Partial Photochemical Oxidation Was a Dominant Fate of Deepwater Horizon Surface Oil

Direct and indirect photodegradation in aquatic systems mitigates photosensitized toxicity in screening-level substance risk assessments of selected petrochemical structures

Photochemical processes are typically not incorporated in screening-level substance risk assessments due to the complexity of modeling sunlight co-exposures and resulting interactions on environmental fate and effects. However, for many substances, sunlight exerts a profound influence on environmental degradation rates and ecotoxicities. Recent modeling advances provide an improved technical basis for estimating the effect of sunlight in modulating both substance exposure and toxicity in the aquatic environment. Screening model simulations were performed for 25 petrochemical structures with varied uses and environmental fate properties. Model predictions were evaluated by comparing the ratios of predicted exposure concentrations with and without light to the corresponding ratios of toxicity thresholds under the same conditions. The relative ratios of exposure and hazard in light vs. dark were then used to evaluate how inclusion of light modulates substance risk analysis. Results indicated that inclusion of light reduced PECs by factors ranging from 1.1- to 63-fold as a result of photodegradation, while reducing PNECs by factors ranging from 1- to 49-fold due to photoenhanced toxicity caused by photosensitization. Consequently, the presence of light altered risk quotients by factors that ranged from 0.1- to 17-fold, since the predicted increase in substance hazard was mitigated by the reduction in exposure. For many structures, indirect photodegradation decreases environmental exposures independently of the direct photolysis pathway which is associated with enhanced phototoxicity. For most of the scenarios and chemicals in the present work, photosensitization appears to be mitigated by direct and indirect degradation from sunlight exposure.

Non-target analysis of crude oil photooxidation products at high latitudes and their biological effects

With new oil and gas lease sales in high-latitude regions, there exists a need to better understand the chemical fate of spilled oil and its effects on biological life. To address this need, laboratory simulations of crude oil spills under sub-Arctic conditions were conducted using artificial seawater and exposure to solar irradiation to create Hydrocarbon Oxidation Products (HOPs). HOPs characterization and their biological effects were assessed using ultra high-performance liquid chromatography (UHPLC) with high resolution mass Orbitrap spectrometry and the aryl hydrocarbon receptor (AhR) chemically activated luciferase gene expression (CALUX) assay. Non-target UHPLC-Orbitrap mass spectrometry analysis identified 251 HOPs that were in greater abundance in light-exposed samples than dark controls. Oxidized polycyclic aromatic hydrocarbons were also detected, including phenanthrene quinone, anthraquinone, hydroxyanthraquinone, and 9-fluoreneone. The composition of HOPs were consistent with photo-products of alkylated two to four ring PAHs, primarily compounds between 1 and 3 aromatic rings and 1-3 oxygens. The HOP mixture formed during photochemical weathering of Cook Inlet crude oil induced greater AhR activity than parent petroleum products solubilized in dark controls, indicating that HOPs, as a complex mixture, may contribute to petroleum toxicity more than the parent petroleum compounds. These non-targeted approaches provide the most comprehensive analysis of hydrocarbon oxidation products to date, highlighting the diversity of the complex mixture resulting from the photooxidation of crude oil and the limitations of targeted analyses for adequately monitoring HOPs in the environment. Taken together, these data identify a critical "blind spot" in environmental monitoring and spill clean-up strategies as there is a diverse pool of HOPs that may negatively impact human and ecosystem health.

Photochemistry of oil in marine systems: developments since the Deepwater Horizon spill

Oil spills represent a major source of negative environmental impacts in marine systems. Despite many decades of research on oil spill behavior, photochemistry was neglected as a major factor in the fate of oil spilled in marine systems. Subsequent to the Deepwater Horizon oil spill, numerous studies using varied approaches have demonstrated the importance of photochemistry, including short-term impacts (hours to days) that were previously unrecognized. These studies have demonstrated the importance of photochemistry in the overall oil transformation after a spill and more specifically the impacts on emulsification, oxygenation, and microbial interactions. In addition to new perspectives, advances in analytical approaches have allowed an improved understanding of oil photochemistry after maritime spill. Although the literature on the Deepwater Horizon spill is extensive, this review focuses only on studies relevant to the advances in oil photochemistry understanding since the Deepwater Horizon spill.

Marine oil spill photodegradation: Laboratory simulation, affecting factors analysis and kinetic model development

Photodegradation significantly influences marine oil spill behavior, yet its role remains underrepresented in current models, impairing predictive accuracy. Addressing this, our study rigorously examined oil properties and environmental determinants affecting marine oil spill photodegradation through laboratory simulations. We identified and quantified key factors and their interactions, noting particularly the positive influence of asphaltene and negative implications of oil density. We also discerned a negative correlation between n-alkane degradation and carbon numbers. Our findings underscored the pivotal roles of temperature and irradiance in photodegradation. All tested oils adhered to first-order kinetics, with rate constants ranging from 0.0348 to 0.0645 day-1. Finally, we introduced a novel model incorporating temperature, irradiance and their interactions, ensuring reasonable simulations for marine oil spill photodegradation, fortifying marine oil spill management strategies.

Effects of aromatic hydrocarbons and evaluation of oil toxicity modelling for larvae of a tropical coral

Application of oil toxicity modelling for assessing the risk of spills to coral reefs remains uncertain due to a lack of data for key tropical species and environmental conditions. In this study, larvae of the coral Acropora millepora were exposed to six aromatic hydrocarbons individually to generate critical target lipid body burdens (CTLBBs). Larval metamorphosis was inhibited by all six aromatic hydrocarbons, while larval survival was only affected at concentrations >2000 μg L-1. The derived metamorphosis CTLBB of 9.7 μmol g-1 octanol indicates larvae are more sensitive than adult corals, and places A. millepora larvae among the most sensitive organisms in the target lipid model (TLM) databases. Larvae were also more sensitive to anthracene and pyrene when co-exposed to ecologically relevant levels of ultraviolet radiation. The results suggest that the application of the phototoxic TLM would be protective of A. millepora larvae, provided adequate chemical and light data are available.

Assessing the effects of sunlight and water on asphalt binder and pavement leachability related to the environment

Extensive global research conducted over 30 years explores asphalt leachability and stormwater runoff. Asphalt's widespread usage in construction materials underscores the importance of understanding its environmental consequences. This study aims to assess the influence of sunlight exposure on water quality, particularly regarding the release of hazardous organic compounds such as polycyclic aromatic compounds. We investigated the effect of concurrent versus sequential exposure to water and sunlight, and dark versus light trials utilizing thin films of asphalt binder as well as old and freshly prepared pavement cores for analysis. Initial laboratory experiments reveal significant water-soluble species when thin asphalt films are exposed to solar simulation while underwater. However, simulating environmental conditions found in roadways by exposing the asphalt binder to solar simulation followed by water immersion leads to a substantial decrease in compound formation. Leachate water from 17-year-old asphalt and 15-year-old concrete pavements exhibits complex compound compositions associated with atmospheric and/or vehicular deposition, posing challenges in deconvoluting their origins. Light and dark trials conducted on freshly prepared asphalt pavement under environmental conditions of sunlight and rain demonstrate minimal runoff variation, with semi-volatile organic compound levels resembling the background. Future investigations will focus on applying insights gained from this study to analyze larger sample sets, with an emphasis on inherent hazardous compound variations.

Changes in Temperature Alter the Toxicity of Polycyclic Aromatic Compounds to American Lobster (Homarus americanus) Larvae

Polycyclic aromatic compounds (PACs) present in the water column are considered to be one of the primary contaminant groups contributing to the toxicity of a crude oil spill. Because crude oil is a complex mixture composed of thousands of different compounds, oil spill models rely on quantitative structure-activity relationships like the target lipid model to predict the effects of crude oil exposure on aquatic life. These models rely on input provided by single species toxicity studies, which remain insufficient. Although the toxicity of select PACs has been well studied, there is little data available for many, including transformation products such as oxidized hydrocarbons. In addition, the effect of environmental influencing factors such as temperature on PAC toxicity is a wide data gap. In response to these needs, in the present study, Stage I lobster larvae were exposed to six different understudied PACs (naphthalene, fluorenone, methylnaphthalene, phenanthrene, dibenzothiophene, and fluoranthene) at three different relevant temperatures (10, 15, and 20 °C) all within the biological norms for the species during summer when larval releases occur. Lobster larvae were assessed for immobilization as a sublethal effect and mortality following 3, 6, 12, 24, and 48 h of exposure. Higher temperatures increased the rate at which immobilization and mortality were observed for each of the compounds tested and also altered the predicted critical target lipid body burden, incipient median lethal concentration, and elimination rate. Our results demonstrate that temperature has an important influence on PAC toxicity for this species and provides critical data for oil spill modeling. More studies are needed so oil spill models can be appropriately calibrated and to improve their predictive ability. Environ Toxicol Chem 2023;42:2389-2399. © 2023 SETAC.

Photochemical degradation in natural attenuation of characteristics of petroleum hydrocarbons (C10-C40) in crude oil polluted soil by simulated long term solar irradiation

Photodegradation process plays an important role in the natural attenuation of petroleum hydrocarbons (PHs) in oil contaminated soil. The photodegradation characteristics of PHs (C10-C40) in topsoil of crude oil contaminated soil irradiated by simulated sunlight in 280 d without microbial action were investigated. The results showed that photodegradation rate of PHs was increased with increasing the light intensity and decreased with increasing the initial concentration of PHs. Moreover, the photodegradation capacity of tested PHs was relevant to the length of carbon chain. The photodegradation rates of C10-C20 were higher than that of C21-C40 in photoperiod. C21-C40 showed an obvious trend of photodegradation after 56 d, although their photodegradation rates were less than 20% at the early stage. And, the redundancy analysis indicated that lighting time was the primary factor for photodegradation of PHs under abiotic conditions. The photodegradation rate was well interpreted by a two-stage, first-order kinetic law with a faster initial photolysis rate. The EPR spectrums showed that simulated solar irradiation accelerated the generation of superoxide radicals, which could react with PHs in soil. Also, the function groups in PHs polluted soil were changed after light exposure, which might imply the possible photodegradation pathway of PHs.

Hydrodynamic Treadmill Reveals Reduced Rising Speeds of Oil Droplets Deformed by Marine Bacteria

In marine environments, microscopic droplets of oil can be transported over large distances in the water column. Bacterial growth on the droplets' surface can deform the oil-water interface to generate complex shapes and significantly enlarge droplets. Understanding the fate of spilled oil droplets requires bridging these length scales and determining how microscale processes affect the large-scale transport of oil. Here, we describe an experimental setup, the hydrodynamic treadmill, developed to keep rising oil droplets stationary in the lab frame for continuous and direct observation. Oil droplets with radii 10 < R < 100 μm were colonized and deformed by bacteria over several days before their effective rising speeds were measured. The rising speeds of deformed droplets were significantly slower than those of droplets without bacteria. This decrease in rising speed is understood by an increase in drag force and a decrease in buoyancy as a result of bio-aggregate formation at the droplet surface. Additionally, we found sinking bio-aggregate particles of oil and bacterial biofilms and quantified their composition using fluorescence microscopy. Our experiments can be adapted to further study the interactions between oil droplets and marine organisms and could significantly improve our understanding of the transport of hydrocarbons and complex aggregates.

Hot and Cold: Photochemical Weathering Mediates Oil Properties and Fate Differently Depending on Seawater Temperature

Photochemical weathering transforms petroleum oil and changes its bulk physical properties, as well as its partitioning into seawater. This transformation process is likely to occur in a cold water marine oil spill, but little is known about the behavior of photochemically weathered oil in cold water. We quantified the effect of photochemical weathering on oil properties and partitioning across temperatures. Compared to weathering in the dark, photochemical weathering increases oil viscosity and water-soluble content, decreases oil-seawater interfacial tension, and slightly increases density. Many of these photochemical changes are much larger than changes caused by evaporative weathering. Further, the viscosity and water-soluble content of photochemically weathered oil are more temperature-sensitive compared to evaporatively weathered oil, which changes the importance of key fate processes in warm versus cold environments. Compared to at 30 °C, photochemically weathered oil at 5 °C would have a 16× higher viscosity and a 7× lower water-soluble content, resulting in lower entrainment and dissolution. Collectively, the physical properties and thus fate of photochemically weathered oil in a cold water spill may be substantially different from those in a warm water spill. These differences could affect the choice of oil spill response options in cold, high-light environments.

Recommendations for advancing test protocols examining the photo-induced toxicity of petroleum and polycyclic aromatic compounds

Photo-induced toxicity of petroleum products and polycyclic aromatic compounds (PACs) is the enhanced toxicity caused by their interaction with ultraviolet radiation and occurs by two distinct mechanisms: photosensitization and photomodification. Laboratory approaches for designing, conducting, and reporting of photo-induced toxicity studies are reviewed and recommended to enhance the original Chemical Response to Oil Spills: Ecological Research Forum (CROSERF) protocols which did not address photo-induced toxicity. Guidance is provided on conducting photo-induced toxicity tests, including test species, endpoints, experimental design and dosing, light sources, irradiance measurement, chemical characterization, and data reporting. Because of distinct mechanisms, aspects of photosensitization (change in compound energy state) and photomodification (change in compound structure) are addressed separately, and practical applications in laboratory and field studies and advances in predictive modeling are discussed. One goal for developing standardized testing protocols is to support lab-to-field extrapolations, which in the case of petroleum substances often requires a modeling framework to account for differential physicochemical properties of the constituents. Recommendations are provided to promote greater standardization of laboratory studies on photo-induced toxicity, thus facilitating comparisons across studies and generating data needed to improve models used in oil spill science.

Bridging the lab to field divide: Advancing oil spill biological effects models requires revisiting aquatic toxicity testing

Oil fate and exposure modeling addresses the complexities of oil composition, weathering, partitioning in the environment, and the distributions and behaviors of aquatic biota to estimate exposure histories, i.e., oil component concentrations and environmental conditions experienced over time. Several approaches with increasing levels of complexity (i.e., aquatic toxicity model tiers, corresponding to varying purposes and applications) have been and continue to be developed to predict adverse effects resulting from these exposures. At Tiers 1 and 2, toxicity-based screening thresholds for assumed representative oil component compositions are used to inform spill response and risk evaluations, requiring limited toxicity data, analytical oil characterizations, and computer resources. Concentration-response relationships are employed in Tier 3 to quantify effects of assumed oil component mixture compositions. Oil spill modeling capabilities presently allow predictions of spatial and temporal compositional changes during exposure, which support mixture-based modeling frameworks. Such approaches rely on summed effects of components using toxic units to enable more realistic analyses (Tier 4). This review provides guidance for toxicological studies to inform the development of, provide input to, and validate Tier 4 aquatic toxicity models for assessing oil spill effects on aquatic biota. Evaluation of organisms' exposure histories using a toxic unit model reflects the current state-of the-science and provides an improved approach for quantifying effects of oil constituents on aquatic organisms. Since the mixture compositions in toxicity tests are not representative of field exposures, modelers rely on studies using single compounds to build toxicity models accounting for the additive effects of dynamic mixture exposures that occur after spills. Single compound toxicity data are needed to quantify the influence of exposure duration and modifying environmental factors (e.g., temperature, light) on observed effects for advancing use of this framework. Well-characterized whole oil bioassay data should be used to validate and refine these models.

Semiconducting mineral induced photochemical conversion of PAHs in aquatic environment: Mechanism study and fate prediction

Semiconducting minerals (such as iron sulfides) are highly abundant in surface water, but their influences on the natural photochemical process of contaminants are still unknown. By simulating the natural water environment under solar irradiation, this work comprehensively investigated the photochemical processes of anthracene (a typical Polycyclic Aromatic Hydrocarbons) in both freshwater and seawater. The results show that the natural pyrite (NP) significantly promotes the degradation of anthracene under solar illumination via 1) NP induced photocatalytic degradation of anthracene, and 2) Fenton reaction due to the NP induced photocatalytic generation of H₂O₂. The material characterization and theoretical calculation reveal that the natural impurity in NP enlarges its band gap, which limits the utilization of solar spectra to shorter wavelength. The contribution of generated reactive intermediates on anthracene degradation follows the order of ¹O₂ >OH > O₂^- in freshwater and O₂^- >¹O₂ >OH in seawater. The photochemically generated H₂O₂ is a vital source for OH generation (from Fenton reaction). The steady-state concentration of OH, ¹O₂ and O₂^- in freshwater were monitored as 3.0 × 10^-15 M, 1.1 × 10^-13 M, and 4.5 × 10^-14 M, respectively. However, the OH concentration in seawater can be negligible due to the quenching effects by halides, and the ¹O₂ and O₂^- concentrations are higher than that in freshwater. An anthracene degradation kinetic model was built based on the experimentally determined reactive intermediates concentration and its second order rate constant with anthracene. Moreover, the anthracene degradation pathway was proposed based on intermediates analysis and DFT calculation, and its toxicity evolution during the photochemical process was assessed by quantitative structure-activity relationship (QSAR) based prediction. This finding suggests that the natural semiconducting minerals can affect the fate and environmental risks of contaminants in natural water.

Elevated nitrate promoted photodegradation of PAHs in aqueous phase: Implications for the increased nutrient discharge

Polycyclic aromatic hydrocarbons (PAHs) are frequently released in aqueous phase by oil spill or from other sources, and photochemical oxidation is one of their major weathering processes. In this study, the photochemical behavior of phenanthrene (PHE, as a representative PAH) were studied and the effects of nitrogenous compounds were evaluated. The results showed that nitrate was an effective photosensitizer for improving the photodegradation of PHE, but the promoting effect was less effective in seawater due to the presence of halogen ions; the ammonia played a negligible role on PHE degradation. The photochemical ionization was a key process for PHE degradation, it can be retarded due to the quenching of triplet excited state by dissolved oxygen, and the inhibition was most prominent in fresh water. The presence of nitrate increased the steady state concentration of •OH from 2.08 × 10^-15 M to 1.04 × 10^-14 M in fresh water, and from 1.5 × 10^-16 M to 2.08 × 10^-15 M in seawater. The secondary-order reaction rate constant between PHE and •OH (k_•OH,PHE) was determined as 5.70 × 10⁹ M^-1 s^-1. Similar trend was observed for ¹O₂. The contribution of •OH to PHE removal was more prominent in fresh water than in seawater due to the quenching effects of halogen, and the increasing of nitrate enlarged the contribution of •OH. Two possible PHE degradation pathways were proposed based on GC-MS analysis and DFT calculation. The Quantitative Structure-activity Relationship (QSAR) evaluation showed that some degradation intermediates were more toxic than PHE, but the total environmental risk was still diminished due to the low percentage of toxic intermediates. This study provided theoretical and experimental insights into the influence of nitrogenous compounds on the photodegradation of PHAs in water environment.

Oil Transport Following the Deepwater Horizon Blowout

The Deepwater Horizon oil spill in the Gulf of Mexico in 2010 was the largest in US history, covering more than 1,000 km of shorelines and causing losses that exceeded $50 billion. While oil transformation processes are understood at the laboratory scale, the extent of the Deepwater Horizon spill made it challenging to integrate these processes in the field. This review tracks the Deepwater Horizon oil during its journey from the Mississippi Canyon block 252 (MC252) wellhead, first discussing the formation of the oil and gas plume and the ensuing oil droplet size distribution, then focusing on the behavior of the oil on the water surface with and without waves. It then reports on massive drifter experiments in the Gulf of Mexico and the impact of the Mississippi River on the oil transport. Finally, it concludes by addressing the formation of oil-particle aggregates. Although physical processes lend themselves to numerical modeling, we attempted to elucidate them without using advanced modeling, as our goal is to enhance communication among scientists, engineers, and other entities interested in oil spills.

Temporal chemical composition changes in water below a crude oil slick irradiated with natural sunlight

Photooxidation can alter the environmental fate and effects of spilled oil. To better understand this process, oil slicks were generated on seawater mesocosms and exposed to sunlight for 8 days. The molecular composition of seawater under irradiated and non-irradiated oil slicks was characterized using ion mobility spectrometry-mass spectrometry and polyaromatic hydrocarbons analyses. Biomimetic extraction was performed to quantify neutral and ionized constituents. Results show that seawater underneath irradiated oil showed significantly higher amounts of hydrocarbons with oxygen- and sulfur-containing by-products peaking by day 4-6; however, concentrations of dissolved organic carbon were similar. Biomimetic extraction indicated toxic units in irradiated mesocosms increased, mainly due to ionized components, but remained <1, suggesting limited potential for ecotoxicity. Because the experimental design mimicked important aspects of natural conditions (freshly collected seawater, natural sunlight, and relevant oil thickness and concentrations), this study improves our understanding of the effects of photooxidation during a marine oil spill.

Complex Mixture Analysis of Emerging Contaminants Generated from Coal Tar- and Petroleum-Derived Pavement Sealants: Molecular Compositions and Correlations with Toxicity Revealed by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

Pavement sealants are of environmental concern because of their complex petroleum-based chemistry and potential toxicity. Specifically, coal tar-derived sealants contain high concentrations of toxic/carcinogenic polycyclic aromatic hydrocarbons (PAHs) that, when weathered, can be transferred into the surrounding environment. Previous studies have demonstrated the effects of coal tar sealants on PAH concentration in nearby waterways and their harmful effects in aquatic ecosystems. Here, we investigate and compare the molecular composition of two different pavement sealants, petroleum asphalt- and coal tar-derived, and their photoproducts, by positive-ion (+) atmospheric pressure photoionization (APPI) and negative-ion (-) electrospray ionization (ESI) coupled with ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry to address species (high-boiling and/or high oxygen content) that lie outside the analytical window of other techniques due to ultra-high molecular complexity. In addition, we evaluate the toxicity of the water-soluble photoproducts by use of Microtox bioassay. The results demonstrate that the coal tar sealant contains higher amounts of PAHs and produces abundant water-soluble compounds, relative to unweathered materials, with a high abundance of PAH-like molecules of high toxicity. By comparison, the asphalt sealant produces fewer toxic water-soluble species, with molecular compositions that are consistent with natural dissolved organic matter. These results capture the mass, chemical diversity, toxicity, and source/photoproduct relationship of these compositionally complex emerging contaminants from the built environment.

Recent advances in chemical and biological degradation of spilled oil: A review of dispersants application in the marine environment

Growing concerns over the risk of accidental releases of oil into the marine environment have emphasized our need to improve both oil spill preparedness and response strategies. Among the available spill response options, dispersants offer the advantages of breaking oil slicks into small oil droplets and promoting their dilution, dissolution, and biodegradation within the water column. Thus dispersants can reduce the probability of oil slicks at sea from reaching coastal regions and reduce their direct impact on mammals, sea birds and shoreline ecosystems. To facilitate marine oil spill response operations, especially addressing spill incidents in remote/Arctic offshore regions, an in-depth understanding of the transportation, fate and effects of naturally/chemically dispersed oil is of great importance. This review provides a synthesis of recent research results studies related to the application of dispersants at the surface and in the deep sea, the fate and transportation of naturally and chemically dispersed oil, and dispersant application in the Arctic and ice-covered waters. Future perspectives have been provided to identify the research gaps and help industries and spill response organizations develop science-based guidelines and protocols for the application of dispersants application.

Oil Irradiation Experiments Document Changes in Oil Properties, Molecular Composition, and Dispersant Effectiveness Associated with Oil Photo-Oxidation

While chemical dispersants are a powerful tool for treating spilled oil, their effectiveness can be limited by oil weathering processes such as evaporation and emulsification. It has been suggested that oil photo-oxidation could exacerbate these challenges. To address the role of oil photo-oxidation in dispersant effectiveness, outdoor mesocosm experiments with crude oil on seawater were performed. Changes in bulk oil properties and molecular composition were quantified to characterize oil photo-oxidation over 11 days. To test relative dispersant effectiveness, oil residues were evaluated using the Baffled Flask Test. The results show that oil irradiation led to oxygen incorporation, formation of oxygenated hydrocarbons, and higher oil viscosities. Oil irradiation was associated with decreased dispersant efficacy, with effectiveness falling from 80 to <50% in the Baffled Flask Test after more than 3 days of irradiation. Increasing photo-oxidation-induced viscosity seems to drive the decreasing dispersant effectiveness. Comparing the Baffled Flask Test results with field data from the Deepwater Horizon oil spill showed that laboratory dispersant tests underestimate the dispersion of photo-oxidized oil in the field. Overall, the results suggest that prompt dispersant application (within 2-4 days), as recommended by current oil spill response guidelines, is necessary for effective dispersion of spilled oil.

Changes in Chemical Composition and Copepod Toxicity during Petroleum Photo-oxidation

Photoproducts can be formed rapidly in the initial phase of a marine oil spill. However, their toxicity is not well understood. In this study, oil was irradiated, chemically characterized, and tested for toxicity in three copepod species (Acartia tonsa, Temora longicornis, and Calanus finmarchicus). Irradiation led to a depletion of polycyclic aromatic hydrocarbons (PAHs) and n-alkanes in oil residues, along with an enrichment in aromatic and aliphatic oil photoproducts. Target lipid model-based calculations of PAH toxicity units predicted that PAH toxicities were lower in water-accommodated fractions (WAFs) of irradiated oil residues ("irradiated WAFs") than in WAFs of dark-control samples ("dark WAFs"). In contrast, biomimetic extraction (BE) measurements showed increased bioaccumulation potential of dissolved constituents of irradiated WAFs compared to dark WAFs, mainly driven by photoproducts present in irradiated oil. In line with the BE results, copepod mortality increased in irradiated WAFs compared to dark WAFs. However, low copepod toxicities were observed for WAFs produced with photo-oxidized oil slicks collected during the Deepwater Horizon oil spill. The results of this study suggest that while oil photoproducts have the potential to be a significant source of copepod toxicity, dilution and dispersion of these higher solubility products appear to help mitigate their toxicity at sea.

Time-dependent molecular progression and acute toxicity of oil-soluble, interfacially-active, and water-soluble species reveals their rapid formation in the photodegradation of Macondo Well Oil

Photodegradation is a significant weathering process that transforms spilled oil, yet, the fate, degradation rate, and molecular transformations that occur through photoinduced pathways remain relatively unknown. The molecular complexity combined with the increased polarity of photoproducts challenges conventional analytical techniques. Here, we catalogue the molecular progression of photochemical transformation products of Macondo Well Oil by negative-ion electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). We track the molecular compositions of oil-soluble, interfacially-active, and water-soluble oil species formed at varying time intervals in photomicrocosm experiments. Short photoirradiation periods (<24 h), not previously reported, are included to reveal rapid photooxidation of native oil components. Surface oil films exposed to solar irradiation were shown to increasingly contribute to the dissolved organic carbon pool as a function of increased irradiation time. FT-ICR MS analysis of acidic species of each fraction identifies tens of thousands of oil-soluble, interfacially-active, and water-soluble phototransformation products, including O_x, NO_x, and SO_x species. Oil-soluble species incorporate oxygen as a function of irradiation periods. After 96 h of irradiation, ~14 wt% of the photooxidized oil film was interfacially active and contained phototransformed species with up to 12 oxygen atoms per molecule. Water-soluble species correspond to highly oxygenated compounds. Importantly, photochemical oxidation is shown to occur within the first hour. Beyond 24 h, photoproducts remain compositionally similar, highlighting the rapid effect of photodegradation to transform oil species into water-soluble compounds. Molecular fingerprints provided by FT-ICR MS highlight the oxygen dependence on oil/water solubility. Microtox® analysis indicates that the toxicity of water-soluble photoproducts rapidly increases at early irradiation time points (first 24 h) compared to the dark control and reaches a maximum at 6 h of irradiation. Results highlight the temporal, molecular progression of photoproducts as they partition from oil-soluble to oil-soluble interfacially-active, and finally to water-soluble species.

Advances in Chemical Analysis of Oil Spills Since the Deepwater Horizon Disaster

Analytical techniques for chemical analysis of oil, oil photochemical and biological transformation products, and dispersants and their biodegradation products benefited significantly from research following the 2010 Deepwater Horizon (DWH) disaster. Crude oil and weathered-oil matrix reference materials were developed based on the Macondo well oil and characterized for polycyclic aromatic hydrocarbons, hopanes, and steranes for use to assure and improve the quality of analytical measurements in oil spill research. Advanced gas chromatography (GC) techniques such as comprehensive two-dimensional GC (GC × GC), pyrolysis GC with mass spectrometry (MS), and GC with tandem MS (GC-MS/MS) provide a greater understanding at the molecular level of composition and complexity of oil and weathering changes. The capabilities of high-resolution MS (HRMS) were utilized to extend the analytical characterization window beyond conventional GC-based methods to include polar and high molecular mass components (>400 Da) and to provide new opportunities for discovery, characterization, and investigation of photooxidation and biotransformation products. Novel separation approaches to reduce the complexity of the oil and weathered oil prior to high-resolution MS and advanced fluorescence spectrometry have increased the information available on spilled oil and transformation products. HRMS methods were developed to achieve the required precision and sensitivity for detection of dispersants and to provide molecular-level characterization of the complex surfactants. Overall, research funding following the DWH oil spill significantly advanced and expanded the use of analytical techniques for chemical analysis to support petroleum and dispersant characterization and investigations of fate and effects of not only the DWH oil spill but future spills.

Nematostella vectensis exhibits an enhanced molecular stress response upon co-exposure to highly weathered oil and surface UV radiation

Crude oil released into the environment undergoes weathering processes that gradually change its composition and toxicity. Co-exposure to petroleum mixtures and other stressors, including ultraviolet (UV) radiation, may lead to synergistic effects and increased toxicity. Laboratory studies should consider these factors when testing the effects of oil exposure on aquatic organisms. Here, we study transcriptomic responses of the estuarine sea anemone Nematostella vectensis to naturally weathered oil, with or without co-exposure to environmental levels of UV radiation. We find that co-exposure greatly enhances the response. We use bioinformatic analyses to identify molecular pathways implicated in this response, which suggest phototoxicity and oxidative damage as mechanisms for the enhanced stress response. Nematostella's stress response shares similarities with the vertebrate oxidative stress response, implying deep conservation of certain stress pathways in animals. We show that exposure to weathered oil along with surface-level UV exposure has substantial physiological consequences in a model cnidarian.

Natural attenuation of oil in marine environments: A review

Natural attenuation is an important process for oil spill management in marine environments. Natural attenuation affects the fate of oil by physical, chemical, and biological processes, which include evaporation, dispersion, dissolution, photo-oxidation, emulsification, oil particle aggregation, and biodegradation. This review examines the cumulative knowledge regarding these natural attenuation processes as well as their simulation and prediction using modelling approaches. An in-depth discussion is provided on how oil type, microbial community and environmental factors contribute to the biodegradation process. It describes how our understanding of the structure and function of indigenous oil degrading microbial communities in the marine environment has been advanced by the application of next generation sequencing tools. The synergetic and/or antagonist effects of oil spill countermeasures such as the application of chemical dispersants, in-situ burning and nutrient enrichment on natural attenuation were explored. Several knowledge gaps were identified regarding the synergetic and/or antagonistic effects of active response countermeasures on the natural attenuation/biodegradation process. This review highlighted the need for field data on both the effectiveness and potential detrimental effects of oil spill response options to support modelling and decision-making on their selection and application.

Sunlight-driven dissolution is a major fate of oil at sea

Oxygenation reactions initiated by sunlight can transform insoluble components of crude oil at sea into water-soluble products, a process called photo-dissolution. First reported a half century ago, photo-dissolution has never been included in spill models because key parameters required for rate modeling were unknown, including the wavelength and photon dose dependence. Here, we experimentally quantified photo-dissolution as a function of wavelength and photon dose, making possible a sensitivity analysis of environmental variables in hypothetical spill scenarios and a mass balance assessment for the 2010 Deepwater Horizon (DwH) spill. The sensitivity analysis revealed that rates were most sensitive to oil slick thickness, season/latitude, and wavelength and less sensitive to photon dose. We estimate that 3 to 17% (best estimate 8%) of DwH surface oil was subject to photo-dissolution, comparable in magnitude to other widely recognized fate processes. Our findings invite a critical reevaluation of surface oil budgets for both DwH and future spills at sea.

Oil fate and mass balance for the Deepwater Horizon oil spill

Based on oil fate modeling of the Deepwater Horizon spill through August 2010, during June and July 2010, ~89% of the oil surfaced, ~5% entered (by dissolving or as microdroplets) the deep plume (>900 m), and ~6% dissolved and biodegraded between 900 m and 40 m. Subsea dispersant application reduced surfacing oil by ~7% and evaporation of volatiles by ~26%. By July 2011, of the total oil, ~41% evaporated, ~15% was ashore and in nearshore (<10 m) sediments, ~3% was removed by responders, ~38.4% was in the water column (partially degraded; 29% shallower and 9.4% deeper than 40 m), and ~2.6% sedimented in waters >10 m (including 1.5% after August 2010). Volatile and soluble fractions that did not evaporate biodegraded by the end of August 2010, leaving residual oil to disperse and potentially settle. Model estimates were validated by comparison to field observations of floating oil and atmospheric emissions.

Assessing the Toxicity of Individual Aromatic Compounds and Mixtures to American Lobster (Homarus americanus) Larvae Using a Passive Dosing System

Aquatic exposures to aromatic compounds (ACs) may be important contributors to biological effects of oil spills. The present study examined the acute toxicity of 11 ACs and 3 binary AC mixtures on stage 1 American lobster larvae using a passive dosing test design. The ACs investigated covered a range of classes and log octanol-water partition coefficient values (K_OW ; 2.5-5.5). Silicone O-rings were used to partition ACs into seawater and maintain stable exposures. Exposed lobster larvae were assessed for mobility and survival at 3, 6, 12, 24, 36, and 48 h. Fluorometry and gas chromatography-mass spectrometry measurements confirmed well-defined substance exposures. Expressing lethality in terms of chemical activities yielded values between 0.01 and 0.1, consistent with a baseline mode of action. Analysis of time-dependent median lethal/effect concentration (L/EC50) values were used to determine incipient values. An expected linear relationship between the incipient log L/EC50 and log K_OW was fit to the empirical toxicity data to derive critical target lipid body burdens for immobilization and lethality endpoints. These values indicate that American lobster larvae fall on the sensitive end of the acute species sensitivity distribution. We used AC toxicity data to successfully predict toxicity of binary mixtures assuming additive toxicity. The observed time-dependent toxicity was inversely related to log K_OW and occurred more quickly than reported previously. The results contribute to improving models for predicting oil spill impacts on American lobster larvae populations. Environ Toxicol Chem 2021;40:1379-1388. © 2021 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Persistence and photochemical transformation of water soluble constituents from industrial crude oil and natural seep oil in seawater

The persistence and transformation of water soluble chemical constituents derived from surface oil from the 2015 Refugio Oil Spill and from a nearby natural seep were evaluated under simulated sunlight conditions. Photoirradiation resulted in enhanced oil slick dissolution, which was more pronounced in spill oil compared to seep oil. Nontargeted analysis based on GC × GC/TOF-MS revealed that photoirradiation promoted oil slick dissolution, and more water soluble compounds were released from spill oil (500 compounds) than from seep oil (180 compounds), most of them (488 in spill oil and 150 in seep oil) still persisting in solution after 67 days of photoirradiation. First-order degradation rate coefficients of humic-like water soluble constituents were found to be 0.26 day^-1 and 0.29 day^-1 for irradiated spill and seep samples, respectively. The decreases in humic-like fluorescence, specific UV absorbance, and aromatic compounds without corresponding decreases in DOC concentration support indirect photochemical transformation in addition to complete photomineralization.

Molecular Comparison of Solid-Phase Extraction and Liquid/Liquid Extraction of Water-Soluble Petroleum Compounds Produced through Photodegradation and Biodegradation by FT-ICR Mass Spectrometry

We apply two widely used extraction techniques, liquid/liquid extraction and solid-phase extraction with styrene-divinylbenzene polymer with a proprietary nonpolar surface priority pollutant (PPL) to water-soluble compounds generated through photodegradation and biodegradation of petroleum. We compare the molecular composition of bio- and photodegraded water-soluble organic (WSO) acids by 21 T negative-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). We highlight the compositional differences between the two extraction techniques for abiotic and biotic degradation processes and identify known toxic species (naphthenic acids) produced through hydrocarbon biodegradation identified by liquid/liquid extraction (LLE) that are not detected with solid-phase extraction (SPE) of the same sample. Photodegraded WSO compounds extracted by SPE-PPL correspond to species with higher O/C ratio and carbon number compared to LLE extracted compounds. Naphthenic acids, a recalcitrant class of nonaromatic carboxylic acids and known acute toxicants formed through biodegradation of oil, are detected in LLE extracts (up to C₃₀ and double-bond equivalents, DBE < 3) but are not detected in SPE-PPL extracts. This suggests that LLE and SPE-PPL retain different water-soluble oil species based on the dominant type of oil weathering process.

Monitoring chemical compositional changes of simulated spilled Brazilian oils under tropical climate conditions by multiple analytical techniques

To comprehensively understand the chemical changes over time of spilled oils subject to tropical climate conditions and the active weathering processes, a spill simulation experiment was conducted along 210 days with two distinct Brazilian oils (19 and 24 API) under irradiation and non-irradiation of sunlight. Isoprenoids and n-alkanes showed a great loss after 40 days for both oils under the two conditions due to evaporation. Diagnostic ratios of saturated biomarkers showed no changes, whereas the polycyclic aromatic hydrocarbons had a decreasing concentration under both conditions mainly due to evaporation. Furthermore, oxygenated polar compounds produced by photooxidation were investigated by ESI(-) FT-ICR MS and showed changes only for the oils exposed to sunlight irradiation. Based on the observed polar compositional changes, new parameters are suggested using heteroatom classes to estimate oil spill time under tropical conditions: NO₃/NO₂; NO₃/(NO + NO₂); ∑NO_x/N₁; (O₄ + O₃)/(O₂ + O₁); O₄/(O₂ + O₁); and O₃/O₂.

Oil Spill Modeling: A Critical Review on Current Trends, Perspectives, and Challenges

Several oil spill simulation models exist in the literature, which are used worldwide to simulate the evolution of an oil slick created from marine traffic, petroleum production, or other sources. These models may range from simple parametric calculations to advanced, new-generation, operational, three-dimensional numerical models, coupled to meteorological, hydrodynamic, and wave models, forecasting in high-resolution and with high precision the transport and fate of oil. This study presents a review of the transport and oil weathering processes and their parameterization and critically examines eighteen state-of-the-art oil spill models in terms of their capacity (a) to simulate these processes, (b) to consider oil released from surface or submerged sources, (c) to assimilate real-time field data for model initiation and forcing, and (d) to assess uncertainty in the produced predictions. Based on our review, the most common oil weathering processes involved are spreading, advection, diffusion, evaporation, emulsification, and dispersion. The majority of existing oil spill models do not consider significant physical processes, such as oil dissolution, photo-oxidation, biodegradation, and vertical mixing. Moreover, timely response to oil spills is lacking in the new generation of oil spill models. Further improvements in oil spill modeling should emphasize more comprehensive parametrization of oil dissolution, biodegradation, entrainment, and prediction of oil particles size distribution following wave action and well blow outs.

The Complexity of Spills: The Fate of the Deepwater Horizon Oil

The Deepwater Horizon oil spill was the largest, longest-lasting, and deepest oil accident to date in US waters. As oil and natural gas jetted from release points at 1,500-m depth in the northern Gulf of Mexico, entrainment of the surrounding ocean water into a buoyant plume, rich in soluble hydrocarbons and dispersed microdroplets of oil, created a deep (1,000-m) intrusion layer. Larger droplets of liquid oil rose to the surface, forming a slick of mostly insoluble, hydrocarbon-type compounds. A variety of physical, chemical, and biological mechanisms helped to transform, remove, and redisperse the oil and gas that was released. Biodegradation removed up to 60% of the oil in the intrusion layer but was less efficient in the surface slick, due to nutrient limitation. Photochemical processes altered up to 50% (by mass) of the floating oil. The surface oil expression changed daily due to wind and currents, whereas the intrusion layer flowed southwestward. A portion of the weathered surface oil stranded along shorelines. Oil from both surface and intrusion layers were deposited onto the seafloor via sinking marine oil snow. The biodegradation rates of stranded or sedimented oil were low, with resuspension and redistribution transiently increasing biodegradation. The subsequent research efforts increased our understanding of the fate of spilled oil immensely, with novel insights focusing on the importance of photooxidation, the microbial communities driving biodegradation, and the formation of marine oil snow that transports oil to the seafloor.

Identification of persistent oil residues in Prince William Sound, Alaska using rapid spectroscopic techniques

Spectroscopic techniques including X-ray fluorescence (XRF) and attenuated total reflectance - Fourier transform infrared spectroscopy (ATR-FTIR) are used to examine oil residues persisting on shorelines in Prince William Sound that originate from the 1989 Exxon Valdez oil spill and oil released as a consequence of the 1964 Great Alaska earthquake. When coupled to classification models, ATR-FTIR and XRF spectral data can be used to distinguish between the two sources of oil with 92% and 86% success rates for the two techniques respectively. Models indicate that the ATR-FTIR data used to determine oil source includes the CO stretch, the twisting-scissoring of the CH₂ group, and the CC stretch. For XRF data, decision tree models primarily utilize the abundance of nickel and zinc present in the oil as a means to classify source. This approach highlights the utility of rapid, field-based spectroscopic techniques to distinguish different inputs of oil to coastal environments.

Biosurfactant electrospun nanofibers exhibit minimal side effects on the structure and function of the liver tissue in male rat model

Oil spills can result in significant damage to marine estuaries, rivers, lakes, wetlands, and shorelines. Electrospun nanofibers containing biosurfactant (ENFs) can be used to clean oil spills up and protect the environmental biology. Present work aimed to study the side-effects of prepared nanofibers on animal models. Screening of the prepared ECNFs on animals showed that three of them (PVA-5, PEO-1, and PEO-5) are safe to hepatic tissues and liver functions. Furthermore, oxidative stress did not change after using these nanofibers. The PVA-1 nanofibers, however, were found to cause major pathological changes in the liver tissue. In addition, PVA-1 nanofibers were proved to alter the total white blood count and the neutrophil percentages significantly in comparison to the control. In conclusion, PVA-5, PEO-1, and PEO-5 are safe to hepatic tissues and liver functions.

Progress in Operational Modeling in Support of Oil Spill Response

Following the 2010 Deepwater Horizon accident of a massive blow-out in the Gulf of Mexico, scientists from government, industry, and academia collaborated to advance oil spill modeling and share best practices in model algorithms, parameterizations, and application protocols. This synergy was greatly enhanced by research funded under the Gulf of Mexico Research Initiative (GoMRI), a 10-year enterprise that allowed unprecedented collection of observations and data products, novel experiments, and international collaborations that focused on the Gulf of Mexico, but resulted in the generation of scientific findings and tools of broader value. Operational oil spill modeling greatly benefited from research during the GoMRI decade. This paper provides a comprehensive synthesis of the related scientific advances, remaining challenges, and future outlook. Two main modeling components are discussed: Ocean circulation and oil spill models, to provide details on all attributes that contribute to the success and limitations of the integrated oil spill forecasts. These forecasts are discussed in tandem with uncertainty factors and methods to mitigate them. The paper focuses on operational aspects of oil spill modeling and forecasting, including examples of international operational center practices, observational needs, communication protocols, and promising new methodologies.

Role of Molecular Structure in the Production of Water-Soluble Species by Photo-oxidation of Petroleum

Asphaltenes are high-boiling and recalcitrant compounds that are generally minor components of crude oil (∼0.1-15.0 wt %) but dominate the composition of heavily weathered spilled petroleum. These solid residues exhibit a high structural complexity, comprised of polycyclic aromatic hydrocarbons (PAHs) that are a mixture of single-core (island) and multicore (archipelago) structural motifs. The mass fraction of each motif is sample-dependent. Thus, knowledge of a potential structural dependence (single- versus multicore) on the production of water-soluble species from asphaltene samples is key to understanding the contribution of photochemically generated dissolved organic matter from oil spills. In this work, asphaltene samples with enriched mass fractions of either island (single-core) or archipelago (multicore) structural motifs are photo-oxidized on artificial seawater by the use of a solar simulator. Molecular characterization of oil- and water-soluble photoproducts, conducted by Fourier transform ion cyclotron resonance mass spectrometry, reveals that island motifs exhibit very limited production of water-soluble species, and their oil-soluble products reflect the molecular composition of the starting material. Conversely, archipelago motifs yield a water-soluble compositional continuum of O_x, S_xO_y, and N_xO_y containing hydrocarbons species that exhibit the typical molecular fingerprint of dissolved organic matter (DOM). The lower carbon number and aromaticity of the archipelago-derived asphaltene photoproducts suggest the occurrence of photofragmentation (or photolysis) reactions. To investigate the possibility of the opposite reaction (photopolymerization), the photo-oxidation of small PAHs isolated from a low-boiling petroleum distillation cut was also performed. It yielded water-soluble compounds with carbon number and aromaticity up to 2-fold higher than the starting material, strongly suggesting that polymerization (addition reactions) occurs. Collectively, the results indicate that the presence of archipelago motifs and the occurrence of cracking/polymerization reactions are central in the production of dissolved organic matter from fossil fuels.

Characterization of an Asphalt Binder and Photoproducts by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Reveals Abundant Water-Soluble Hydrocarbons

Road asphalt is comprised of aggregate (rocks) mixed with a binder composed of high-boiling petroleum-derived compounds, which have been thought to be relatively inert (unreactive) and thus leach small amounts of polyaromatic hydrocarbons (PAHs) into water from the built environment. However, recent studies have demonstrated that petroleum readily undergoes photooxidation and generates water-soluble oxygen-containing hydrocarbons. Therefore, here, we investigate the effects of solar irradiation on an asphalt binder. Upon irradiation in a photooxidation microcosm, thin films of the asphalt binder produce abundant oil- and water-soluble oxygenated hydrocarbons, which we hypothesize are also leached from roads and highways through photooxidation reactions. Ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) enables extensive compositional characterization of the virgin asphalt binder, irradiated asphalt binder, and the water-soluble photoproducts. The results reveal the production of water-soluble species that resemble the molecular composition of petroleum-derived dissolved organic matter, including abundant hydrocarbons and S-containing species with up to 18 oxygen atoms. The results also confirm photo-induced oxidation, fragmentation, and potentially polymerization as active processes involved in the production of water-soluble organic pollutants from asphalt.

The effects of ultraviolet radiation and climate on oil toxicity to coral reef organisms - A review

Oil pollution remains a significant local threat to shallow tropical coral reef environments, but the environmental conditions typical of coral reefs are rarely considered in oil toxicity testing and risk assessments. Here we review the effects of three environmental co-factors on petroleum oil toxicity towards coral reef organisms, and show that the impacts of oil pollution on coral reef taxa can be exacerbated by environmental conditions commonly encountered in tropical reef environments. Shallow reefs are routinely exposed to high levels of ultraviolet radiation (UVR), which can substantially increase the toxicity of some oil components through phototoxicity. Exposure to UVR represents the most likely and harmful environmental co-factor reviewed here, leading to an average toxicity increase of 7.2-fold across all tests reviewed. The clear relevance of UVR co-exposure and its strong influence on tropical reef oil toxicity highlights the need to account for UVR as a standard practice in future oil toxicity studies. Indeed, quantifying the influence of UVR on toxic thresholds of oil to coral reef species is essential to develop credible oil spill risk models required for oil extraction developments, shipping management and spill responses in the tropics. The few studies available indicate that co-exposure to elevated temperature and low pH, both within the range of current daily and seasonal fluctuations and/or projected under continued climate change, can increase oil toxicity on average by 3.0- and 1.3-fold, respectively. While all three of the reviewed environmental co-factors have the potential to substantially increase the impacts of oil pollution in shallow reef environments, their simultaneous effects have not been investigated. Assessments of the combined effects of oil pollution, UVR, temperature and low pH will become increasingly important to identify realistic hazard thresholds suitable for future risk assessments over the coming century.

How the 2010 Deepwater Horizon spill reshaped our understanding of crude oil photochemical weathering at sea: a past, present, and future perspective

The weathering of crude oil at sea has been researched for nearly half a century. However, there have been relatively few opportunities to validate laboratory-based predictions about the rates, relative importance, and controls of oil weathering processes (e.g., evaporation, photo-oxidation, and emulsification) under natural field conditions. The 2010 Deepwater Horizon (DWH) spill in the Gulf of Mexico provided the oil spill science community with a unique opportunity to evaluate our laboratory-based predictions in nature. With a focus on photochemical weathering, we review what we knew prior to the DWH spill, what we learned from the DWH spill, and what priority gaps in knowledge remain. Three key findings from the DWH spill are discussed. First, the rate and extent of photochemical weathering was much greater for the floating surface oil than expected based on early conceptual models of oil weathering. Second, indirect photochemical processes played a major role in the partial oxidation of the floating surface oil. Third, the extensive and rapid changes to the physical and chemical properties of oil by sunlight may influence oil fate, transport, and the selection of response tools. This review also highlights findings and predictions about photochemical weathering of oil from several decades ago that appear to have escaped the broader scientific narrative and ultimately proved true for the DWH spill. By focusing on these early predictions and synthesizing the numerous findings from the DWH spill, we expect this review will better prepare the oil spill science community to respond to the next big spill in the ocean.

Assessing the prevalence, products, and pathways of dissolved organic matter partial photo-oxidation in arctic surface waters

In sunlit waters, photodegradation of dissolved organic matter (DOM) yields completely oxidized carbon (i.e., CO₂) as well as a suite of partially oxidized compounds formed from oxygen incorporation (i.e., partial photo-oxidation). Of these two groups of DOM photo-products, more studies focus on CO₂ (a greenhouse gas) than on partially oxidized DOM, which is likely a diverse group of compounds with poorly constrained roles in aquatic carbon cycling or biogeochemistry. The objective of this study is to address knowledge gaps on the prevalence, products, and pathways of DOM partial photo-oxidation. Here we traced the photochemical incorporation of isotopically labelled ¹⁸O₂ into DOM isolated from Alaskan Arctic surface waters using high-resolution mass spectrometry. Complete and partial photo-oxidation of DOM was also quantified as CO₂ production and O₂ consumption. The majority of ¹⁸O-containing partial oxidation photo-products were classified as carboxylic rich alicyclic molecules (CRAM) and overlapped in composition with previously reported photo-products known to result from the oxidation of DOM by singlet oxygen. These results support a previously proposed hypothesis that photo-oxidation by singlet oxygen may contribute to the formation of CRAM, a compound class of DOM ubiquitously observed in surface waters. The novel application of an isotopic tracer for oxygen incorporation with a mass balance approach to quantify complete and partial photo-oxidation of DOM revealed that less than one mol of O₂ is required to produce one mol of CO₂. A sensitivity analysis based on this new knowledge demonstrated that the magnitude of DOM partial photo-oxidation may be underestimated by up to four-fold. Consequently, partial photo-oxidation likely plays a more prominent role in shaping DOM composition in sunlit waters of the Arctic than previously understood. Therefore, partial photo-oxidation should be increasingly incorporated into the experimental framework of studies focused on DOM composition in surface waters.

A data-driven framework for defining stages of oil weathering

Oil weathering is often described subjectively after a spill. Adjectives like "moderate" and "severe" help define the extent of oil loss but fail to communicate quantitatively and reproducibly the degree of weathering. The use of subjective weathering terms often leads to misperceptions about persistence and toxicity of oil residues in the environment. The weathering of MC252 oil from the Deepwater Horizon spill started immediately after release during the 1500-m ascent to the sea surface and continued as it was transported on the surface and reached the shoreline. Weathering processes included evaporation, dissolution, photo-degradation, and biodegradation, among others. With extensive sample collection and detailed chemistry and source fingerprinting analyses, the Deepwater Horizon data provide a unique opportunity to evaluate weathering processes semi-quantitatively. An objective method of defining the degree of oil weathering is developed based on the detailed chemical results for over 700 MC252 oil samples from the environment.

Environmental significance of lubricant oil: A systematic study of photooxidation and its consequences

Lubricant (lube) oil discharge from ships has been widely considered as normal "operational consumption", but is now deemed to be oil pollution. Despite the chronic contamination of the marine environment by lube oil, the number of studies related to its environmental impact, characteristics, and toxicity is limited. This study is the first attempt to investigate the environmental fate of lube oil subjected to photooxidation using in situ mesocosms. A tiered approach using thin-layer chromatography-flame ionization detection (TLC-FID), Fourier-transform infrared spectroscopy (FTIR) and gas chromatography-mass spectrometry (GC-MS) demonstrated compositional changes in lube oil and the water-soluble fraction (WSF). Total polycyclic aromatic hydrocarbons (ΣPAHs) in lube oil after 96 h of photooxidation were measured at 79.8 and 41 μg/g in the control (Con) and exposure (Exp) groups, respectively. Meanwhile, the ΣPAHs concentration in WSF after 96 h was very low, at 0.25 and 0.45 μg/L in Con and Exp, respectively. FTIR and GC-MS helped identify bond changes and photoproducts in WSF. A wide range of photoproducts, including carboxylic acids, esters, anhydrides, aldehydes and ketones, were identified in WSF. Toxic effects of WSF in both the Con and Exp groups obtained after 96 h of photooxidation were evaluated on olive flounder (Paralichthys olivaceus) embryos. Morphological defects, especially tail fin fold defects, were found to be significantly elevated in both the Con and Exp groups, with marginally higher frequency in Exp. The results of this study demonstrate the need for further research on lube oil weathering, including monitoring over prolonged periods of time.

Fate of diluted bitumen spilled in the coastal waters of British Columbia, Canada

There is public concern about the behaviour of spilled diluted bitumen (dilbit) in marine and estuarine waters. We provide a preliminary assessment of the results of laboratory experiments and models, in the context of environmental conditions in the coastal waters of British Columbia. Most dilbit spilled within this region would likely float at the surface and be transported to shore by winds and currents. Fresh dilbit is too light to sink in coastal waters. Highly weathered dilbit could sink where salinity is less than 14, typically only near river mouths and in the top 1-3 m of fjords after heavy rainfall. Subsurface plumes of weathered dilbit could re-emerge at the surface. Sinking oil-particle aggregates are unlikely to form in coastal waters. However, dilbit could be entrained below the surface by wave mixing during storms and to depths of 150 m by coherent mixing in the Haro Strait tidal convergence zone.

Biodegradation, Photo-oxidation, and Dissolution of Petroleum Compounds in an Arctic Fjord during Summer

Increased economic activity in the Arctic may increase the risk of oil spills. Yet, little is known about the degradation of oil spills by solar radiation and the impact of nutrient limitation on oil biodegradation under Arctic conditions. We deployed adsorbents coated with thin oil films for up to 4 months in a fjord in SW Greenland to simulate and investigate in situ biodegradation and photo-oxidation of dispersed oil droplets. Oil compound depletion by dissolution, biodegradation, and photo-oxidation was untangled by gas chromatography-mass spectrometry-based oil fingerprinting. Biodegradation was limited by low nutrient concentrations, reaching 97% removal of nC_13-26-alkanes only after 112 days. Sequencing of bacterial DNA showed the slow development of a bacterial biofilm on the oil films predominated by the known oil degrading bacteria Oleispira, Alkanindiges and Cycloclasticus. These taxa could be related to biodegradation of shorter-chain (≤C₂₆) alkanes, longer-chain (≥C₁₆) and branched alkanes, and polycyclic aromatic compounds (PACs), respectively. The combination of biodegradation, dissolution, and photo-oxidation depleted most PACs at substantially faster rates than the biodegradation of alkanes. In Arctic fjords during summer, nutrient limitation may severely delay oil biodegradation, but in the photic zone, photolytic transformation of PACs may play an important role.

Molecular-Level Composition and Acute Toxicity of Photosolubilized Petrogenic Carbon

To examine the molecular-level composition and acute toxicity per unit carbon of the petroleum-derived dissolved organic matter (DOM_HC) produced via photo-oxidation, heavy and light oils were irradiated over seawater with simulated sunlight. Increases in dissolved organic carbon concentrations as a function of time were associated with changes in the DOM_HC composition and acute toxicity per unit carbon. Parallel factor analysis showed that the fluorescent dissolved organic matter (FDOM) composition produced from the heavy oil became more blue-shifted over time, while the light oil produced a mixture of blue- and red-shifted components similar to FDOM signatures. Ultrahigh-resolution mass spectrometry reveals that the composition of the DOM_HC produced from both heavy and light oils was initially relatively reduced, with low O/C. With time, the composition of the DOM_HC produced from the heavy oil shifted to unsaturated, high-oxygen compounds, while that produced from the light oil comprised a range of high O/C aliphatic, unsaturated, and aromatic compounds. Microtox assays suggest that the DOM_HC initially produced is the most toxic (62% inhibition); however, after 24 h, a rapid decrease in toxicity decreased linearly to 0% inhibition for the heavy DOM_HC and 12% inhibition for the light DOM_HC at extended exposure periods.

Decomposition of sediment-oil-agglomerates in a Gulf of Mexico sandy beach

Sediment-oil-agglomerates (SOA) are one of the most common forms of contamination impacting shores after a major oil spill; and following the Deepwater Horizon (DWH) accident, large numbers of SOAs were buried in the sandy beaches of the northeastern Gulf of Mexico. SOAs provide a source of toxic oil compounds, and although SOAs can persist for many years, their long-term fate was unknown. Here we report the results of a 3-year in-situ experiment that quantified the degradation of standardized SOAs buried in the upper 50 cm of a North Florida sandy beach. Time series of hydrocarbon mass, carbon content, n-alkanes, PAHs, and fluorescence indicate that the decomposition of golf-ball-size DWH-SOAs embedded in beach sand takes at least 32 years, while SOA degradation without sediment contact would require more than 100 years. SOA alkane and PAH decay rates within the sediment were similar to those at the beach surface. The porous structure of the SOAs kept their cores oxygen-replete. The results reveal that SOAs buried deep in beach sands can be decomposed through relatively rapid aerobic microbial oil degradation in the tidally ventilated permeable beach sand, emphasizing the role of the sandy beach as an aerobic biocatalytical reactor at the land-ocean interface.

Molecular-Level Characterization of Oil-Soluble Ketone/Aldehyde Photo-Oxidation Products by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Reveals Similarity Between Microcosm and Field Samples

We present a solid-phase extraction method followed by derivatization with a charged tag to characterize ketone/aldehyde-containing functionalities (proposed photo-oxidation transformation products) in weathered petroleum by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). A photo-oxidation-only microcosm mimics solar irradiation of crude oil in the environment after an oil spill. A biodegradation-only microcosm enables independent determination as to which of the two weathering processes contributes to the formation of oil-soluble ketone/aldehyde species. Results confirm that photo-oxidation produces ketones/aldehydes in crude oil when exposed to solar radiation in laboratory experiments, whereas biodegraded oil samples do not produce ketone/aldehyde compounds. Field samples collected after different time periods and locations after the Deepwater Horizon oil spill are also shown to contain ketones/aldehydes, and comparison of field and photo-oxidation-only microcosm transformation products reveal remarkable similarity. These results indicate that the photo-oxidation microcosm comprehensively represents ketone/aldehyde-formation products in the field, whereas the biodegradation microcosm does not. Solid-phase extraction coupled with derivatization leads to selective identification of ketone/aldehyde species by MS. Although improved dynamic range and slightly reduced mass spectral complexity is achieved by separation/derivatization, comprehensive molecular characterization still requires mass resolving power and mass accuracy provided by FT-ICR MS.