of two fundamental processes.
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From page 143... ...
usually include the effects of shear dispersion, it is interesting to consider this component separately and evaluate how it varies with sea state and dispersant effectiveness. One type of shear dispersion that was discussed previously involves larger droplets that become vertically entrained into the water column and later rise to the surface.
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From page 144... ...
. These include evaporation, dissolution, dispersion of whole oil droplets into the water column (entrainment)
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From page 145... ...
After that, the fate of physically and chemically entrained oil droplets in the water column is considered. In evaluating the fate of entrained oil droplets, the primary focus is on biodegradation of dispersant-treated oil and the interaction of both physically entrained and dispersant-treated oil droplets with suspended particulate material.
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From page 146... ...
, and it was validated for Alaska North Slope crude oil by National Oceanic and Atmospheric Administration (NOAA) and Minerals Management Service (MMS)
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From page 147... ...
the oil slick after 48 hours of weathering, showing loss of the most volatile compounds; C) the oil slick after 12 days of weathering, showing the loss of all compounds with molecular weights less than n-C11 (Kovats Index 1100)
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From page 148... ...
, who has proposed a simplified pseudo-component (SPC) model relating molar volume, vapor pressure, and molecular weight to the boiling point of the component.
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From page 149... ...
. As a result, it may be inappropriate to always model oil as a well-mixed phase, and algorithms for both well-mixed and diffusion-controlled fluids may need to be sequentially utilized as a function of oil weathering-dependent viscosity changes to better approximate spilled oil evaporative behavior.
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From page 150... ...
(1998) studied photooxidation of PAH in a variety of crude oils, and a general overview is presented in NRC (2003)
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From page 151... ...
. Secondly, it has been shown that the lower-molecular-weight alkane and aromatic components in fresh crude oils serve as solvents to control the in-situ solubility and precipitation behavior of these highermolecular-weight constituents within the oil phase.
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From page 152... ...
Unstable emulsions are those that rapidly decompose to separate water and oil phases after mixing energy is removed, generally within a few hours. Some water (usually <10 percent)
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From page 153... ...
153 TRANSPORT AND FATE FIGURE 4-3 Maximum water uptake in water-in-oil emulsions versus initial parent oil viscosity. The mixing time is 24 hours at 6 and 13° C
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From page 154... ...
Photolyzed components were believed to facilitate the accumulation of waxes at the oil-water interface, which were believed to "block" the access and penetration of the dispersant's surfactant into the oil phase. Without such stabilizing agents the water droplets in a water-in-oil emulsion will tend to coalesce and separate from the oil phase.
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From page 155... ...
, and the more recent data have significantly expanded the viscosity-limited range of dispersant effectiveness (and the concomitant time window available for responding to an oil spill at sea)
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From page 156... ...
Noting the requirement for the loss of lower-molecular-weight alkane and aromatic solvents to precipitate asphaltenes for the formation of stable mousse with Alaska North Slope (ANS) crude oil, S.L.
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From page 157... ...
Presumably, this loss resulted from the increased surface-area-to-volume ratio of the smaller physically entrained oil droplets compared to the more continuous surface oil slick. After 12 days with continued constant turbulent mixing in the wave tank there was little evidence of physically dispersed oil droplets in the water column.
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From page 158... ...
In their breaking wave experiments, Delvigne and Sweeney (1988) measured droplet sizes from 6 to >800 µm with the highest number concentrations in the 650 µm size range with greater numbers of the smaller droplets driven deeper into the water column and greater numbers of larger droplets near the surface.
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From page 159... ...
In the case of an oil spill, true dissolution of individual components is controlled by the mole fraction of each component in the slick, the oil/ water partition coefficient, the oil-water interfacial surface area (which significantly increases with successful dispersant application) , and the interphase mass transfer coefficient.
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From page 160... ...
At higher shear stresses and dispersed oil droplet loadings, the PAH concentrations in the water column were influenced primarily by the mole fraction of the individual PAH in the entrained oil droplets. To assess the time-series water-column concentrations in the subarctic flow-through wave-tank studies discussed above, Payne et al.
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From page 161... ...
161 TRANSPORT AND FATE A 765 860 894 996 1187 1024 1299 1317 981 B 1299 1024 1317 1187 1412 996 765 880 1386 1447 1463 1558 1773 1746 C 894 1317 1299 860 1746 1187 Kovats Retention Index Compound 765 toluene 860 ethylbenzene 1463 894 o-xylene D 981 1,3,5-trimethylbenzene 996 C3-benzene 1024 C4-benzene 1187 naphthalene 1299 2-methylnaphthalene 1317 1-methylnaphthalene 1386 1,1-biphenyl 1447 1412 2,6-dimethylnaphthalene 1746 1447 C2-naphthalene 1463 C2-naphthalene 1558 2,3,5-trimethylnaphthalene 1746 dibenzothiophene 1773 phenanthrene FIGURE 4-5 FID gas chromatograms of filtered water samples showing dissolvedphase components obtained from 2,800-liter flow-through open-air summer wavetank experiments using 16 liters of Prudhoe Bay crude oil after: A) 5 minutes; B)
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From page 162... ...
even though there was little or no physical dispersion of oil droplets occurring at that time. There was no evidence of significant aromatic hydrocarbon dissolution into the water column after 13 months in the wave-tank systems.
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From page 163... ...
Although the use of chemical dispersants will clearly increase the upper water column concentration of entrained oil droplets, and theoretically should lead to enhanced dissolution of water-soluble PAH components (French-McCay and Payne, 2001) , no field measurements of this phenomenon have been successfully completed to date.
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From page 164... ...
The measured concentrations of dispersed oil droplets were very heterogeneous and reflected the patchy distribution of oil on the water surface before dispersant application. Nevertheless, the
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From page 165... ...
Instead, the effects of surfactants or commercial dispersant mixtures on the biodegradation rates of crude oil and defined hydrocarbons appear to depend on the chemical characteristics of the surfactants, the hydrocarbons, and the composition of the microbial community. Other factors, such as nutrient concentrations, oil-water ratios, and mixing energy, can also be expected to affect the observed biodegradation rate of dispersed oil.
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From page 166... ...
. Corexit 9500 had no effect on the rate of n-alkane biodegradation in Alaskan North Slope crude oil when compared to physically dispersed oil (Davies et al., 2001)
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From page 167... ...
are kept constant. A survey of thirteen dispersants and two surfactants showed that none significantly stimulated the biodegradation rate of Norman Wells crude oil by an oil-degrading enrichment culture in an artificial freshwater medium (Foght et al., 1987)
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From page 168... ...
. In this study, the maximum rate of biodegradation of Alaska North Slope crude oil by sludge from a refinery wastewater treatment system was obtained at a dispersant HLB of about 8, which is significantly lower than the HLB that was most efficient for the homologous series of Igepals (Van Hamme and Ward, 1999)
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From page 169... ...
calcoaceticus ATCC 31012 by about 40 percent relative to physically dispersed oil, but the nonionic surfactants stimulated the oil biodegradation rate under the same conditions when they were tested alone. These surfactants apparently acted directly at the cell surfaces, possibly interacting with proteins in the cytoplasmic membranes, because they also affected transport and/or oxidation of acetate, a completely water-miscible substrate (Bruheim et al., 1999)
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From page 170... ...
-phenanthrene in oil by three pure cultures of PAH-degrading bacteria, which presumably accumulated the phenanthrene directly from the aqueous phase, in the same study. Although it is clear that surfactants can interfere with attachment of hydrophobic bacteria to oil droplets, the overall effects of chemical dispersion of crude oil on its biodegradation rate are likely to be very complex.
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From page 171... ...
. Also, treatment of ANS crude oil with Corexit 9500 only marginally affected bacterial colonization of oil droplets, with 40 percent of chemically dispersed oil droplets being colonized by at least one bacterium after four days compared to colonization of 60 percent of physically dispersed oil droplets after about one week (Davies et al., 2001)
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From page 172... ...
Microbial growth on open-ocean oil slicks is likely to be nutrient limited and may be slow relative to processes that lead to formation of water-in-oil emulsions, which tend to be extremely resistant to biodegradation. The only way to predict the contribution of biodegradation to the fate of dispersed crude oil is to incorporate this process as a term in a comprehensive fate and transport model.
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From page 173... ...
will result in removal of the target compound when GC-MS is the primary analytical tool, but the biotransformation products may not be significantly less toxic than the parent substrate, and in fact could be much more toxic. For example, whereas some studies have demonstrated relatively rapid biodegradation of 4-ring PAH, such as pyrene and chrysene, in chemically dispersed crude oil when the process was monitored by GC-MS (Yamada et al., 2003)
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From page 174... ...
Specifically, as entrained oil droplets interacted with SPM and their density increased, the agglomerates were removed from the upper water column and were no longer able to recoalesce and rejoin the surface slick. Their studies also showed that oil/ SPM aggregate formation occurs primarily with dispersed oil droplets in the water column interacting with SPM rather than SPM scavenging oil from the surface.
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From page 175... ...
oil droplets and individual dissolved constituents from fresh and weathered Alaska North Slope (ANS) crude oil and commercially available No.
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From page 176... ...
The model results showed that the sticking efficiency between oil droplets and sediment particles is a significant factor in oil/SPM aggregate formation. Not as much is known about the longer-term fate of oil-SPM agglomerates while still in suspension in the water column; however, Wood et al.
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From page 177... ...
Dispersant-Treated Oil Droplet/SPM Interactions Mackay and Hossain (1982) completed one of the earliest exploratory studies of naturally and chemically dispersed oil in which several crude oils were dispersed with varying amounts of chemical dispersants in seawater in the presence of differing quantities of sedimenting mineral and organic matter.
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From page 178... ...
, was important for stable aggregate formation. In a competitive sense, however, it is also known that as asphaltene content in the crude oil increases, its propensity to form higher viscosity water-in-oil emulsions also increases, and this inhibits dispersant effectiveness and oil droplet formation.
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From page 179... ...
Oil appears as the darkest areas; B) multiple oil droplet aggregate formed at intermediate clay concentration; and C)
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From page 180... ...
Models of the trajectory of floating oil are regularly used, along with field measurements, in real time during a spill. To date in the United States, models of the transport and fate of dispersed oil have primarily been used in pre-planning exercises to simulate hypothetical spills in order that response measures, including use of chemical dispersants, can be evaluated before the fact, or to assist with natural resource damage assessment of real spills after the fact.
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From page 181... ...
Sensitivity Study In order to understand the effects of various processes on the transport and fate of spilled oil, a series of sensitivity tests was performed with the NOAA surface oil fate model ADIOS2 (Lehr et al., 2002)
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From page 182... ...
As the wind becomes stronger, more oil is naturally entrained into the water column -- from 0 volume percent at 2 m/s wind to 3 volume percent at 10 70 60 Oil volume as volume percent 2-m/s Wind 10-m/s Wind 50 25-m/s Wind 40 30 20 10 0 Evaporation Chemical Natural Floating On Beach Dispersion Dispersion FIGURE 4-10 Predicted oil distributions 24 hours after the release of Alaskan North Slope crude oil (no dispersant applied) under 2-, 10-, and 25-m/s wind in nearshore off Florida Keys.
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From page 183... ...
The oil spill location is marked by "+." Black spots represent oil floating on the water surface, and the shaded areas show different ranges of oil concentrations in the top 1 m of the water column. The oil plume in the top 1 m of the water column is following a different trajectory at a different speed than the oil on the surface that is moved by the wind and the current.
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From page 184... ...
Figure 4-12 presents the predicted composition (a relative volume fraction of each distillation cut) of these three oils floating on the water surface 0 and 6 hours after the spill.
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From page 185... ...
Because these refined oils have a low percentage of low-temperature distillation cuts, the IFO 300 and diesel evaporated only 10 and 18 volume percent, respectively, over 24 and 14 hours (much less than Alaska North Slope crude oil)
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From page 186... ...
Because chemical dispersants help transport oil into the water col umn, realistic simulation of subsurface transport becomes more impor tant when evaluating the use of chemical dispersants, and the same for mulation may not be sufficient. It is recommended that a range of 3-D
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From page 187... ...
These processes may play a significant role from the instant the oil enters the environment, and they constrain a number of operational decisions and play a significant role in evaluating potential impacts of whole and dispersed oil on sensitive species or habitats. Fate and Weathering of Oil Oil on the Surface Better information is still needed to determine the window of opportunity and percent effectiveness of dispersant application for different oil types and environmental conditions.
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From page 188... ...
When dispersed oil plumes become diluted by the transport processes that act in the surface layer of the ocean, however, the surfactants present in the dispersant will partition out of the oil into the surrounding seawater. If this partitioning is fast relative to the kinetics of bacterial attachment to oil droplets, the dispersant may not interfere with microbial uptake of the petroleum hydrocarbons (i.e., the dispersed oil droplets will behave like physically dispersed oil except the oil-water interfacial area will be larger due to entrainment of a larger number of small droplets in the water column)
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From page 189... ...
High-molecular-weight PAH, on the other hand, are likely to persist in the residual oil droplets, which may be ingested by animals in the water column or benthos where they can exert chronic effects. Therefore, the biodegradation kinetics and ultimate biotransformation products of high-molecular-weight PAH should be investigated using indigenous microbial communities from seawater.
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From page 190... ...
Present and Possible Role of Models As discussed previously, various processes constrain a number of operational decisions and play a significant role in evaluating potential impacts of whole and dispersed oil on sensitive species or habitats. Models are, therefore, powerful and necessary tools to support decisionmakers during all phases of oil spill planning, response, and assessment.
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From page 191... ...
In discussions with NOAA modelers, it was noted that predicting the three-dimensional flow distribution as a part of the oil transport and fate modeling within several hours after an oil spill is difficult. A real-time model application uses actual environmental conditions and oil properties, but, because of time limitations, uses simple approaches for approxi
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From page 192... ...
comes from obtaining appropriate horizontal and vertical diffusivities. It is difficult to integrate all interacting transport and fate processes and oil properties to predict how much oil will be found in specific areas during an actual oil spill without the use of models.
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