Composition and fate of gas and oil released to the water column during the Deepwater Horizon oil spill

Quantitative information regarding the endmember composition of the gas and oil that flowed from the Macondo well during the Deepwater Horizon oil spill is essential for determining the oil flow rate, total oil volume released, and trajectories and fates of hydrocarbon components in the marine environment. Using isobaric gas-tight samplers, we collected discrete samples directly above the Macondo well on June 21, 2010, and analyzed the gas and oil. We found that the fluids flowing from the Macondo well had a gas-to-oil ratio of 1,600 standard cubic feet per petroleum barrel. Based on the measured endmember gas-to-oil ratio and the Federally estimated net liquid oil release of 4.1 million barrels, the total amount of C₁-C₅ hydrocarbons released to the water column was 1.7 × 10¹¹ g. The endmember gas and oil compositions then enabled us to study the fractionation of petroleum hydrocarbons in discrete water samples collected in June 2010 within a southwest trending hydrocarbon-enriched plume of neutrally buoyant water at a water depth of 1,100 m. The most abundant petroleum hydrocarbons larger than C₁-C₅ were benzene, toluene, ethylbenzene, and total xylenes at concentrations up to 78 μg L^-1. Comparison of the endmember gas and oil composition with the composition of water column samples showed that the plume was preferentially enriched with water-soluble components, indicating that aqueous dissolution played a major role in plume formation, whereas the fates of relatively insoluble petroleum components were initially controlled by other processes.     

Chemical data quantify Deepwater Horizon hydrocarbon flow rate and environmental distribution

Detailed airborne, surface, and subsurface chemical measurements, primarily obtained in May and June 2010, are used to quantify initial hydrocarbon compositions along different transport pathways (i.e., in deep subsurface plumes, in the initial surface slick, and in the atmosphere) during the Deepwater Horizon oil spill. Atmospheric measurements are consistent with a limited area of surfacing oil, with implications for leaked hydrocarbon mass transport and oil drop size distributions. The chemical data further suggest relatively little variation in leaking hydrocarbon composition over time. Although readily soluble hydrocarbons made up ∼25% of the leaking mixture by mass, subsurface chemical data show these compounds made up ∼69% of the deep plume mass; only ∼31% of the deep plume mass was initially transported in the form of trapped oil droplets. Mass flows along individual transport pathways are also derived from atmospheric and subsurface chemical data. Subsurface hydrocarbon composition, dissolved oxygen, and dispersant data are used to assess release of hydrocarbons from the leaking well. We use the chemical measurements to estimate that (7.8 ± 1.9) × 10⁶ kg of hydrocarbons leaked on June 10, 2010, directly accounting for roughly three-quarters of the total leaked mass on that day. The average environmental release rate of (10.1 ± 2.0) × 10⁶ kg/d derived using atmospheric and subsurface chemical data agrees within uncertainties with the official average leak rate of (10.2 ± 1.0) × 10⁶ kg/d derived using physical and optical methods.     

From the Cover: Science in support of the Deepwater Horizon response

This introduction to the Special Feature presents the context for science during the Deepwater Horizon oil spill response, summarizes how scientific knowledge was integrated across disciplines and statutory responsibilities, identifies areas where scientific information was accurate and where it was not, and considers lessons learned and recommendations for future research and response. Scientific information was integrated within and across federal and state agencies, with input from nongovernmental scientists, across a diverse portfolio of needs—stopping the flow of oil, estimating the amount of oil, capturing and recovering the oil, tracking and forecasting surface oil, protecting coastal and oceanic wildlife and habitat, managing fisheries, and protecting the safety of seafood. Disciplines involved included atmospheric, oceanographic, biogeochemical, ecological, health, biological, and chemical sciences, physics, geology, and mechanical and chemical engineering. Platforms ranged from satellites and planes to ships, buoys, gliders, and remotely operated vehicles to laboratories and computer simulations. The unprecedented response effort depended directly on intense and extensive scientific and engineering data, information, and advice. Many valuable lessons were learned that should be applied to future events.     

Acoustic measurement of the Deepwater Horizon Macondo well flow rate

On May 31, 2010, a direct acoustic measurement method was used to quantify fluid leakage rate from the Deepwater Horizon Macondo well prior to removal of its broken riser. This method utilized an acoustic imaging sonar and acoustic Doppler sonar operating onboard a remotely operated vehicle for noncontact measurement of flow cross-section and velocity from the well’s two leak sites. Over 2,500 sonar cross-sections and over 85,000 Doppler velocity measurements were recorded during the acquisition process. These data were then applied to turbulent jet and plume flow models to account for entrained water and calculate a combined hydrocarbon flow rate from the two leak sites at seafloor conditions. Based on the chemical composition of end-member samples collected from within the well, this bulk volumetric rate was then normalized to account for contributions from gases and condensates at initial leak source conditions. Results from this investigation indicate that on May 31, 2010, the well’s oil flow rate was approximately 0.10 ± 0.017 m³ s^-1 at seafloor conditions, or approximately 85 ± 15 kg s^-1 (7.4 ± 1.3 Gg d^-1), equivalent to approximately 57,000 ± 9,800 barrels of oil per day at surface conditions. End-member chemical composition indicates that this oil release rate was accompanied by approximately an additional 24 ± 4.2 kg s^-1 (2.1 ± 0.37 Gg d^-1) of natural gas (methane through pentanes), yielding a total hydrocarbon release rate of 110 ± 19 kg s^-1 (9.5 ± 1.6 Gg d^-1).     

From the Cover: Applications of science and engineering to quantify and control the Deepwater Horizon oil spill

The unprecedented engagement of scientists from government, academia, and industry enabled multiple unanticipated and unique problems to be addressed during the Deepwater Horizon oil spill. During the months between the initial blowout on April 20, 2010, and the final well kill on September 19, 2010, researchers prepared options, analyses of tradeoffs, assessments, and calculations of uncertainties associated with the flow rate of the well, well shut in, killing the well, and determination of the location of oil released into the environment. This information was used in near real time by the National Incident Commander and other government decision-makers. It increased transparency into BP’s proposed actions and gave the government confidence that, at each stage proposed, courses of action had been thoroughly vetted to reduce risk to human life and the environment and improve chances of success.     

Deepwater Horizon crude oil impacts the developing hearts of large predatory pelagic fish

The Deepwater Horizon disaster released more than 636 million L of crude oil into the northern Gulf of Mexico. The spill oiled upper surface water spawning habitats for many commercially and ecologically important pelagic fish species. Consequently, the developing spawn (embryos and larvae) of tunas, swordfish, and other large predators were potentially exposed to crude oil-derived polycyclic aromatic hydrocarbons (PAHs). Fish embryos are generally very sensitive to PAH-induced cardiotoxicity, and adverse changes in heart physiology and morphology can cause both acute and delayed mortality. Cardiac function is particularly important for fast-swimming pelagic predators with high aerobic demand. Offspring for these species develop rapidly at relatively high temperatures, and their vulnerability to crude oil toxicity is unknown. We assessed the impacts of field-collected Deepwater Horizon (MC252) oil samples on embryos of three pelagic fish: bluefin tuna, yellowfin tuna, and an amberjack. We show that environmentally realistic exposures (1–15 µg/L total PAH) cause specific dose-dependent defects in cardiac function in all three species, with circulatory disruption culminating in pericardial edema and other secondary malformations. Each species displayed an irregular atrial arrhythmia following oil exposure, indicating a highly conserved response to oil toxicity. A considerable portion of Gulf water samples collected during the spill had PAH concentrations exceeding toxicity thresholds observed here, indicating the potential for losses of pelagic fish larvae. Vulnerability assessments in other ocean habitats, including the Arctic, should focus on the developing heart of resident fish species as an exceptionally sensitive and consistent indicator of crude oil impacts.The Deepwater Horizon disaster resulted in the release of more than 4 million barrels (636 million L) of oil into the offshore waters of the northern Gulf of Mexico between April 10 and July 14, 2010 (1). Although subsurface application of dispersant near the wellhead resulted in retention of a considerable portion of oil in the bathypelagic zone (2), oil also traveled to the upper surface waters where it formed a large and dynamic patchwork of slicks (e.g., covering an estimated 17,725 km² during May 2010) (3). In the decades following the last major US oil spill (the 1989 Exxon Valdez spill in Alaska), developing fish embryos have been shown to be especially vulnerable to the toxicity of crude oil (4). The northern Gulf provides critical spawning and rearing habitats for a range of commercially and ecologically important pelagic fish species, and the timing of oil release into the ecosystem from the damaged Deepwater Horizon/MC252 well coincided with the temporal spawning window for bluefin and yellowfin tunas, mahi mahi, king and Spanish mackerels, greater and lesser amberjack, sailfish, blue marlin, and cobia (5–13). Yellowfin tuna (Thunnus albacares) and greater amberjack (Seriola dumerili) contribute to important commercial fisheries (48,960,000 pounds in 2010 and 4,348,000 pounds in 2004, respectively) (14, 15). The Atlantic bluefin tuna (Thunnus thynnus) population from the Gulf of Mexico is currently at a historically low level (16), and was recently petitioned for listing under the US Endangered Species Act. For these and other pelagics, the extent of early-life stage loss from oiled spawning habitats is an important outstanding question for fisheries management and conservation.The developing fish heart is known as a sensitive target organ for the toxic effects of crude oil-derived polycyclic aromatic hydrocarbons (PAHs) (4). Of the multiple two- to six-ringed PAH families contained in crude oil, the most abundant three-ringed compounds are sufficient to drive the cardiotoxicity of petroleum-derived PAH mixtures. These compounds (fluorenes, dibenzothiophenes, and phenanthrenes) directly disrupt fish cardiac function (17, 18), thereby interfering with the interdependent processes of circulation and heart chamber formation. Exposure of fish embryos to PAH mixtures derived from crude oil slows the heartbeat (bradycardia) and reduces contractility (17, 19–21). The underlying mechanism was recently shown to be blockade of key potassium and calcium ion channels involved in cardiac excitation-contraction coupling (22). These collective effects of PAHs during embryonic and larval stages can influence the structure and function of the adult fish heart in ways that permanently reduce cardiac performance (23), potentially leading to delayed mortality. Consistent with this, mark-recapture studies on pink salmon following the Exxon Valdez spill found that transient and sublethal exposures to crude oil at very low levels during embryogenesis reduced subsequent marine survival to adulthood by 40% (24, 25). Exposures to relatively higher PAH concentrations cause embryonic heart failure and death soon after fish hatch into free-swimming larvae (19, 20, 23). These effects occur at a total PAH concentration range as low as 1–10 µg/L for more sensitive species (26, 27), levels as much as an order-of-magnitude lower than those measured in some samples collected both at depth and at the surface during the Deepwater Horizon active spill phase (28, 29).The above crude oil cardiotoxicity syndrome has been extensively characterized in zebrafish embryos exposed to several geologically distinct oils (17, 21, 23, 30, 31), including the Mississippi Canyon 252 (MC252) crude oil released from the blown out Deepwater Horizon wellhead (20, 32). Similar effects have been reported for temperate marine and anadromous species, such as Pacific herring (19, 26, 27, 33) and pink salmon (34, 35), following exposure to Alaska North Slope crude oil. Although zebrafish are a tropical freshwater model species, the embryos of herring and salmon assessed in the aftermath of the Exxon Valdez spill develop at cold temperatures (4–12 °C) over relatively long intervals (weeks to months). In contrast, pelagic species spawning in the warm surface waters of the northern Gulf of Mexico (e.g., 24–29 °C) develop rapidly (24–48 h to hatch) (36, 37). The influence of development duration on PAH uptake and toxicity, if any, is not well understood. The higher temperatures characteristic of waters in the Gulf of Mexico may also influence how the chemical composition of crude oil in surface habitat(s) changes over time (i.e., weathers). Processes that determine weathering are generally accelerated at higher temperatures, potentially influencing the fraction of cardiotoxic PAHs that is bioavailable for uptake by floating fish embryos in the mixed layer and thermocline regions. To address these information gaps, controlled laboratory exposures are necessary to determine the sensitivity of Gulf species to Deepwater Horizon crude oil.To assess potential early life-stage losses from large pelagic predator populations that were actively spawning in habitats affected by the Deepwater Horizon spill, we determined the effects of field-collected MC252 oil samples on the development of embryos from representative warm water open-ocean fish species. Our approach extended earlier work in zebrafish, a laboratory model species and Pacific herring, a marine nearshore spawner (19, 27, 38). Zebrafish and herring both produce large demersal embryos that are relatively easy to manipulate (i.e., collect, dechorionate, and image at consistent ontogenetic intervals). In contrast, Gulf pelagic species produce small, fragile, buoyant embryos that develop relatively rapidly (on a timescale of hours relative to days or weeks for zebrafish and herring, respectively) and are not amenable to dechorionation. Moreover, the embryos hatch into buoyant larvae. Normally present in infinite-volume pelagic habitats, they are very sensitive to any form of physical contact, thereby complicating conventional embryology in small-volume laboratory cultures. Finally, access to embryos is difficult, with only a few land-based facilities capable of maintaining spawning broodstocks throughout the world.In the present study we overcome the aforementioned challenges for focal pelagic species that included yellowfin tuna, Southern bluefin tuna (Thunnus maccoyii), and yellowtail amberjack (or kingfish, Seriola lalandi). The yellowfin tuna are the same species that spawn in the Gulf of Mexico, and the other two species are closely related congenerics to T. thynnus and S. dumerili, respectively. Controlled bluefin tuna spawning is exceptionally difficult to achieve in a husbandry facility, and we used the only land-based captive broodstock available in the world for experiments. Similarly, we relied on a commercial broodstock of yellowtail amberjack and a research broodstock of yellowfin tuna, the latter the only worldwide source of fertilized embryos for this species. Embryos were exposed to high-energy water-accommodated fractions (HEWAFs) (20) that generated PAH concentrations and compositional profiles closely matching water samples collected during active MC252 crude oil release phase.