Approximately 3 million gallons (10,000 metric tons [tonnes]) of oil or refined petroleum product1 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.
OVERARCHING CHALLENGE TO EFFECTIVE DECISIONMAKING
In general, the information base used by decisionmakers 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 (as discussed in Chapter 1, 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 (see Chapter 6 for more detail).
SETTING PRIORITIES IN DISPERSANT RESEARCH
Key components of an effective and integrated research effort should include efforts to further improve understanding of dispersant effectiveness and the potential impact of dispersed oil at meaningful scales to support decisionmaking in a broader array of spill scenarios, especially those scenarios where potential impacts on one portion of the ecosystem (e.g., water column) must be weighed against benefits associated with reducing potential impact on another (e.g., coastal wetland). In an effort to provide some prioritization, the following research recommendations are presented in order of significance. The most pressing or widely relevant issues are listed first, with less pressing or narrowly relevant issues raised later.
With the proposed USCG regulations requiring the availability of dispersants in pre-approval zones, the issue of availability will no longer be a limiting factor; thus the main questions to be addressed by responders in the pre-approval zones are: (1) Will mechanical recovery be effective and sufficient? (2) If not, is the oil dispersible? (3) If so, are the environmental conditions conducive to the successful application of dispersant and its effectiveness? and (4) If so, will the effective use of dispersants reduce the impacts of the spill to shoreline and water-surface resources without significantly increasing impacts to water-column and benthic resources? Better information is needed to determine the window of opportunity and percent effectiveness of dispersant application for different oil types and environmental conditions. Relevant state and federal agencies, industry, and appropriate international partners should develop and implement a focused series of studies that will enable the technical support staff advising decisionmakers to better predict the effectiveness of dispersants for different oil types and environmental conditions based on climatological data supplemented with real-time in-situ observations. (Detailed and specific recommendations are discussed at length in Chapters 3 and 4.)
Oil trajectory and fate models used by the technical support staff advising on-scene decisionmakers for dispersed oil behavior are not adequate in terms of: (1) their representation of the natural physical process involved, (2) verification of the codes, and (3) validation of the output from these models in an experimental setting or during an actual spill. Thus, their ability to predict the concentrations of dispersed oil and dissolved petroleum hydrocarbons of concern in the water column with sufficient
accuracy to aid in real-time spill decisionmaking has yet to be fully determined. Oil trajectory and fate models used by government agencies during spill response to predict the behavior of dispersed oil should be improved, verified, and then validated in an appropriately designed experimental setting or during actual spills. Two general types of modeling efforts and products should be recognized: (1) output intended to support decisionmaking during preplanning efforts, and (2) output intended to support emergency response to provide “rough-cut” outputs in hours. (Detailed and specific recommendations are discussed at length in Chapters 4 and 5.)
The mechanisms of both acute and sublethal toxicity from exposure to dispersed oil are not sufficiently understood. Recent studies in the literature suggest that toxicity from physically and chemically dispersed oil appears to be primarily associated with the additive effects of various dissolved-phase polynuclear aromatic hydrocarbons (PAH) with additional contributions from heterocyclic (N, S, and O) containing polycyclic aromatic compounds. Additional toxicity may be coming from the particulate, or oil droplet, phase, but a particular concern stems from potential synergistic effects of exposure to dissolved components in combination with chemically dispersed oil droplets. Relevant state and federal agencies, industry, and appropriate international partners should develop and fund a series of focused toxicity studies to determine the mechanisms of both acute and sublethal toxicity to key organisms from exposure to dispersed oil. With a better understanding of the mechanisms of toxicity, toxicity tests can be refined to generate data on toxic levels and thresholds for use by decisionmakers. (Detailed and specific recommendations are discussed at length in Chapters 5 and 6.)
The factors controlling rates of the biological and physical processes that determine the ultimate fate of dispersed oil are poorly understood. Of particular concern is the fate of dispersed oil in areas with high suspended solids and areas of low flushing rates. There is insufficient information to determine how chemically dispersed oil interacts with suspended sediments, both short- and long-term, compared to naturally dispersed oil. There are many important, unanswered questions about how dispersed oil might be consumed by plankton and deposited on the seafloor with fecal matter or otherwise passed through the food chain. Relevant state and federal agencies, industry, and appropriate international partners should develop and fund a focused series of studies to quantify the weathering rates and final fate of chemically dispersed oil droplets compared with undispersed oil. (Detailed and specific recommendations are discussed at length in Chapters 3 and 4.)
There is insufficient understanding of the actual concentrations and temporal or spatial distributions and behavior of chemically dispersed oil
from field settings (from either controlled experiments or actual spills). Data from field studies (both with and without dispersants) are needed to validate models and provide real-world data to improve knowledge of oil fate and effects. In the future, wave-tank or spill-of-opportunity studies should include efforts to measure total petroleum hydrocarbon (TPH) and PAH concentrations in both the dissolved phase and particulate/oil droplet phase for comparison to TPH and PAH thresholds measured in toxicity tests and predicted by computer models. Relevant state and federal agencies and industry should develop and implement detailed plans (including pre-positioning of sufficient equipment and human resources) for rapid deployment of a well-designed monitoring effort for actual dispersant applications in the United States. (Detailed and specific recommendations are discussed at length in Chapters 2, 3, and 4.)
To date, there have been no wave-tank or laboratory studies that can be used reliably to predict the performance of dispersants on water-in-oil emulsions (i.e., mousse) generated from the weathering of oil on the water surface. Relevant state and federal agencies, industry, and appropriate international partners should initiate a detailed investigation of wave-tank studies that specifically address the chemical treatment of weathered oil emulsions. (Detailed and specific recommendations are discussed at length in Chapters 3 and 4).
One of the most significant weaknesses in correlating laboratory-scale and meso-scale experiments with conditions in the open ocean results from a lack of understanding of the turbulence regime in all three systems. Likewise, one of the biggest uncertainties in computer modeling of oil spill behavior (with and without dispersant application) comes from obtaining horizontal and vertical diffusivities. Relevant state and federal agencies, industry, and appropriate international partners should initiate a detailed investigation of upper sea-surface turbulence with particular emphasis on quantifying horizontal and vertical diffusivities and the rate of energy dissipation. (Detailed and specific recommendations are discussed at length in Chapter 4.)
Finally, serious consideration should be given to determining the value and potential role of field testing. The body of work done to date has provided important, but still limited understanding of many aspects of the efficacy of dispersants in the field and the behavior and toxicity of dispersed oil. Developing a robust understanding of these key processes and mechanisms to support decisionmaking in nearshore environments will require taking dispersant research to the next level. This new work will require systematic analysis using rigorous experimental design and execution, making use of standard chemical and other measurement techniques carried out by trained, certified personnel. Many factors will need to be systematically varied in settings where accurate measurements can
be taken. It is difficult to envision the proper role of field testing in a research area where investigators have yet to reach consensus on standard protocols for wave-tank testing. The greater complexities (and costs) of carrying out meaningful field experiments suggest that greater effort be placed, at least initially, on designing and implementing a thorough and well-coordinated bench-scale and wave-tank research program. Such work should lead to more robust information about many aspects of dispersed oil behavior and effects. When coupled with information gleaned through more vigorous monitoring of actual spills (regardless of whether dispersants are used effectively in response), this experimental work should provide far greater understanding than is currently available. Upon completion of the work recommended in this report, the value of further field-scale experiments may become obvious. If deemed valuable, such field-scale work would certainly be better and more effectively designed and executed than is currently possible. Future field-scale work, if deemed necessary, should be based on the systematic and coordinated bench-scale and wave-tank testing recommended in this report. (Detailed and specific recommendations are discussed in Chapters 3, 4 and 5.)