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The science behind dispersant use policies Typically many variables at play when making decision over whether to deploy chemical dispersants in response to offshore oil spill Alan Bailey Petroleum News
The incorporation of chemical dispersants into the toolbox for responding to an offshore oil spill has become a controversial subject over the years. While dispersants can quite rapidly break up an oil slick, people worry about the environmental impact of the dispersed oil and of the dispersant chemicals themselves. But what is the scientific basis for dispersant use? And how does this science impact a decision to use dispersants when tackling a spill?
On Oct. 12, during the U.S.-Canada Northern Oil and Gas Research Forum in Anchorage, Richard Bernhardt, scientific support manager in Alaska’s Division of Spill Prevention and Response, reviewed dispersant science and how this science impacts dispersant policies in Alaska.
Alaska policies In 2016 the Alaska Regional Response Team, the advisory board for coordinating the government response to Alaska oil spills, approved a new plan, setting policies for the use of dispersants around the coast of the state. That plan includes a dispersant pre-authorization zone in a region north and south of the Alaska Peninsula and extending around southern Alaska to Prince William Sound.
Essentially, a dispersant works by breaking an oil slick into minute droplets that enter the water column, where they are then devoured by oil consuming microbes. Left to its own devices, a slick will disperse naturally. However, dispersant use greatly speeds up that process, thus potentially enabling the relatively rapid removal of a slick, depending on the circumstances surrounding the slick’s location.
Bernhardt said that voluminous scientific literature has been published from research into dispersant use. A key finding is that dispersant effectiveness and the environmental impacts of dispersants both depend on a multitude of factors - policies for dispersant use and individual decisions on the application of dispersants depend on evaluations of these factors in individual circumstances.
Oil characteristics The character of the oil itself impacts dispersant effectiveness. In particular, it is important to know how dense the oil is, and hence whether it will tend to float on the sea surface. The oil’s pour point, the temperature above which the oil is thin enough to flow, is also critical. North Slope crude, for example, will tend to float and has a pour point of minus 8 C, Bernhardt said.
Weather conditions can also be important. Natural dispersion of the oil actually becomes more effective than chemical dispersion when wind strengths exceed around 27 miles per hour, Bernhardt said.
One area of research has involved determining the optimum ratio of dispersant to oil in a dispersant operation. The optimum ratio has been found to be around one to 20. However, trying to maintain this ratio is difficult because of significant variations in the concentration of oil between different parts of a slick, Bernhardt said.
Chemical dispersants typically break up the oil into much smaller droplets than does natural dispersion. But a portion of the droplets resurface, a phenomenon that may result in only 50 percent or so of the oil remaining dispersed. Although this tendency may raise questions over the point of using the dispersant technique, it is important to realize that, by comparison, the mechanical recovery of oil from a sea-surface slick is typically only about 5 to 15 percent effective, Bernhardt commented.
Harmful chemicals Crude oil consists of a cocktail of chemicals. From an environmental perspective, components called polycyclic aromatic hydrocarbons, or PAHs, are particularly harmful to ocean wildlife. And the natural dispersion of oil tends to selectively disperse less of these chemicals than do chemical dispersants. However, research has shown that, while under natural dispersion the PAHs may remain in the water for extended periods of time, chemical dispersion will result in the microbial removal of 85 percent of the PAHs within two weeks, Bernhardt said.
And the environmental impacts of chemicals such as PAHs critically depends on the life stage of any organisms that the chemicals encounter. While exposure of embryonic organisms to minute concentrations of PAHs can cause serious birth defects, adult fish, for example, may only be sensitive to much higher concentrations. Moreover, the impact on an animal depends on whether animal actually absorbs the chemicals: Some oil on an animal’s skin may not, for example, significantly impact the animal, if the oil is not incorporated into the animal’s metabolism.
Timing may also be important: Some fish species, for example, migrate and may only exist in certain regions at certain times of the year.
Research has also shown that oil dispersed at the sea surface does not normally penetrate the water column below a depth of about 30 feet, Bernhardt said. Recognizing that the majority of ocean species around Alaska spawn in the shallower coastal waters, Alaska dispersant use policy prohibits dispersant use in water depths of less than 60 feet, he said. Moreover, Alaska’s dispersant pre-authorization zone lies at least 20 miles offshore and avoids critical wildlife habitats.
Response decision making In addition to the use of science in determining policies for dispersant use, science will also come into play when a response team is evaluating the potential use of dispersants when dealing with an actual oil spill. The team will conduct a net environmental benefit analysis before a decision on dispersant use is made, Bernhardt said. This analysis will involve considering the various factors at play, such as wind conditions and the proximity of rivers and estuaries, to weigh up the odds of success and the environmental risks if dispersants are applied. And state policy requires a small-scale pilot test of dispersant application, before a full-scale application can be authorized.
In addition, state policy requires the dispersant to be deployed within seven hours of a dispersant use decision being made. The presence of a 75,000-gallon stockpile of dispersant in the state is critical to being able to meet that deployment time limit, Bernhardt said.
And, although there are risks associated with dispersant use, these risks must be balanced against the risks of alternative response techniques. Oil spilled on the water is a bad situation, whatever approach is used to deal with it.
“Once oil’s spilled, the genie’s out of the bottle,” Bernhardt said. “It’s going to disperse naturally, whether or not we choose to use chemicals.”
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