weathering patterns of oil residues eight years after the exxon valdez oil spill

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Weathering Patterns of Oil ResiduesEight Years after the Exxon Valdez OilSpillJACQUELINE MICHEL* and MILES O. HAYESResearch Planning, Inc., PO Box 328, Columbia, SC 29202, USA

Eight years after the Exxon Valdez spill, oil residues inPrince William Sound, Alaska ranged from moderately toextremely weathered. The least weathered residues werefound in samples collected from gravel beaches with well-established armor. There had been little to no change inweathering stage for the oil from these sites since 1994.There was evidence of some physical erosion, but littlechemical change for this deeply penetrated oil. In contrast,most other oil residues have increased in weathering,compared to 1994. Only one asphalt pavement was at amoderate weathering stage. All other samples containedhydrocarbons which were at advanced or extreme weath-ering stages. Ó 1999 Published by Elsevier Science Ltd.All rights reserved.

Keywords: Exxon Valdez Oil Spill; Oil Weathering.

Introduction

Our research team has been studying the weatheringtrends of oil residues from the Exxon Valdez at selectedstations in Prince William Sound, Alaska since Sep-tember 1989. This work is part of an integrated, long-term study of the physical, biological, and chemicalchanges at di�erent shoreline habitats a�ected by thespill (Shigenaka, 1997). Intensive shoreline cleanup wasconducted in 1989 using high-pressure, hot-waterwashing followed by nutrient addition. In 1990 surfaceresidues were manually removed, and berm relocationimplemented at 25 sites. In 1991, there was limitedmanual removal of surface residues, a large-scale stormberm relocation project along 2 km of gravel beach atPoint Helen, and mechanical removal of oiled sedimentsnear salmon spawning streams. In 1997, cleanup of oilresidues was conducted using a shoreline cleaning agent

at sites important to the people of Chenega Bay village.Our stations include several which were set-aside andnot treated by any cleanup techniques. The precedingpaper (Hayes and Michel, in preparation) describes theresults of studies on the physical fate of the residual oil,emphasizing gravel beaches. This paper focuses on rel-ative weathering patterns of oil residues in 1997, 8 yearsafter the spill.

Sampling and Analytical Methods

Sediment samples were collected from the intertidalzone by digging trenches and sampling a clean side ofthe trench. Sampling depth varied widely, depending onthe shoreline type and depth of oil penetration andpersistence. The objective was to sample persistent oilresidues, not characterize the overall rate of oil removal,thus samples were collected at the same locations on thebeach face, over time but at varying depths dependingon visual observations of where the heaviest oil oc-curred.

Chemical analyses were carried out by the Environ-mental Studies Institute at Louisiana State University.Total extractable organics (TEO) was determinedgravimetrically after solvent extraction with freon(USEPA Standard Method 503). The July 1997 sampleswere also analyzed by gas chromatography and FlameIonization Detection (GC-FID) and the results reportedas total petroleum hydrocarbons (TPH). For the typi-cally heavily contaminated samples, TEO is a good in-dicator of bulk oil contamination. TPH by GC-FIDprovides a more quantitative measure of mid-range hy-drocarbons and a chromatographic picture which pro-vides more information on the source of hydrocarbonsand the degree of weathering. All samples were alsoanalyzed by gas chromatography/mass spectroscopy(GC/MS), targeting 40 PAHs known to characterizepetroleum hydrocarbons, as well as hopanes and

Marine Pollution Bulletin Vol. 38, No. 10, pp. 855±863, 1999

Ó 1999 Published by Elsevier Science Ltd. All rights reserved

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*Corresponding author.

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steranes. The Exxon Valdez source oil (North Slopecrude) contains 13 800 mg/kg PAH using the methodsand equipment consistently used throughout most ofour studies. A detailed discussion of the analyticalmethods is provided in Henry et al. (1997). Table 1 listsall the samples collected in 1997, the visual descriptionsof oiling as recorded in the ®eld, and the TEO, TPH,and total targeted PAH results in milligrams of oil perkilogram of sediment (mg/kg), dry weight. Table 2 listsTEO and PAH concentrations in two of the gravelbeaches for the period 1989±97 for samples collectedfrom the same tidal elevation. These results indicate howconcentrations have generally changed during this pe-riod. Although the exact same site was not re-sampled

each time, the samples represent the general area listedfor each station. We emphasize that all sampling e�ortsfocused on areas with visible or previously known oilcontamination. Refer to Fig. 1 in Hayes and Michel (inpreparation) for station locations.

Weathering Patterns

The chemical results are presented using two formats.First, double-ratio plots were generated, using the C2-and C3-homologues of the PAH groups of phenanthreneand dibenzothiophene. These PAH groups are boththree-ringed PAHs which undergo preferential removalby degradation of the lighter homologues. As the oilweathers, the ratios shift toward zero, allowing com-parison of the degree of weathering among samples.

The second format is a histogram plot of the concen-trations of individual targeted PAH alkyl homologues,normalized to hopane. Hopanes are highly resistant tobiodegradation, thus can be used as an internal standardfor normalizing PAH concentrations among sampleswith widely varying levels of petroleum. The PAH pat-tern is often used to characterize oils, for ®ngerprinting,weathering, and toxicity assessment. Table 3 lists theabbreviations used on the PAH histogram plots. Theplots are compared against the plot for the originalsource oil, North Slope Crude (NSC), to characterizethe weathering stage of the oil in each sample. Com-parisons of the weathering stages of the 1997 samplescan be made using a scheme proposed by Sauer et al.(1993), as described below and listed in Table 1:

TABLE 1

Results of chemical analyses of sediment samples collected during the July 1997 survey.

Station no./shorelinetype1

Depth(cm)

Visual oil2

descriptionTEO

(mg/kg)TPH

(mg/kg)Total PAH(mg/kg)

Weatheringstage3

Point Helen1-1 Tr. A, gravel beach 50±55 HOR 32000 19 000 570 II1-2 Tr. B, gravel beach 40±45 HOR 21000 9600 490 IISmith Island3-1 Tr. A, gravel beach 28±35 HOR 27000 17 000 450 II3-2 Tr. B, gravel beach 10±15 MOR 17000 8900 450 IIBay of Isles6-1 Tr. A, rocky shore 0±5 AP 19 000 6700 150 IV6-2 Tr. B, rocky shore 0±5 AP 44 000 14 000 340 IIINE Knight Island7-1 Tr. B, gravel beach 35±40 LOR 6500 3700 6.8 IV7-2 Tr. C, gravel beach 50±60 LOR 4200 1500 13 IVBlock Island9-1 Tidal ¯at 2±10 LOR 840 770 23 III9-2 Rocky ledge 0±2 AP 13 000 5400 210 IIHerring Bay10-1 Tr. A, pebble beach 65±75 LOR 2300 340 8.4 IIIHerring Bay13-1 Tr. A, rocky rubble shore 15±20 LOR 400 130 2.3 IVNorthwest Bay14-1 Pebble beach 5±10 Clean ± 4.5 Trace IVLatouche Island15-1 Tr. B, gravel beach 12±15 OF 1600 550 7.5 IV

1Tr� trench. All samples were collected from the upper intertidal zone.2HOR�heavy oil residue; MOR�medium oil residue; LOR� light oil residue; OF� oil ®lm; AP� asphalt pavement.3 See text for de®nition of weathering stages.

TABLE 2

TEO and PAH concentrations in sediment samples collected fromselected gravel beach stations in Prince William Sound with persistentoil. Samples are from the bottom of trenches, at depths >25 cm.

Survey date TEO (mg/kg) PAH (mg/kg)

Point Helen (N-1) Upper Platform1991 12 580 N/A1992 13 960 1091994 18 000 3601997 32 000 570

Knight Island (N-7) Upper Platform1990 8430 ±1991 6480 1211992 18 710 36

15 870 361994 7700 171997 4200 13

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Marine Pollution Bulletin

Stage I (initial weathering) indicates oil in whichevaporation has been the dominant process for the lossof PAH compounds. A small amount of loss may alsohave occurred from dissolution. Stage I is characterizedby loss of the parent compounds of the di-aromatichydrocarbons. Stage I oil would have been removedfrom natural weathering processes once the oil strandedonshore. No sediment samples collected in either 1994or 1997 from Prince William Sound were characterizedas being at Stage I weathering.

Stage II (moderate weathering) indicates oil in whichthe C0- C1- and C2-alkyl groups of the two- and three-ringed PAH compounds have been progressively re-duced. Total naphthalenes have been reduced so thatthey are about equal to total phenanthrenes, compared

to the original oil where the naphthalenes are aboutthree times more abundant than total phenanthrenes.These oil residues have been exposed to biodegradationand photo-oxidation over the last 8 years, though atreduced rates. Five of the 14 samples collected in 1997were at Stage II weathering.

Stage III (advanced weathering) indicates oil in whichthe PAH pattern shows continued losses of the naph-thalene, ¯uorene, and phenanthrene alkyl homologues.Because they are more resistant to weathering, thenaphthobenzothiophenes have increased in relativeabundance and can even be the dominant PAH group inthe pattern. Three of the 1997 samples were at Stage IIIweathering.

Stage IV (extreme weathering) indicates oil which hasundergone extensive transformation. The PAH patternis nearly devoid of the two- and three-ringed PAHcompounds. Naphthobenzothiophenes are the domi-nant PAH group, and chrysenes have increased in rel-ative abundance. However, the hydrocarbons are stillconsidered to be of a petrogenic, rather than a pyrogenicorigin because of the presence of naphtho-benzothiophenes and the dominance of the alkyl ho-mologues of pyrene and chrysene over the parentcompounds. Six of the 1997 samples were classi®ed as atStage IV.

Figure 1 shows the double-ratio plot for the 14 sam-ples collected from the sNOAA stations in 1997, com-pared with the 1997 analysis for NSC. Most samples fallalong a line from the upper right, where the NSC sourceoil plots, toward the origin. The two samples which plotfarthest from this line (14-1 and 15-1) contained verylow levels of PAHs, probably at background. Thesamples that clustered close to the source oil contained

Fig. 1 Double-ratio plot for the 14 1997 samples from NOAA sta-tions. NSC is the source oil from the Exxon Valdez. Weatheringpreferentially degrades the C2-homologues, shifting the pointstoward the origin. See Table 1 for sample descriptions.

TABLE 3

Key for the abbreviations used on the PAH histogram plots.

Abbreviation PAH name

NAPH NaphthaleneFLU FluoreneDBTP DibenzothiophenePHEN PhenanthreneANT AnthraceneNBT NaphthobenzothiopheneFLANT FluoranthenePYR PyreneB(a)ANT Benz(a)AnthraceneCHRY ChryseneB(b+k)F Benzo(b,k)FluorantheneB(e)P Benzo(e)PyreneB(a)P Benzo(a)PyrenePERYL PeryleneINDPYR Indeno(1,2,3-c,d)pyreneBENZP Benzo(g,h,i)peryleneDIBENZ Dibenz(a,h)anthracene

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the highest PAH concentrations, at 340±570 mg/kg.Four out of the ®ve samples that plot near the source oilrepresent the deeply penetrated oil in the gravel beachesat Point Helen (N-1) and Smith Island (N-3). They weremoderately weathered (Stage II), and the least weath-ered of all samples collected in 1997.

In contrast, the two sediment samples (7-1 and 7-2)from the gravel beach at N-7 had PAH patterns whichwere extremely weathered (Stage IV) and the mostweathered of all samples collected in 1997. Sheens hadnever been reported at this beach after 1990. The PAHshad been almost completely degraded. These sampleshave been the most weathered for each period since1992. Several factors are suggested as the cause of thishigh degree of degradation: (1) special application of thefertilizers Customblen and Inipol in 1990 when thebeach was used as a test site for use of nutrients to speeddegradation of subsurface oil; (2) the open nature of thesurface armor (as discussed in Hayes and Michel, in

preparation); and (3) erosion during episodic stormswhich lowered the beach pro®le by tens of centimeters,eroding the oiled sediments and exposing the deeperoiled layers to weathering processes.

Station N-9 on Block Island is one of a very few tidal¯ats that were oiled during the spill. Penetration andpersistence of oil was unexpected in a water-saturatedtidal ¯at; however, intensive washing of the adjacentbeach and rocky outcrops may have ¯ushed oil andoiled sediments onto the ¯at, and o�shore rocky out-crops could have created sheltered areas when oil slickspersisted and eventually contaminated the sediments.Total PAH concentrations in the part of the tidal ¯at inthe lee of a large bedrock outcrop have remained thesame from 1990 to 1997, ranging between 23 and 29 mg/kg, whereas `background' levels in the middle of thetidal ¯at sediment were 0.2 mg/kg PAH in 1994. In clamtransplant studies at this site in 1991±93, Shigenaka(1997) reported uptake of whole oil by clams, with clean

Fig. 2 PAH histogram plots for the 12 1997 samples from NOAAstations, normalized to hopane.

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transplanted clams reaching total PAH levels of nativeclams in 5 months. Eight years later, the PAHs in thesandy sediments on tidal ¯at on Block Island were atStage III (advanced) weathering, though there were stillsome two-ringed PAH present.

The least weathered cluster in Fig. 1 includes the as-phalt pavement collected from the peaty sediments indepressions on the rocky outcrop at N-9 on Block Is-land. This pavement was much less weathered, com-pared to other 1997 pavements that contained muchmore oil. Asphalt pavements have undergone a widerange of weathering, and it is not clear which factors aremost important in controlling the rate of degradation.

Oil loading is obviously an important factor in¯u-encing PAH weathering rates, but not the only one.Henry et al. (1997), using the nC-18/phytane ratio as anindicator of the degree of biodegradation, suggest thatthere is a reduction in the degradation rate above20 000 mg/kg TEO, 10 000 mg/kg TPH, and 250 mg/kgPAH. That is, samples with concentrations above these

threshold levels 8 years after the spill still had some n-alkanes present. Since n-alkanes are the most readilydegraded compounds in oil, their persistence for 8 yearsindicates that these oil residues are still only moderatelyweathered. PAHs are more resistant to biodegradationthan n-alkanes, as well as the major contributors to oil'stoxicity, so they are valuable indicators of the potentialfor on-going impacts from residual oil.

Figure 2 shows histograms of the PAHs in eachsample, normalized to hopane. The determination ofweathering stage shown in Table 1 was based primarilyon the pattern of PAHs as shown in these histograms.The samples are listed in Fig. 2, grouped by station type,with the NSC reference oil shown on the bottom of eachgrouping for comparison.

The ®rst four samples shown in Fig. 2 are of thedeeply buried oil in the gravel beaches at Point Helen(N-1) and Smith Island (N-3). Total PAHs in thesemixed sand and gravel samples in 1997 were very high,450±570 mg/kg. Chronic sheening has been reported at

Fig. 2 (continued)

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N-3 on Smith Island since 1989, and during the July1997 site visit, sheens were still observed draining outwith the ground water on the falling tide. Shigenaka(1997) reported the results of mussel transplant studiesin 1991±93 at N-3 which documented the bioavailabilityof oil being released from the gravel beach. In 14 days,PAH levels in transplanted mussels increased by twoorders of magnitude and matched the ®ngerprint of thesubsurface oil.

In 1997, samples of the subsurface oil in the gravelbeach at Smith Island were at Stage II weathering, asindicated by the reduction in total naphthalenes so thatthey are about equal to total phenanthrenes, comparedto the original oil where the naphthalenes are aboutthree times more abundant than total phenanthrenes.The C0- C1- and C2-alkyl groups of the two- and three-ringed PAH compounds have been progressively re-duced, compared to the fresh oil. Note how similar thepatterns of the four- and ®ve-ringed PAH compounds inthese gravel beach samples are to the fresh oil; they are

essentially unchanged. In comparison, the next twosamples shown in Fig. 2 are from the gravel beach at N-7, where the oil has undergone extreme weathering. Theonly PAHs remaining are the four- and ®ve-ringedcompounds, and they total only 6.8 and 13 mg/kg. Thisoil was at Stage IV weathering. However, the hydro-carbons are still considered to be of a petrogenic, ratherthan a pyrogenic, origin because of the presence ofnaphthobenzothiophenes and the dominance of the al-kyl homologues of pyrene and chrysene over the parentcompounds. Since these samples were collected fromdeep within gravel beaches heavily oiled during theExxon Valdez spill, it is likely that the oil residues arefrom the spill, rather than from other sources of chronicoil pollution or oil seeps. Fuels used by vessels in theSound are primarily diesel which would not contain thefour- and ®ve-ringed PAH compounds.

The sample from the gravel beach at N-15 on La-touche Island contained very low PAH levels (7.5 mg/kg), but also had a petrogenic (crude oil) PAH pattern at

Fig. 2 (continued)

860


Stage IV weathering (same as in 1994). This beach hadundergone extensive sediment reworking in 1990 whichsuccessfully removed a layer of heavily oiled sedimentsbelow a thick gravel armor.

The next sample shown in Fig. 2 is from the pebblebeach in Northwest Arm (N-14), which contained only0.02 mg/kg total PAH. Even though oil levels were ex-tremely low, the PAH pattern still resembled a crude oilpetrogenic source, although there may be some dieselfuel contamination, which is likely because NorthwestArm is a popular anchorage site. The sample from 65 to75 cm deep in the pebble beach at N-10, in Herring Bay,contained hydrocarbons which were clearly of crude oilsource and at Stage III weathering. Note that the nap-hthobenzothiophenes had increased in relative abun-dance to the point that they are the dominant PAHgroup in the pattern, yet the two- and three-ringedPAHs were still present.

The next sample, N-13-1, is from a rocky rubble shorein Herring Bay which was one of the set-aside sites

where no cleanup was conducted. Total PAHs were2.3 mg/kg, and the pattern indicated Stage IV weather-ing. Yet, black oil droplets and sheens were still ob-served on the water table in the trench from which thesample was collected. Two samples collected in 1994were at Stages II+ and III, indicating that weatheringhas continued at this site.

The next three samples are of asphalt pavementscollected from Bay of Isles (N-6) and Block Island (N-9). Note that they range in PAH weathering from StageII to Stage IV. The least weathered was from Block Is-land and contained the lowest oil content. The twosamples from the set-aside rocky shore in Bay of Isleswere collected only a few meters apart. The mostweathered pavement (N-6-1) was at the higher tidal el-evation and hard throughout, whereas N-6-2 was col-lected about mid-tide and had a soft, mousse-likeinterior.

There is relatively good agreement between the twoapproaches to characterize the degree of weathering

Fig. 2 (continued)

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among the 1997 sample set. All of the samples with aPAH pattern indicative of Stage II weathering areplotted in the upper right corner of Fig. 1. The threesamples at Stage III PAH weathering are plotted in amiddle grouping. The Stage IV samples are plotted in alarger area than the other groups, primarily because thetotal PAHs were very low, close to background. Thedouble-ratio plot method starts to vary when PAHlevels approach the detection level.

In summary, 8 years after the spill, the least weath-ered oil residues in Prince William Sound occurred asdeeply penetrated oil in gravel beaches with well-estab-lished armor. The causes for the slow physical removalrates are discussed in Hayes and Michel (in prepara-tion). The slow chemical weathering rates are likely dueto the heavy oil loading. Yet, it is surprising that onsome of the most exposed gravel beaches in PrinceWilliam Sound the oil has not been ¯ushed out of highlypermeable coarse sediments during vigorous tidal in-undations (with a 5-m tidal range) over the past 8 years.

Oil/®ne particle interaction has been proposed as animportant mechanism for natural oil removal, and itwas ®rst hypothesized based on experiments conductedwith oiled sediments from the Sound in 1989 (Bragg andYang, 1995). Owens and Sergy (1997) state that the onlyconditions under which these processes are not e�ectivein the breakdown and removal of stranded oil are: (1)where the oil strands above the high-tide line; and (2)when an asphalt skin has formed on the oil, preventingthe oil/®ne particles from breaking away from the oilsurface. Neither of these exceptions apply to the deeplypenetrated oil in the gravel beaches of Prince WilliamSound. The oil in these beaches is liquid enough to forma slick on the water table in trenches, so it seems itwould be susceptible to ¯ushing, particularly if oil/®neparticle interactions were occurring. Thus, it appearsthat deeply penetrated oil is another exception to theconditions under which oil/®ne particles interaction ise�ective. The result of our studies can be used to iden-tify those parts of shoreline where natural removal

Fig. 2 (continued)

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processes are likely to be slow and thus oil residuespersistent.

This study was sponsored by the Hazardous Materials Response andAssessment Division of NOAA, Jerry Galt, Division Chief; RobertPavia, Contracting O�cer Technical Representative; and GaryShigenaka, Project Technical Manager. Chemical analyses were car-ried out at the Environmental Studies Institute of Louisiana StateUniversity by Charles B. Henry, Jr. and Pauline O. Roberts. We wereassisted in the ®eld by numerous individuals over the 8-year period, towhom we are extremely grateful. Graphics and other technical supportwere provided by Joe Holmes, Dot Zaino, and Becky Cox of the sta�at Research Planning, Inc.

Bragg, J. R. and Yang, S. H. (1995) Clay-oil ¯occulation and its e�ectson the rate of natural cleansing in Prince William Sound followingthe Exxon Valdez oil spill. In Exxon Valdez Oil Spill Ð Fate andE�ects in Alaskan Waters, eds P. G. Wells, J. N. Butler and J. S.Hughes, pp. 178±214. ASTM STP 1219, American Society forTesting and Materials, Philadelphia, PA.

Hayes, M. O. and Michel, J. (in preparation) Factors determining thelong-term persistence of Exxon Valdez oil in gravel beaches. MarinePollution Bulletin 38, 92±101.

Henry, Jr., C. B., Roberts, P. O., Overton, E. B., Frederick, M.,LeBlanc, R. and East, R. (1997). Summary of chemical results: 1991Prince William Sound Geology cruise. Institute for EnvironmentalStudies, Louisiana State University, Baton Rouge, LA.

Owens, E. H. and Sergy, G. A. (1997) Application of recent technicaladvances to the decision process for shoreline treatment. InProceedings, 1997 International Oil Spill Conference, pp. 289±295,American Petroleum Institute, Washington, DC.

Sauer, T. C., Brown, J. S., Boehm, P. D., Aurand, D. V., Michel, J.and Hayes, M. O. (1993) Hydrocarbon source identi®cation andweathering characterizations of intertidal and subtidal sedimentsalong the Saudi Arabian coast after the Gulf War oil spill. MarinePollution Bulletin 27, 117±134.

Shigenaka, G. (ed.) (1997). Integrating physical and biological studiesof recovery from the Exxon Valdez oil spill. NOAA TechnicalMemo. NOS ORCA 114. National Oceanic and AtmosphericAdministration, Seattle, Washington.

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weathering patterns of oil residues eight years after the exxon valdez oil spill

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