Whether the result of an oil well blowout, a vessel collision or grounding, a leaking pipeline, or some other incident at sea, each marine oil spill will present unique circumstances and challenges. The oil type and properties, location, time of year, duration of spill, water depth, environmental conditions, affected biomes, potential human community impact, and available resources may vary significantly. In addition, each spill may be governed by policy guidelines, such as those set forth in the National Response Plan, Regional Response Plans, or Area Contingency Plans. To respond effectively to the specific conditions presented during an oil spill, spill responders have used a variety of response options, including mechanical recovery of oil using skimmers and booms, in situ burning of oil, monitored natural attenuation of oil,1 and dispersion of oil by chemical dispersants. Because each response method has advantages and disadvantages, it is important to understand specific scenarios where a net benefit may be achieved by using a particular tool or combination of tools.
Typically, oil spill response tools are used to reduce the amount of floating oil at the surface through direct removal (skimmers), in situ burns, or dispersion into the water column. Floating oil may pose health risks for people (especially spill responders) as well as for seabirds and air-breathing marine species such as sea turtles and marine mammals. Winds may drive floating oil ashore into vulnerable habitats such as salt marshes where oil cannot be removed without causing additional damage. The primary objective of dispersant use is to reduce the amount of floating oil by promoting the formation of small droplets that remain or become entrained in the water column, where they are subjected to greater dissolution and dilution. Under conditions conducive to microbial growth (e.g., the presence of oxygen, adequate nutrients, and sufficient microbial seed population), the small droplets formed by dispersants may also biodegrade more rapidly.
This report builds on two previous National Research Council (NRC) reports on dispersant use (NRC, 1989, 2005) to provide a current understanding of the state of science and to inform future marine oil spill response operations.2 The response to the 2010 Deepwater Horizon (DWH)
1 Monitored natural attenuation refers to tracking the environmental processes that break down oil, including biodegradation.
2 Since the release of the prepublication version, the text was edited for clarity and references have been checked and modified as necessary.
spill included an unprecedented use of dispersants via both surface application and subsea injection. The magnitude of the spill stimulated interest and funding for research on oil spill response, and dispersant use in particular. This report considers and synthesizes much of that work, as well as other literature, to address the Statement of Task (see Chapter 1). Furthermore, the focus of this report is on marine oil spill scenarios for which dispersants would be considered a potential response option. In the United States, that is limited to areas beyond 3 nautical miles from shore and in depths greater than 10 m. Although the focus of this report is spills occurring off the coast of the United States, the expectation is that the report will have broad application internationally.
OIL SPILL RESPONSE DECISION MAKING
Human life is the first priority in marine oil spill response. Hence, the Federal On-Scene Coordinator and Area Contingency Plans place top priority on decisions affecting human health and safety. After human safety, the next priority is development of a response strategy that most effectively reduces environmental consequences, offers the greatest protection, or promotes the fastest recovery.
Determining whether the use of dispersants is appropriate for a given oil spill scenario requires decision-making tools for assessing the relative benefits of the various response options. These tools incorporate available information to estimate the likely fate and transport of oil and dispersant components and to assess the effects associated with human and environmental exposure to oil and dispersant components.
A number of approaches, collectively known as Net Environmental Benefit Analysis (NEBA), help decision makers select the response option(s) most likely to minimize the net environmental impacts of oil spills. NEBA must account for the variable nature of oil spills and a broad range of natural resources that could be impacted. This requires flexibility to allow for “real-time” alignment with changing field conditions.
Three tools that could be used to support the NEBA approach for oil spills are described:
- Consensus Ecological Risk Assessment (CERA): uses a detailed, semiquantitative risk ranking square to perform comparative analyses of response methods;
- Spill Impact Mitigation Assessment (SIMA): similar to CERA, but utilizes a single score for extent of exposure and duration of recovery and adds a weighting factor for resource values based on local priorities established through a stakeholder consensus-building process; and
- Comparative Risk Assessment (CRA): uses an integrated model to simulate the fates and effects of a spill scenario and employs a weighting function to represent the relative exposure, susceptibility, and importance of resources.
Each process involves a structured approach used by the response community and stakeholders to compare the impact mitigation potential of the available response options.
All three decision-making tools (CERA, SIMA, and CRA) have value for supporting contingency plan development, strategic planning during the initial stages of a spill response, or tactical execution during the active phase of a response. Because a CRA relies on an integrated model adapted for a particular spill scenario, it takes considerable time before results are available; hence, it typically has more value for contingency planning. An integrated model consists of various sub-models that simulate the transport, degradation, mitigation efforts, and ultimate fate of the hydrocarbon, and in some cases, the model may even use this information to estimate the effects on important components of the local biota. With further development, the NEBA process also could be used to estimate human health and socioeconomic impacts. Because CRAs evaluate the
relative risks and benefits of various response options, there is greater tolerance for uncertainties in the modeling. Importantly, each tool can be used to engage stakeholders, an essential element for providing input on local or regional priorities, expanding awareness, and building confidence and trust in the decision-making process.
Recommendation: Decision makers should further evaluate surface and subsea spill scenarios using NEBA tools (i.e., CERA, SIMA, or CRA) to better define the range of conditions (e.g., oil type, sea state, depth, location, resources at risk) where dispersant use may be an appropriate and/or a feasible response option for reducing floating oil.
Although CERA and SIMA can be adapted for situations in which limited information is available to inform the analysis, all tools used in the NEBA process rely to some extent on the ability to estimate both a series of processes that influence where the oil goes and how oil composition changes over time (fate and transport) as well as the effects of oil on species throughout the affected ecosystem (aquatic toxicology and biological effects). The report is organized to first address the state of the science related to fate and transport followed by aquatic toxicology and biological effects. It then covers the human health considerations that are critical concerns for decision makers. Based on this information, the report discusses the trade-offs associated with dispersant use versus other response options under various spill conditions and explains how these trade-offs are weighed using the NEBA approaches described above.
FATE AND TRANSPORT OF DISPERSANTS AND OIL
Fate and Transport of Dispersants
Modern dispersant products (e.g., Dasic Slickgone NS, Finasol® OSR 52, Corexit® EC9500A) are a mixture of solvents and surface active agents (surfactants) with different physicochemical properties and therefore potential fates in the environment. Once released into the aquatic environment, dispersants are subject to rapid dilution, dissolution, biodegradation, and photodegradation processes. Consequently, there is just a brief time window in which ocean biota might encounter the full dispersant formulation. When a dispersant is introduced at depth by subsea injection, dispersant components will differentially dilute and dissolve, with some being retained at depth (e.g., in intrusion layers and sequestered in sediments). In this situation, deepwater3 biota could be exposed to dilute concentrations of the more persistent and water-soluble dispersant components, such as the anionic surfactant dioctyl sodium sulfosuccinate (DOSS).
In laboratory experiments, dispersant components (including the solvents and surfactants) degrade rapidly, within hours to days. In field conditions, the few studies on the effects of dilution on dispersant fate and transport have shown that concentrations of dispersants reach a maximum of 5-13 parts per million (ppm) after surface applications and generally decrease to less than 1 ppm within minutes to hours.
Research examining the long-term fate of dispersant constituents indicates that only trace amounts of DOSS persist, even after the large volumes of dispersants used in the DWH spill. This indicates that dilution, dissolution, biodegradation, and photodegradation likely acted in that case to limit the long-term exposure of aquatic species to dispersant components.
3 The committee recognizes there are varying definitions for the terms “deep water” or “deep-water,” which largely depend on the context of their use; however, for the purposes of this report, the committee generally considers “deep water” to be greater than 500 m.
Fate and Transport of Untreated and Chemically Dispersed Oil
Many types of oils, including crude oil and refined products, may be released into the marine environment, at which point their composition begins to change. The oil type or chemical composition determines the long-term behavior of oil as modified by such processes as evaporation, aerosolization, photochemical oxidation, dissolution, biodegradation, aggregation, and adhesion. Key determinants of physical behavior include the molecular weight distribution of hydrocarbons, the abundance of other elements (e.g., N, S, and O), and the relative abundance of saturates, aromatics, resins, and asphaltenes. The oil’s chemical composition also influences the action of dispersants: lighter oils are more dispersible, while dispersants may have limited effectiveness on high-viscosity oils.
In a deepwater blowout, release of gas bubbles and oil droplets creates a buoyant, multiphase plume. As the plume rises, gas bubbles and soluble oil components dissolve into the entrained seawater, decreasing the buoyancy of the plume. A lateral intrusion layer forms, enriched in hydrocarbons, where the dissolved components and microdroplets4 encounter currents and the ambient density stratification of the water column (see Figure S.1).
On the sea surface, oil slicks become dispersed through the action of breaking waves. While this occurs naturally, it can be amplified by the application of dispersants. The small droplets formed by dispersion become entrained below the surface by waves, turbulence, and Langmuir circulation.
Oil droplet size is a primary determinant of both subsurface and surface oil transport; hence, understanding the dynamics associated with droplet formation, size distribution, and transport is foundational for improving studies of oil fate, including the effects of dispersants. Dispersants lower the interfacial tension of oil, thereby promoting the formation of small droplets and microdroplets. With regard to surface oil, droplets form when turbulence drives oil beneath the surface. The depth of penetration and the resurfacing time depend in part on the droplet size. In a deepwater release, droplets form at the source and rise through the water column as a function of their size. Oil type and the densities of the oil and surrounding seawater will influence rise velocity, but generally, larger droplets have greater buoyancy and hence rise more quickly than smaller droplets.
Because of their slower rise rate, smaller oil droplets will lose more soluble components before surfacing and thus release fewer volatiles to the atmosphere. Smaller oil droplets may also be transported further from the source and surface over a broader area, potentially reducing atmospheric concentrations of volatiles. This has implications for oil spill response because inhalation of volatile organic compounds (VOCs) is a major health concern for responders working in the area of the spill. Furthermore, under favorable conditions, small droplets may enable greater biodegradation to occur because of the increased surface area and longer residence in the water column. In the case of microdroplets, insufficient buoyancy prevents their surfacing, and they become trapped at depth with the soluble oil components. The purpose of using dispersants is to enhance the formation of these small oil droplets and thereby increase dissolution and biodegradation while decreasing exposure.
4 The committee recognizes that the term “microdroplets” is loosely defined, but in this report the term typically means droplets that are approximately 70 microns or less.
Droplet Models and Experiments
Models of droplet formation and transport have been developed to improve predictions of the fate of spilled oil and effects of dispersants. Experiments and models can provide insight on droplet formation and distribution. For example, models can explore different spill scenarios by varying parameters such as oil properties, flow rate, depth, or the dispersant-to-oil ratio (DOR). Experiments can test how well models perform at different scales and can examine the effects of various oil types, proportions of methane, dispersant formulations, and DORs. The combination of experiments and models provides a powerful tool for understanding factors that determine droplet size and behavior as well as the sensitivity of a system to certain parameters and processes.
For any particular spill, unforeseen factors may impact droplet size and complicate reconstruction of the actual conditions. Field trials and actual spills (spills of opportunity) could help reveal processes that influence oil fate and transport beyond those incorporated into current models and laboratory experiments.
Since the DWH spill, models have been developed to better represent the processes determining droplet size and transport for both surface and subsurface spills. However, sources of uncertainty remain, including processes such as tip streaming, pressure gradients, and out-gassing. Therefore, additional modeling and field-scale experimentation will be required for more accurate predictions of oil fate and transport. Because it can be difficult to obtain permits for experimental field studies, a spill of opportunity is another option for obtaining the observations necessary to improve models. A spill of opportunity involves being prepared and coordinated in advance so that should a spill occur, scientists are in a position to collect samples and data. Any field-scale study will be inherently restricted because of logistical challenges and open boundaries. Thus, it would be highly desirable to develop a large-scale laboratory facility with the ability to include high ambient pressure and observation of droplets as they evolve over time.
AQUATIC TOXICOLOGY AND BIOLOGICAL EFFECTS
Oil can present an immediate hazard to ocean life, both at the surface and below. At the surface, oil can harm animals such as seabirds, turtles, and marine mammals through physical smothering from direct contact, ingestion, inhalation, and aspiration of oil. Dispersants have been used in part to reduce the hazards of surface oil, both at the offshore site of the spill and through wind-driven transport to nearshore habitats. However, the action of dispersants in a surface spill increases the amount of oil in the water column, both as dissolved oil constituents and as small droplets, where fish and other species may be exposed through absorption or ingestion.
Concerns over the substantial use of dispersants during the DWH spill triggered an expansion of research on the toxicity of oil, dispersed oil, and dispersants. Toxicity studies have been conducted by exposing biota to various oil and oil/dispersant mixtures under laboratory conditions. In most experiments, the conditions in the laboratory are not designed to be analogous to conditions in the field. Instead, the experiments are designed to identify threshold concentrations for a variety of marine species to evaluate potential effects of dispersant use on water column species.
However, the results of laboratory studies have been equivocal, due—at least in part—to a lack of consistency in the media preparation, exposure procedures, and chemical analyses, despite earlier recommendations to employ standardized toxicity testing protocols (NRC, 2005). This lack of consistency has reduced the ability to compare results across studies and develop a comprehensive picture of the toxicity of oil and dispersants. As described below, the committee suggests an approach for using results from many studies to develop a coherent analysis of the toxicity of dispersants and chemically dispersed oil.
Dispersant Only Toxicity
Modern dispersants (e.g., Dasic Slickgone NS, Finasol® OSR 52, and Corexit® EC9500A) have been formulated with less toxic chemical constituents, employing ingredients found in common consumer products such as cleaners and cosmetics. However, lack of full disclosure of substances comprising the dispersant formulations following use in the DWH spill contributed to public concern about toxicity, although the Centers for Disease Control and Prevention released the statement that the “ingredients [of Corexit® 9500A and Corexit® 9527A] are not considered to cause chemical sensitization; the dispersants contain proven, biodegradable and low toxicity surfactants.”5
Toxicity is a function of both concentration and exposure duration. Based on operational dispersant application rates at the surface, the dispersant-only concentrations (i.e., from a
noncontinuous dispersant application) are expected to range between 1 and 15 mg/L in the first minutes to several hours. Species sensitivity analysis, based on toxicity tests of dispersant alone, yielded an HC56 at 65.8 mg/L when field conditions were simulated with a spiked flow-through test (~2.5-hour half-life). Hence, under field conditions with possible exposures of a few hours, the dispersant concentration would be roughly 10-fold lower than the level that would be toxic to the most sensitive 5% of tested species. As underscored in the previous NRC reports, the concern with dispersant use is whether dispersed oil is more toxic than untreated oil is, not the toxicity of current dispersant formulations.
Dispersed Oil Toxicity
To determine the relative toxicity of dispersed oil, many laboratory studies have compared solutions of oil equilibrated with seawater to oil and dispersant mixtures equilibrated with seawater. Toxicity testing protocols consist of three main elements: media preparation, exposure, and chemical characterization. Preparing a dose of oil (media preparation) is not as simple as preparing a dose of a single miscible compound, because oil components vary in solubility and partition into both the oil and the aqueous phase. Two different methods have typically been used for preparing a range of concentrations: variable loading and variable dilution.
In this approach, a water-accommodated fraction (WAF; aqueous phase separated from the oil after mixing) is prepared for each concentration of oil to be tested: for example, 100 mg oil/L. When a dispersant is included, a chemically enhanced water-accommodated fraction (CEWAF) is produced at the same oil concentration. Both WAFs and CEWAFs contain microdroplets, but CEWAFs contain a higher concentration of microdroplets for the same initial loading of oil. WAF and CEWAF have the same dissolved oil concentration because at equilibrium the dissolved concentration depends on the oil-to-water ratio, not the amount of oil present in microdroplets. An analysis using available variable loading toxicity tests comparing CEWAFs to WAFs shows that the higher concentration of microdroplets in the CEWAF does not increase toxicity until the oil loading is above approximately 100 mg oil/L. Hence, variable loading experiments indicate that at or below approximately 100 mg/L, dispersed oil is no more toxic than is untreated oil. Above approximately 100 mg oil/L the increase in toxicity with dispersants is due to increased generation of oil microdroplets.
An alternative approach, commonly applied in oil toxicity tests, uses a single stock solution prepared at a high oil loading that is serially diluted to create a set of decreasing concentrations. However, there is a fundamental problem with this test design. When the WAF or CEWAF is diluted, the concentration of the dissolved oil components decreases and is no longer in equilibrium with the oil in the microdroplets. This causes further dissolution of oil components from the microdroplets until the solution reaches equilibrium. However, the dissolved concentration will be higher than predicted by the proportion of the dilution. Because dispersants create more microdroplets, the dissolved concentration in the CEWAF dilutions will be higher than in the equivalent WAF
6 Acute HC5 refers to the concentration at which 5% of the tested species have their LC50 (concentration lethal to half of the test population for a 96-hour exposure). At this or lower concentrations, 95% of the species have an LC50 above the HC5. Note that toxicity is greater when the LC50 or HC5 is lower.
dilutions. This mismatch in the dissolved oil concentrations and composition can be corrected by direct measurement of the dissolved oil concentration in each dilution. However, without correction for the actual dissolved oil concentrations, a direct comparison of WAF and CEWAF toxicity will not produce meaningful results.
Recommendation: Funding agencies, research consortia, and other sponsoring groups should require that research teams use standardized toxicity testing methods, such as those developed by the Chemical Response to Oil Spills: Ecological Effects Research Forum (CROSERF) program, and analytical chemistry protocols to fully characterize hydrocarbon composition and concentrations in the exposure media. For testing the effect of dispersant, the variable loading test design is recommended.
Effect of Exposure Time
The duration of the exposure is another determinant of toxicity.7 The typical progressive decrease in LC50 for tests of 24-hour, 48-hour, and 96-hour duration indicates that toxicity increases with longer exposure times. In addition to acute mortality, sublethal effects affecting early life stages and adults can reduce fitness and species abundance. Acute and chronic tests typically employ different endpoints: mortality for the acute tests, and growth and reproduction or other endpoints for chronic tests. The lower toxicity thresholds for acute and chronic effects arise from both longer exposure time and the difference in endpoints. Nevertheless, this wide variation needs to be considered when evaluating oil toxicity.
Another consideration for assessing the use of dispersants is phototoxicity. Exposure to sunlight enhances the toxicity of certain polycyclic aromatic hydrocarbons (PAHs) absorbed by the organism. The result is a 10- to 100-fold increase in toxicity for these photoactive PAHs. Hence, a reduction in the amount of oil at the surface with dispersant use would lower the potential aquatic toxicity of the oil. Exposure to sunlight also increases the rate of photodegradation, which can affect the resulting toxicity by producing new compounds. Both of these effects need to be considered when assessing the effect of exposure to sunlight. Typically, short-duration toxicity tests do not consider phototoxic effects.
Determining Effects of Dispersant Use
To compare the toxic effects of untreated and chemically dispersed oil on marine life, it is necessary to evaluate the following four factors:
- Concentration exceeding known acute or chronic toxicity thresholds for the specific oil;
- Duration of exposure above toxic thresholds;
- Spatial and temporal distribution of marine life; and
- Species sensitivity to oil exposure above the acute or chronic toxicity thresholds.
7 “Acute” exposure typically refers to an exposure of 96 hours or less. “Chronic” exposures are longer and, in some cases, span multigenerations of the organism. LC50 refers to “lethal concentration” causing 50% mortality of the tested organisms. The term “acute test” denotes a short duration test with mortality as the endpoint. A “chronic test” is a longer duration test usually with sublethal endpoints, although chronic mortality is also observed in these tests.
In addition, it is necessary to quantify the toxicity of the mixture of the dissolved hydrocarbons of crude oil that result during an oil spill. The necessary parameter is the toxic unit (TU): TU equals the ratio of the dissolved aqueous concentration of the compound to the toxic concentration, either LC50 or HC5, of that compound. It has been shown that the toxicity of a mixture of the dissolved hydrocarbons can be estimated by adding the TUs of each component. If the sum of the TUs is greater than one, the mixture will exhibit the toxicity of the level of the LC50 or HC5 used to define the TU. Because TUs are based on the composition of the mixture, it is possible to compare the toxicity of various mixtures of PAHs from different source oils and from mixtures that results from the differential solubility of oil constituents in seawater. Because PAHs vary widely in toxicity, the TU provides a more accurate measure than do the more commonly reported total PAHs, which represent the sum of the PAH concentrations without the LC50 or HC5 normalization.
Recommendation: The use of toxic units should be integrated into revised oil toxicity testing standards, evaluation criteria for models, and response option risk analysis. This represents a paradigm shift away from developing toxicity tests that attempt to reproduce field exposure conditions and toward developing a consistent means of using toxicity metrics such as HC5 and LC50 for toxicity models used with fate and transport models to compare the exposure and toxicity of various response options, including dispersants.
HUMAN HEALTH CONSIDERATIONS
Human health and safety represent the first priority in oil spill response decision making. Surprisingly, significant research effort on the direct human health impacts of oil spills is relatively recent, beginning with the Exxon Valdez and Sea Empress oil spills and expanding after the Prestige oil spill in 2002. The potential health effects of dispersant use during oil spills were not subject to epidemiological investigation until the DWH spill in 2010.
The key questions with regard to human health are whether dispersant use alters the health risk imposed by an oil spill by (1) dispersant use directly causing adverse effects, (2) effects of dispersant and oil mixtures, or (3) indirect effect of dispersant use changing the extent or duration of the spill.
During oil spill response, primary exposure pathways of concern are inhalational and dermal exposure of response workers. Direct effects on response workers can be mitigated through a proper worker health and safety program that focuses on personal protective equipment and monitoring. Community health concerns arising from exposure to oiled shorelines; socioeconomic effects, such as disruption of commercial and subsistence fisheries; and concerns over contaminated seafood also need to be considered as factors in oil spill response.
Human Exposure and Toxicity of Oil
With regard to human exposure to crude oil, the primary oil constituents of concern are the VOCs (benzene, toluene, ethylbenzene, and xylene [BTEX]) and PAHs. The carcinogenicity of benzene and PAHs, particularly benzo(a)pyrene, is well characterized. Dispersants may affect exposure to these oil constituents by altering their fate, transport, and biodegradation. Far less is known about the potential toxicity of weathered crude oil, which has much lower concentrations of the lower molecular weight components of concern, but it is reasonable to consider that it should be lower than the toxicity of fresh oil.
In addition to exposure to VOCs at the response site, VOCs released during an oil spill can contribute to the formation of secondary air pollutants, such as ozone, which could lead to inhalational exposure downwind from the spill location. In a deepwater blowout, subsea use of dispersants
could reduce the potential for inhalational exposure by increasing the dissolution of VOCs during the slower transit of dispersed oil droplets to the surface.
Dermal exposure to oil constituents has been shown to cause skin irritation and skin cancer (EPA, 2017). At present, there is insufficient evidence to determine if dispersant use increases the transdermal absorption of crude oil components.
Although responders could be exposed to oil and/or dispersants through accidents or improper use of protective gear, broader community exposure to dispersants or dispersant-oil mixtures is much less likely because dispersant use is limited to offshore spills. Possible routes of exposure include ingestion, inhalation, and dermal contact. Exposure via ingestion could occur through consumption of seafood contaminated with PAHs or dispersant components during or after an oil spill. Protocols for closing and reopening fisheries during and after an oil spill are designed to protect public health from this exposure route.
If a response tool, such as dispersants, shortens the intensity and duration of response activities, and proper health and safety measures are in place, exposure risk would be lower, particularly for responders. This factor merits inclusion as part of the trade-off considerations with regard to decisions on dispersant use.
Assessment of Exposure to Workers and Community Members
To date, exposure assessment during oil spills has been hampered by the lack of protocol development and hence unknown baselines for the constituents of oil and dispersants. To improve assessments of exposure, a standardized, analytical chemistry protocol will be needed to monitor the levels of dispersant components and dispersant-oil mixtures in environmental media and biota in advance of the next spill.
Two studies of DWH spill responders have attempted to disentangle the direct effects of dispersants from other worker health risks. While these studies noted similar adverse effects associated with dispersant exposures, both have limitations in their ability to validate exposure to dispersants based on self-reporting by workers.
Investigators from the National Institute of Environmental Health Sciences and collaborative programs attempted to assess the impact of exposure to dispersants based on respiratory, dermal, and eye irritation symptoms previously reported as part of an extensive health study of DWH response workers. The second study consisted of a cross-sectional evaluation of 4,855 U.S. Coast Guard personnel involved in the DWH response.
In both of these epidemiological studies, limitations in the exposure assessment for dispersants affect the strength of the conclusions. The delayed initiation of the studies and the lack of a dispersant/dispersed oil biomarker necessitated reliance on self-reporting, making it difficult to accurately estimate exposures and thus the effects of dispersant/dispersed oil versus untreated oil.
Indirect Human Health Effects
Often, the adverse health effects noted in studies of communities near an oil spill, including the DWH, have been associated with psychosocial and economic impacts rather than toxicity associated with direct exposure to chemicals. Communities at particular risk are those that already have relatively poor health and a past history of environmental injustice, which characterizes many of the communities affected by the DWH disaster. Health impacts in both workers and community members likely are at least partly dependent on the duration of the oil spill recovery period. If
dispersants shorten this duration, presumably overall impacts on worker and community health would lessen. A spill can also lead to prolonged closure of fisheries, causing secondary effects on community psychological and socioeconomic well-being.
Recommendation: Selection of biomarkers to improve human exposure assessment should consider the toxicity of dispersant and oil components and degradation products (produced by both biological and photodegradation), persistence in the environment, and bioaccumulation potentials. Biomarkers and analytical protocols should be established for each dispersant formulation listed on the U.S. Environmental Protection Agency’s (EPA’s) National Contingency Plan Product Schedule.
Recommendation: In advance of the next significant oil spill, the reporting requirements for details of injury and illness reporting for worker health and safety should be improved, with a clear focus on whether workers were exposed to dispersant. To that end, publication and ready availability of well-defined DWH worker health and safety statistics is needed. Exposure assessment and toxicological evaluation should recognize that response workers may not be from a healthy worker population and may not know how to minimize exposure.
SELECTION OF RESPONSE OPTIONS
Making the best decision possible during an oil spill requires balanced consideration of the potential consequences of the spill under a natural recovery scenario versus the consequences associated with each response strategy. It can be difficult to make trade-off decisions during an ongoing spill based on field data because observations may be limited. Efforts to ensure human safety, contain the oil, and minimize environmental damage take priority over monitoring and scientific studies. Pre-spill planning and scenario development prior to a spill provide the knowledge base on which decisions can be made during a spill event, as long as human health considerations are included in the NEBA tools as discussed above.
The primary response options considered in this report include surface dispersant operations, subsea dispersant injection, at-sea mechanical recovery, controlled (in situ) burning, biostimulation, and monitored natural attenuation. Typically, a response strategy will require a combination of response methods to adapt to constraints presented by the oil type, physical environment, weather, and health and safety considerations. The advantages and limitations of various response options have been described in detail elsewhere, including previous NRC reports; consequently, this discussion focuses on dispersants.
Surface Dispersant Operations
Dispersants can be applied to surface oil from vessels or aircraft. Aerial application allows for a high coverage rate and for treatment of large volumes of oil. Potential advantages include reduction of VOCs at the surface, no requirements for storing recovered oil, low manpower requirements, enhanced biodegradation, and application to a wide variety of spill situations. A disadvantage is the limited time frame for dispersant application; there is a relatively short “window of opportunity” for treating the spilled oil before it weathers and may become too viscous. Also, aerial dispersant operations are limited to favorable weather conditions, daylight hours, and sufficient turbulence (from waves) to mix the dispersant into the oil, although the operational window for use is expected to be broader than for mechanical containment and recovery techniques. Surface dispersant use requires specialized equipment and expertise as well as special approvals and meeting regulatory requirements.
Subsea Dispersant Injection
A notable advantage of subsea injection is the increased efficiency in treating large volumes of oil, thus requiring less dispersant compared to surface applications. At depth, dispersed oil will be subject to greater loss of soluble components and increased dispersion than will oil treated through surface application of dispersant. Furthermore, subsea injection operations can take place continuously, while surface application is limited to daylight hours and favorable wind and sea state conditions. Subsea injection requires less manpower than other response options and may reduce the VOCs at the surface.
As with other response options, there are potential limitations and trade-offs associated with subsea dispersant injection. Like surface application, subsea dispersant injection requires special approvals, is subject to regulatory requirements, and requires specialized equipment and expertise. It is more difficult to monitor dispersant effectiveness in the subsea than at the surface. Furthermore, by entraining oil within the water column, it may have greater impacts on marine biota present in the water column. In addition, less is known about the long-term effects of subsea dispersant injection.
COMPARATIVE STUDIES OF RESPONSE METHODS
A limited number of comparative studies have evaluated the effectiveness, benefits, and limitations of various response methods. This report highlights five comparative studies.
The first, Tropical Oil Pollution Investigations in Coastal Systems (TROPICS), established three shallow-water study sites from 1983 to 2015 in Panama to evaluate the impacts of untreated and dispersed oil relative to a control site. The purpose of the TROPICS study was to evaluate the relative health of the ecosystem at each site. In the first 10 years, the plot exposed to dispersed oil had recovered to pre-spill conditions, while the site exposed to undispersed oil still showed negative effects on the mangroves (Renegar et al., 2017b).
The second set of studies involved two CRAs. The CRAs rely on integrated numerical modeling to predict which environmental and human health impacts may arise in various response scenarios.
The first CRA, referred to as CRA-1 to differentiate it from the generic term CRA, was a simulation of a single site with DWH-like oil in the northeastern Gulf of Mexico. It compared oil mass distributions and ecological impact assuming four response options: no response, traditional responses (mechanical, burning, and surface dispersants), mechanical only, and subsea dispersant injection (SSDI) plus traditional responses. For this particular scenario and set of assumptions, SSDI appeared to be at least as effective in reducing impacts on the selected species of concern as all the traditional responses combined (Bock et al., 2018; French-McCay et al., 2018b; Walker et al., 2018a). CRA-2, an extension of CRA-1, explored the sensitivity of the fates to changes in flow rate and blowout location (e.g., distance from shore and water depth). Two sites were considered: a shallower site at 500 m and a deeper site at 1,400 m. Overall, CRA-2 indicates that at 500 m depth, SSDI generally will be less effective in reducing oiling at the surface and at the shore than at 1,400 m depth; at some threshold water depth, SSDI benefits will become negligible (French-McCay and Crowley, 2018).
The third study involves a comparison of VOCs emitted to the atmosphere near the well during a DWH-like blowout using an integrated oil-fates model for the ocean and a numerical model for the atmosphere to compare SSDI with no response. The inputs were similar to those used in CRA-1. The study concludes that SSDI reduces peak VOCs by factors of 100- to 200-fold depending on the winds (Crowley et al., 2018).
The fourth comparison study of note used an alternative integrated fate and effects model to evaluate the effectiveness of SSDI during the DWH relative to no dispersant use. The model was validated using observed concentrations of oil constituents. It was then used to estimate the distribution of oil through the water column with and without SSDI. A DOR of 1:250 was assumed.
In this modeling exercise, dispersant increased the volume of oil retained in the lower water column by 55% and reduced the volume of oil that surfaced resulting in 28% fewer VOCs in the atmosphere (Gros et al., 2017). A follow-up study by Socolofsky and Gros using the same methodologies (see Appendix E) found that a DOR of 1:100 virtually eliminated surfacing of oil for the DWH spill scenario.
The fifth comparison involved a SIMA prepared for an exploration drilling project in offshore Nova Scotia that focused on a source control event (Slaughter et al., 2017). Based on the resources of concern identified in this exercise, dispersant use compared favorably to other response options.
Based on results from these field and modeling studies, surface and subsurface dispersant application represents a useful tool for oil spill response. When used appropriately, dispersants can decrease the amount of oil at the surface, thereby reducing the potential exposure of response personnel to VOCs and decreasing the extent of oiled areas encountered by marine species at the surface. Each response method has a complex suite of advantages and disadvantages, including and not limited to encounter rate, effectiveness, and ecosystem and human health effects that should be considered when developing and executing oil spill response plans. These complex trade-offs are best addressed using NEBA tools such as CERA, SIMA, and CRA.
Recommendation: The NEBA tools (CERA, SIMA, and CRA) should be expanded to consistently address the health of response personnel, community health, and socioeconomic considerations (e.g., beach closures). Furthermore, these tools should be used to gain stakeholder input on local or regional priorities, expand awareness, and gain trust in the decision-making process.
Finding: Experience with historical spills and integrated models consistently indicate that for large spills, dispersants (both SSDI and surface) are a response option that can substantially reduce surface oil.
Finding: The understanding of the impacts of dispersant as a response tool has been greatly advanced by laboratory experiments and modeling, but these efforts are often limited by their inability to capture the complexity or scale found in the field. Important issues that are best answered in a field study or future spill (spill of opportunity) cover a broad spectrum of topics, including validation of integrated models and their sub-models, especially scaling of droplet size; better understanding of health impacts on response workers (unintentional releases only); validation of response decision-making approaches; and discovery of previously unknown linkages in complex ecosystems affected by oil.
Recommendation: Efforts to take detailed scientific measurements during future spills (spills of opportunity) and/or to conduct dedicated field experiments should be strongly encouraged. In the case of a spill of opportunity, preplanning and pre-deployment as well as focusing on the priorities for such observations are essential to avoid delays in the start of taking these measurements. Given its long-term funding and mandate, the National Academies Gulf Research Program,8 or a foundation with similar long-term funding, would be in an ideal position to work with the Interagency Coordinating Committee on Oil Pollution Research to coordinate a field experiment or scientific efforts for deployment in a spill of opportunity.
8 As a result of settlements from the DWH spill, $500 million were designated to the development and 30-year endowment of the National Academies Gulf Research Program, whose mission is “catalyzing advances in science, practice, and capacity to generate long-term benefits for the Gulf of Mexico region and the Nation.” In furtherance of its mission, the National Academies Gulf Research Program funds grants, fellowships, and activities.