Distribution and Attenuation of Polycyclic Aromatic Hydrocarbons in

Article pubs.acs.org/est

Distribution and Attenuation of Polycyclic Aromatic Hydrocarbons in Gulf of Mexico Seawater from the Deepwater Horizon Oil Accident Paul D. Boehm,* Karen J. Murray, and Linda L. Cook Exponent, Inc., 1 Clock Tower Place, Suite 150, Maynard, Massachusetts 01754, United States S Supporting Information *

ABSTRACT: The extended duration of the oil release from the Deepwater Horizon accident (April 20−July 15, 2010) triggered a need to characterize environmental exposures in four dimensions through sampling and tracking the changes in distributions, concentrations, and compositions of oil and total polycyclic aromatic hydrocarbons (TPAH) in the Gulf of Mexico over time and space. More than 11 000 water samples were collected offshore during more than 100 cruises and were measured for 50 parent and alkylated polycyclic aromatic hydrocarbons (PAHs). Elevated concentrations (greater than 1 ppb) of TPAH were largely limited to an area within about 20 km of the wellhead in the subsurface deepwaters at 1000−1200 m depth to the southwest of the wellhead and in the top 3 m underlying the surface oil. Concentrations decreased with distance and time, and changes in the PAH composition indicate that these changes were due to differential solubilization, photodegradation, evaporation, and/or biodegradation of individual PAH compounds. These limited areas of elevated PAH concentrations disappeared within weeks after the release was stopped.

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INTRODUCTION

subjected to additional losses due to evaporation and photodegradation.12 The extended duration of the release (April 20−July 15) drove a need and presented an opportunity to measure environmental exposures of marine resources to released petroleum and petroleum constituents and, at the same time, provided a unique situation to capture samples during an active release and to track the changes in distributions, concentrations, and compositions over time and space. What resulted was an unprecedented sampling, analysis, and data interpretation effort involving many organizations and individuals and more than 100 individual cruises.13 The development of a fourdimensional (area × depth × time) understanding of the chemical distributions became one focal point of the interpretation of data from these monitoring efforts. At the same time, investigations of deep sea “plumes”10,14 and of areas in the water column with the highest concentrations2 drove many sampling efforts. While previous studies and publications on water chemistry of oil spills involved more limited sampling, the data presented here are part of a large, multifaceted oceanographic sampling effort, with more than 11 000 water samples collected, beginning on May 5, 2010, and continuing through the end of 2010. One outcome of these sampling efforts during the DWH release was the reporting of a

On April 20, 2010, an explosion at the Deepwater Horizon (DWH) drilling platform resulted in the subsequent release of a large volume of light crude oil (API gravity 37; specific gravity 0.8)1 from the wellhead in the Gulf of Mexico (GOM) 103 km southeast of Southwest Pass, Louisiana, USA.2 From that day, until the well was completely capped on July 15, 2010, petroleum was continuously released into this deepwater environment. Estimates of the overall magnitude of the release vary and several estimates have been reported in the literature and determined by the court.3−5 The release took place at a depth of about 1500 m with larger droplets of oil rapidly rising to the surface, while finer oil droplets became effectively neutrally buoyant due to their small size and remained at a depth of approximately 1000−1200 m advecting with the water currents.6 In an effort to mitigate fouling of shorelines along the GOM by the oil, the chemical dispersant COREXIT 9500 was injected into the oil stream at the release point and was also applied to surface slicks via aerial and surface application.7,8 Observations during periods of subsurface dispersant application and periods without dispersant noted a decrease in the surfaced oil above the wellhead with dispersant use.9 In the water, the released oil underwent a series of chemical and physical changes. Chemical and physical dispersion of the oil into droplets promoted the dissolution of the most soluble components from the oil phase to the water.10 Microbial activity further metabolized the hydrocarbons through degradation.9,11 Oil that reached the sea surface was then © XXXX American Chemical Society

Received: July 27, 2015 Revised: September 16, 2015 Accepted: September 25, 2015

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DOI: 10.1021/acs.est.5b03616 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

Figure 1. Location of the offshore water sampling stations during and after the DWH accident during 2010. The black triangle represents the location of the MC252 wellhead. Differently shaded backgrounds show the division of the samples into four geographical quadrants: NW, NE, SE, and SW.

time frame in 2010 when oil was released and/or when surface oil was observed (i.e., the release period). We report here on data collected from May 5 through August 3, 2010, the last date with documented recoverable oil on the surface (the “release period”), and from August 4 through December 2010 (the “post-release period”).19 Though multiple chemical parameters were measured, emphasis is placed here on examining the hydrocarbon levels found at depth southwest of the wellhead, with particular attention to PAHs, which are a suite of aromatic chemicals with varying toxicity and chemical properties that are of specific interest in injury assessment investigations. These compounds comprise approximately 1.4% of the total source oil released from the DWH (Table S1) although PAH content of Macondo oil has previously been reported at concentrations up to 4% using a different measurement basis.10

hydrocarbon maximum at a depth of 1000−1200 m to the southwest of the wellhead, sometimes referred to as the southwest or subsurface plume.2,6,10,14 Although most oil spill studies involve some element of water column sampling, there have been few historical oil spills for which comparable large, comprehensive data sets were obtained. Other large water column chemistry investigations were conducted for the Ixtoc 1 in the GOM in 1979 and the Exxon Valdez in Prince William Sound off Alaska in 1989.15,16 The water column has also been investigated in a more limited extent as part of the North Cape spill off the coast of Rhode Island in 1996.17 Data collected during these events have shown that chemicals associated with the spills, such as polycyclic aromatic hydrocarbons (PAHs), reach maximum water concentrations early on during the release and decrease rapidly with time and distance from the release point and that maximum concentrations are kinetically determined and never approach equilibrium (i.e., solubility) concentrations.15,16 Such “ephemeral data” are not always obtainable due to the unpredictable and often short duration of petroleum spills, but they are important to truly characterize the extent and magnitude of the oil exposure.18 In cases where the empirical ephemeral data are absent, models are used to fill in the gaps. However, due to the many variables involved in modeling, such as the release location, release rates, release conditions, oil type, and oceanography, the models require extensive calibration and knowledge of the specific oceanographic system. The objective of this paper is to examine the overall DWH water data set for trends relating to the distribution of PAHs during and after the

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MATERIALS AND METHODS Sampling. Water samples were collected during more than 100 discrete cruises during the release and postrelease periods as part of the collaborative response and natural resource damage assessment (NRDA) efforts undertaken by government agencies, academic researchers, and BP. Different study plans and their cruises were designed with different missions, resulting in varied approaches in choosing sample station locations and depths. Some cruises used a “fixed station sampling strategy,” while others focused on tracking of the oil. In the fixed sampling, water was collected at a predetermined series of geographical locations and depths. As part of these cruises, water column profiling using a conductivity, temperB


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Quality Assurance Plan (AQAP).22,24 Possible degradation of chemical concentrations in sample bottles, after collection, but prior to extraction, was examined by several methods including comparison of refrigerated samples, per the NRDA Quality Assurance Plan, with samples filtered and/or frozen in the field and comparison of acid-preserved samples with refrigerated samples. Results showed that any degradation of PAHs in samples after collection was minimal. Comparable concentrations in data sets, including refrigerated and additionally preserved samples, were determined to be well within any sampling variability itself. In some cases, additional parameters such as dispersant chemical markers, volatile hydrocarbons, and biomarkers were analyzed as part of the analytical chemistry component of this sampling, but those results are not discussed here. When hopane biomarker data were available, they were used along with other largely insoluble compounds as indicators of the presence of a separate oil droplet phase, due to the insolubility and relative recalcitrance of hopane on short time scales.23,25 Data Sources, Quality Control, and Validation. Chemical data used in this analysis were obtained from the publically available water data posted on BP’s Gulf Science Data Web site (gulfsciencedata.bp.com). Prior to being publically posted, these data were validated by independent data validators, using procedures consistent with U.S. Environmental Protection Agency National Functional Guidelines for Data Review26 procedures and the analytical control criteria found in the AQAP24 (specifically in Table 6.1a contained therein). After formal validation, which focused on adherence to analytical QA/QC specifications per the AQAP, data were further reviewed for scientific accuracy and completeness by data analysts and data users familiar with both the analytical, petroleum geochemical, and environmental aspects of these types of data. Data that were rejected by the data validators based on lack of adherence to analytical quality control criteria or by data users based on unverifiable positional or other information were excluded from the data posting and our analysis. More details on the specifics of the included samples, analyses, and validation procedures can be found in the Data Posting Summary Report included in the data package of the Gulf Science Data Web site. The posted water data set includes water data from both federal and state waters. For the purpose of this analysis, only samples within federal waters were considered. Additionally, the samples were limited to those collected in 2010 and categorized as from the “release period” or “post-release period” as previously defined. To understand the spatial distribution, samples were sometimes further defined by the geographical quadrant from which they were collected, relative to the wellhead (Figure 1).

ature, and depth (CTD)-dissolved oxygen (DO)-fluorescence array was used to characterize the water column and samples were collected at multiple depths, surface to near-bottom, including the depth of any fluorescence maximum at each station. On the oil tracking cruises designed to locate and track the oil and to determine the highest concentrations, fluorescence measurements were also used to determine locations of detectable hydrocarbons, which were then specifically sampled for chemical analysis. Several different types of fluorescence sensors were used during the oil tracking cruises and were set at various wavelengths within the crude oil absorption and emission spectra range. Sensitivity levels for these instruments varied by up to 2 orders of magnitude, based on manufacturer’s information. A final group of samples was collected as part of the effort to monitor the surface of the water column prior to and after the application of chemical dispersants from the sea surface, with surface samples collected from a single oil-slicked water parcel before and after aerial dispersant application. The net result of all of these cruises is a sample set which is represented by several complementary sampling strategies, sampling over time and distance from the wellhead, and comprised more than 10 100 discrete water samples and can be examined in its entirety. Sampling occurred on 83 of 90 days during the May 5 through August 3 time frame. The sampling locations for all of the data are presented in Figure 1. Samples collected on different surveys were collected using similar, written protocols based on standard oceanographic methods and written study plans.20 Offshore water column samples were collected by teams of scientists comprised of government, academic, and BP researchers, using documented sampling and decontamination procedures to minimize potential shipboard contamination from equipment and from surface oiling. Rosettes with CTD sensors were submerged with either Go-Flo or Niskin bottles, which were triggered to sample at various depths. Go-Flo bottles are submerged closed, opened and closed at depth, while Niskin bottles are submerged open and then closed at depth. Sampling protocols were designed to avoid contamination of the sampling apparatus by surface oil slicks. For the bulk of the samples, one liter water samples were transferred directly from the Niskin or Go-Flo bottles into precleaned glass containers with Teflon-lined lids. The bottles were labeled and refrigerated before being shipped to analytical laboratories under chain of custody. About 25% of the samples was preserved with hydrochloric acid in the field. The time between sample collection and extraction was generally between 4 and 7 days for samples that were not acid preserved and up to 14 days for acid preserved samples. Analytical Chemistry. Several analytical laboratories performed the analyses for total hydrocarbon and PAHs. Protocols and target analytes were largely consistent between laboratories, and both parent and alkylated PAHs (2- to 6ringed parent PAHs and alkylated homologues of naphthalene, fluorene, phenanthrene, dibenzothiophene, fluoranthene/pyrene, and chrysene)21,22 were measured using gas chromatography−mass spectrometry (GC/MS). GC/MS response factors for alkylated PAHs were based on those of the parent PAHs.16,21−23 Analytical detection limits were generally in the range of 1−5 ng/L (0.001−0.005 parts per billion = ppb). These limits varied because of differences in sample preparation factors and interlaboratory variability. Samples were stored, extracted, and analyzed according to published methods and those compiled in the Deepwater Horizon NRDA Analytical

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RESULTS AND DISCUSSION Results from this large data set allow for a comprehensive analysis of how the oil and its components were distributed in space and time and of many of the weathering processes at work. The focus of this work is limited to the PAH compounds. The total PAH (TPAH) was calculated as the sum of 50 parent and alkyl PAHs (Table S3) measured in each sample. For this analysis, the data set includes a total of 11 407 offshore (defined as in Federal waters: 3 nm off Louisiana, Mississippi, and Alabama, 9 nm off Florida and Texas) water column samples collected on the various cruises and measured for PAHs. Approximately 39% of the samples had no detectable TPAH. C


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Table 1. Summary of TPAH Concentrations (μg/L = ppb) and Distributions During the Release Period (May−Aug 3)a concentration distribution (%)

depth

distance

quadrant

count

geomean (ppb)

median (ppb)

range (ppb)

ND−0.1 ppb

0.1−1 ppb

1−10 ppb

0−1 m 1−10 m 10−200 m 200−950 m 950−1300 m 1300+ m

609 712 1067 1102 1455 463

0.087 0.091 0.011 0.011 0.027 0.012

0−1 miles (0−1.9 km) 1−5 miles (1.9−9.3 km) 5−10 miles (9.3−18.5 km) 10−20 miles (18.5−37 km) 20+ miles (37+ km)

370 2422 859 448

10−100 ppb

>100 ppb

0.069 0.102 0.012 0.013 0.024 0.013

ND−105 000 ND−44.9 ND−8.93 ND−3.09 ND−73.7 ND−151 000

60 50 86 84 68 87

21 29 12 15 20 10

15 20 2 1 8 2

3 2

0.076 0.029 0.020 0.014

0.062 0.032 0.020 0.006

ND−151 000 ND−6370 ND−44.0 ND−105 000

57 69 74 76

25 20 18 13

11 9 7 8

5 2 2 2

1309

0.015

0.015

ND−95.3

84

13

3

0.4

NE NW SE SW

1057 1576 736 2039

0.021 0.033 0.015 0.022

0.017 0.037 0.014 0.020

ND−151 000 ND−105 000 ND−42.9 ND−53.8

74 69 78 73

18 21 15 17

6 8 6 7

1 2 1 2

1 0.4

all samples

5408

0.023

0.023

ND−151 000

73

18

7

2

0.3

1

4 2 3 0.2 1

a

TPAH is the sum of 50 parent and alkylated PAH compounds as defined in Table S3. Samples are represented in each category (depth, distance, and quadrant), and there are 5408 samples in each category. For calculation of the geomean, TPAH nondetects (ND) are set to 0.001 ppb.

Table 2. Summary of TPAH Concentrations and Distributions (μg/L = ppb) after the Release Period (Aug 4−December 2010)a concentration distribution (%)

depth

distance

quadrant

count

geomean (ppb)

median (ppb)

range (ppb)

ND−0.1 ppb

0.1−1 ppb

1−10 ppb

10−100 ppb

0−1 m 1−10 m 10−200 m 200−950 m 950−1300 m 1300+ m

773 655 1133 1181 1543 714

0.007 0.004 0.004 0.004 0.003 0.003

0.007 ND ND ND ND 0.0003

ND−6.68 ND−17.9 ND−11.9 ND−9.25 ND−6.23 ND−23.2

91 95 95 97 97 97

8 3 5 3 2 2

1 1 1 0.3 0.5 1

0−1 miles (0−1.9 km) 1−5 miles (1.9−9.3 km) 5−10 miles (9.3−18.5 km) 10−20 miles (18.5−37 km) 20+ miles (37+ km)

30 357 359 455 4798

0.014 0.008 0.004 0.004 0.003

0.018 0.009 0.001 0.001 ND

ND−0.090 ND−6.76 ND−11.9 ND−9.25 ND−23.2

100 93 97 95 96

6 2 3 3

1 2 1

NE NW SE SW

533 850 692 3924

0.005 0.009 0.004 0.003

0.002 0.011 0.002 ND

ND−23.2 ND−17.9 ND−7.20 ND−6.23

93 89 97 97

5 9 2 2

1 1 1 0.6

0.6 0.1

all samples

5999

0.004

ND

ND−23.2

96

4

1

0.07

>100 ppb

0.3 0.1

0.1

0.3 0.06

a

TPAH is the sum of 50 parent and alkylated PAH compounds as defined in Table S3. Samples are represented in each category (depth, distance, and quadrant), and there are 5999 samples in each category. For calculation of the geomean, TPAH nondetects (ND) are set to 0.001 ppb.

In the entire data set, only 18 samples (or less than 0.2% of the samples) exceeded TPAH concentrations of 100 ppb, with the highest concentration measured at 151 000 ppb at the wellhead itself at 1524 m depth in the midst of the active release. The maximum TPAH concentration (189 ppb) observed by Diercks et al.2 was lower and also very close to the wellhead, indicating the high variability in concentrations in the immediate proximity of the release.2 These high TPAH samples were all measured during the early portion of the

Tables 1 and 2 present a summary of these results broken down by depth, distance, and spatial quadrant relative to the wellhead, during and after the release. The geometric mean (geomean) is used to compare the TPAH concentrations because the geomean is a robust estimate of the central tendency of a skewed data set.27 Median values of the measured data are also presented. When geomeans were calculated, nondetects were set at 0.001 ppb, which represents the low end of the detected concentration range. D


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Environmental Science & Technology release (May/June), prior to any recovery of the oil from the wellhead,10 and with the exception of samples collected from surface slicks, these highest concentrations of TPAH were determined to be less than 2.0 km from the wellhead (Table 1). Of those samples with greater than 100 ppb TPAH, nine were collected in the 0−1 m surface layer likely due to entrained slick oil from surface mixing and nine were taken at depths greater than 1300 m. Those deepwater samples with greater than 100 ppb TPAH were collected by the remotely operated vehicle Skandi Neptune near the point of active oil release (i.e., the wellhead), in a specific effort to capture freshly released oil. While the highest concentrations were noted during the release near the wellhead, over 70% of the measured values from these sampling events, many of which were targeted at the highest levels of fluorescence, were within a concentration range consistent with GOM background, suggesting a highly nonuniform distribution of PAHs in the water column. It is important to note that TPAH background in the GOM is impacted by the presence of numerous oil seeps in the area.28 Additionally, the Mississippi River is a source of TPAH input from terrestrial sources. Background TPAH data in the GOM are sparse though they have been reported to be in the range of about 0.03 to 0.4 ppb and could be higher in close proximity to seep areas.28−30 On the basis of the lower end of the GOM background range, fewer than 30% of the samples collected during the DWH investigations exceeded the low end of reported background concentrations. PAH Concentrations over Time and Distance. While there was a wide distribution of TPAH concentrations (nondetect to 151 000 ppb TPAH) measured during and after the DWH accident, an examination of overall trends shows that maximum TPAH concentrations were measured at the beginning of the release (Figure 2) and seldom exceeded 100 ppb. With the exception of some of the 464 samples collected in May 2010, which targeted the immediate wellhead release area, the great majority of samples collected each month thereafter contained less than 0.1 ppb TPAH. In May 2010, the majority of the samples (75%) contained less than 1 ppb TPAH. While the May surveys were largely focused near or within 20 km of the wellhead, samples in the same area taken in June and July exhibited somewhat lower concentrations. Subsea use of chemical dispersant began in early May, but recovery of oil through insertion into the riser pipe and other mechanisms began in early June12 and may have mitigated the highest TPAH concentrations. During the sampling period from May to August 3, 2010 (release period), the TPAH concentrations were, not surprisingly, higher than during the postrelease period (Figure 2, Tables 1 and 2). Overall, in the postrelease period, 99% of the samples contained less than 1 ppb PAH (Table 2). During and after the release period, concentrations of TPAH decreased rapidly with distance from the wellhead. The occurrence of TPAH concentrations greater than 1 ppb in the deeper (greater than 10 m depth) waters was infrequent beyond 20 km from the wellhead. This finding is underscored by the fact that the overall sampling was nonrandom, with roughly half of the sampling specifically targeting areas with suspected oil and associated PAHs. The rapid attenuation of PAH concentrations both with time and distance are attributable not only to dilution but also to the microbial biodegradation activity that occurred in the water, with reported aliphatic hydrocarbon half-lives on the order of several days.9,11 Aromatic biodegradation rates

Figure 2. Total polycyclic aromatic hydrocarbon (TPAH) concentrations of all water samples as a function of distance (A) and time (B). Samples with nondetectable TPAH were set to 0.0001 ppb for plotting on a log-scale graph.

have not yet been published, though the extent of PAH biodegradation has been reported.9 Subsurface PAHs in the Southwest Quadrant. Many of the sampling surveys used in situ fluorescence to detect the probable presence of aromatic hydrocarbons in the water column.2,10 Fluorescence and other data indicated the presence of a hydrocarbon “plume” to the southwest of the wellhead at roughly 1000−1200 m depth.2,10,14 The reference to plume actually refers to a layer of dilute, but detectable, hydrocarbons at depth at concentrations that are higher than those detected above and below those in the plume. Further sampling efforts were concentrated in that quadrant at those depths by the Response and NRDA efforts (Figure 1; Table 1). This direction is consistent with reported subsurface current directions in this area of the GOM.5 Approximately 2000 samples were obtained from southwest of the wellhead at the depth range of 950− 1300 m encompassing this plume, 710 of which were collected between May and August 3, 2010. Sampling of this depth range to the southwest was undertaken on 50 of the 81 days during the release, starting in mid-May through August 3, 2010; thus, the sample set represents a very robust empirical data set. When compared with the entire water set, the TPAH distribution in the southwest quadrant was similar to the TPAH distribution in all other directions. To examine the PAH concentrations and compositions in the plume at 1000 to 1250 m in the southwest quadrant further, this subset of samples was examined separately from the rest of the data (Figure 3). The TPAH results obtained during the release of oil are plotted in the upper panel (Figure 3A). A delineation between concentrations above 1 ppb TPAH and E


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Figure 4. Comparison of subsurface (greater than 10 m depth) TPAH trends with distance from the wellhead during the release (A) and the postrelease (B) periods. Figure 3. Depth distributions of TPAH concentrations from the wellhead to 100 km, in the southwest direction during the release (A) and postrelease (B) periods.

However, concentrations 1 m below the slick increased after dispersant application and were overall higher than those measured 10 m below the slick, consistent with mean values seen in the larger data set. This is an indication that dispersants were effective in moving oil from the surface slick to the water column. Analysis of PAH compositional data in the water samples indicated that it is likely that entrainment of surface oil into the upper water column was limited to the upper 4 to 5 m as evidenced by the relative depletion ratios of chrysene and C1- to C4-chrysenes relative to recalcitrant biomarker hopane. Photodegraded surface oil was identified by higher depletion ratios of C3- and C4-chyrsenes as compared to chrysene and C1−C2-chrysenes.31 While data show that dispersants were effective, concentrations of TPAH in samples taken outside of the area of active chemical dispersant use were as high or often higher than those where chemical dispersants were used, suggesting that physical mixing and entrainment of surface oil was as important or more important than chemical dispersion in reintroducing oil constituents to the surface waters. In the surface layers, the individual PAH compositional data indicate the presence of both dissolved and weathered droplet oil-related PAHs, as might be expected in the vicinity of entrained surface slicks. PAH Compositions and Partitioning. With regards to environmental impact, all PAHs are not the same. While total PAH is a convenient parameter to report, the corresponding toxicity of individual PAHs varies.32−34 Moreover, the environmental significance of PAHs in water draw not only on their toxicity but also on their physical form (i.e., dissolved, associated with particles or in liquid droplets) with much of the exiting literature drawing on PAH in the dissolved form to trigger and assess bioavailability and associated toxicity.34−36 The sampling methods used here to measure TPAH in whole water samples do not directly indicate the physical form of the

below 1 ppb related to subsurface oil can be seen at a distance of about 18.5 km (10 nautical miles) from the wellhead during the release (Figures 3A and 4A), although this distance was reduced in other directions away from the southwest plume. After the release ended, the concentration field in the plume was rapidly attenuated, and results showed substantially lower TPAH concentrations after the well was shut (Figures 3B and 4B). This finding suggests that an exposure zone of concentrations greater than 1 ppb TPAH in deeper waters was limited to an area within about 15−20 km from the wellhead during all times of the release. This finding is similar to that from the Ixtoc 1 blowout, where the elevated concentration field was limited to about 20 km from the blowout site.15 PAH in the Surface Water. While the subsurface plume to the southwest of the wellhead is arguably the most well-studied feature of this oil release, the highest water column TPAH concentrations outside of the subsurface plume were found in the uppermost surface layers of the water column. Overall, the highest mean concentrations were found in the surface waters in the top 10 m (Table 1) and within this range specifically within approximately the top meter (Table 1). The buoyant nature of the released oil caused larger droplets to rise to the surface within a few hours. Once on the surface, a slick was formed, and the water chemistry data indicate that oil was reintroduced to the surface layer through physical entrainment due to wave action as well as the use of chemical dispersants. In a single study targeting surface slicks to which chemical dispersant was being applied, an examination of water at 1 and 10 m depth showed no statistical difference in PAH concentration before and after dispersant application. 7 F


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oil (MC252 source oil, Figure 5A) analyzed at the same laboratory and a laboratory-generated dissolved PAH assemblage (water accommodated fraction, WAF, of the source oil, Figure 5B)37,38 are presented for comparison. The MC252 oil, exhibiting a PAH profile completely in the oil phase, and the WAF, illustrating a PAH profile largely in the aqueous phase, are representative of the possible end points for the distribution of PAH between phases in a given water sample. The PAH assemblage in the sample taken at 1030 m is similar to a mix of largely WAF (dissolved) material and, to a smaller extent, liquid or droplet oil. Hopane, a highly insoluble compound, and the higher molecular weight five-ringed PAHs, also largely insoluble, are not detected above detection limits in this sample. In contrast, the sample taken at 1105 m shows an example of weathered oil droplets, with clear evidence of loss of the lighter, more soluble compounds as might be expected as the oil partitions and weathers. The insoluble compound hopane is detected in this droplet-dominated sample, confirming that this sample contains nondissolved oil. The differences in this partitioning behavior are not a simple function of the TPAH concentration. In this case (Figure 5C), the mix of dissolved components and small amounts of droplet oil is associated with a sample with a lower overall TPAH than that in Figure 5D. Similar examples of compositional heterogeneity between proximal samples, including samples appearing to contain solely dissolved TPAH, are found throughout the deepwater data set. These differences are indicative of a complex system in which physical, chemical, and biological processes affected the oil, sometimes on small scales. The deepwater plume samples, for example, contained a mixture of dissolved and particulate/droplet oil during the release with the composition of individual samples highly variable and geographically patchy. While the extended nature of the DWH release and the depth at which it occurred are unusual, the overall trends in PAH distribution with distance and time are consistent with those seen in other marine crude oil releases, including the other well-studied Ixtoc-1 subsea blowout in the GOM.15,16 The comprehensive nature of the large DWH data set examined in the present research resulted in the detection of many transient, small- and large-scale features in the water column PAH data. Oil residues from this incident were detected in the water column at locations distant from the initial release point, but PAH concentrations rarely exceeded 1 ppb beyond 20 km from the wellhead, and rapid decreases in TPAH concentrations were seen after the release was stopped on July 15, 2010. Processes that contribute to the PAH loss at depth include biodegradation and physical transport (dilution).19 Biodegradation and dissolution impart unique signatures to the PAH fingerprint, which can be used to trace the fate and transport of the released oil.39 Besides providing further information about the extent of the release, additional in-depth investigations of the PAH compositional data in this data set will provide information on specific processes related to the partitioning and attenuation of PAH in this system. Further investigation of the specific processes which alter the oil and result in the attenuation of its components will provide insight into the potential environmental impact of the release.

PAH, whether dissolved PAH or PAH present as part of a separate oil phase. This latter phase could consist of droplets, microdroplets, or colloidal or other particulate-associated oil. Previously studied subsurface blowouts have been shown to result in the formation of oil−water dispersions due to the mixing energy present at the release.15 The addition of chemical dispersant at depth in the DWH accident undoubtedly promoted the formation of additional small (less than 70 μm) droplets, which remained suspended in the water column for extended periods of time.12 The compositional PAH profiles in the subsurface water samples indicate the presence of an identifiable and quantifiable mix of droplet and dissolved PAH, including the material in the 1000−1250 m plume.37 Not surprisingly, the PAH compositional data indicate variations in compositions within the water sample set. However, these compositions varied widely even between closely spaced, adjacent samples taken at depth (Figure 5). This figure shows PAH profiles from two samples obtained less than 100 m apart on a single vertical Hydrocast, on June 2, 2010, at a distance of 12.6 km from the wellhead. A profile from a control

Figure 5. Comparison of PAH profiles in control oil (A), a laboratorygenerated water accommodated fraction (B), and two adjacent samples on the same cast to the southwest of the wellhead during the spill (C and D). Bracketed groups indicate parent and increasingly alkylated compounds. N = naphthalenes, F = fluorenes, P = phenanthrenes/anthracenes, D = dibenzothiophenes, F/P = fluoranthenes/pyrenes, and C = chrysenes. The full compound abbreviation list is provided in Table S3. Solubility of compounds generally decreases between groups from left to right and within groups with increasing alkylation (left to right).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.5b03616. G


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S1: Control oil characterization. S2: Distribution of sampling by depth and distance for offshore GOM water by month. S3: List of individual PAH and alkyl-PAH included in the calculated total PAH parameter with abbreviations. (PDF)

AUTHOR INFORMATION

Corresponding Author

*Phone: 978-461-4601; fax: 978-461-4699; e-mail: pboehm@ exponent.com. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by BP Exploration and Production Co. We thank the many researchers from multiple consulting firms, laboratories, agencies, and institutions who participated in these sample collections and sample analyses.

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REFERENCES

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Distribution and Attenuation of Polycyclic Aromatic Hydrocarbons in

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