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4.2.4 Improving Dispersion of Oil

Chemical dispersants are most effective when applied during or quickly after a spill or sub-sea release event, before dilution, weathering and emulsification of the oil reduces the effectiveness of the dispersants.  Modern dispersants are mixtures of solvents consisting of organic carbon chains that are oleophilic and surfactants that are hydrophilic.  The combination of oleophilic and hydrophilic components change the surface viscosity of the oils and create small droplets of oil that are released from the surface water and move into the water column or from deep water releases into adjacent deep pelagic environments.  These small oil droplets have greatly increased surface area that increases the rate of microbial degradation compared to the oil prior to dispersant application.  Dispersant mixtures have been evaluated by numerous organizations to determine their toxicity and efficiency of dispersion under many different environmental conditions.  By breaking up the oil and creating micron-sized droplets, chemical dispersion reduces the persistence of a surface slick or the potential for sub-sea discharges to reach the surface and thereby minimizes potential encounters by marine mammals and offshore bird populations. Application of chemical dispersant to sub-surface and to surface oil slicks reduces the amount of oil that becomes stranded on the shoreline and prevents oil from transforming into weathered oil-in-water emulsions that are resistant to further biodegradation (Lewis and Daling 2001). 

However, dispersing oil into the water column from surface slicks or deep water releases is most effective when the oil is fresh and unweathered.  Mitigating damage to the shoreline and to organisms that may encounter surface slicks means exposing the near surface and shallow or deep pelagic communities to elevated concentrations of dispersed oil for short periods of time. 

The literature reviewed focused on dispersant applied at the sea surface and it contains information on the toxicity of oil chemically dispersed into the water column and effects on those marine organisms from laboratory studies.  Information on the behavior of dispersants applied below the upper surface layer during blow-out scenarios were sparse during this review period.  However, it is expected that post-spill studies of the recent Macondo well blow-out and explosion that occurred in 2010 in the Gulf of Mexico will contribute greatly to our knowledge of subsea application of dispersants as well as natural dissolution and biodegradation processes that can occur in the deep ocean environment and at very cold temperatures that are similar to arctic temperatures.

There are limited holistic assessments that combine the toxicity of chemically dispersed oil data with information on specific assessments on toxic impacts associated with Arctic communities or the comparative damage resulting if oil persists on the surface or comes ashore.  Therefore, the rationale for application of chemical dispersants should be based on the comparison of the possible extent and duration of impacts to the organisms living in the water column (e.g. fish, shellfish, plankton, etc.) resulting from the use of dispersants, and the extent and duration of potential damage which would result if dispersants were not used, i.e. from a persistent surface slick (e.g. effects to birds, mammals, and fish and invertebrates that live in the very surface of the water) and from the stranding of the weathered oil on the shoreline (e.g. effects to coastal shoreline species and benthic organisms).  This type of information must be factored into the tradeoffs associated with Arctic dispersant use, and is considered in the section “Monitoring Natural Recovery (no active response)”. Impact of Chemically Dispersed Oil

Most toxicity studies evaluate the impact of increasing the exposure of pelagic organisms to oil as a result of dispersing the material into the water column.  Considering the toxicity toward water column organisms, it is recognized that the observed toxicity effects from chemically dispersed oil is due to the effects of the increased quantity of dispersed oil into the water and are not caused by­­­­ the dispersant itself, as modern dispersant formulations are much less toxic than oils (Hemmer et al. 2011).

In assessing dispersed oil toxicity, determinants of adverse effects for a given species are exposure concentration and duration of exposure (see also a more detailed review of peer-reviewed literature presented in Section 6, Ecotoxicology of Oil and Treated Oil).  A review of field studies found that small-scale field tests have demonstrated that the concentration of dispersant in water falls to less than 1 mg/L within hours (NRC 2005).  The available data suggest that in general, maximum dispersed oil concentrations after a spill are less than 50 mg/L immediately after dispersion into the upper water column (top 3 m) and that dispersed oil concentrations dilute rapidly, dropping to 1 to 2 mg/L in less than 2 h throughout the water column (Cormack and Nichols 1977, Daling and Indrebo 1996, McAuliffe et al.1980).  These low concentrations are generally below estimated toxicity threshold concentrations derived from exposure experiments for most common water column organisms (McFarlin et al. 2011, Gardiner et al. 2013). 

The BIOS experiment conducted in sub-Arctic nearshore areas in the 1970s studied oil dispersion impact on nearshore environments and concluded that the results offer no compelling ecological reasons to prohibit the use of chemical dispersants on oil slicks in nearshore areas (Potter et al. 2012).  Secondly, the results provide no strong ecological reasons to undertake an intrusive effort to cleanup stranded oil (on certain shoreline types).

During an experimental oil spill in the Barents Sea in 2009, 2000 L of crude oil were dispersed six hours after release (Potter et al. 2012). Two hours later, measurements of oil in water were performed at depths of 1, 2 and 3 m. The maximum concentration of oil in water was measured to 5.5 ppm (at 2 m depth) with an oil droplet size smaller than 10 µm, 30 minutes after mixing energy was added by the ship thrusters.  The monitoring indicated background concentrations were restored shortly after these measurements, as the plume had most likely drifted and diluted with the currents (Merlin and Le Floch 2012).  After the Sea Empress incident, a major spill in nearshore waters at the port of Milford Haven, UK, dispersed oil concentrations were monitored and quantified in the field.  Results showed 10 ppm dispersed oil immediately after the dispersant application, decreasing to 1 ppm 2 days after, 0.5 ppm 1 week after and 2 ppb 1 month after (SEEEC 1998). 

Such a decrease can be modeled with the following relationship:

                                   C = C0 e-1.35                                                                                                 Equation 1

Where C equals oil concentration at time (t in hours);

C0 is the initial concentration;

e-1.35 represents an exponential decline in oil concentration 

Application of the equation yields a half-life of 12 h for the dispersed oil concentrations [every 12 hours the concentration is divided by 2 (Merlin and Le Floch 2012)]. This reflects a dilution rate for a sustained spill response implemented over several days in a deep, but nearshore environment.  In more recent toxicology studies carried out in several laboratories in North America (Aurand and Coelho 2005), the exposure duration was modeled after a single dispersant application to offshore, open water habitats establishing a half-life of 4 hours. These representations of ‘spiked’ exposures are more environmentally realistic (closer to real field conditions) than standard laboratory ‘constant’ exposures, and result in a reduced level of effects (Gardiner et al. 2013). 

The effect and toxicity of a water soluble fraction (WSF) versus chemically dispersed oil was studied by using realistic exposure concentrations based on the WSF concentrations  monitored during an offshore field experiment (i.e. initial TPAH concentration of less than 7 ppb; NRC 2005).  The Arctic amphipod Gammarus setosus was used as test species in a continuous flow experiment. Body burden measurements showed higher level of PAHs in the gammarids exposed to oil and dispersant for 12 days than in those exposed to oil alone, consistent with the higher concentrations of oil that would be present when dispersant are used.  Several biomarkers were monitored, and gammarids exposed to oil and dispersant also showed moderate signals of exposure after recovery in clean seawater.

In a recent study on adult and juvenile fish and bivalve species conducted at elevated concentrations (up to 70 mg/L), the observed effects were sublethal and temporary.   After 2 weeks, sublethal bioindicators did not show any differences between animals exposed to the chemically dispersed oil and mechanically dispersed oil (Merlin and LeFloch 2012).  This demonstrates that exposure to chemically dispersed oil is not more toxic than the physically dispersed oil.  However, the same research team reported that fish kept in a natural environment after exposure did show residual responses (persistent) in terms of growth (Merlin and LeFloch 2012).  In conjunction with the previous study, experiments conducted with herring embryos in a wave tank showed abnormalities after constant exposure to elevated concentrations (to 10 ppm), but no effect when a more realistic and rapid dilution exposure regime was generated (McIntosh et al. 2010). 

Dispersant toxicity research has been conducted recently on specific Arctic species of concern as part of a laboratory toxicity testing program conducted in Barrow, Alaska.  It was found that Arctic species that were tested have similar or greater tolerance to representative concentrations of dispersed oil compared to the numerous temperate species that have been tested (Word and Gardiner in prep.).  Also, the acute toxicity of exposures to dispersant alone only occurs at concentrations that are greater than concentrations proposed for application of dispersant products in OSR (McFarlin et al. 2011; Gardiner et al. 2013).  For most species that have been tested, dispersed-oil acute toxicity thresholds are on the order of 1 mg/L based on laboratory tests that expose test organisms for periods of 2 to 4 days.  Water column concentrations above toxicity thresholds in an actual spill are limited to the top few meters and exposures at potentially toxic concentrations are limited in duration due to rapid dilution kinetics. Conclusions on Chemical Dispersion

The available body of laboratory data, experimental field studies and monitoring following actual spills shows that dispersed oil may potentially cause environmental impacts but these will be limited to the organisms in the immediate vicinity of dispersed oil plume and in cases when the rate of dilution of the dispersed oil plume is slow.  This would be the case for sensitive areas with limited water exchange,e.g. close to the shore.  Even in such cases, these impacts would generally be limited to non-mobile organisms.  For example, monitoring following dispersant use at major oil spill incidents over the past 40 years has never reported significant losses of mature fish populations at sea following dispersant applications. 

Laboratory and field research as well as monitoring following actual incidents assessing the impact of use of dispersants in OSR, demonstrate that:

  • The toxicity of oil/dispersant mixtures is related to the oil in the mixture and not the dispersant.
  • The toxicity of the oil is directly related to the amount of oil that organisms are exposed to.  That is, when dispersants are applied to oil the increase in response of pelagic organisms is directly related to the exposure concentration and duration of exposure to the oil.
  • The toxicity of dispersed oil is relatively low and often not observable in real environment as long as there is no restriction to the rapid dilution process of the plume of dispersed oil (e.g. open-ocean).
  • There is no evidence that Arctic species are more or less sensitive than other temperate climate species that have been tested with dispersed oil. Dispersing Oil using Oil Mineral Aggregates (OMA)

The use of fine mineral particle (such as clay minerals) is an alternative response method to dispersant used to break up an oil slick into small droplets and stabilize the oil dispersion in the water column.  When applied to physically dispersed oil, oil droplets aggregate readily with suspended particulate matter (SPM) such as clay minerals and organic matter to form [oil-SPM] aggregates called oil mineral aggregates (OMA; Le Floch et al. 2002).  It is important to distinguish the use of OMA from sinking agents.  Rather than bind to bulk oil as dense sediment and cause the oil droplets to sink, OMA will cause the oil to be suspended in the water as micron-sized droplets associated with a complex of mineral material in much the same result as chemical dispersants generate micron sized droplets (Khelifa 2005, Khelifa et al. 2005).  The simplest form of OMA consists of an oil droplet coated with micrometer-sized solid mineral particles that prevent the droplets from sticking to each other and reforming a slick.  When OMA forms, the dense mineral fines (small but 2.5 to 3.5 times denser than most oils) adhering to the oil droplets will reduce the overall buoyancy of the droplets, retarding their rise to the surface but keeping them somewhat buoyant so they do not sink.  This promotes oil droplet dispersion throughout the water column to low concentrations, and ultimately enhancing their biodegradation by natural bacteria (Lee et al. 2011).

Positive lab and basin tests of the concept led to a field test in 2008 (Lee et al. 2011).  The field test was designed to evaluate the concept of using an icebreaker’s propeller and application of mineral chalk fines with seawater to create OMA. Visual observations confirmed that the oil stayed physically dispersed in the upper water column and did not resurface (Potter et al. 2012).   Attempts were made to combine chemical dispersant use with fine mineral application.  The dispersant was added to promote the dispersion of micron-sized droplets into the water column while the addition of fines attempted to stabilize this dispersion.  The result was not especially convincing, as dispersant presence seemed to inhibit the formation of OMA complex with the oil droplets.

Preventing the re-surfacing of the droplets under the adjacent ice in the Arctic would be a significant environmental benefit since OMA also enhances natural biodegradation of spilled oil.  The application of fine minerals seems well adapted to ice infested conditions as the presence of ice reduces the sea surface agitation; chemically dispersed oil may tend to resurface over prolonged time periods if not stabilized by OMA formation.  It is also beneficial that the types of fine minerals needed for OMA dispersion are those that are commonly stockpiled in oil exploration facilities as drilling mud components; consequently, a source would be readily available in the event of a spill. Environmental Impact of OMA formation

Most of the studies on this topic were devoted to the mechanism and efficiency of the technique to optimize the application conditions; very few considered the environmental impact of the use of OMA.  A lab study dealing with the use of dispersant in estuaries assessed the toxicity of dispersed oil with presence of Montmorillonite clay (Merlin and Le Floch 2012).  It was shown that the presence of this fine grained material reduced the observed impact on biota exposed to the dispersed oil plume to the level of impact commonly seen from oil that is mechanically dispersed (without chemical dispersant addition).  To date, sparse information has been identified on the environmental impacts and relative toxicity of OMA in the Arctic.

Laboratory and field research as well as monitoring following actual incidents assessing the impact of use of OMA in OSR, demonstrate whether:

  • The toxicity of oil/OMA mixtures is altered or if the oil in the mixture is the predominant cause of any toxicity observed with OMA use.
  • The association of oil and OMA may alter the toxicity of the oil by decreasing bioavailability due to the adsorptive process that occurs to the OMA. Conclusions on OMA

The environmental advantages of using OMA to stabilize oil dispersion in the upper water column are similar to those expected from the use of chemical dispersant.   Mineral fines are nontoxic to marine life.  The main impact expected from addition of the mineral fines could be a temporary increase of the sea turbidity which should be similar to the level of turbidity promoted by chemical dispersion.  Other mechanisms of impact would be similar to the environmental/biological impacts discussed with chemically dispersed oil.  The description of the flocculation of fines to the outer surface of small oil droplets leads to the following questions prior to its acceptance as an OSR option for the Arctic.

  • What is the optimum rate of OMA application to oil to maximize its benefits (OMA may need a 1:1 ratio with oil to provide its benefits.
  • Does OMA surface coating of oil droplets reduce potential microbial degradation or use of the oil droplets?
  • Does OMA surface coating of oil droplets reduce the potential toxicity of the droplets or decrease the solubility and exposure of the more soluble/toxic components of the oil?
  • Are OMA coated oil droplets available to suspension feeding organisms in an unweathered, potentially more toxic form?
  • The remaining questions regarding the long term environmental fate of the OMA aggregates are: do they tend to sink progressively with time? What is the impact of settled OMA to the exposed area of bottom resources that could be large but at very diffuse concentrations?  Does the mineral separate from the oil droplet?   What is the impact of OMA or separated mineral exposures to dilute inorganic particulate matter and would it be any different than that endured from settling of ocean particulate matter?