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Critical Review

Integrating dispersants in oil spill response in Arctic and other icy environments Alun Lewis, and Roger C. Prince Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b06463 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on May 3, 2018

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Integrating dispersants in oil spill response in Arctic and other icy environments

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Alun Lewis1 and Roger C. Prince*2

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Staines, Middlesex, TW18 2EG, UK [email protected]

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Stonybrook Apiary, Pittstown, NJ 08867, USA [email protected]

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Abstract

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Future oil exploration and marine navigation may well extend into the Arctic Ocean, and

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government agencies and responders need to plan for accidental oil spills. We argue that

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dispersants should play an important role in these plans, since they have substantial logistical

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benefits, work effectively under Arctic conditions, and stimulate the rapid biodegradation of

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spilled oil. They also minimize the risk of surface slicks to birds and mammals, the stranding

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of oil on fragile shorelines and minimize the need for large work crews to be exposed to

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Arctic conditions.

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1. Introduction

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In 2009, the US Geological Survey estimated that the Arctic might house 30% of the world’s

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undiscovered natural gas reserves and 13% of its undiscovered oil, mostly offshore under less

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than 500 m of water1. The decreasing extent and duration of Arctic ice cover suggests that

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extraction of these resources will be increasingly commercially viable2,3. Oil exploration

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activities are most often regulated by national and state authorities that mandate response

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plans for spills and well blow-outs. Subsequent oil production will involve shipping by

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tankers or pipelines, and these too will require planning for the event of a spill229.

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Response plans typically rely on skimming oil as the preferred option, but this is rarely

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successful with large spills in remote areas – despite the massive scale of the response to the

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Deepwater Horizon blow-out for example, much less than 10% of the oil that surfaced was

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collected4, and even less was collected from the water following the spill from the Exxon

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Valdez5. Ice will certainly interfere with skimming operations6. Since biodegradation and

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combustion are the only other processes that remove a majority of the hydrocarbons from the

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biosphere, there has been considerable interest in harnessing these natural processes in oil

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spill response7,8,. The most effective way to stimulate oil biodegradation at sea is to use

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dispersants to facilitate the disruption of the oil slick to tiny droplets. Here we review issues

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to be considered when using this technology in the Arctic and in other cold-water

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environments.

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Arctic regions are characterised by very low air temperatures (winter minima 20 would be produced.

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As their name implies, refined products are commercial products produced in refineries. They

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are typically sold based on physical properties, such as ease of ignition, viscosity or lubricity,

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rather than on chemical composition. Refining begins with distillation, and many streams are

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subsequently treated with catalysts to generate valuable molecules32. Some of the residual

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material is converted to Bunker fuels for ships, and such fuels power almost all large

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commercial vessels33. Bunker fuels are far more viscous than most crude oils, and need

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heating to be pumpable within the ship.

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Crude oils encompass a broad range of viscosities, from close to that of water (a few

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centipoise) to viscous oils that at ambient temperature have the viscosity of peanut butter

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(100,000 cP). Oils with API gravities >20 typically have viscosities27 at ambient temperatures

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of 0°C; they pose a challenge to

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both mechanical recovery and dispersion227.

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The fate and behaviour of oil accidentally released in the Arctic will depend on the

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circumstances of the release and prevailing ice conditions at the time34. Oil released onto the

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sea surface may encounter open water, partial ice cover or total ice coverage depending on

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the season and location. Oil deposited onto near total ice coverage would accumulate on top 5 ACS Paragon Plus Environment

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of the ice and snow. Oil released subsea will rise to the sea surface. If a high level of ice

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coverage is present, a substantial percentage of the oil will be trapped under the ice, and

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potentially become incorporated into the ice as it grows. As oil spreads on the surface of

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water with up to 30% ice coverage, it will drift with the wind at approximately 3.5% of wind

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speed and leave a trail of very thin oil, or sheen, on the sea surface35. As the ice coverage

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exceeds 30%, the oil on the surface between the floes begins to move less independently of

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the surrounding ice36, and at above 50% ice cover the spreading of oil is mostly constrained

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by the ice. The intrinsic properties of crude oils begin to change as soon as they are released

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into the environment, especially on water. As oil spreads on water it loses the lightest

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components by evaporation and begins to incorporate water; both processes gradually

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increase the viscosity and pour point.

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Oil spill response in cold waters may include mechanical recovery with booms and skimmers,

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but will likely rely on in situ burning and dispersants37. Our purpose is to review the latter in

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the context of a potential spill in cold and/or ice-infested waters.

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3. Dispersant effectiveness on spilled oil under Arctic conditions.

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3.1

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Except under the very calmest sea conditions, some spilled oil will disperse into the water

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column as small oil droplets under the influence of waves and other turbulence38. Natural

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dispersion is promoted by increased mixing energy (rougher seas), but resisted by the

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rheology (viscosity and pour point) of the oil, and the oil/water interfacial tension. Natural

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dispersion will initially remove some spilled oil from the sea surface in most sea conditions,

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but soon stops as the oil viscosity increases due to ‘weathering’39. Ice on the sea dampens

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wave action40-42 and will suppress the natural dispersion of oil.

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Natural dispersion can be enhanced by the addition of oil spill dispersants43,44. Dispersants

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are blends of surfactants in solvents and when applied to spilled oil on the sea, the surfactants

Natural dispersion and dispersant use

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in the dispersant orientate at the oil/water interface and lower the oil/water interfacial

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tension45. When mixing energy is applied a high proportion of the oil volume will be

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converted into oil droplets that are small enough to be maintained in the water column by the

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prevailing turbulence225, even if dispersants were applied several days earlier46, or before oil

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was trapped in ice47. The concentration of dispersed oil droplets in water will be rapidly

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diluted to low levels in most sea conditions48-51 and the majority of the dispersed oil will be

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biodegraded promptly (see below).

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While dispersants can be applied with vessels, a major advantage is that they can be applied

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by aircraft52. Initial access to a spill location at the speed of an airplane, and then rapid

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treatment of oil slicks that are spreading over large areas, are important advantages over

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mechanical recovery or vessel application because mechanical recovery has to proceed at the

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speed of vessels. This is particularly important in remote locations; dispersant application is

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far less logistically challenging than mechanical recovery, which requires more personnel and

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equipment. Nevertheless, aircraft use for dispersant application may still be limited by

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weather and the lack of infrastructure in some regions of the Arctic.

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In addition, dispersants are a response option that can treat oil released from a subsea blowout

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at the source53 if the oil cannot be recovered by mechanical means. This is important for

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contingency planning of what is usually the worst-case oil spill scenario for offshore drilling,

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and if implemented has the advantage of treating the oil before it distributes at the surface,

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and immediately enhancing the biodegradation process.

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Removing the spilled oil from the sea surface, or preventing the oil from reaching the sea

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surface by subsea dispersant use, stops the oil from becoming emulsified54 and becoming

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persistent on the sea surface. Emulsified oil will most likely form tarballs55 that may

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eventually drift ashore and persist for years.

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Dispersant effectiveness is most often taken to mean the proportion (percentage) of oil

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dispersed into the water column as the result of dispersant addition. Dispersant effectiveness

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has been studied at sea, in laboratory-scale test methods, and in wave tanks, and a

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comprehensive list of experiments under Arctic conditions is given in Table S1. As discussed

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elsewhere44, dispersant effectiveness at sea can be sub-divided into separate components of

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operational effectiveness (probability of dispersant achieving desired treatment rate in oil),

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chemical effectiveness (proportion of oil dispersed following dispersant addition) and

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hydrodynamic effectiveness (transport and dilution of dispersed oil in the water column).

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Each of these aspects of dispersant effectiveness is influenced by several factors.

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3.2

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A number of large-scale experiments at sea have been conducted over many years49,56 (and

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see Supplementary Materials). Such experiments are very costly, logistically complex, must

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conform to regulatory requirements and are subject to severe restraint by the prevailing

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weather and sea conditions. While it is possible to gain a lot of information about the fate and

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behaviour of spilled oils, and this is extremely useful in calibrating smaller-scale studies, it is

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currently impossible to accurately quantify dispersant effectiveness at sea. Oil dispersed into

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the water column has a very heterogeneous spatial distribution and is rapidly diluted to low

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concentrations. It is therefore not possible to construct a ‘mass balance’ from which the

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dispersant effectiveness at sea can be derived. A variety of monitoring methods, such as the

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SMART (Special Monitoring of Applied Response Technologies)57 protocol, has been

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proposed, but none accurately quantify dispersant effectiveness at sea.

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3.3

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In the absence of the ability to quantitatively determine dispersant effectiveness at sea, many

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tests have been devised to measure dispersant effectiveness in the laboratory58, including the

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Warren Spring Laboratory (WSL) LR 448 Rotating Flask method59, Mackay-Nadeau-

Dispersant effectiveness at sea

Dispersant effectiveness in the laboratory

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Steelman (MNS) method60, Swirling Flask Test (SFT)61, Baffled Swirling Flask Test (BFT)62

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and the Institut Français du Pétrole (IFP)63 dilution method. These test methods only assess

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the chemical effectiveness aspect of dispersion, the proportion of oil dispersed by dispersant

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addition, and are typically used or proposed to assess dispersant effectiveness for regulatory

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purposes in various countries. An important misconception is that dispersant effectiveness

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tests used for regulatory listing accurately predict field performance. Regulatory tests

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normally use methods and test oils that will produce a mid-range effectiveness such as 45%64,

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60%65 or 75%66 so that the relative performance of dispersants, under the conditions

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employed in the test method, can be discriminated. These tests do not predict dispersant

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effectiveness in the field, or even in large wave tanks, where effectiveness is routinely >90%,

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even at 1°C 60. The apparent dispersant effectiveness in any method depends on several

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factors, including the intensity of the mixing action (which varies greatly between the

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methods), the reduction in oil/water interfacial tension achieved by dispersant addition

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(dependant on dispersant composition and treatment rate), and the resistance to dispersion

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caused by the flow properties (rheology, including viscosity and pour point) of the oil tested,

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under the conditions of the test. A major disadvantage of laboratory methods is that it is not

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possible to scale the results to predict dispersant performance at sea. In part, this is because

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laboratory tests do not include the operational and hydrodynamic elements of dispersant

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effectiveness at sea, especially the enormous potential dilution available at sea. There have

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been attempts to measure the energy dissipation rates in some laboratory test methods67 to

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provide a more fundamental basis for comparisons between different methods, but these have

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yet to be conclusive, and comparisons between apparent effectiveness in the laboratory and at

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sea remain elusive.

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3.4

Dispersant effectiveness in wave tanks

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The use of wave tanks of various sizes addresses some of the artificialities of laboratory scale

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testing. The waves produced in a wave tank can be well-characterized and they produce a

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dispersion mechanism that resembles dispersion of oil at sea more closely than the mixing in

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the various laboratory tests. Experimental parameters can be controlled in a way that is not

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possible at sea and, being a closed system, oil that is not dispersed can be recovered to

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provide a quantitative estimate of dispersant effectiveness. The effectiveness of ‘good’

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dispersants in wave tank experiments is much higher than in laboratory tests, routinely

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>90%68,69.

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3.5

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Arctic conditions, in the context of dispersant effectiveness, refers to low surface water

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temperatures near 0°C, the possible presence of ice, and as a more minor effect, water

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salinities transiently differing from temperate oceans.

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The effect of low water temperature

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The low water temperatures of the Arctic gave rise to concerns that dispersants would be

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inherently less effective than in temperate climes because the lower water temperature would

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increase oil viscosity. This concern has proved to be unjustified in most cases. In general, low

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viscosity oils can be dispersed easily while higher viscosity oils are more difficult to disperse,

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but there is no linear decrease of dispersant effectiveness with oil viscosity. Instead, the

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dispersant effectiveness is approximately constant until a high viscosity value is reached at

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which the dispersant effectiveness drops. In the late 1970s it was considered that an oil

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viscosity of higher than 2,000 cP (equivalent to liquid honey at room temperature) would be

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the limit for dispersant effectiveness70, but subsequent studies71-73 have shown that modern

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dispersants can disperse some weathered and emulsified oils with a significantly higher

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viscosity of 20,000 cP or more (equivalent to chocolate syrup at room temperature). A single

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viscosity value is an inadequate description of the flow behaviour (rheology) as almost all

Dispersant effectiveness in Arctic conditions

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‘weathered’ oils, and all emulsified oils, exhibit non-Newtonian flow behaviour, and

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weathering varies within a floating slick. Nevertheless, the concept of a generally-applicable

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‘limiting’ oil viscosity for effective dispersion persists in dispersant effectiveness studies74,75.

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To aid contingency planning for oil spill response, some ‘rules of thumb’ have evolved, and it

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is considered unlikely that dispersant will be effective on ‘weathered’ oils that have a

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viscosity of more than 10,000 cP or a pour point (the temperature at which it just flows228) of

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5-10°C above sea temperature227. As oil viscosity increases with the time the spilled oil is on

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the sea surface, the decrease in likely dispersant-effectiveness can be expressed as a time

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“window of opportunity” for dispersant use76.

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A consolidated body of work, built up over many years, shows that low water temperatures

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do not preclude dispersant effectiveness. Early work used laboratory-scale methods with

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water temperatures of 0°C and 20°C to assess the effectiveness of different dispersants77-80.

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The lower water temperature caused higher oil viscosity, but did not result in significant

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changes in the measured dispersant effectiveness; the oil viscosity was not limiting dispersant

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effectiveness. Tests using a small wave tank in the 1980’s81 also concluded that dispersion of

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crude oils with water temperatures of 0°C was feasible, and subsequent wave tank

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experiments showed that Hibernia crude oil (API Gravity 35°) can be dispersed in cold water

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(0°C and 1°C) until the oil has lost ~10% by evaporation. Further studies68,82,83 with a wider

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range of crude oils indicate that dispersant effectiveness can be >90% with water

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temperatures of 0°C to 2°C. Oils with viscosities of 3,000 cP or less were dispersed with a

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very high level of effectiveness, although this decreased to ~50% for oils with viscosities of

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10,610 and 18,690 cP. Cold-water tests conducted at OHMSETT in 201684 with four

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dispersants showed a range of dispersant effectiveness, and some achieved high levels of

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effectiveness.

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The effect of ice

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Studies on the effect of the presence of ice on dispersant effectiveness using laboratory test

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methods also began in the early 1980s, but results with different protocols were inconsistent.

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The presence of ice in the MNS test85 caused a reduction in dispersant effectiveness because

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the ice caused total damping of the air-induced waves. Similarly, tests with the oscillating

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hoop method 86 resulted in reduction of dispersant effectiveness with up to 50% ice coverage.

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Yet testing in a laboratory-scale wave tank with a paddle-board wave generator87 suggested

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that the presence of ice caused higher dispersant effectiveness than when ice was absent.

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Tests were conducted outdoors in the Canadian winter in a wave tank with broken ice of

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different types to resolve the ambiguity88. Low energy wave activity caused extensive

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dispersion (>90%) of crude oil treated with dispersant when broken ice was present, but not

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in a lead created between two ice sheets. Dispersant effectiveness tests with cold water and

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brash ice were also conducted at OHMSETT89. The oils tested were Chayvo crude oil from

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Sakhalin (API gravity 35.5°) and Hibernia crude oil (35°) and the dispersant was Corexit

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9527. Three-meter diameter boomed areas of oil and ice were subjected to long period, non-

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breaking, low waves. Very low energy waves (15 cm height, 6 s period) did not cause

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dispersion of oil in ice-free conditions, but as a mixture of blocks and small fragments of ice

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was added to the cold water, higher ice concentrations consistently produced higher

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dispersant effectiveness. A similarly extensive study using a meso-scale flume with varying

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amounts of ice was conducted to determine the weathering and dispersibility of weathering

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Statfjord crude oil (API gravity 39.4°) under simulated Arctic conditions90. Weathering of oil

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was slower in Arctic conditions than at higher water temperatures with no ice present. It

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seems the low water temperature led to a higher oil viscosity and slower spreading, and the

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presence of ice further restricted the spreading of the oil so that the oil layer on the sea

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surface was thicker than it would have been at higher temperatures. This resulted in slower

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evaporative loss of volatile oil components, less water-in-oil emulsification, and the time

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“window of opportunity” for effective dispersant use was thereby increased. The limited

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mixing energy available due to wave damping in high ice coverage becomes the limiting

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factor for dispersant effectiveness, and additional mixing energy would need to be applied

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after dispersant application to achieve maximal dispersion of the oil, perhaps by propeller

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wash from icebreakers or support vessels91 if these are available.

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Dispersant effectiveness at sea with ice

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The findings of the laboratory and wave tank studies with regard to the ‘weathering’ of oils

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and dispersant effectiveness in Arctic conditions have been confirmed by measurements and

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observations at sea92,93. For example, dispersants were successful in dispersing five different

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oils (API gravity 23.7-43.3°) in the marginal ice zone of the Barents Sea with ice coverage of

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70-90%. Even after six days the oils were dispersible with Corexit 9500, providing agitation

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was supplied by the water-jet thrusters of small boats; less than 10% of the oil remained on

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the sea surface after dispersant treatment93, compared to the majority of oil remaining in the

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untreated slicks.

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The effect of salinity variation

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The salinity of Arctic seawater can be lower than standard open ocean salinity (~35 ppt or

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psu) due to extensive freshwater from the Mackenzie River delta into the Canadian Beaufort

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Sea and the Ob and Yenisey rivers into the Kara Sea94 (which can also deliver large amounts

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of suspended sediments). The temperature and salinity in the very top layer of a calm sea (a

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few centimetres) also varies as ice cover forms and melts; salt is expelled from ice into the

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water during freeze-up and low salinity water is released into the sea when the ice melts,

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although such discrepancies are usually eliminated with any wave action. Laboratory testing

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under simulated Baltic Sea conditions95 with low salinities concluded that different

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dispersants varied in their effectiveness, and similar results have been found in other

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studies96,97. Most dispersants have been formulated to be most effective at salinities above

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20ppt, and their performance is slightly reduced in brackish water, but oil spill dispersants

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that are effective in low salinity and fresh water can be readily formulated98, if required.

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4. Toxicity of dispersants and dispersed oil

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Dispersants were first used on a large scale in the response to the 1967 Torrey Canyon

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disaster. While the products were judged successful at sea, dispersants and degreasing agents

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were also applied directly to oiled shorelines, and flame-throwers99 were used as part of

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shoreline clean-up. There were relatively persistent ecological effects from such treatment100,

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and this precedent-setting event has challenged perceptions about dispersant use ever since43.

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It led to development of new dispersant formulations101 with minimal acute toxicity45 and

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restrictions on where dispersants are applied (i.e., not on shorelines). Further there has been a

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revolution in understanding acute and chronic toxicity tests102-104. Modern dispersants are

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typically formulated from chemicals already used as food additives and/or cosmetics105, and

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they have acute toxicities106,107,218 (LC50 values for 48 - 96 hour exposures of tens to hundreds

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of ppm) essentially equivalent to common shampoos and dish cleaners106,108 (including those

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used to clean oiled sea-birds109). Standard aerial application of dispersants219 (47 l/ha) hitting

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water with no oil would result in a transient concentration of dispersant in the top meter of

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seawater of ~5ppm. When it successfully disperses oil, however, there will be a many fold

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higher concentration of oil in the water.

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The acute toxicity of oil and dispersed oil falls in a range of LC50 values (48-96 hour

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exposure) of 2 to 50 ppm102104, and Arctic species are no more sensitive than their temperate

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cousins111,112. Studies reporting that dispersed oil is more acutely toxic than an undispersed

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slick are the result of reporting nominal concentrations, and when toxicity test results are

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based on measured oil concentrations in the water, dispersant-treated oil is no more toxic than

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physically dispersed oil; the components in the oil drive the toxicity and dispersants do not

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cause a synergistic effect110. Thus, as with all toxicants, it is ‘the dose (concentration x time

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of exposure) that makes the poison’113, and the potential for acute effects rests on exposure to

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more than a few ppm oil or dispersant for some time.

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To understand the potential for acute toxic effects after application, we need to know the

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concentration of dispersed oil in a dispersing plume, and how this changes with time. The

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concentration immediately after dispersion could be several hundred ppm114, but dilution and

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diffusion rapidly reduce the concentration to around 1 ppm within a few hours as the plume

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increases in volume. It is worth noting that without this dilution, the droplets would

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eventually aggregate, as they do in laboratory vessels, and re-form a floating slick – it is the

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turbulence and enormous dilution available in the sea that makes dispersion a viable

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technology114. Once dispersion has occurred, coalescence becomes less and less likely as the

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droplets dilute and have fewer collisions. Whether the surfactants remain with the droplets

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once they have formed, or diffuse away, has no influence on the fate of the oil. Dilution

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continues, of course, but most field trials failed to find oil after several hours. In any case we

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can expect that oil concentrations continue to fall as time proceeds, and it is noteworthy that

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despite the ongoing release of oil into the Gulf of Mexico from the tragic Deepwater Horizon

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blowout, the vast majority (84%) of the >20,000 water samples taken had oil concentrations

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below 1 ppb115, even though sampling was biased towards locations where oil was expected

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based on winds and currents.

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We can thus conclude that oil dispersion – whether by natural wave action or enhanced by

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dispersants - will likely have a very local and short duration toxic influence on the indigenous

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meiofauna and flora – small and developing fish, copepods, algae etc. – but that the volume

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impacted is likely only the first meter or so beneath the dispersed slick, and only for a short

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time. Measured concentrations of dispersed oil in the deepwater ‘plume’ from the Deepwater

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Horizon wellhead were all less than 1 ppm oil, and most were much lower115, so again the

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acute toxicity was likely low.

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Chronic effects of dispersant-facilitated dispersed oil are more difficult to predict, not least

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because there have been few studies at the dilutions that occur in the sea, and in any case

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biodegradation removes dispersed oil and dispersant quite promptly (see below). The

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dispersants used in the Deepwater Horizon response show neither androgen- nor estrogen-

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receptor activity116, and their components have been in widespread commercial use for so

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long that unexpected chronic effects seem unlikely. The widespread use of dioctyl

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sulfosuccinate in commerce, for example, led Hayworth and Clement117 to conclude that the

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trace amounts of this compound in nearshore waters in the Gulf of Mexico came from

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terrestrial sources rather than dispersant applications. Chronic effects of the hydrocarbons

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themselves undoubtedly include the initiation of detoxification mechanisms, such as the

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potential induction of cytochrome P450s118,119. Since hydrocarbons have been part of the

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biosphere for millions of years, such responses may simply indicate a healthy response to

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environmental exposures.

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This is not to say that oils spill have no adverse environmental impacts – of course they do,

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and there has been a wealth of work curated by NOAA on research following the Deepwater

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Horizon tragedy120. Responders and responsible parties will debate how extensive these

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effects are, or will be in a different location, and these discussions in turn will inform Net

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Environmental Benefit Assessments in the Arctic. Fodrie et al.121 point out that there is a

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discrepancy between consistently negative impacts on individual organisms in the laboratory,

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and the absence of measurable negative impacts among populations of estuarine fishes

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exposed to oil. A recent paper considering the risk to Arctic cod populations in the Beaufort

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Sea suggests that even very large spills are unlikely to have a detectable population effect122.

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5. Oil and dispersant biodegradation under Arctic conditions

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The biodegradation of oil has been studied for almost a century123 and by 1941 it was clear

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that marine microorganisms were able to degrade a long list of hydrocarbons: ‘gasoline,

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kerosene, lubricating oils, crude oils,…’124. There has been enormous progress since then,

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and the biodegradation pathways of most of the broad classes of hydrocarbons are reasonably

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well understood in aerobic environments125-130. Aerobic biodegradation in seawater is

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remarkably extensive, with the essentially complete disappearance of nearly all the

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hydrocarbons in a few weeks at dilutions that reflect environmental concentrations of

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dispersed oil131. The slowest to degrade are cyclic saturated hydrocarbons such as the

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hopanes and steranes131, and the asphaltenes and resins that are the pigments of petroleum.

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There is some evidence that the latter components are at least partially biodegradable132-135,

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but the biodegradation of road asphalt is very slow, even with canine nutrient amendments.

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The biodegradation of hopanes and steranes is known136,137,131, but their environmental

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persistence is such that Ourisson138 concluded that there was of the order of “1011-12 tonnes in

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the biosphere, which is as much or more than the total mass of organic carbon in all living

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organisms.” Once the majority of the hydrocarbons have been degraded131,200, the residue is a

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tar-like material rather akin to road tar – a non-oily solid that probably amounts to no more

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than 15% of an oil with API gravity >20°. In small quantities, as the remainder from the

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biodegradation of small oil droplets, the residue is difficult to distinguish from humic and

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fulvic substances in sediments139. We note in passing that the polycyclic aromatic

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hydrocarbons of most toxicological concern222 (e.g. benzo[a]pyrene, 5-methylchrysene, etc.)

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are pyrogenic, and are not found beyond miniscule amounts in crude oils27.

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The rate of biodegradation has been the subject of much confusion for many years, with

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estimates of half-lives in the environment ranging from days to years140. A major

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confounding factor in understanding biodegradation is the insolubility of most hydrocarbons

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– for example decane is only soluble to 49 ppb, and longer alkanes are even less soluble141,

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while the solubility maximum for phenanthrene in seawater is 710 ppb142 and of

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perhydrophenanthrene (its fully saturated form) is 20 ppb141. Thus almost all the components

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Environmental Science & Technology

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of a dispersed oil are present as a separate phase, oil droplets, in seawater, and microbial

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attack is restricted to the surface of the oil at the oil-water interface. The surface area is

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maximized with the smallest droplets, and as expected Brakstad et al.142 have shown that oil

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droplets of 10µm diameter are degraded subtly faster than those of 30µm under identical

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conditions. A major goal of modern dispersant application is thus to facilitate the generation

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of the smallest possible droplets. When oil is dispersed as dilute droplets of 1km) shoreline led to measurable oil concentrations on the shore of only 1-2ppm215.

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