From the National Academy of Sciences :
" 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. "
" The benthic zone is the ecological region at the lowest level of a body of water such as an ocean or a lake, including the sediment surface and some sub-surface layers. Organisms living in this zone are called benthos. They generally live in close relationship with the substrate bottom; many such organisms are permanently attached to the bottom. "
Here's the whole article from the National academy of the Sciences.
Approximately 3 million gallons (10,000 metric tons [tonnes]) of oil or refined petroleum product are spilled into the waters of the United States every year (NRC, 2003). This amount represents the total input from hundreds of spills, many of which necessitate timely and effective response. When these oil spills occur in the United States, the primary response methods consist of the deployment of mechanical on-water containment and recovery systems, such as booms and skimmers.
Under the Oil Pollution Act of 1990 (OPA 90), the U.S. Coast Guard (USCG) passed rules for vessel and facility response plans that specified the minimum equipment and personnel capabilities for oil containment and recovery. This requirement has significantly expanded mechanical response capability above that which existed in 1989 at the time of Tanker Vessel (T/V) Exxon Valdez spill (the event that led to passage of OPA 90). Mechanical recovery, however, is not always sufficient because conditions at the spill are often outside of the effective operating conditions of the equipment. OPA 90 also called for national and regional response teams to develop guidelines to address the use of other on-water response strategies, specifically the use of chemical dispersants and in-situ burning.
Throughout the Unites States, many regional response teams have identified zones where dispersants and in-situ burning are “pre-approved” for use. This pre-approval means that the response and re-
source agencies have determined that the Federal On-Scene Coordinator has the authority, as outlined under the pre-approval definitions, to decide to use dispersants without additional consultation. In general, these pre-approval zones are in waters beyond 3 nautical miles (nm; roughly 5 kilometers [km]) of the shoreline and in water depths greater than 30 feet (10 meters). Even with establishment of these pre-approval zones, dispersant use has been infrequent, in part reflecting the difficulty of mobilizing available equipment and dispersants within a narrow window of opportunity in which they can be effective. In areas where dispersants are not often considered, it takes more time to identify, contract, and mobilize the specialized resources needed for dispersant application.
To address the concerns regarding requisite equipment and personnel capabilities, the U.S. Coast Guard in 2002 proposed changes to the oil spill contingency planning regulations measuring the minimum capabilities for dispersant application in all pre-approved zones within acceptable time frames. With implementation of the regulations, dispersant application resources will become more readily available. The potential, therefore, for using dispersants in nearshore and shallow waters, when appropriate, will increase as well.
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.
Each spill is a unique event that unfolds over a variety of time scales. Properties of petroleum hydrocarbons immediately start to change when spilled onto water. This natural “weathering” makes the oil more difficult to disperse through time; consequently, the window of opportunity for effective dispersant application is early, usually within hours to 1–2 days after a release under most conditions, though there are exceptions. The decision to apply dispersants is thus time sensitive and complex. 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. Thus, decisionmaking regarding the use of dispersants falls into two broad temporal categories: (1) before the event during spill contingency planning; and (2) shortly after the initial event, generally within the first 12 to 48 hours.
In recognition of the increased potential to use dispersants in a variety of settings, the Minerals Management Service (MMS), the National Oceanic and Atmospheric Administration (NOAA), the USCG, and the American Petroleum Institute (API) asked the National Academies to form a committee of experts to review the adequacy of existing information and ongoing research regarding the efficacy and effects of dispersants as an oil spill response technique in the United States.2 Emphasis was placed on understanding the limitations imposed by the various methods used in these studies and on recommending steps that should be taken to better understand the efficacy of dispersant use and the effect of dispersed oil on freshwater, estuarine, and marine environments. Specifically, the committee’s task was to:
- review and evaluate ongoing research and existing literature on dispersant use (including international studies) with emphasis on (a) factors controlling dispersant effectiveness (e.g., environmental conditions, dispersant application vehicles and strategies, and oil properties, particularly as the spilled oil weathers), (b) the short- and long-term fate of chemically or naturally dispersed oil, and (c) the toxicological effects of chemically and naturally dispersed oil;
- evaluate the adequacy of the existing information about dispersants to support risk-based decisionmaking on response options for a variety of spatially and temporally defined oil spills;
- recommend steps that should be taken to fill existing knowledge gaps, with emphasis to be placed on how laboratory and mesoscale ex-periments could inform potential controlled field trials and what experimental methods are most appropriate for such tests.
A similar request was put to the National Academies in the mid 1980s, leading to the publication of the 1989 NRC report Using Oil Spill Dispersants on the Sea. The current report is not truly an update of the 1989 report, as it selectively revisits some topics while including discussions on issues that have emerged since that time. Many readers may, therefore, find the assessments and summaries in Using Oil Spill Dispersants on the Sea of value.
OVERARCHING CHALLENGE TO EFFECTIVE DECISION MAKING
In general, the information base used by decision makers dealing with spills in areas where the consequences of dispersant use are fairly straightforward has been adequate (for example, situations where rapid dilution has the potential to reduce the possible risk to sensitive habitat enough to allow the establishment of pre-approval zones). Many of the technical issues raised in this report, however, deal with settings where greater confidence is needed to make effective decisions regarding potential benefits or adverse impacts associated with dispersant use. In many instances where a dispersed plume may come into contact with sensitive water-column or benthic organisms and populations, the current understanding of key processes and mechanisms is inadequate to confidently support a decision to apply dispersants. Thus, such decisions regarding the potential use of dispersants in nearshore settings are creating a demand for additional information.
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. Given the limited funding available to carry out needed research in this area, it is particularly important that research be carried out as efficiently as possible and that the research process focuses on efforts that result in sound, reproducible results that support decisionmaking. In many instances, efforts to reduce experimental complexity to ensure reproducibility or to secure cost savings have led to results that have very limited utility for making decisions in natural settings. NOAA, the Environmental Protection Agency (EPA), the Department of the Interior (including MMS and U.S. Geological Survey), USCG, relevant state agencies, industry, and appropriate international partners should work together to establish an integrated research plan which focuses on collecting and disseminating peer-reviewed information about key aspects of dispersant use in a scientifically robust, but environmentally meaningful context.
Dr. Jim Clark Head of Oil Spill Research Program ... (Exxon Mobil Corporation; Head of ExxonMobil's Oil; Exxon Biomedical Sciences , Inc.) has the following to say about COREXIT products in one of his presentations, titled :
DISPERSANT BASICS : Mechanism, Chemistry, and Physics of Dispersants in Oil Spill Response.
The following was a water tank test done in Alaska, concerning various COREXIT products, but strangely done in water with a very low salinity content, where sea application dispersants are almost singularly reliant on salt to maintain the H/B balance, this test was flawed, IMHO. Jim Clark specifically states that dispersants in low salt environments lose their effectiveness( stated in the above pdf )
Report on visit to OHMSETT to observe Exxon/MMS Cold-Water Dispersant Tests
March 5-6, 2002