Science of the Total Environment 573 (2016) 189–202

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Science of the Total Environment

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Weathering patterns of polycyclic aromatic hydrocarbons contained insubmerged Deepwater Horizon oil spill residues when re-exposedto sunlight

Gerald F. John, Yuling Han, T. Prabhakar Clement ⁎Environmental Engineering Program, Department of Civil Engineering, Auburn University, Auburn, AL 36849, USA

H I G H L I G H T S G R A P H I C A L A B S T R A C T

• PAHs in DWH oil spill residues weath-ered significantly when re-exposed tosunlight.

• Sunlight induced weathering effectswere higher for lower molecular weightPAHs.

• Sand particles in oil spill residues havethe potential to inhibitphotodegradation.

E-mail address: clement@auburn.edu (T.P. Clement).

http://dx.doi.org/10.1016/j.scitotenv.2016.08.0590048-9697/© 2016 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 29 March 2016Received in revised form 7 August 2016Accepted 8 August 2016Available online xxxx

Editor: D. Barcelo

The Deepwater Horizon (DWH) oil spill event released a large amount of sweet crude oil into the Gulf of Mexico(GOM). An unknown portion of this oil that arrived along the Alabama shoreline interacted with nearshore sed-iments and sank forming submerged oil mats (SOMs). A considerable amount of hydrocarbons, including poly-cyclic aromatic hydrocarbons (PAHs), were trapped within these buried SOMs. Recent studies completed usingthe oil spill residues collected along the Alabama shoreline have shown that several PAHs, especially higher mo-lecularweight PAHs (four ormore aromatic rings), are slowlyweathering compared to theweathering levels ex-perienced by the oil when it was floating over the GOM. In this study we have hypothesized that theweatheringrates of PAHs in SOMs have slowed down because the buried oil was isolated from direct exposure to sunlight,thus hindering the photodegradation pathway. We further hypothesized that re-exposing SOMs to sunlightcan reactivate various weathering reactions. Also, SOMs contain 75–95% sand (by weight) and the entrappedsand could either block direct sunlight or form large oil agglomerates with very little exposed surface area;these processes could possibly interferewith weathering reactions. To test these hypotheses, we completed con-trolled experiments to study the weathering patterns of PAHs in a field recovered SOM sample after re-exposingit to sunlight. Our experimental results show that the weathering levels of several higher molecular weight PAHshave slowed down primarily due to the absence of sunlight-induced photodegradation reactions. The data alsoshow that sand particles in SOM material could potentially interfere with photodegradation reactions.

© 2016 Elsevier B.V. All rights reserved.

Keywords:PAHsPhotodegradationOil spill weatheringDWH oil spillBP oil spillTar balls

gineering, Auburn University, Auburn, AL 36849, USA.

⁎ Corresponding author at: Environmental Engineering Program, Department of Civil En

190 G.F. John et al. / Science of the Total Environment 573 (2016) 189–202

1. Introduction

TheDeepwater Horizon (DWH) accident, which occurred on April 20,2010 released an estimated 5.3 × 108 kg of oil (Aeppli et al., 2012;McNutt et al., 2012) from the Macondo well (MC252) over a period of87 days (Hayworth et al., 2015). It was estimated that about 26% of re-leased oil remained in the water phase (Atlas and Hazen, 2011; Lehret al., 2010; OSAT-1, 2010). This oil is expected to have distributedinto various forms: a fraction would have been distributed into thewater column and eventually transported into the deeper waters ofGulf of Mexico (GOM) (Chanton et al., 2015), a fraction deposited inthe vicinity of the MC252 well, a fraction sank to the ocean bottom asmarine snow (Gong et al., 2014; Passow et al., 2012), and the remainingoil was deposited along various GOM beaches (Lehr et al., 2010).

The oil spill residues deposited along Alabama's beaches were pre-dominantly in the form of water-in-oil emulsion, a highly viscous andneutrally buoyant brownish material known as “mousse”. An unknownamount of thismousse interactedwith suspended solids near the shore-line and sank, forming submerged oil mats (SOMs). SOMs were laterfragmented bywaves and other transport processes to formmobile sur-face residual oil balls (SRBs) (Dalyander et al., 2014; Hayworth et al.,2015; Liu et al., 2012; Michel et al., 2013; Urbano et al., 2013). TheSRBs and SOMs are predominantly composed of sand (about 75 to95% by mass) (Hayworth and Clement, 2011; OSAT-1, 2010; OSAT-2,2011). The remnant oil trapped in SOMs and SRBs was found to containvarious hazardous chemicals including polycyclic aromatic hydrocar-bons (PAHs), which can be toxic to both human and ecological systems(Liu et al., 2012; Mulabagal et al., 2013; Urbano et al., 2013; Yin et al.,2015a).

PAHs are a group of organic compounds that contain two or morefused aromatic rings and are one of the most toxic classes of environ-mental compounds present in crude oils (Neff, 1979). Due to their car-cinogenic and mutagenic properties, PAHs are classified as hazardousorganic compounds (ATSDR, 1995; Mumtaz et al., 1996; Yin et al.,2015b). Several recent studies have shown that PAHs present in SOMsand SRBs found along the northern Gulf of Mexico shoreline appear tohave very low weathering rates (Hayworth et al., 2015; McDanielet al., 2015; Mulabagal et al., 2013; Natter et al., 2012; Turner et al.,2014; Yin et al., 2015b). The reason for these slow weathering rates isstill not clear.

When crude oil is released into the marine environment it un-dergoes various natural weathering processes including spreading, ad-vection, volatilization, dissolution, photodegradation, emulsification,sedimentation and microbial degradation (Dutta and Harayama, 2000;Ehrhardt et al., 1992; Garrett et al., 1998; Guo and Wang, 2009; Hallet al., 2013; Hamoda et al., 1989; NRC, 2003; Prince et al., 2003; Wanget al., 2005). Volatilization is one of themost importantweathering pro-cesses inmany oil spill incidents and it can greatly reduce the volume ofthe spilled oil (Fingas, 1997).

Photodegradation is another important weathering process that cantransform PAHs present in crude oil. Photodegradation typically yieldsby-products that have higher polarity and water solubility and thusphotodegradation reactions can enhance the net dissolution rates ofPAHs and other chemicals (Dutta and Harayama, 2000; Lee, 2003).Photodegradation can also increase the susceptibility of crude oil to mi-crobial degradation (Dutta and Harayama, 2000). However,photodegradation could also increase the net environmental toxicityby making the oil more bioavailable to marine biota (Lee, 2003).

Past studies have shown that PAHs trapped in the DWH oil spill res-idues are amenable to photodegradation (Aeppli et al., 2012; Hall et al.,2013; Radović et al., 2014). A recent field study completed by Yin et al.(2015b) using DWH oil spill residues recovered from the Alabamashoreline has indicated that PAHs in these residues should have under-gone considerable weathering including volatilization, dissolution andphotodegradation when the oil was floating over the GOM waterswhile transiting from the wellhead to the shoreline. This study has

also shown that the net weathering rate appears to have slowed downconsiderably once the oil was submerged forming SOMs (Yin et al.,2015b).

In this study we have hypothesized that the weathering rates ofPAHs in SOMs have slowed down because the buried SOMswere isolat-ed from the sunlight hindering one of the major weathering pathways,photodegradation. Note, we use the phrases “weathering levels or rateshave slowed down” or “low weathering levels or rates” to imply that theweathering processes have slowed down when compared to those ex-perienced by the DWH oil when it was floating over the open oceanwhile transiting from the wellhead to the GOM shoreline. We havealso hypothesized that re-exposing SOMs to sunlight would reactivatephotodegradation reactions, which will increase the net weatheringrate. Furthermore, SOMs contain 75% to 95% inert sand by weight, andthe inert sand could either physically block sunlight or could allow oilto agglomerate over sand particles and reduce the surface area;these processes could possibly inhibit photodegradation and otherweathering processes. The objective of this effort is to test these hypoth-eses by conducting controlled laboratory experiments using a SOMsample recovered from a field site in Gulf Shores, Alabama, USA. Whileothers have investigated photodegradation of various types of PAHs inthe past, our study is unique because, to the best of our knowledge,this is the first study that has conducted sunlight re-exposure experi-ments using SOM samples related to the DWH oil spill. This work is sig-nificant because DWH oil spill residues in the form of SOMs and SRBscontinue to be present along the GOM shoreline (Hayworth et al.,2015; Yin et al., 2015b). Therefore, it is important to understand variousenvironmental processes that can degrade these residues. The results ofthis study can help understand how the PAHs trapped in SOMs and SRBscould degrade over timewhenperiodically re-exposed to sunlightwhilethey are transported in shallowwaters or deposited along the shoreline.

2. Experimental

2.1. Samples

MC252 reference crude oil was supplied by British Petroleum (BP)and is referred to as “DWH reference oil.” The SOM (commonly knownas tarmat) sample usedwas collected by our teamduring a joint excava-tion survey completed by the U.S. Coast Guard, Alabama Department ofEnvironmental Management (ADEM) and Auburn University. This sur-vey was completed about three weeks after the Tropical Storm Lee,which mobilized buried oil and deposited large amounts of SRBs onAlabama's beaches, as it made landfall during the first week of Septem-ber 2011. Following this meteorological event, field observations indi-cated the presence of a buried SOM within 50 m from the shorelinelocated about a mile west of the Lagoon Pass located in Gulf Shores, Al-abama, USA. A field survey was completed at this site on September 24,2011 to excavate this SOM using a long-arm excavator. The GPS coordi-nates for this site are: N30.24 andW87.74. After about 4 h of excavationour teamwas able to dig out a SOM buried under 1 to 2m of sand about20 to 30 m from the shoreline (see Figure S1). Multiple split samples(field sample name: TRB2011) of the SOM material were shared be-tween the three teams. The Auburn University sample, designated asTRB2001-AUClement, was used as the reference SOM sample in thisstudy. This SOM is an ideal sample for the sunlight re-exposure experi-ment since this oil spill sample was excavated from a depth of over 1 of2m; it is highly unlikely that the residual oil in this sample was ever ex-posed to the sunlight after it was submerged to form the SOM.

2.2. Materials

Dichloromethane and hexane used in this study were of analyticalgrade or higher. The solvents, silica gel (60–200 μm), and anhydroussodium sulfate (ACS grade) were purchased from VWR International(Suwanee, GA). C30 ββ-hopane (17β(H),21β(H)-hopane) standard

191G.F. John et al. / Science of the Total Environment 573 (2016) 189–202

was purchased from Chiron (Trondheim, Norway). PAH standardmixtures consisting of twenty seven PAHs (naphthalene, 1-methylnaphthalene, 2-methylnaphthalene, 2,6-dimethylnaphthalene,2,3,5-trimethylnaphthalene, biphenyl, acenaphthylene, acenaphthene,fluorene, phenanthrene, 1-methylphenanthrene, anthracene,dibenzothiophene, fluoranthene, pyrene, benzo[a]anthracene,chrysene, benzo[b]fluoranthene, benzo[j]fluoranthene,benzo[k]fluoranthene, benzo[e]pyrene, benzo[a]pyrene, perylene,dibenz[a,c]anthracene, dibenz[a,h]anthracene, indeno[1,2,3-cd]pyreneand benzo[ghi]perylene) were purchased from Agilent Technologies(Wilmington, DE). A mixture of four deuterated standards containingnaphthalene-d8, acenaphthene-d10, phenanthrene-d10 andbenzo[a]pyrene-d12 was purchased from Ultra Scientific Analytical So-lutions (North Kingstown, RI). These four deuterated compoundswere used as surrogates. p-Terphenyl-d14 (purity 98.5%) was purchasedfrom AccuStandard (New Haven, CT, USA) and was used as the internalstandard. Chromatographic separation of various PAH compounds wasachieved using Agilent Technologies' J&W DB-EUPAH column(20m× 180 μm× 0.14 μm). The back flush setup used Agilent Technol-ogies' inert fused silica column (0.7 m × 150 μm × 0 μm).

Activated silica gelwas prepared according to the procedure given inWang et al. (1994a). About 150–200 g of silica gel was serially rinsedthree times with 250 mL acetone, hexane and dichloromethane andthen dried for 12 h in a fume hood. After drying, silica gel was heatedin an oven at 40–50 °C for 8 h and then activated at 180 °C for 20 h. An-hydrous sodium sulfate was purified by heating at 400 °C for 4 h andthen cooled and stored in tightly sealed glass containers.

2.3. Estimation of oil content in SOM samples

The oil content in SOMmaterial was determined using the methodsdescribed in previous studies (Mulabagal et al., 2013; Yin et al., 2015b).The SOM sample was initially broken into smaller fragments and airdried to remove moisture. About 1 g of the homogenized sample wasextracted using 10 mL of dichloromethane. The extraction step was re-peated four times. The solid residues that remained were dried andweighed to determine the oil content in the SOM sample. The averageoil content in the SOM was estimated to be 13%.

2.4. Preparation of various types of oil spill samples used in this study

Three types of oil samples namely “DWH sample” (original MC252source oil), “SOM sample” (excavated tarmat with sand) and “ESOMsample” (extracted oil from the SOM; this is residual oil without anysand particles) were used in this study. The DWH samples were pre-pared by dissolving 2.5 g of source oil in 5 mL of dichloromethane to

Fig. 1. Three types of oil spill samples used in this study (duplicate samples are shown toillustrate possible sample variabilities).

yield an effective concentration of 0.5 g/mL of oil. Exactly 100 μL ofthis DWH oil solution was added and spread uniformly on a petri dish.The solvent was allowed to evaporate to form a thin oil coating on thepetri dish (see Fig. 1a); and the weight of the source oil in all theDWH samples is 50 mg.

To prepare SOMsamples, about 390mg (50mgoil equivalent) of ho-mogenized SOM (with sand)wasweighed onto each petri dish andwasbroken into tiny fragments and spread uniformly on the petri dish (seeFig. 1b).

In order to study the direct effects of sunlight on remnant oil when itwas devoid of sand, we first extracted the residual oil from the SOMusing the following procedure. About 19.5 g of homogenized SOM(2.5 g oil equivalent) sample was mixed with equal quantities of anhy-drous sodium sulfate (USEPA, 2007a; USEPA, 2007b). About 50mLof di-chloromethane was added and the sample was sonicated for 10 min.The extract was collected and then the remaining solids were extractedtwice using similar steps. The extracts were combined and then the sol-vent was removed under a gentle stream of nitrogen. The extract wasweighed and then reconstituted in dichloromethane to obtain a final ex-tract that had an oil concentration of 0.25 g of oil/mL. This extract wasused to prepare ESOM samples. Exactly 200 μL of this SOM extract solu-tion was spread uniformly on a petri dish. The solvent was allowed toevaporate to form a thin layer of residual oil on the petri dish (see Fig.1c); estimated weight of SOM extract in all the ESOM samples is50 mg. Multiple sets of DWH, SOM and ESOM samples were prepared.

2.5. Design of sunlight exposure experiments

In all our outdoor weathering experiments, the petri dishes contain-ing various type of samples were divided into two sets – (1) sunlight-control (SC) samples, which include a total of 21 petri dishes that pro-vided data at seven time points for three types of samples containingDWH, SOM and ESOM; and (2) sunlight-exposed (SE) samples, whichinclude a total of 42 petri dishes that provided duplicate data at seventime points for the three types of samples (DWH, SOM and ESOM). Inorder to prevent deposition of dust and other external contaminants,the SC samples were placed in a box covered with a black sheet, andthe SE samples were placed in a box covered by a standard photo-framing glass plate (Figure S2). The sides of the boxes were ventilatedwith multiple openings covered with a mesh. We completed initialscreening experiments to verify that the glass used in this study didnot block the photodegradation reaction pathways of various targetPAHs (see Figure S3). The outdoor weathering experiments were de-signed such that the control (SC) samples will be exposed to almostall natural outdoor environmental conditions except direct sunlight.Both SC and SE samples (one covered with a black sheet and otherwith a glass plate) were placed under direct sunlight right next toeach other. However, due to direct exposure to sunlight the tempera-ture within SE samples was higher than the SC samples. The tempera-tures inside the SC and SE boxes were measured using a laserthermometer and they were 40.6 ± 0.5 °C and 56 ± 1 °C, respectively.During this period, the average air temperature (standardmeasurement

Table 1Cumulative amount of solar radiation received by the samples at different exposure times.

Time(hours)

DWH SOM & ESOM

Total amount of solar radiation(W-h/m2)

Total amount of solar radiation(W-h/m2)

0 0 01 551 8192 910 15254 1943 25838 5470 395016 9379 866532 18,278 21,41364 37,080 43,798

192 G.F. John et al. / Science of the Total Environment 573 (2016) 189–202

made under shade) recorded at a local weather stationwas about 32 °C.Each day the petri-dishes were exposed to outdoor conditions from10AM to 4 PM.During evenings/nights and also during rainy and cloudydays the samples were stored in a freezer at −15 °C (Nicodem et al.,1998). The entire oil present in each petri-dish was extracted for analy-sis after 1, 2, 4, 8, 16, 32 and 64 h of exposure to sunlight. Hourly solarradiation and temperature datawere obtained fromAWISWeather Ser-vices for aweather station located in Auburn, AL, USA. Total hourly solarradiationwas computed for the respective exposure times and are sum-marized in Table 1. The average solar radiation per daywas estimated tobe 660 ± 90 W/m2. The temperature reported during the experimentranged between 22 and 34 °C.

2.6. Design of lab control experiments

A set of petri dishes containing DWH oil were placed in a fume hoodand were allowed to continuously weather in the lab for 64 h; thesesamples were designated as DWH-lab-controls (DWH-LC). The labora-tory temperature under which these experiments were completedranged between 20 and 22 °C.

2.7. Extraction and column chromatographic fractionation methods

To quantify PAH concentrations, the DWH-LC, DWH-SC, DWH-SE,ESOM-SC and ESOM-SE samples were spiked with 20 μL of 50mg/L sur-rogate mixture, and the sample was extracted with 1 mL of dichloro-methane and transferred to a 12 mL vial. The extraction step wasrepeated twice. The extracts were then concentrated under a gentlestream of nitrogen to remove dichloromethane. For SOM-SC and SOM-SE samples, the solutions were first transferred to a 12 mL vial andthen spiked with the surrogate mixture.

Column chromatographic fractionation was completed using a pub-lished procedure (John et al., 2014;Wang et al., 1994a). A glass column(10.1 mm diameter and 200 mm length) was plugged with glass wooland about 3 g of activated silica gel was added to the column followedby 1 g of anhydrous sodium sulfate. The column was then chargedwith 20 mL of hexane. The sample in the 12 mL vial was extractedwith 1 mL of hexane and transferred to the column. The vial was thensequentially washed with 2 mL of hexane, with 1 mL in each step, andthe contents were transferred to the column. All solvents eluted fromthe column prior to this step were discarded. Exactly 12 mL of hexanewas then used to elute aliphatic hydrocarbon fractions from the sample.The hexane-eluted fraction was labeled as F1. Next, 15 mL of 50% hex-ane and 50% dichloromethane solvent mixture was used to elute all ar-omatic hydrocarbon fractions, and this fraction was labeled as F2. Thefractions, F1 and F2 were concentrated under a gentle stream of nitro-gen gas and then reconstituted in 10 mL of hexane andhexane:dichloromethane solvent mixture (ratio of 1:1) for F1 and F2,respectively. The F1 and F2 fractions were spiked with internal stan-dards C30ββ-hopane and p-Terphenyl-d14, respectively.

2.8. GC/MS analytical procedures

The F1 and F2 fractions were analyzed using Agilent TechnologiesGas Chromatograph (7890) coupled to Triple Quadrupole Mass Spec-trometer (7000B). The F1 fraction was analyzed for hopanes andsteranes using the selected ion monitoring (SIM) mode. Five groups ofalkylated PAHs and seventeen other PAHs (in the F2 fraction) were an-alyzed using SIM andmultiple reactionmonitoring (MRM)methods, re-spectively, using previously established analytical procedures (Yin et al.,2015a; Yin et al., 2015b). The GC andMS parameters andmethods usedfor analyzing the F1 and F2 fractionswere identical to those given in ourprevious study (Yin et al., 2015b).

The five groups of alkylated PAHs were considered in this study; thestandards used for quantifying these chemicals are as follows: for Group1, C0-naphthalene was quantified using naphthalene; C1-naphthalenes

using 2-methylnaphthalene; C2-naphthalenes using 2,6-dimethylnaphthalene; and C3- and C4-naphthalenes using 2,3,5-trimethylnaphthalene. For Group 2, C0-phenanthrene was quantifiedusing phenanthrene; and C1- to C4-phenanthrenes using 1-methylphenanthrene. For Group 3, C0- to C3-dibenzothiophenes werequantified using dibenzothiophene. For Group 4, C0- to C3-fluoreneswere quantified using fluorene. For Group 5, C0- to C4-chryseneswere quantified using chrysene. Seventeen other PAHs that werequantified include: biphenyl, acenaphthylene, acenaphthene,anthracene, fluoranthene, pyrene, benzo[a]anthracene,benzo[b]fluoranthene, benzo[j]fluoranthene, benzo[k]fluoranthene,benzo[e]pyrene, benzo[a]pyrene, perylene, dibenz[a,c]anthracene,dibenz[a,h]anthracene, indeno[1,2,3-cd]pyrene and benzo[ghi]perylene.The chromatographic peak area of the internal standard p-Terphenyl-d14 was used to normalize the areas of PAH chromatographic peaks inthe F2 fraction.

2.9. Quality assurance and quality control

The quality assurance and quality control protocol used was similarto the one used in our previous study (Yin et al., 2015b), and is brieflydescribed below. To compensate for GC/MS instrument variations,each F1 and F2 fraction samples were spiked with internal standardsC30ββ-hopane and p-terphenyl-d14, respectively, prior to analysis. Fur-ther to monitor the recovery of the extraction process, each samplewas spiked with 20 μL surrogate standard mixture containing 50 mg/Lof naphthalene-d8, acenaphthene-d10, phenanthrene-d10 andbenzo[a]pyrene-d12 prior to extraction process. The measured recoverylevels ranged from 70 to 93%, 72–89%, 61–84% and 62–101% for the foursurrogate chemicals, respectively. The limit of detection (LOD) and limitof quantitation (LOQ) established for the PAH quantification methodwere 0.20 to 3.65 ng/mL and 0.24 to 4.32 ng/mL, respectively (Yinet al., 2015b).

2.10. Assessment of total oil weathering levels using hopane ratios

C30αβ-hopane was used as an internal conservative petroleum bio-marker compound (Mulabagal et al., 2013;Wang et al., 1994c; Yin et al.,2015a; Yin et al., 2015b) to determine the net weathering of SOM andESOM with respect to the source oil. The C30αβ-hopane response inthe original source crude oil sample was quantified by first computinga ratio Hoil, which is the ratio of the peak area of C30αβ-hopane incrude oil to the peak area of C30ββ-hopane. Similarly, the hopane ratiofor an equivalent amount of weathered sample, Hweathered, which is thepeak area of C30αβ-hopane in a weathered oil spill sample to the peakarea of C30ββ-hopane, was computed. The percentage weatheringlevel was estimated using a modified version of a formula reported inMulabagal et al. (2013):

%weathering ¼ 1−Hoil

Hweathered

� �� 100 ð1Þ

The net weathering level of residual oil in the SOM and ESOM sam-ples with respect to that of DWH oil were estimated to be, 34% and41%, respectively.

2.11. Estimation of PAHs remaining in SOM and ESOM sampleswith respectto the DWH reference oil

Organic compounds present in crude oil weather concurrently;therefore, it is necessary to use a non-degrading compound as a conser-vative marker to normalize the concentration of compounds measuredin environmental samples (Douglas et al., 1996). C30αβ-hopane wasused as a conservative marker in the normalization step as it is knownto be recalcitrant to both biodegradation and photodegradation reac-tions (Aeppli et al., 2014; Prince et al., 1994; Radović et al., 2014). The

193G.F. John et al. / Science of the Total Environment 573 (2016) 189–202

C30αβ-hopane response in the original source crude oil sample wasquantified by first computing a ratio HDWH, which is the ratio of thepeak area of C30αβ-hopane in crude oil to the peak area of the internalstandard C30ββ-hopane. Similarly, the hopane ratio for the weatheredsamples,Hweathered, which is the peak area of C30αβ-hopane in a weath-ered oil sample (SOM or ESOM) to the peak area of C30ββ-hopane wasalso computed. These ratios were used to compute the hopane normal-izing factor, HDWH/Hweathered, which was then used to estimate the per-centage of target PAHs remaining in the samples, with respect to theDWH reference oil, using the following equation (Douglas et al., 1996;Radović et al., 2014; Yin et al., 2015b).

%PAHs remaining in weathered oil ¼ PAH in weathered samplePAH in DWH oil

� HDWH

Hweathered

� �� 100

ð2Þ

2.12. Estimation of PAHs remaining during sunlight-exposure experiments(compared to the PAHs in respective reference samples at time zero)

The PAHs remaining in DWH, SOM or ESOM samples afterweathering experiments were estimated as follows. H0 and Ht, theratio of peak area responses of C30αβ-hopane and C30ββ-hopane insamples at time zero and at times 1, 2, 4, 8, 16, 32 and 64 h were esti-mated. PAH0 and PAHt, the amount of PAHs present in samples at time0 h and at various times t (1, 2, 4, 8, 16, 32 and 64 h) were also estimat-ed. The %PAHs remaining in samples were computed using the equa-tion:

%PAHs remaining ¼ PAHt

PAH0� H0

Ht

� �� 100 ð3Þ

3. Results

3.1. Source identification

The SOM sample was analyzed first to confirm its origin. It has beenwell established that DWH oil spill residues found along Alabama'sbeaches have unique physical characteristics – they are brownish,sticky, oily material with strong petroleum odor and contain consider-able amount of sand (Mulabagal et al., 2013; OSAT-2, 2011; Yin et al.,

Fig. 2. (a) Radar plots of hopane diagnostic ratios of DWH and SOM samples. (b) Comparisonsamples.

2015b). The SOM sample used in this study matched all of these physi-cal characteristics. The sample was further analyzed for hopanes andsteranes, in order to analyze their chemical fingerprints, using the ana-lytical procedures described in previous studies (Mulabagal et al., 2013;Wang et al., 1994a;Wang et al., 1994b; Yin et al., 2015b). The followingsource-specific hopane diagnostic ratioswere used in this study to iden-tify the source: Ts/Tm, C29/C30, C31(S)/C31(S + R), C32(S)/C32(S + R),C33(S)/C33(S + R), C34(S)/C34(S + R) and C35(S)/C35(S + R)(Mulabagal et al., 2013). The source-specific hopane diagnostic ratiosestimated for both DWH and SOM samples are plotted on a radar plotshown in Fig. 2a. The data show that the source-specific diagnostic ra-tios match DWH oil fingerprints, confirming that the residual oil inSOMmust have originated from the DWH oil spill.

The steranes dataset can be used to provide additional confirmationfor identifying the source. The extracted ion chromatogram for steranes(m/z of 217) for both DWH and SOM is compared in Fig. 2b. These chro-matograms match well and these results provide additional evidencethat the oil in SOM originated from the DWH oil spill.

3.2. Comparison of PAHs measured in DWH, SOM and ESOM samples

The estimated values of PAHs present in DWH, SOM and ESOM sam-ples prior to sunlight exposure are summarized in Table 2. The percentlevels of PAHs remaining in SOM and ESOM compared to those presentin the DWH sample are also presented in Table 2. The total amount oftarget PAHs in the DWH sample was estimated to be about15,900 mg/kg oil. PAHs in the DWH source oil sample were dominatedby C0- to C4-naphthalenes.

The PAHs measured in SOM (and ESOM) are the residual amount ofPAHs remaining in the sunken oil; this oil was impacted by ocean-scaleweathering processes and by other bio-chemical transformation pro-cesses forwell over a year (fromburial in Summer 2010until its excava-tion on September 24, 2011). The total amount of PAHs in the SOMsample was estimated to be about 3080 mg/kg oil (also, as expectedthe total PAHs in ESOM was close to this value and was 2860 mg/kg;note some differences in PAH concentrations in SOM and ESOM shouldbe expected due to sample variabilities and analytical variations). ThePAHs in the SOM and ESOM samples were primarily dominated by C0-to C4-phenanthrenes. Both samples had very little C0-, C1-, C2- and C3-naphthalenes, C0-fluorene, biphenyl, acenaphthylene andacenaphthene.

of extracted ion chromatogram fingerprints of steranes (m/z of 217) for DWH and SOM

Table 2Initial concentration of various PAHs in DWH, SOM and ESOM samples.

Compound/homologuesMolecular weightg/mol

DWH oil SOM ESOM

Concentrationmg/kg-oil

Concentrationmg/kg-oil

PAHs remaining in SOM%

Concentrationmg/kg-oil

PAHs remaining inESOM %

C0-naphthalene 128 580 ± 20 3.48 ± 0.02 0% 3.35 ± 0.02 0%C1-naphthalenes 142 1940 ± 40 3.47 ± 0.03 0% 3.264 ± 0.005 0%C2-naphthalenes 156 3100 ± 100 5.0 ± 0.2 0% 4.47 ± 0.02 0%C3-naphthalenes 170 2400 ± 100 52 ± 4 2% 38.9 ± 0.4 1%C4-naphthalenes 184 2100 ± 100 140 ± 10 5% 109 ± 1 3%C0-phenanthrene 178 213 ± 5 54 ± 2 16% 48.4 ± 0.7 13%C1-phenanthrenes 192 930 ± 20 500 ± 10 36% 441 ± 5 30%C2-phenanthrenes 206 1020 ± 20 780 ± 20 51% 707 ± 8 43%C3-phenanthrenes 220 620 ± 10 430 ± 10 46% 409 ± 5 41%C4-phenanthrenes 234 321 ± 4 218 ± 4 45% 210 ± 3 41%C0-dibenzothiophene 184 53 ± 2 8.36 ± 0.06 9% 9.5 ± 0.3 7%C1-dibenzothiophenes 198 75 ± 2 32 ± 1 28% 27.2 ± 0.3 22%C2-dibenzothiophenes 212 99 ± 4 75 ± 1 52% 64.4 ± 0.9 42%C3-dibenzothiophenes 226 74 ± 2 64 ± 1 60% 55.7 ± 0.9 50%C0-fluorene 166 131 ± 5 5.4 ± 0.1 1% 4.93 ± 0.05 1%C1-fluorenes 180 333 ± 9 50 ± 3 10% 40.2 ± 0.9 7%C2-fluorenes 194 420 ± 15 127 ± 7 20% 106 ± 2 16%C3-fluorenes 208 360 ± 17 149 ± 9 27% 149 ± 2 25%C0-chrysene 228 54.7 ± 0.8 55.5 ± 0.9 67% 64.0 ± 0.5 75%C1-chrysenes 242 121 ± 1 80 ± 2 43% 99.1 ± 0.9 51%C2-chrysenes 256 152 ± 2 62 ± 3 26% 81.2 ± 0.7 33%C3-chrysenes 270 107 ± 2 30 ± 2 16% 38.7 ± 0.4 21%C4-chrysenes 284 65 ± 2 19 ± 1 16% 22.3 ± 0.3 19%Biphenyl 154 200 ± 6 2.83 ± 0.01 1% 2.67 ± 0.02 1%Acenaphthylene 152 61 ± 1 3.38 ± 0.07 4% 3.21 ± 0.04 3%Acenaphthene 154 70 ± 1 4.12 ± 0.07 4% 3.81 ± 0.01 3%Anthracene 178 12.5 ± 0.7 5.8 ± 0.2 30% 5.4 ± 0.1 26%Fluoranthene 202 15.8 ± 0.4 11.5 ± 0.2 48% 11.21 ± 0.07 42%Pyrene 202 21.1 ± 0.6 13.5 ± 0.4 42% 13.6 ± 0.2 38%Benzo[a]anthracene 228 16.4 ± 0.4 5.34 ± 0.05 22% 5.11 ± 0.04 18%Benzo[b]fluoranthene 252 13.9 ± 0.4 14.3 ± 0.1 67% 13.9 ± 0.2 59%Benzo[k]fluoranthene 252 5.93 ± 0.08 5.75 ± 0.02 64% 5.46 ± 0.03 54%Benzo[j]fluoranthene 252 5.8 ± 0.1 5.04 ± 0.03 57% 4.82 ± 0.02 49%Benzo[e]pyrene 252 18.3 ± 0.6 16.42 ± 0.08 59% 16.13 ± 0.08 52%Benzo[a]pyrene 252 6.49 ± 0.07 5.56 ± 0.02 57% 5.24 ± 0.02 48%Perylene 252 7.7 ± 0.4 5.63 ± 0.02 48% 5.31 ± 0.04 41%Dibenz[a,c]anthracene 278 10.2 ± 0.3 7.59 ± 0.03 49% 7.19 ± 0.01 42%Dibenz[a,h]anthracene 278 8.88 ± 0.07 8.00 ± 0.02 59% 7.60 ± 0.02 50%Indeno[1,2,3-cd]pyrene 276 8.7 ± 0.2 7.79 ± 0.02 59% 7.36 ± 0.03 50%Benzo[ghi]perylene 276 8.6 ± 0.1 7.90 ± 0.03 60% 7.55 ± 0.04 52%Total PAHs 15,900 ± 200 3080 ± 30 13% 2860 ± 10 11%

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3.3. Results of lab-control experiments

The percent PAHs remaining in DWH-LC samples at different timesare presented in Fig. 3. The following set of PAHs showed weatheringin DWH-LC samples – C0- to C4-naphthalenes, C0-phenanthrene, C0- toC1-dibenzothiophenes, C0- to C2-fluorenes, biphenyl, acenaphthylene,acenaphthene, anthracene and perylene. Other higher molecularweight PAHs present in the DWH oil do not show weathering inDWH-LC samples.

3.4. Results of sunlight exposure experiments

The sunlight exposure study took 27days to complete,which includ-ed several rainy and cloudy dayswhen the sampleswere not exposed tosunlight. Overall, the samples accumulated a total of 64 h of sunlight bythe end of the experiment. The percent PAHs remaining in DWH, SOMand ESOM samples after exposure to sunlight at various times are sum-marized in Figs. 4 through 9. In the following sections, we discuss thesedata and compare theweathering levels of PAHs in control (SC) and ex-posed (SE) samples. Note that the SC samples were exposed to all out-door weathering conditions except direct sunlight and hence theyshould account for all the non-photodegradation processes. The SE sam-ples were exposed to direct sunlight and they represent both

photodegradation and non-photodegradation processes. As discussedearlier, the SE samples experienced higher temperatures compared toSC samples due to direct exposure to sunlight, and hence could have ex-perienced slightly higher levels of volatilization. These temperature ef-fects had minor impact in this study since most of the highermolecular weight target PAHs of interest were relatively recalcitrantto volatilization.

3.4.1. Weathering of PAHs in DWH source oil samplesThe percent PAHs remaining in DWH-SC and DWH-SE samples are

presented in Figs. 4 and 5, respectively. In DWH-SC samples, the follow-ing PAHs showedweathering – C0- to C4- naphthalenes, C0- to C2-phen-anthrenes, C0- to C3-dibenzothiophenes, C0- to C3-fluorenes, biphenyl,acenaphthylene, acenaphthene, anthracene, fluoranthene, pyrene andperylene. Several PAHs including C1- to C2-phenanthrenes, C2- to C3-dibenzothiophenes, C3-fluorenes and fluoranthene and pyrene, whichshowed almost noweathering in DWH-LC samples, showedweatheringin DWH-SC samples. Further, the weathering of PAHs in DWH-SC ishigher compared to that of DWH-LC.

In DWH-SE samples all five groups of alkylated PAHs and the otherseventeen target PAHs showed weathering with time. At the end of64 h of exposure to sunlight, the net weathering for all the PAHs weregreater than 90% except for C0- and C1-chrysenes.

Fig. 3. Each figure shows temporal changes in PAH concentrations (normalized based on initial concentration) remaining in theDWH reference crude oil at various timeswhen the oil wasallowed to evaporate and weather in a fume hood. The data show temporal changes in: a) C0-N to C4-N; b) C0-P to C4-P; c) C0-D to C3-D; d) C0-F to C3-F; e) C0-C to C4-C; f) B(a)A, B(b)F,B(k)F, B(j)F, B(e)P and B(a)P; g) Bip, Acy, Acp, Anth, Flan, Py, Pery, D(a,h)An, D(a,c)An and I(1,2,3-cd)Py (abbreviations used are given at the end).

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Fig. 4. Each figure shows the temporal changes in PAH concentrations (normalized based on initial concentration) remaining in the DWH reference crude at various times when it wasexposed to sunlight. Each figure has two plots: SC sunlight control sample and SE sunlight exposed sample. Figures in every horizontal row show a family of alkylated PAHs. Row 1:C0-N to C4-N; row 2: C0-P to C4-P; Row 3: C0-D to C3-D; Row 4: C0-F to C3-F and Row 5: C0-C to C4-C (abbreviations used are given at the end).

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3.4.2. Weathering of PAHs in extracted SOM samplesThe percent PAHs remaining in ESOM-SC and ESOM-SE samples are

presented in Figs. 6 and 7. As discussed earlier, the ESOM sample hadvery low concentrations of C0- to C3-naphthalenes, C0-fluorene, biphe-nyl, acenaphthylene and acenaphthene. Therefore, these PAHs werenot monitored in ESOM samples.

The following PAHs showed weathering in ESOM-SC samples – C4-naphthalenes, C0- to C4-phenanthrenes, C0- to C3-dibenzothiophenes,C1- to C3-fluorenes, anthracene, fluoranthene, pyrene and perylene. InESOM-SE samples, all the PAHs show weathering with time. At theend of 64 h of exposure to sunlight, the net weathering of almost allthe PAHs were greater than 90%, except for C0- and C1-chrysenes.

3.4.3. Weathering of PAHs in SOM samplesThe percent PAHs remaining in SOM-SC and SOM-SE samples are

presented in Figs. 8 and 9. As discussed earlier, the SOM sample hadvery low concentrations of C0- to C3-naphthalenes, C0-fluorene, biphe-nyl, acenaphthylene and acenaphthene. Therefore, these PAHs werenot monitored in SOM samples.

The following PAHs showed weathering in SOM-SC samples – C4-naphthalenes, C0- to C2-phenanthrenes, C0- to C2-dibenzothiophenesand C1- to C3-fluorenes. In SOM-SE samples, the following PAHsweath-ered with time – C4-naphthalenes, C0- to C4-phenanthrenes, and C0- toC3-dibenzothiophenes and C1- to C3-fluorenes.

4. Discussion

4.1. DWH oil experiments

The experimental data for DWH oil control samples (both DWH-LCand DWH-SC samples) indicate that one of the major weathering path-ways for PAHs, especially for lowermolecular weight PAHs, was volatil-ization. This result can be inferred from the C0- to C4-naphthalene data(see Figs. 3 and 4). These data also show that theweathering rates of C0-to C4-naphthalenes decreased with increase in alkylation. It is wellknown that the rate of volatilization decreases with decrease in vaporpressure (also, vapor pressure will decrease with increase in molecularweight; for example, at room temperature the vapor pressure of naph-thalene is about 10 Pa (ILO-ICSC, 2015) and benzo[a]pyrene is 10−7 Pa

Fig. 5. Each figure shows the temporal changes in PAH concentrations (normalized based on initial concentration) remaining in the DWH reference crude oil at various timeswhen it wasexposed to sunlight. Eachfigurehas twoplots: SC sunlight control sample and SE sunlight exposed sample. Every horizontal rowhas a set of PAHs. Row1: Bip, Acn, Acp, Anth and Flan; Row2: Py, Pery, B(a)A and B(b)F; Row 3: B(k)F, B(j)F, B(e)P and B(a)P and Row 4: D(a,c)An, D(a,h)An, I(1,2,3-cd)Py and B(ghi)P (abbreviations used are given at the end).

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(TOXNET, 2016)). Alkylation increases themolecular weight and there-fore the rate of volatilization is expected to decrease with increase in al-kylation (Mekenyan et al., 1995; Sage and Sage, 2000). Whenweathering data for PAHs in DWH-LC and DWH-SC are compared, it isclear that both the weathering rates and net removal of PAHs inDWH-SC are higher than those observed for DWH-LC samples. Fromthese results, it is evident that outdoor conditions should have increasedPAH weathering rates. This increase was due to direct radiative heatingexperienced by the DWH-SC samples, which resulted in higher temper-ature and hence higher volatilization (Mekenyan et al., 1995).

For sunlight exposed DWH samples, the data shown in Figs. 4 and 5indicate that the weathering rates of PAHs in DWH-SE samples wererelatively higher than those of control (DWH-SC) samples. Theweathering rates of C0- to C4-naphthalenes decreased with increase inalkylation, implying that these groups of PAHs weathered primarilythrough volatilization even when they were exposed to sunlight. Thehigher molecular weight PAHs, C0- to C4-chrysenes showed noweathering in both laboratory and outdoor-control samples (DWH-LCand DWH-SC). However, all the chrysenes weathered considerably inDWH-SE samples indicating that sunlight can play a significant role inweathering of these compounds. The weathering rates of C0- to C4-chrysenes increasedwith increase in alkylation (note this trendwas op-posite to the one observed for alkylated (C0- to C4-) naphthalenes), im-plying that chrysenes weathered through photodegradation processes.Note, for chrysene compounds, increased weathering with increases inalkylation is an expected trend (Garrett et al., 1998).

The data presented in Figs. 4 and 5 indicate that after 64 h of sunlightexposure almost all of the PAHs in DWH-SE samples, except C0- and C1-chrysenes, weathered by over 90%. It likely that if the sunlight exposure

times were increased well beyond the 64 h, the C0- and C1-chryseneswould have also weathered. Yin et al. (2015b) noted that all the PAHsin DWH oil experienced considerable weathering while the oil wasfloating over the GOMwaters. Theywent on to postulate that volatiliza-tion and photodegradation mechanisms (compared to other naturalprocesses such as dissolution and biochemical reactions) should haveplayed a significant role inweathering of DWHoil spill residues. Our ex-perimental results show that exposure to sunlight can lead to increasedrate of volatilization of lower molecular weight PAHs (two to three aro-matic rings), and it would also result in photodegradation of highermo-lecular weight PAHs (four or more aromatic rings) that are relativelynon-volatile.

4.2. ESOM oil experiments

The ESOM samples were prepared using the residual oil extractedfrom SOM. This residual ocean-weathered oil has already lost most ofthe volatile PAHs. Therefore, as expected, the ESOM samples onlycontained about 11% of total PAHs that was originally present inthe source crude. In the sunlight exposure experiments, the extract-ed oil was uniformly spread over the petri-dish, similar to the DWHsamples (see Fig. 1). Since the physical form (i.e., spreading pattern)of ESOM-SC and ESOM-SE samples were identical to DWH-SC andDWH-SE samples, respectively, these two sets of experiments exhib-ited a similar weathering patterns. In ESOM-SC samples, theweathering rates of lower molecular weight PAHs decreased with in-crease in alkylation, for C0- to C4-phenanthrene, C0- to C3-dibenzothiophenes and C1- to C3-fluorenes, indicating that theweathering process was dominated by volatilization. Other PAHs

Fig. 6. Each figure shows the temporal changes in PAH concentrations (normalized based on initial concentration) remaining in ESOM samples at various times when it was exposed tosunlight. Each figure has two plots: SC sunlight control sample and SE sunlight exposed sample. Every horizontal row has family of alkylated PAHs. Row 1: C4-N; Row 2: C0-P to C4-P; Row3: C0-D to C3-D; Row 4: C0-F to C3-F and Row 5: C0-C to C4-C (abbreviations used are given at the end).

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that showed significant weathering in ESOM-SC samples are: anthra-cene, fluoranthene, pyrene and perylene. The temporal variations inweathering patterns observed for ESOM-SC samples were similar tothose observed for DWH-SC samples.

When the extracted residual oil was exposed to sunlight (ESOM-SEsamples), the weathering rates of several lower molecular weightPAHs, including C0- to C4-phenanthrene, C0- to C3-dibenzothiophenesand C1- to C3-fluorenes, decreasedwith increase in alkylation, indicatingthat these PAHs weathered primarily through volatilization. On theother hand, the weathering rates of C0- to C4-chrysenes increasedwith increase in alkylation, once again indicating that these higher mo-lecular weight PAHs must have weathered through photodegradation(Garrett et al., 1998). It is clear from the data shown in Figs. 6 and 7that all of the PAHs present in the residual ESOM oil have weathered ei-ther via volatilization, photodegradation, or a combination of these twoprocesses. Similar to the DWH-oil dataset, the total amount of PAHs re-moved within a given amount of time was always higher for sunlightexposed (ESOM-SE) samples as opposed to the control (ESOM-SC)

samples, indicating that exposure to sunlight aided various weatheringprocesses.

4.3. SOM experiments

The physical form of the SOM samples, which were broken andspread as small particles over the petri-dish, was different from boththe DWH and ESOM samples which were spread as a thin coatingover the petri-dish (see Fig. 1). Natural SOMs found along the Alabamashoreline only have about 5–25% oil by weight (Yin et al., 2015b) andthe remaining is mostly composed of sand. SOMs are typically buriedunder 1 to 2 m of sand away from the shoreline. Therefore, it is likelythat the residual oil present in SOM was not exposed to sunlight afterit was submerged. In our experiments, we homogenized SOMmaterialinto small particles and re-exposed the oil coated sand particles to sun-light. Theweathering data for SOM-SC and SOM-SE samples (Figs. 8 and9) show that theweathering levels of lower molecular weight PAHs, in-cluding C0- to C4-phenanthrenes, C0- to C3-dibenzothiophenes and C1-

Fig. 7. Each figure shows the temporal changes in PAH concentration (normalized based on initial concentration) remaining in ESOM samples at various times when it was exposed tosunlight. Each figure has two plots: SC sunlight control sample and SE sunlight exposed sample. Every horizontal row has a set of PAHs. Row 1: Anth, Flan, Py and Pery; Row 2: B(a)A,B(b)F, B(k)F and B(j)F; Row 3: B(e)P, B(a)P, D(a,h)An and D(a,c)An, and Row 4: I(1,2,3-cd)Py and B(ghi)P (abbreviations used are given at the end).

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to C3-fluorenes, decreasedwith increase in alkylation. Also, as expected,the overall weathering levels of PAHs are higher in SOM-SE samples asopposed to control (SOM-SC) samples. The weathering patterns ob-served for lower molecular weight PAHs are similar to those observedfor DWH and ESOM samples, indicating that these PAHs have weath-ered primarily through volatilization. However, unlike the DWH-SEand ESOM-SE samples (where almost all the PAHs degraded after 64-hours of sunlight exposure), several higher molecular weight PAHs inSOM-SE samples showed little or no weathering. While C0-to C4-chrysenes showed a minor amount of weathering (about 5 to 15%), allother higher molecular weight PAHs showed no weathering (see Fig.9) in our 64-hour sunlight exposure experiment. The experimentaldata for the ESOM samples indicated that photodegradation can play asignificant role in weathering all higher molecular weight PAHs. Al-though the chemical characteristics of the residual oil in ESOM andSOM samples were identical, photodegradation appears to have playedvery little role in weathering higher molecular weight PAHs in SOMsamples which were exposed 64 h of sunlight. It appears that the sandparticles in SOM could have interfered with sunlight-inducedweathering effects. We postulate that since ESOM samples were

uniformly spread as a thin film over the petri-dish (see Fig. 1), a largersurface area of oil was exposed to sunlight. On the other hand, the oilin SOM samples was distributed in the form of agglomerates of oil stick-ing onto sand grains and this resulted in very little oil surface area beingexposed to direct sunlight. Furthermore, it is possible that sand grainsthemselves could have either blocked or absorbed some solar energy.The data shown in Figs. 8 and 9 indicate that sand particles haveinhibited photodegradation reactions of several higher molecularweight PAHs and prevented them from weathering within the timespan of this experimental study. It is possible that prolonged exposureto sunlight could have weathered some of these chemicals and there-fore more long-term studies are needed to better quantify theweathering times scales.

The homogenized SOM material used in this experiment had an ef-fective size of 1 to 2mm.Under field conditions, SOMs are homogenizedby natural shoreline forces into SRBs typically ranging from 1 to 10 cm(Hayworth et al., 2015; Yin et al., 2015b). The natural SRBs will have asmaller effective surface area compared to the homogenized labsamples and therefore it is likely that photodegradation and otherweathering processes such as microbial degradation could be relatively

Fig. 8. Eachfigure shows the temporal changes in PAH concentration (normalized based on initial concentration) remaining in SOM at various timeswhen itwas exposed to sunlight. Eachfigure has two plots: SC sunlight control sample and SE sunlight exposed sample. Every horizontal row has a set of alkylated PAHs. Row 1: C4-N; row 2: C0-P to C4-P; row 3: C0-D to C3-D;row 4: C0-F to C3-F and row 5: C0-C to C4-C. (abbreviations used are given at the end).

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slower in field-scale SRBs. Since the burial process has inhibitedphotodegradation pathways, perhaps the only other major pathwayfor degrading higher molecular weight PAHs present in SOM is mostlikely be microbial degradation processes. Therefore, future researchefforts should focus on microbial degradation pathways of higher mo-lecular weight PAHs present in SOMs. Finally, the influence of sandinhibiting naturalweathering processes is an issue that is almost uniqueto theDWHoil spill (becauseDWHSRBs and SOMs contain over 75–95%sand). Our preliminary experimental results show that sand particles inSOMs and SRBs could interfere with natural weathering processes, par-ticularly photodegradation reactions; however further investigationsare needed to fully understand these effects.

List of abbreviations

DWH Deepwater HorizonBP British PetroleumPAHs Polycyclic aromatic hydrocarbonsSOM Submerged oil matsSRB Surface residual oil balls

GPS Global positioning systemESOM Extracted oil from SOMSC Sunlight-controlSE Sunlight-exposedLC Lab controlLOD Limit of detectionLOQ Limit of quantitationC0-N C0-naphthaleneC1-N C1-naphthalenesC2-N C2-naphthalenesC3-N C3-naphthalenesC4-N C4-naphthalenesC0-P C0-phenanthreneC1-P C1-phenanthrenesC2-P C2-phenanthrenesC3-P C3-phenanthrenesC4-P C4-phenanthrenesC0-D C0-dibenzothiopheneC1-D C1-dibenzothiophenesC2-D C2-dibenzothiophenes

Fig. 9.Eachfigure shows the temporal changes in PAH concentrations (normalizedbased on initial concentration) remaining in SOMat various timeswhen itwas exposed to sunlight. Eachfigure has two plots: SC sunlight control sample and SE sunlight exposed sample. Every horizontal row has a set of PAHs. Row 1: Anth, Flan, Py and Pery; Row 2: B(a)A, B(b)F, B(k)F andB(j)F; Row 3: B(e)P, B(a)P, D(a,h)An and D(a,c)An and Row 4: I(1,2,3-cd)Py and B(ghi)P (abbreviations used are given at the end).

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C3-D C3-dibenzothiophenesC0-F C0-fluoreneC1-F C1-fluorenesC2-F C2-fluorenesC3-F C3-fluorenesC0-C C0-chryseneC1-C C1-chrysenesC2-C C2-chrysenesC3-C C3-chrysenesC4-C C4-chrysenesBip BiphenylAcn AcenaphthyleneAcp AcenaphtheneAnth AnthraceneFlan FluoranthenePy PyrenePery PeryleneD(a,h)An Dibenz[a,h]anthracene.D(a,c)An Dibenz[a,c]anthracene.I(1,2,3-cd)Py Indeno[1,2,3-cd]pyrene

B(a)A Benzo[a]anthraceneB(b)F Benzo[b]fluorantheneB(k)F Benzo[k]fluorantheneB(j)F Benzo[j]fluorantheneB(e)P Benzo[e]pyreneB(a)P Benzo[a]pyreneB(ghi)P Benzo[ghi]perylene

Acknowledgments

This work was supported, in part, by the background tarball assess-ment study funded by the Alabama Emergency Management Agency.The GC/MS/MS facility and the characterization methods were devel-oped using grant funds received from NSF. We like to thank US CoastGuard and ADEM for their support during field excavation survey. Wewould also like to thank Dr. Fang Yin for all his suggestions, and Dr.Joel Hayworth for his help, guidance and support with GFJ's graduatework. Editorial support provided by Sarah Gustitus is kindly acknowl-edged. GFJ conducted experimental work, developed ideas, wrote themanuscript, and revised the work; YH helpedwith experimental efforts

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and reviewed the manuscript; and TPC provided ideas, completed thefield sampling study, supervised experimentalwork, developed fundingsupport, co-wrote themanuscript and helpedwith revisions. Overall, allthree authors have contributed to this effort and their relative responsi-bilities, computed using the Clement (2013) approach, are: GFJ (50%),YH (15%) and TPC (35%). We also like to thank all five STOTEN journalreviewers and the associate editor for their detailed, constructive re-view comments.

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.scitotenv.2016.08.059.

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