Physical and Chemical Characterization of In-Situ Burn Residue Encountered by a Deep-Water Fishery in the Gulf of Mexico

An unprecedented volume of crude oil was burned in-situ during the Deepwater Horizon oil spill response. Much of the Deepwater Horizon surface oil was found to readily ignite, and as a result, a robust open water response program of in-situ burning was initiated eight days after the fire and explosion on the drilling rig, and burns were conducted for over a ten-week period thereafter. In addition to being the first operational implementation of the response technique on open water during a spill, it also figured significantly in estimates of overall oil fate. The Federal Interagency Solutions Group (2010) estimated that 5–6% of the Macondo (also referred to as Mississippi Canyon 252, or MC252, its lease designation) oil released from the well was consumed by the in-situ burn operations. A total of 411 individual burns took place during the active spill response (April 28–July 19). The total amount of oil consumed in the burns was estimated to have been between 220,000 and 310,000 bbl (Allen et al., 2011). Figure 1 shows the locations of in-situ burn operations in the Gulf of Mexico (GoM) relative to the Louisiana coast and the location of the Macondo wellhead.

Oil residues from the Deepwater Horizon incident assumed many different physical and chemical forms, from dark and fluid to brightly-colored, thick emulsions to nearly mineralized tarballs. These different manifestations reflected the range of weathering processes and the physical interactions with the environment to which the oil residues had been subjected to over time. With respect to in-situ burning, Stout and Payne (2016) estimated that the 411 burn events generated between 38,800 to 54,700 bbl of residue—a considerable volume of petroleum-related hydrocarbons introduced into the environment, under any circumstances. Most of the resultant burn residues from these large-scale operations sank in the relatively deep waters of the GoM. While not formally monitored, operational workers and members of the U.S. Coast Guard Atlantic Strike Team did observe changes in the physical characteristics of the Macondo oil once it had been burned: its viscosity and density both appeared to increase. In fact, combustion apparently increased the density of the residual oil to the point where most of the post-burn residue became heavier than seawater, resulting in it slowly sinking (Figure 2).

In late 2010, the deep-water royal red shrimp fishery operating north of the Macondo wellhead and the primary burn zone encountered tarballs at 200 m. Figure 1 shows that the locations where burns were conducted were typically in water depths of hundreds of meters to over a thousand meters. As most commercial fishing activity in the GoM occurs in shallower nearshore waters or in the upper portion of the pelagic water column, the possibility of offshore in-situ burning affecting fisheries seemed remote. However, Figure 1 also shows that the 200 m bathymetric contour borders the burn zone to the north. The slow descent of burn residue through the water column potentially allowed small currents and eddies to move it into shallower areas of the shelf. There, it could be encountered by a deep water fishery in the GoM, that for royal red shrimp. This species represents a niche market among harvested shrimp, accounting for less than one percent of total GoM shrimp landings (Cascorbi, 2007). However, it is prized for its size and flavor.

At the time the tarballs were pulled up in the royal red shrimp trawls, they were tentatively sourced as Deepwater Horizon in-situ burn residues. However, the recovered material was physically and chemically very different from the MC252 oil that flowed from the unsecured wellhead. The tarballs recovered from the royal red shrimp nets in 2010 represented a unique form factor with distinct chemical characteristics, likely as a result of the thermal transformation of the oil in addition to typical weathering processes.

The focus of this paper is the tarballs sampled by royal red shrimp trawl gear along the 200 m bathymetric line (see Figure 1). These shrimp trawl tarballs were one component of a larger in-situ burn residue research project conducted by the authors (Shigenaka et al., 2015). Comprehensive descriptions of the physical and chemical characterizations, and details of the complete research project can be found in the 2015 report. In the current paper, we will describe the physical and chemical characteristics of this unusual form of MC252 oil, compare it to other MC252 oil residues, and describe approaches used for oil source-fingerprinting. Finally, we will discuss broader implications for spill response when in-situ burning is being considered.

Following the November 2010 closure of the royal red shrimp fishery in the GoM due to the trawl encounters with tarballs at depth, NOAA chartered two fishing vessels to sample royal red shrimp in the closed area over three cruises in 2010 and 2011. These tarballs were large, viscous, and dense masses of oily residue encapsulated within a hardened exterior. The tumbling of these tarballs in the trawl nets incorporated shrimp and other trawl debris into the residue (see Figure 3). A total of 26 shrimp trawl tarballs were physically characterized for density and percent asphaltenes, and chemically characterized by gas chromatography/mass spectrometry operated in selected ion monitioring mode (GC/MS-SIM). The GC/MS data was used to qualitatively to determine any characteristic differences present in the normal alkane profiles and to calculate concentrations for a targeted list of polycyclic aromatic hydrocarbons (PAHs) to evaluate if changes in the chemical composition were indicative of in-situ burning. The PAH concentrations were used to calculate a modified Fossil Fuel Pollution Index (FFPI) and the Wang Pyrogenic index for each shrimp trawl tarball. Both of these indices were used to determine if the PAH signatures in the shrimp trawl tarballs was petrogenic or pyrogenic. The last phase of chemical characterization was oil source fingerprinting by diagnostic ratio analysis and chemometric analysis. The oil source fingerprinting component of this research utilized the GC/MS data for four oil biomarker groups: tri- and pentacyclic hopanes, diasteranes and regular steranes, 14β(H)-steranes, and the triaromatic steroids. Peak height ratios were calculated within each of these four oil biomarker groups for all 26 shrimp trawl tarballs and these ratios were statistically compared to the same MC252 ratios. Also, GC/MS peak intensity data for each of the four oil biomarker groups was used for chemometric analysis as an additional fingerprinting approach.

The physical and chemical properties of the trawl-derived tarballs differed from the other oil matrices analyzed during the Deepwater Horizon response. The average density and percent asphaltenes of the shrimp trawl tarballs were significantly different from other MC252 tarballs (p=0.02 and <0.001, respectively). The average density of the shrimp trawl tarballs was 1.04 g/mL and the average percent asphaltenes was 20%—compared to the density of 0.73 g/mL and percent asphaltenes of 2% for other Macondo tarballs. The average density of the shrimp trawl tarballs was not significantly higher than the average density of in-situ burn samples collected after burning (p=0.27); however, the asphaltenes content was significantly higher (p=0.02) than the percent asphaltenes of the in-situ burn samples collected during burn operations. Figure 4 displays the average densities (left) and the average % asphaltenes for the shrimp trawl tarballs, MC252 in-situ burn residues after burning, and other MC252 tarballs.

The normal alkane profile of the shrimp trawl tarballs (Figure 5a) was uniquely different from other Macondo oil residues (e.g., Figures 5b and 5d). This is not unusual, and in fact the differences in features within chromatograms of different oil samples is frequently used by forensic chemists to denote differences in degree and type of weathering. Figure 5d, for example, with its larger “hump,” or unresolved complex mixture (UCM), is typical of an oil sample that has been extensively weathered. The shrimp trawl tarball profile (Figure 5a) has a UCM peak around n-C₂₉, which was also noted by Stout and Payne (2016) in their analyses of Deepwater Horizon in-situ burn samples. Stout and Payne also analyzed a putative in-situ burn residue sample recovered by a remotely operated vehicle at 1430 m in the Gulf of Mexico, and it showed a similar UCM feature. Review of the chromatograms from all 26 shrimp trawl tarballs shows that most, but not all, samples have a UCM peak comparable to that in Figure 5a. We suggest that the characteristic chromatographic UCM profile may be at least partially diagnostic for Deepwater Horizon in-situ burn residues recovered in the environment.

Polycyclic aromatic hydrocarbons are a relatively smaller proportion of crude oil mixtures. Regardless of their proportions in crude oil, PAHs are underscored by toxicity and are frequently the focus of chemical analyses and toxicological assessments. A primary goal of spill response and remediation is to reduce the overall loading of oil in the environment. In discussing the viability and implementation of in-situ burning as a response method, Garrett et al. (2000) and Wang et al. (1999) noted that burning spilled oil reduces the total amounts of PAHs in the environment, although shifts and even increases in some specific PAHs do occur. The combustion of oil changes the distribution of aromatic hydrocarbons in such a way that chemists can use the shifts to document that, in fact, burning occurred, as well as to assess the origin of environmental PAH residues—i.e., are they derived largely from oil or from the burning of organic matter (i.e., fossil fuel combustion, industrial plants, forest fires).

Deepwater Horizon in-situ burn results from May and July, 2010, reflect this shift. Total targeted PAHs decreased by 50% in MC252 oil samples collected pre- and post-burn (total average PAHs before burning = 5315 ± 902 mg/kg, n=8; total average PAHs after burning = 2631 ± 1913 mg/kg, n=10). In addition to the overall reduction of PAHs, there were also statistically significant concomitant increases in total concentrations of benzo[b] and [k] fluoranthene, benzo[e] and [a] pyrene, perylene, indeno[1,2,3-cd]pyrene, dibenz[a,h]anthracene, and benzo[g,h,i]perylene (i.e., enhancement of the less-volatile aromatics that are also considered as pyrogenic PAHs). With the exception of indeno[1,2,3-cd]pyrene, these compounds were already present in the Macondo oil; however, their concentrations increased after in-situ burning.

The Fossil Fuel Pollution Index (FFPI) (Boehm and Farrington, 1984) and the Wang pyrogenic index (Wang et al., 1999) utilize the combustion-related chemical changes to assess percent contribution of petrogenic and pyrogenic compounds to a mix of hydrocarbons. FFPI and Wang were both calculated for the shrimp trawl tarballs and compared to other MC252 oil residues to determine if the PAH signatures of in-situ burn residues changed from a petrogenic signature to a more pyrogenic signature. The FFPI was modified by LSU-RCAT to incorporate an expanded list of target aromatic compounds. A modified FFPI value closer to one (1.0) is defined to represent petrogenic/oil-derived PAHs, while a value less than 0.6 represents pyrogenic/combustion-derived PAHs. The Wang pyrogenic index was developed after characterizing PAHs in burn residues and soot samples from experimental in-situ burning of diesel fuel (Wang et al., 1999). The Wang pyrogenic index was modified for this study since biphenyl, acenaphthylene, and acenaphthene were not included in the project target compound list. Wang et al. (1999) calculated their pyrogenic index for diesel to be 0.004, between 0.009 and 0.019 for experimental residues, and greater than 0.08 for soot samples. They interpreted a higher index value to represent a pyrogenic source, a lower index value, a petrogenic source.

Results for before and after in-situ burning of Macondo oil (Table 1) are consistent with expected trends for both FFPI and the Wang index. That is, the average FFPI of the before in-situ burning samples was 0.77 and the average FFPI of the after in-situ burning samples decreased to 0.65. This difference was significant, reflecting the expected shift to more pyrogenic profiles. The shrimp trawl tarball results were roughly equivalent to or lower than post-burn FFPI values obtained for the in-situ burn residues. The pre-burn/post-burn Wang index values showed the expected increase (from 0.01 to 0.07, and 0.01 to 0.05), with the average Wang index result for the shrimp trawl tarballs (0.06) about the same as the values obtained for post-burn samples.

Table 2 summarizes a comparison of the diagnostic ratio results of the shrimp trawl tarballs, Macondo source oil, and EPA South Louisiana Crude (SLC) oil standard. All 26 shrimp trawl tarballs were a match (scored 15 out of 15, or 100%) to MC252 oil based on the diagnostic biomarker ratio analyses. The EPA-SLC standard scored 8 out of 15, or 53%, and would be considered a non-match to the other two sample types. The ability to discriminate between the two ostensibly similar Louisiana crude oils, while at the same time being able to link a chemically altered burn residue to its source oil, is a demonstration of the utility of diagnostic ratio analyses employed for this study

Chemometrics is an exploratory data analysis technique that recognizes patterns using multivariate pattern recognition algorithms and classifies samples into related groupings, often termed tribes and families (Peters et al., 2005; Peters et al., 2007; Peters et al., 2008; Lorenson et al., 2011; Peters et al., 2013). The benefits of chemometrics are that there are no assumptions about the distribution of data, and large amounts of data can be quickly processed to understand natural groupings present in the data set. The two most common chemometric approaches are hierarchical cluster analysis (HCA) and principal component analysis (PCA).

In this study, chemometric analysis was performed using peak intensity data extracted from the GC/MS analyses of MC252 source oil analyzed with each batch of samples, surface oil samples collected from before and after in-situ burning, and for eight of the sub-surface shrimp trawl tarballs suspected to be ISB residues. The samples undergoing chemometric analysis were analyzed within a similar time frame and with comparable instrumental conditions. Peak intensity data points for other sub-surface tarball samples did not line-up, preventing them from being included in this analysis.

The chemometric analysis separated surface residues from sub-surface residues based on peak intensity data extracted from the hopanes. Hopane peak intensity data was used because the hopanes were the most discriminating diagnostic biomarker ratios for differentiating MC252 oil from other south Louisiana crude oils (Meyer, 2016). HCA resulted in three families (i.e., clusters) which are shown in the corresponding 2-D PCA plot in Figure 6. The sub-surface samples, (i.e., the black cluster), are on the opposite side of the PCA plot than the samples collected on the surface (i.e., the blue and green clusters) which indicates that they are different from these clusters. HCA and PCA analysis even revealed a distinction, despite some overlap, between before and after in-situ burning. This overlap is logical because not all the oil being burned is going to be consumed and altered at the same rate. The spreading nature of these two clusters also indicates an increase in variation among these surface samples, also an artifact of differing burn conditions. The sub-surface samples, on the other hand, are a much tighter cluster indicating less variation among these samples. All three clusters have MC252 source oil within them. The apparent separation in Figure 6 further corroborates that the sub-surface trawl tarballs are related to MC252 oil, yet are different from other residues related to the Deepwater Horizon ISB operations.

The scale and duration of the in-situ burn operations conducted during the Deepwater Horizon response provided an opportunity to learn a great deal about the changes that occur to crude oil when it is burned as a remedial option. One of the key objectives of this study was to determine if the tarballs recovered from the royal red shrimp trawls could be identified as Macondo oil, and the analysis of biomarker ratios supported this link. Comparison of the results for pre- and post-in-situ burn oil samples also showed that the diagnostic biomarkers maintained their integrity, and thus, their utility, in the face of the very high temperatures generated by large volumes of oil burning on the surface of the water.

Another objective was to determine if the shrimp trawl tarballs could be linked to combustion; that is, might other weathering or degradation processes have resulted in the substantial physical and chemical changes distinguishing the semi-solid, dense oil residue recovered from 200 m depth from the Macondo source oil? Analysis of Deepwater Horizon surface post-burn samples showed changes in both density and asphaltenes content that were consistent with the measured parameters of the trawl-recovered material—although not to the extent found in the shrimp trawl tarballs. In addition, the shrimp tarball chemistry showed a hydrocarbon distribution characterizing the shifts that occurred in known in-situ burn samples. This was reflected in both chromatographic profiles as well as pyrogenic indices to determine the influence of combustion-generated PAHs in hydrocarbon mixes.

The Deepwater Horizon spill experience validated the operational feasibility and practical application of in-situ burning as an open water response method. The scale of in-situ burn operations was unprecedented, with the amount of oil consumed roughly equivalent to the total volume for the Exxon Valdez oil spill in 1989. Using the standard in-situ burn efficiency estimates of 90–95 percent, the amount of residue resulting from the operations potentially ranged from 11,000–33,000 bbl. Stout and Payne (2016) used a lower efficiency assumption and estimated 38,800–54,700 bbl of residues.

The fate of the burn residue from the Deepwater Horizon in-situ burn operations might never have been known, were it not for the chance oil residue encounters of the deep water fishery operating in the same general area as most of the burn operations and ROVs assessing the impact of the spill (i.e., Stout and Payne, 2016). Even though the impacts to the royal red shrimp fishery were minimal (i.e., federal fisheries managers and seafood safety specialists quickly assessed and then re-opened the fishery), these residue and fishery interactions represent a cautionary footnote for large-scale in-situ burn operations that should be considered in planning, and factored into tradeoff analyses during future spill responses. Questions that the spill response community could consider in advance of the next incident include: Can modeling of physical processes be used to predict potential movements of sinking residues? Are water column or benthic resources at risk from exposure to residues? Are in-situ burn residues toxic to exposed organisms, either in the short- or long-term? Are in-situ burn residue containment or recovery efforts feasible? If so, what are the options?

Funding for this project was provided by the U.S. Department of the Interior, Bureau of Safety and Environmental Enforcement, under Interagency Agreement E13PG00001. Alan A. Allen and Neré Mabile, who directed the burn operations during Deepwater Horizon, provided some of the field in-situ burn samples for this study. Reference to any specific commercial products, process, or service by trade name, trademark, manufacturer, or otherwise, does not constitute or imply its endorsement, recommendation, or favoring by the United States Government. The views and opinions of authors expressed do not necessarily state or reflect those of the United States Government, and shall not be used for advertising or product endorsement purposes.