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4.2.1 Natural Attentuation

Several field tests with experimentally released oil have been completed in the Arctic (see SL Ross, 2012 for an overview).  However, except for the Baffin Island Oil Spill (BIOS) experiment these tests were devoted to studying the behavior and environmental fate of the oil in icy conditions and not the environmental impact at the spill location or other ECs.  Nonetheless, this information helps frame the physical and chemical factors that affect the nature of oil released in an Arctic environment and help to identify the challenges to be encountered in implementation of any type of OSR activity.  In fact, this sets the basic expectations for what might occur if natural attenuation were the only response option implemented. 

The specific challenge encountered in the Arctic during OSR is the presence of permanent or seasonal ice, which has many consequences (Potter et al. 2012).   Ice reduces the sea surface agitation which coupled with the low prevailing temperature, slows down the spreading of the oil slick reducing physical weathering and emulsification that occurs with more active surface water disruption.  Ice also limits oil spreading when it is between ice blocks, or when beneath or on top of the ice.  This helps keep the oil relatively concentrated reducing the rate of oil weathering.  Large quantities of oil can be trapped either in snow, or on or under the ice within spaces found in the unevenness of the ice surface.  When ice is forming, oil can be encapsulated in the new ice and thus kept unaltered during the winter season.  Oil trapped in ice can then be released to surrounding waters when the ice melts, possibly reappearing as fresh, unweathered oil.  Oil has been observed to migrate through the ice, at a rate that is dependent on oil viscosity.  Experimental studies have determined that oil components separate within the ice and undergo degradation during the winter in the ice brine channels, as discussed below and in Section 5. Potential Environmental Impact of Untreated Oil

If spilled oil is not recovered or treated, heavier oils may persist on the surface of water or surface of ice and can affect biological communities that are utilizing these interfaces, especially birds or mammals with fur (due to the potential loss of thermal insulation) whereas lighter oils may naturally disperse into the water column.  A reduced rate of oil weathering may occur if oil is encapsulated by ice and the oil may not be biologically available until the spring thaw.    Another unique attribute of the Arctic is that oil can strand on the shore during the ice-free season, whereas at other times the shoreline may be protected by landfast ice, which prevents oil from coming ashore.  Oil stranding on shoreline substrates during the ice free period is subjected to the strong erosional forces of the ice on shoreline substrates during the next ice build-up and ice break-up seasons.  Only deeply buried oil that might occur in intertidal cobble fields would be sequestered for extended periods of time (e.g., decades).  Therefore in many cases the persistence of oil onshore will be governed by the physical erosional forces that occur in many shorelines which minimize its retention.  However, the weathering and recovery processes may be longer (e.g. occupy those area for more than one year) in cases where oil is sequestered in spaces among cobble or boulder fields or where oil may be trapped in isolated nearshore water bodies.  The practicalities of staging recovery operations in remote locations are also a consideration.

The effect of oil that is left to natural attenuation on the shoreline depends upon:

  • The environmental resources of concern that are present when the oiling occurs and during subsequent seasons when extended oil exposures could be possible.
  • The duration certain ECs of shoreline contamination, which may not persist for more than one year in while in others it may be extended for longer periods.
  • The resiliency of the populations of various species that were impacted as a result of the spill and treatment methods that were used.  The resiliency of these populations and communities of organisms is controlled by fecundity, immigration from unaffected areas, and the diversity of organisms present within the affected habitats.

McAuliffe et al. 1980 reported on the effects of oil on under-ice meiofauna as a part of the BIOS project (McAuliffe et al. 1980).  Effects of this experimental spill on ice algae are summarized in a report by SL Ross (2010).  In that study, the bottom 10 cm of ice had decreased density of meiofauna, no adverse effects were observed on the ice algal community, and under-ice invertebrates showed no mortality but did drift away from the oil impacted area for days following the spill.  All the findings of the BIOS Project are summarized in Li et al. (1992).

A separate study conducted on first-year sea ice off Svalbard showed that there is a migration of bioavailable water soluble components (WSC) from encapsulated oil through the ice to the underlying water (Dickins et al. 2006, Faksness et al. 2012).  The estimated toxicity of these dissolved oil components in the ice was calculated using toxic units and the findings indicated that the concentration of WSC in the brine channels might be acutely toxic to the ice fauna.   Results from another field study of an experimental release of 7000 L crude oil in the Barents Sea showed low concentrations of dissolved hydrocarbons (maximum concentrations were 4 ppb dissolved hydrocarbons and 32 ppb total hydrocarbons) in subsurface water.  Predicted toxicity to the exposed community in the upper layers of the water, expressed as toxic units, was 0.11 or less, indicating that the potential for acute toxicity was low in subsurface bulk water (Faksness et al. 2012).  However, the effects of surface oil on organisms using the surface layer, polynyas, ice-edges or adjacent shorelines where oil compounds can be re-concentrated was not assessed.

These studies indicate that there could be effects on the local ice biota if the oil is encapsulated in the ice or trapped underneath the ice. The organisms associated directly with the ice could be exposed to potentially toxic dissolved hydrocarbons over the course of several months, causing potentially toxic oil components to enter the Arctic marine food web. On the other hand, the measured concentrations of dissolved hydrocarbons in the water column, or underneath an untreated oil slick, have been lower than potentially toxic concentrations, perhaps indicating that severe effects to organisms residing in the water column would be negligible.  However, this does not take into account the reconcentration processes that occur at interfaces such as the surface of the water, at ice water interfaces, convergence zones and shorelines (refer to Sections 2 and 3). Conclusions on Natural Attenuation

Oil remaining in Arctic habitats without treatment will behave similarly to non-Arctic situations, although chemical processes such as dissolution, volatilization, and biodegradation may occur at a slower rate resulting in increased persistence.  In non-ice periods oil spills on the sea surface will remain at the sea surface and be transported in slicks by winds and currents to shorelines, convergence zones, and offshore surface waters.  During that process some of the oil will dissolve into the water column or be physically dispersed into the water column as droplets, some will volatilize into the atmosphere, while the majority of the oil may remain on the surface where it will weather, biodegrade, emulsify and accumulate in zones of reconcentration.  Subsurface releases of untreated oil will generally rise towards the sea surface but during that transport it may also be rapidly biodegraded based on the increased surface area of oil droplets created by the turbulence of the release.  Oil that remains on the sea surface can be stranded on shorelines or concentrate in convergence zones but the oil may also be encapsulated by ice as it forms.  In order to facilitate the forecasting of the seasonal dynamics of oil in these compartments, it is important that data are available for NEBA evaluations. This will facilitate the decision making process regarding the most appropriate response option under various conditions. 

Such a NEBA process would evaluate the trade-offs of untreated oil containment by ice and treatment efficiency with decreased impact on pelagic environments by dispersant treated oil in non-ice environments.  In addition the increased effects of surfaced oil as it is captured and released by formation and melting of ice on seabirds, marine mammals, annual ice fauna and flora should be evaluated. Also the biological significance of overwintered oil and ice must be determined.

Many data that serves as a basis for such evaluations is already available, but improvement of the information base would result in further reduction of uncertainties. Suggested topics for such studies are: 

  1. Biodegradation
    • Measure the biodegradation of oil in ice and trapped within leads or under ice over a winter season.  Compare to biodegradation of oil in pelagic waters and surface layers during non-ice periods.
    • Does frazzle ice increase biodegradation of oil released from ice by physical grinding and disturbance of oil, creating larger surface area for microbes to degrade the oil?
  2. 2. Presence of VECs
    • Determine avoidance behavior for fish and invertebrate VEC’s associated with oil trapped with ice.  Indications are that they will move away from oil.
    • Evaluate the use of polynyas or leads by VEC fish, invertebrates, sea birds, and marine mammals and the potential for oil effects in these critical habitats. Compare oil within broken ice fields and open waters as an attraction to seabirds, marine mammals, fish and invertebrates.
    • Summarize the same types of information for seabirds, shorebirds, marine mammals. 
  3. Considering the uniqueness of Arctic shorelines influenced by landfast ice, it will be important to understand the dynamic processes controlling the fate and persistence of oil on such shorelines.  This will require an assessment of the potential for lingering oil releases and the assessment of the natural decontamination rate resulting from different responses