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The known and unknown on dispersants Chemicals cause spilled oil to dissipate quickly in the ocean but what may be the impact of dispersed oil on the wildlife? Alan Bailey Petroleum News
The massive use of chemical dispersants to deal with oil spewing from the Macondo wellhead during the 2010 Deepwater Horizon disaster brought the use of this particular oil spill response technique to the forefront of many people’s attentions. But what are the benefits of using dispersant chemicals in this way? And what are the downsides to a technique that can result in the relatively rapid dissipation of an ocean-surface oil slick?
On Feb. 9 during the Alaska Forum on the Environment some dispersant experts talked about the practicalities of dispersant use, and about the benefits and risks associated with the technique.
Dealing with a bad situation Tom Coolbaugh from ExxonMobil Research and Engineering Co. stressed that an oil spill is a bad situation that people hope will not happen. A dispersant is one component of a set of response tools designed try to make that situation less bad, he said. And in choosing which tool to use, oil spill responders need to consider the net environmental benefit of using one technique versus another.
Dispersants work in a very similar manner to domestic detergents, converting a continuous oil slick into a cloud of minute oil droplets that dissipate through the water column.
By greatly increasing the contact surface area between the oil and the water, dispersion accelerates the rate at which naturally occurring microbes consume the oil, with the microbes using the oil as an energy source and a source of carbon. However, initially at least, the oil remains in the water, in droplet form, following dispersant application. So, a decision on dispersant use will depend on an evaluation of the ecological outcome of leaving the oil on the water surface versus dispersing it into the water column, taking into account factors such as places that need protection from the oil, Coolbaugh commented.
Dispersant research Coolbaugh said that a joint industry program working through IPIECA, an international oil industry association, has been testing the effects of applying dispersants subsea, rather than on the water surface. In the Deepwater Horizon disaster nearly half of the dispersant used was injected subsea directly into the oil flowing from the Macondo well. The industry sponsored research has found that treating the source of a subsea oil plume in this way, rather than treating the resulting surface oil slick, can reduce the volume of dispersant required by a factor of five, thus greatly reducing the total volume of dispersant that needs to be applied to the water column, Coolbaugh said.
Another research initiative is investigating techniques for locating areas where oil is thickest in an oil slick, to enable the maximization of benefits gained from the application of response techniques such a dispersant use. People are also developing new dispersant delivery platforms. One company, for example, is developing a dispersant platform based on a Boeing 727 aircraft, to reduce the dispersant transportation time relative to the traditional use of slower aircraft such as the Hercules C-130, Coolbaugh said.
Consumption by microbes Kelly McFarlin, a research student at the University of Alaska Fairbanks, has been researching the impact of dispersants on the rate at which microbes consume oil in seawater. Worldwide, more than 180 bacteria genera have species that can degrade petroleum by breaking down oil components into carbon dioxide and water. In fact, marine micro-organisms have long been exposed to natural oil seeps - it has been estimated that between 1990 and 1999 natural oil seeps released about 600,000 tonnes of petroleum per year into the world’s oceans, McFarlin said. There are known natural oil seeps off the North Slope of Alaska and naturally occurring organisms in Arctic seawater that degrade petroleum, she said.
Although low water temperatures tend to increase oil viscosities and hence reduce the availability of oil to micro-organisms, researchers have found bacteria that have adapted to maintain their metabolic rates in extreme cold, McFarlin said.
Speeds up the process McFarlin said that she has conducted tests using oil mixed into seawater samples collected near Barrow, on Alaska’s North Slope. Using Corexit 9500, the dispersant used in the Deepwater Horizon response, and working at relatively low temperatures, the impact of the dispersant on the rate of microbial decomposition of the oil when the oil was vigorously mixed with the water was quite modest, McFarlin said. However, other published research has demonstrated that the impact of dispersant on the biodegradation on a simulated surface oil slick is very marked - the application of Corexit during the research resulted in the loss of 84 percent of the oil after 40 days, as compared with a 33 percent oil loss if no dispersant is used, she said.
And measurement of the carbon dioxide emitted by the microbial action on the oil indicates that most oil consumption takes place in the first 10 days of dispersant application.
The laboratory research does confirm that micro-organisms indigenous to the Arctic can biodegrade North Slope crude oil. However, biodegradation does not completely remove all oil components, with some compounds within the cocktail of oil chemicals being hard to degrade. And research is needed into the effectiveness of biodegradation in and under sea ice, McFarlin said.
Toxicity concerns John Incarnoda from the National Marine Fisheries Service commented on what is known about the toxicity impacts on marine ecosystems of applying dispersants to spilled oil. The active ingredients in commercial dispersants are also widely used in domestic soap products and have quite low toxicity, Incarnoda said. However, the toxicity impact of applying dispersants to an oil slick is something of a two edged sword: On the one hand the dispersant increases the rate of microbe degradation of the oil and removes the oil from the sea surface, where some marine animals are active, while on the other hand the dispersion of huge numbers of oil droplets into the water column may increase the rate of uptake of the oil by marine organisms, damaging those organisms and potentially impacting the marine food chain.
Particularly at risk are fish eggs and invertebrate organisms that tend to absorb oil that they come into contact with, Incarnoda said.
Ingested oil Sarah Allen from the NOAA Office of Response and Restoration in Anchorage said that a 2012 study into the impact of dispersed oil on one species of crustacean had shown ingested oil in the organism’s gut and in fecal pellets. The excretion of the oil by the organism could move the oil to greater depths in the ocean, Allen said. More research is needed into the impact of chemical dispersion on the uptake of oil in organisms’ digestive tracts, she said.
Incarnoda said that one area of concern is the potential impact of dispersed oil on fish embryos, organisms that are typically translucent and hence susceptible to enhanced toxicity because of the effects of sunlight. Researchers have consistently found that changes in acidity resulting from oil exposure causes the embryos to die from heart failure, he said.
However, much research remains to be done on the potential impacts of both dispersants and dispersed oil on the marine ecosystem. For example, nobody knows why oil impacts the acidity of a fish embryo, and there is an unanswered question regarding whether the presence of dispersant changes the rate of uptake of oil droplets by organisms such as fish eggs, Incarnoda said.
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