About the invisible oil spill – and the chemicals that made it disappear

Summary:   Chemical dispersants have played a critical role in the Gulf oil spill, minimizing both the oil slick and the resulting public outcry.  But these are chemicals, not magic dust.  Much of the oil is hidden — dispersed, diluted — throughout the volume of the Gulf instead of spreading on the surface.   This protects life on the coast, but risks life in the sea.  Since there is little research on their effects, we’re (again) conducting a test on the largest possible scale (welcome fellow lab rats).  The doomsters’ fears (the Gulf will die!) are probably exaggerated, but the public is too complacent about the dangers.


  1. Use of dispersants in the Deepwater Horizon oil spill
  2. About the pros and cons of dispersants chemicals
  3. Findings of the National Academies report about dispersants
  4. Fact sheets about dispersants
  5. Other posts about the Deepwater Horizon disaster and an Afterword

(1)  Use of dispersants in the Deepwater Horizon oil spill

(a)  Aprox 436 thousand gallons of various dispersants have been put on or in the Gulf, as of May 12.

(b)  Use this application to see and understand its size:  How big is the Deepwater Horizon oil spill?, by Paul Rademacher.

(c)  “More on the Scale of the Gulf Spill“, James Fallows, blog of The Atlantic, 12 May 2010 — Excerpt:

One thing I think needs to be added to it, however, is the concept of volume rather than two-dimensional overlays. Imagine that the spill not only lay on the ground in those locations, but drifted from ground level to 5000 feet in altitude–killing and sickening wildlife wherever the oil was encountered. Then we begin, I think, to grasp some of the horror of what is happening. Most of it is where we can’t even see it right now.

(2)  About the pros and cons of dispersants chemical

(a)  “Chemical dispersants being used in Gulf clean-up are potentially toxic“, Tom Philpott, Grist, 6 May 2010

(b)  Excerpt from “Deep-sea chemical dispersants weighed for cleanup of Gulf of Mexico oil spill“, Washington Post, 10 May 2010:

The decision on whether to use chemical dispersants deep below the sea’s surface to break up the Gulf of Mexico oil spill boils down to two central questions: Is it worth taking this unprecedented step to protect the region’s sensitive and ecologically valuable wetlands, even at the potential expense of its marine life? And because the scientific literature on this question is so sparse, should federal officials conduct extensive new research before making the leap?

“It’s sort of the devil you know versus the devil you don’t,” said Linda Greer, a senior scientist at the Natural Resources Defense Council. “It’s really shocking to me how little research has been done into these basic questions.”

Responders to the downed Deepwater Horizon rig have spent days applying more than 253,000 gallons of oil dispersant — Nalco’s Corexit 9500 — to break up the tens of thousands of gallons of oil that have reached the ocean’s surface.  But these compounds have never been used at depth. Federal officials have conducted two rounds of tests to determine dispersants’ effects hundreds of feet underwater, and they are consulting with state and federal agencies as well as local community leaders before making a decision to proceed.

On a basic level, dispersants work the same way dishwashing liquid works on grease: They break up the oil into tiny droplets by attaching to the oil, which then becomes diluted in the water. Scientists and policymakers agree that the oil from the spill poses a greater threat to wildlife and vegetation than the chemicals in the dispersants.  “You’re putting something into the water, it’s less toxic than the oil, so it’s a trade-off,” said Coast Guard Commandant Thad Allen, national incident commander for the BP spill, in an interview.

But the question of the broader trade-off — whether these compounds will wreak havoc on the marine system over time — remains unanswered. Five years ago, the National Academy of Sciences issued a nearly 400-page study on oil dispersants, which cautioned that “the current understanding of key processes and mechanisms is inadequate to confidently support a decision to apply dispersants.”

The NAS panel urged additional scientific inquiry into the matter, but little of that research has taken place. And now, the federal government will have to make a significant decision without it.  “It’s a hard call,” said Carys L. Mitchelmore, one of the authors of the 2005 NAS report and an associate professor at the University of Maryland Center for Environmental Science’s Chesapeake Biological Laboratory.

Applying dispersants at depth could kill fish larvae — such as those from the imperiled Atlantic bluefin tuna that use the Gulf of Mexico for spawning grounds — and threaten filter-feeders, such as whale sharks, that pass through those waters. It could also harm commercially valuable oysters and mussels, as well as organisms low on the food chain that sustain larger marine creatures. “You would be killing off their food,” Mitchelmore said.

There are other unknowns: Almost no research has been done on whether the dispersants will undermine the water repellency of birds, which is essential for regulating their body temperature. And most of the testing has been conducted in laboratories rather than in the field, which might mean that scientists have underestimated the toxic threat a mixture of oil and chemicals could pose. New research suggests that natural light enhances oil’s toxicity, Mitchelmore said, which would threaten translucent organisms such as fish larvae.

In addition, Greer said, scientists do not know whether chemically treated oil degrades as quickly as oil that’s dispersed through wind and wave, and if it’s more toxic.

(3)  National Academies report

Summary of the “Oil Spill Dispersants: Efficacy and Effects“, National Academies, 2005:

Pros and Cons

Oil spill dispersants do not actually reduce the total amount of oil entering the environment. Rather, they change the inherent chemical and physical properties of oil, thereby changing the oil’s transport, fate, and potential effects. Small amounts of spilled oil naturally disperse into the water column, through the action of waves and other environmental processes.

The objective of dispersant use is to enhance the amount of oil that physically mixes into the water column, reducing the potential that a surface slick will contaminate shoreline habitats or come into contact with birds, marine mammals, or other organisms that exist on the water surface or shoreline. Conversely, by promoting dispersion of oil into the water column, dispersants increase the potential exposure of water-column and benthic biota to spilled oil.

Dispersant application thus represents a conscious decision to increase the hydrocarbon load (resulting from a spill) on one component of the ecosystem (e.g., the water column) while reducing the load on another (e.g., coastal wetland). Decisions to use dispersants, therefore, involve trade-offs between decreasing the risk to water surface and shoreline habitats while increasing the potential risk to organisms in the water column and on the seafloor. This trade-off reflects the complex interplay of many variables, including the type of oil spilled, the volume of the spill, sea state and weather, water depth, degree of turbulence (thus mixing and dilution of the oil), and relative abundance and life stages of resident organisms.

… . Given the potential impacts that dispersed oil may have on water-column and seafloor biota and habitats, thoughtful analysis is required prior to the spill event so that decisionmakers understand the potential impacts with and without dispersant application.

Research on their effects

… Research funds in the United States to support oil spill response options in general are extremely limited and declining the total amount is less than $10 million annually). Consequently, despite the complex and numerous variables involved in risk-based decisionmaking regarding the potential use of dispersants, efforts to fill knowledge gaps must be thoroughly grounded in the recognition that no amount of research or environmental monitoring will eliminate uncertainty entirely. Failure to make a timely decision regarding dispersant application is in actuality a decision not to use dispersants, and in some instances may place some natural resources at an increased and unnecessary risk.

… The mechanisms of both acute and sublethal toxicity from exposure to dispersed oil are not sufficiently understood. … The factors controlling rates of the biological and physical processes that determine the ultimate fate of dispersed oil are poorly understood. … There is insufficient understanding of the actual concentrations and temporal or spatial distributions and behavior of chemically dispersed oil.

What happens to the oil:  biodegradation  (Chapter 4)

… The effects of surfactants and commercial oil dispersant mixtures on the rate and extent of biodegradation of crude oil, petroleum products, and individual hydrocarbons have been intensively investigated for over thirty years with mixed results. In some studies, biodegradation is stimulated, others find evidence of inhibition, and others observe no effects attributable to the presence of surfactants or commercial dispersants. Experimental systems have used a wide variety of substrates (e.g., crude oil, individual hydrocarbons), surfactants (e.g., commercial dispersant mixtures, pure surfactants), and microbial communities (e.g., natural seawater microbiota, microbial communities enriched by growth on crude oil, pure cultures). None of these factors appear to systematically affect the outcome.

Chapter 5:  Toxicological Effects of Dispersants and Dispersed Oil

… One of the most difficult decisions that oil spill responders and natural resources managers face during a spill is evaluating the environmental trade-offs associated with dispersant use. The objective of dispersant use is to transfer oil from the water surface into the water column. When applied before spills reach the coastline, dispersants will potentially decrease exposure for surface dwelling organisms (e.g., seabirds) and intertidal species (e.g., mangroves, salt marshes), while increasing it for water-column (e.g., fish) and benthic species (e.g., corals, oysters).

Decisions should be made regarding the impact to the ecosystem as a whole, and this often represents a trade-off among different habitats and species that will be dictated by a full range of ecological, social, and economic values associated with the potentially affected resources. Comparing the possible ecological consequences and toxicological impacts of these trade-offs is difficult.

(4)  Fact sheets for the dispersants

(5a)  Other posts about the Deepwater Horizon disaster

  1. Valuable background information about oil slicks: excerpts from Science, 2 May 2010
  2. Important background information about the oil spill (an example of real journalism), 2 May 2010
  3. Sources of reliable information about the Gulf Oil Spill, 4 May 2010
  4. We know what happened at the Deepwater Horizon rig. Here’s why it happened., 5 May 2010
  5. We’re at a key point in the Gulf Oil spill, while urban legends breed and circulate among the credulous, 7 May 2010

(5b)  Afterword

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