Towards an Understanding of the Evolution (Fate and Transport) of the 2010 Deepwater Horizon Oil Spill

The Deepwater Horizon (DWH) well blowout at Mississippi Canyon Lease Block 252 (MC252) occurred on April 20, 2010, at approximately 1,500 m water depth and approximately 103 km from Southwest Pass, on the Louisiana coast. Over an 87-day period, until the well was capped on July 15, 2010, large volumes of natural gas and oil having an API gravity of 37 (specific gravity 0.8) (Oil Budget Calculator Science and Engineering Team, 2010) were released into the waters of the Gulf of Mexico (GOM). Reported estimates of the amount of liquid oil released have varied, but the value has been set by the Federal Court at 3.19 million barrels (United States District Court for the Eastern District of Louisiana, 2015). To lessen the ecological impact from surface oiling and prevent the fouling of shorelines, Corexit 9500, a chemical dispersant, was injected into the oil stream at the release point at depth and applied aerially to surface slicks (Kujawinski et al., 2011). The chemical dispersion at depth, combined with the physical dispersion resulting from the force of the gas and oil escaping from the well, resulted in the formation of oil droplets in a range of sizes. Larger droplets of oil rose rapidly to the surface; while smaller neutrally buoyant oil microdroplets (approximately 10–60 microns in diameter) remained in the 1,000–1,200 m-depth range advecting with the water currents (Camilli et al., 2010). The enhanced formation of these small droplets resulting from dispersant application promoted the dissolution of the most soluble components of the oil, and microbial action further degraded the petroleum hydrocarbons in the water column (Atlas and Hazen, 2011; Hazen et al., 2010). While the liquid oil itself, even with evaporative weathering, did not reach a specific gravity allowing it to sink outright, as the non-dissolved oil within the water column moved away from the wellhead, it interacted with particles forming bacterial aggregates known as “marine snow” which resulted in deposition of oil remnants to the surrounding sediments, where it was further degraded. On the ocean surface, the oil was subjected to evaporation and photodegradation of susceptible components, and some fraction reached the shorelines of Louisiana and the other Gulf states.

The overall magnitude and duration of the DWH incident prompted an extensive environmental sampling effort to characterize the fate of the released petroleum (Figure 1). In 2010, the Oil Budget Calculator (OBC; Oil Budget Calculator Science and Engineering Team, 2010) was developed as a tool to estimate the amounts of oil from the Deepwater Horizon oil spill that were transported to various environmental locations. Because the focus of the OBC was on estimating the fraction of the spilled oil that might still be amenable to response actions, the OBC tool was explicitly focused only on aiding the response activities. As stated “(t)he Calculator does not track the final fate of the spilled oil. Instead it estimates oil that may be amenable to response decisions as opposed to oil that is not (e.g., dissolved or evaporated oil)” (OBC, 2010); therefore no attempt was made to estimate the amount of dispersed oil that reached the shoreline or the offshore sediments nor was the fate of oil in the water (e.g., biodegradation) evaluated quantitatively.

In the six years since the OBC was published, a number of researchers have studied the various processes involved in determining the fate of oil from the Deepwater Horizon oil spill (DWHOS). Processes which acted on the oil, such as biodegradation, deposition via marine snow and other processes, and dispersion, are better understood. However, the relative magnitudes of these environmental fates is less clear. Additionally, the spill volume of 3.19 million barrels entering the GOM (United States District Court for the Eastern District of Louisiana, 2015) is lower than initial estimates used in the OBC.

The goal of this paper is to provide an overview of the fate of the oil and to synthesize the available information and apply a framework similar to that of the OBC to develop a more complete conceptual model of the environmental fate of the oil released during the DWHOS. As in the published OBC, only the liquid fraction of the oil and gas release is considered. However, unlike the OBC, the goal of this discussion is not only to consider the recoverable fractions of the oil but also to understand the non-recoverable fractions. While there is some discussion of the progression of the oil with time in this paper (e.g., weathering and biodegradation), a full discussion of individual processes is beyond the scope of this paper. The results presented here describe the extent of oiling from this event, but the reader should bear in mind that the “mere presence of oil will not constitute an injury” (Federal Register, 1996), and the resulting areas of potential exposure described here do not imply that the MC252 oil caused harm in that area and further investigations need to be undertaken to quantify injury (Boehm et al. 2016; Deepwater Horizon Natural Resource Damage Assessment Trustees, 2016).

As with fingerprinting approaches to understanding the fate of DWH oil in sediments (e.g., Murray et al., 2016; Stout et al., 2016), developing a comprehensive picture of where oil went after the DWHOS requires synthesizing multiple lines of evidence, including chemical data for water, sediment and oil samples, observational data for the locations of surface and shoreline oiling, and reports of response activities such as skimming and burning (OBC, 2010).

Water, sediment, and oil samples were collected throughout the GOM from 2010 through 2014 and together show the distribution of petroleum-related compounds in the various media. Details of the sampling programs and analysis methods can be found in published databases and associated documentation (BP Gulf Science Data 2016a,b). In addition to examining trends in concentration and occurrence, the composition of the various chemical fractions was analyzed. To separate dissolution from the process of biodegradation, ratios of compounds with similar solubilities but different biodegradation rates are examined. N-alkane to branched alkane ratios are compared for the saturated oil fraction while phenanthrene to dibenzothiophene ratios are used for the aromatic fraction (Bayona 1986). Additionally, depletion of high-molecular-weight alkanes and high-molecular-weight polycyclic aromatic hydrocarbons (PAHs) are compared to the relatively recalcitrant biomarker hopane. Shoreline oiling was documented via Shoreline Cleanup Assessment Technique (SCAT) surveys and confirmed with additional chemistry data (Challenger and Murray, 2016). In total, SCAT teams surveyed more than 4,300 miles of shoreline in the Gulf. The results of these observations were compiled into a database.

To examine fate and transport, it is more helpful to examine the various components of the oil in the OBC separately, as the potential for degradation or transport is based on both the location and the physical state of the oil. A conceptual model is shown in Figure 2. The basic framework of the OBC was used to estimate the amounts of oil recovered, that which remained in the subsurface, and that which may have reached shorelines (Figure 3a and 3b). These estimated numbers are approximations based on the reported totals described in the OBC for the entire spill period, but they provide some insight on the relative amounts of oil which may have affected the various media. Despite smaller volumes of oil being available for treatment via dispersion resulting in higher dispersant to oil ratios, the percentage dispersed was assumed to remain constant. Overall, the residual “other” category in the OBC is likely to reflect oil available for shoreline stranding, as little biodegradation occurred within the surface oil material (slicks, sheens, etc.; Atlas et al., 2011).

The gas and oil mixture that escaped from the Macondo wellhead into the Gulf of Mexico contained a wide range of chemical compounds. The majority of compounds in the liquid oil were less dense than the surrounding water, so the droplets were positively buoyant, with the largest fraction rising to the surface in approximately four hours (Ross 2010). During the passage through the water column, the more water soluble compounds entered the water phase such that the chemical composition of the oil that reached the surface was different from the oil escaping the wellhead. For example, partitioning of the water soluble PAH compounds occurred that led to ~18% depletion of the PAHs compared to the source oil by the time it reached the surface above the wellhead (Brown, 2011). The larger and more complex chemical compounds such as the sterane and hopane biomarkers have low water solubilities and were able to reach the water surface with relatively little change in their concentration (Brown, 2011).

Despite the skimming, burning, and dispersing of the oil at sea, a proportion of the released oil eventually reached the shore. The oil slicks and sheens rarely travelled directly from the source towards the coast but were circulated around the offshore waters for several days before making landfall. This increased the oil’s exposure to UV light (facilitating photodegradation) and allowed more compounds to evaporate, dissolve, or disperse before reaching the shorelines. The chemical composition of the oil that reached the shore was significantly different from the source oil, as most of the alkanes with a chain length of 16 were absent along with most of the two-ring PAHs. Most of the offshore oil samples near the coast were weathered before landfall, having lost at least 80% of their total PAHs relative to the original oil. Oil that reached the shoreline had been depleted of more than 90% of the total PAHs initially in the oil (Figure 4; Brown, 2011). However, biodegradation had little effect on the oil as it remained on the surface and only accelerated once the oil was stranded on the shorelines (Atlas et al., 2011), with greater extents of biodegradation occurring in marshes compared to sandy beaches.

A subsurface oil layer, an anomalous layer of dissolved oil and oil droplets transported at depth from the wellhead release point, was observed (Camilli et al., 2010). Known popularly as “the plume,” this material was largely transported at a water depth of 1,000–1,300 m to the southwest of the wellhead. It was initially detected as part of shipboard fluorescence monitoring (e.g., Camilli et al., 2010) and was evident in the elevated total PAH (TPAH) concentrations seen in the samples from the 950–1,300 depth range (Boehm et al., 2016). The presence of the insoluble compound hopane and other highly insoluble PAHs in this layer is consistent with the extensive presence of non-dissolved oil, possibly neutrally-buoyant microdroplets. This finding, based on hundreds of observations, contrasts with initial reports that suggested this layer comprised primarily dissolved petroleum components (Camilli et al., 2010) but consistent with the description of the oil transport in the OBC. To confirm the presence of non-dissolved oil, a physical separation method was used in the field on a subset of water samples to help differentiate between dissolved and non-dissolved components in a single water sample (Payne et al., 1999). The filters and filtrates from about 400 samples were analyzed separately to characterize the operationally defined “dissolved” (i.e., constituents which pass through a 0.7 μm glass fiber filer) and non-dissolved (material retained on the glass fiber filter) phases, respectively. Overall, the proportions of non-dissolved (undifferentiated particulates and droplets) and “dissolved” TPAH ranged from >90% dissolved to >90% non-dissolved, indicating a wide range of heterogeneity in the distribution of these phases within the water column. As expected from solubility data, the dissolved phase was enriched in the lighter, more soluble 2-and 3- ring PAHs (light polycyclic aromatic hydrocarbons, LPAH), while the non-dissolved phase was enriched in the heavier 4- and 5- ring PAH (heavy polycyclic aromatic hydrocarbons, HPAH). Figure 5 shows PAH profiles from two samples, both containing approximately 10 ppb TPAH, which illustrate two extremes of dissolved/non-dissolved TPAH distribution.

The extent of the subsurface layer of oil was limited in both time and space. The majority of elevated TPAH concentrations were found within 18.5 km (10 nmi) during the active release, with some low levels of oil-containing water samples found up to 55.6 km (30 nmi) to the southwest. This feature, however, rapidly disappeared after the release from the wellhead was stopped in July 15, 2011 (Boehm et al., 2016). Some researchers have described the extent of the plume as occurring more than 256 miles from the release point (Passow, 2016; Payne, 2015D). This description is not based on direct measurement of MC252 oil or any oil component. Instead, researchers relied on indirect measurements such as oxygen depletion as indicators of the presence of oil. However, this method is inappropriate to track the oil as the oxygen depletion resulting from the presence of oil may persist long after the oil has been removed from a parcel of water, either by sinking or through biodegradation.

The oil within the subsurface plume was actively biodegraded (Hazen, 2010), as evidenced not only by microbial measurements but also by using the diagnostic ratios for alkanes and PAHs (Bayona, 1986). Ratios in water samples were lower farther from the wellhead, consistent with degradation along the flow path (Figure 6). Alkane degradation occurred more quickly than aromatic degradation, consistent with usual patterns of oil biodegradation.

The available sediment chemistry data support that the oil reaching the sediments did so as a result of one primary and two secondary mechanisms. The primary mechanism of oil transport and deposition in offshore sediments was adsorption of sub-surface plume material containing partially degraded oil droplets to marine detritus, followed by sinking of these aggregates to the seafloor. This mechanism was dominant in the areas southwest of the wellhead, where the majority of the subsurface plume was found. Deposition on the seafloor immediately surrounding the wellhead, however, occurred via mixing of physically dispersed oil droplets with heavy synthetic-based drilling muds used during the Top Kill efforts. In addition, the creation and sinking of in situ burn residues as a secondary mechanism of oil transport resulted in spatially patchy deposition resulting in highly localized or minor deposition (Stout et al., 2016). A fourth mechanism involved the sinking of oil which had already reached shorelines and had been remobilized from beaches and marshes via erosion. That mechanism is more related to shoreline oiling and resulted in depositions known as “submerged oil material” or SOMs (Deepwater Horizon Natural Resource Damage Assessment Trustees, 2016). It was limited in extent, affecting only a narrow ribbon immediately adjacent to oiled shores.

Chemical dispersants were used at the wellhead location at about 1,500 m depth to break up large oil masses into small (10–60 μm diameter) droplets in an effort to facilitate oil degradation and prevent shoreline oiling. In addition to promoting degradation, the small droplet size had the effect of decreasing the buoyancy of the oil droplets, which instead of rising to the water surface, maintained their depth and moved with subsurface currents to the south of the wellhead. Promoting transport of oil in the subsurface water column likely not only promoted extensive biodegradation but also contributed to the formation of oiled marine aggregates. Released oil appears to have been transported to offshore sediments through adsorption to suspended particulate matter (Loh et al., 2014). While some of these particles were likely inorganic (e.g., suspended minerals transported from the coastal shelf), a large fraction comprised aggregates of organic material and active microbial communities, some of which have been shown to be able to degrade hydrocarbons (Passow, 2016; Hazen, 2010). When these aggregates settle through the water column, they are collectively called “marine snow” (Passow et al., 2012). While this process occurs naturally in the absence of an oil release and results in deposition of detrital material in sediments throughout the world’s oceans, the carbon source afforded by the oil accelerated this process in the Gulf after the spill in areas where bacteria could access subsurface oil droplets.

For measurable amounts of petroleum hydrocarbons associated with marine snow to have reached the sea floor, the sedimentation rate of the oiled particles needed to be fast enough that natural oil degradation processes were unable to completely remove the oil compounds before deposition. The presence of oil-containing marine snow is consistent with the footprint of the deposited oil within approximately 37 km (20 nmi) of the wellhead (Valentine et al., 2014; Stout, 2016; Murray et al., 2016) and with descriptions of a localized subsurface plume in the water column.

Oil deposition via marine snow over a wider area of the GOM, including areas to the north and east of the wellhead, has been theorized by some. These hypotheses are based in part on early anecdotal visual information from deep submersible dives (S. Joye, media reports), from laboratory experiments indicating the formation of flocculent material during biodegradation studies (e.g., Hazen et al., 2010), from sediment trap data (Passow, 2016), and from data from several cores collected from the DeSoto Canyon area with dark layers allegedly indicative of anomalous sedimentation during the 2010 period (Hollander and coworkers as part of MOSSFA/GoMRI). However, while these small data sets and laboratory observations are intriguing, sediment chemistry data sets do not appear to support the hypothesis of widespread deposition of oil over the geographic areas hypothesized. In 2014, a cooperative cruise between BP and Trustee scientists sought to better define the area of subsurface oil deposition by collecting samples in areas not covered by initial cruises in 2010–2011. Additionally, some locations previously sampled were revisited to look for evidence of hydrocarbon degradation. Multiple chemistry samples collected in DeSoto canyon in 2014 found no evidence of marine snow-related oil deposition in that area (Murray et al., 2016). The location of elevated sediment TPAH concentrations and fingerprinting results support deposition in a limited area due to the prevailing sub-surface current as the major transport mechanism for offshore sediments (Murray et al., 2016; Stout, 2016).

During the Top Kill process, heavy particulates were expelled into the water column during the pumping of synthetic-based drilling muds into the wellhead, leading to a second oil transport pathway to sediments. In this case, these particles mixed with oil in the surrounding water column and precipitated onto sediments within 3 km or less of the wellhead, due to their relatively higher density. Sediment chemistry samples collected from within this area have elevated TPAHs compared to expected background concentrations, and a chemical analysis indicates that these PAHs are similar to that of Macondo oil (Murray et al., 2016). Additionally, chemical indicators of the drilling muds, such as olefins and elevated barium, support the mud transport pathway in this area. It should be noted that the injury resulting from this transport mechanism is spatially restricted to a small area adjacent to the wellhead; therefore, it is considered a secondary transport mechanism of oil to offshore sediments.

Finally, in situ burning of surface oil was conducted as an offshore response activity following the DWH accident. It has been estimated that 5% of the released oil was subjected to burning (U.S. Coast Guard 2011). As the lighter hydrocarbons preferentially burned, the resulting residues formed dense agglomerations that were observed to slowly sink in the water. The identification of burn residues on the sea floor has been made by several researchers (e.g., Shigenaka et al., 2015; Stout, 2016; Murray, 2016). However, these residues are relatively small, highly weathered (due to burning), solid, and readily identifiable through visual observations and their chemical signature. As in the water column, extensive biodegradation is seen within the sediments of the GOM. This is evident in both the concentration and the composition of hydrocarbons in sediment samples (Murray et al., 2016a,b). Evidence for the degradation of highly insoluble, recalcitrant steranes is seen in the fingerprints of the offshore sediments, and overall surface sediment PAH concentrations in 2014 were lower relative to the same locations in 2010–2011 (Murray 2016).

To understand fate and transport of the DWH oil, it helps to subdivide the categories in the OBC into more specific categories (Figure 7). The oil in 2010–2011 can be grouped into three broad categories: 1) oil which was immediately recovered and never entered the GOM, 2) Oil which remained in the subsurface, and 3) oil which was transported to the surface. However, within these groups, the ultimate fate of the oil varied based on the physical and chemical characteristics of the oil and the environment. With the possible exception of oil within the surface slicks, oil throughout the GOM (on the shoreline, in the sediment, and within the water column) showed evidence of biodegradation at rates and to an extent greater than predicted based on previous spills (Atlas and Hazen, 2010; Atlas 2015). Oil which remained in the subsurface water column was eventually degraded, diluted to below detectable concentrations, or deposited to the sediment. On the surface, oil could also be degraded or dispersed (Passow, 2016), but up to 14% of the initial oil released may have persisted to be transported to shorelines. Six hundred forty-two million pounds of oiled debris were removed from the shorelines through response activities (Deepwater Horizon Natural Resource Damage Assessment Trustees, 2016), of which 2% were estimated to be oil, accounting for about 13% of the shoreline oil, while the remainder was susceptible to biodegradation and other weathering processes.

Based on the lack of observed oil on the sea surface after August 3, 2010 and within the water column after early 2011, the remaining DWH oil from 2012 on was limited to the offshore sediments and the shoreline areas in U.S. waters. Before response-related removal and biodegradation, this accounted for approximately 16% of the total oil released (Figure 8), and studies have shown rapid biodegradation in both of these environments; therefore, the quantities of released oil which currently remain in 2016 as oil, as opposed to degraded oil materials (i.e., “petrocarbon”) in any environmental compartment are negligible, with the possible exception of the sediment immediately adjacent to the wellhead.

We thank the many researchers from multiple consulting firms, laboratories, agencies, and institutions who participated in these sample collections and analyses.