Slightly Weathered Exxon ValdezOil Persists in Gulf of Alaska BeachSediments after 16 YearsJ E F F R E Y W . S H O R T , * , † G A I L V . I R V I N E , ‡

D A N I E L H . M A N N , § J A C E K M . M A S E L K O , †

J E R O M E J . P E L L A , ⊥

M A N D Y R . L I N D E B E R G , †

J A M E S R . P A Y N E , |

W I L L I A M B . D R I S K E L L , # A N DS T A N L E Y D . R I C E †

Auke Bay Laboratory, Alaska Fisheries Science Center,National Marine Fisheries Service, NOAA, 11305 GlacierHighway, Juneau, Alaska 99801-8626, U.S. Geological Survey,Alaska Science Center, 1011 East Tudor Road, Anchorage,Alaska 99503, Institute of Arctic Biology, 907 Yukon Drive,University of Alaska, Fairbanks Alaska 99775,Post Office Box 210332, Auke Bay, Alaska 99821,Payne Environmental Consultants, Inc., 1991 Village ParkWay, Suite 206 B, Encinitas, California 92024;6536 20th Avenue NE, Seattle, Washington 98115

Oil stranded by the 1989 Exxon Valdez spill has persistedin subsurface sediments of exposed shores for 16 years.With annualized loss rates declining from ∼68% yr-1

prior to 1992 to ∼4% yr-1 after 2001, weathering processesare retarded in both sediments and residual emulsifiedoil (“oil mousse”), and retention of toxic polycyclic aromatichydrocarbons is prolonged. The n-alkanes, typically veryreadily oxidized by microbes, instead remain abundant inmany stranded emulsified oil samples from the Gulf of Alaska.They are less abundant in Prince William Sound samples,where stranded oil was less viscous. Our results indicatethat, at some locations, remaining subsurface oil may persistfor decades with little change.

IntroductionOil weathering processes, including evaporation, dissolution,and microbial oxidation, eventually remove even recalcitrantconstituents in situ (1), while physical dispersion mediatedby wave impacts accelerates these processes. But once oilfrom accidental spills percolates into stable, porous beaches,transformation through weathering in anoxic sediments isslowed (1). In sediments protected from physical dispersion,weathering rates are also considerably reduced. As a result,oil buried in anoxic sediments may retain toxic componentssuch as polycyclic aromatic hydrocarbons (PAH) on timescales of years to decades (2-5). In contrast, our study oflingering oil from the 1989 Exxon Valdez spill demonstratesoil preservation in aerated sediments for more than a decadeby a different mechanism. Evidence presented here suggests

preservation can be a result of the viscous, often emulsifiedstate of the oil when deposited.

The Exxon Valdez spill deposited ∼20 000 metric tons ofoil on beaches, with ∼85% of this inside Prince William Sound(PWS) (6). Some was incorporated into porous sediments,occasionally to depths of 1 m or more (7). High annual ratesof subsurface oil loss from beaches within PWS (∼68% yr-1

from 1991 to 1992; ref 7) initially implied that remainingsubsurface oil would soon be reduced to negligible amounts(8). However, by 2001, ∼100 tons of oil remained on beacheswithin PWS, mostly buried within the uppermost half-meter(9, 10), implying loss rates declined within PWS to ∼20% yr-1

during the period 1992-2001 (9). An unknown additionalamount of oil remained elsewhere along the northern Gulfof Alaska (GOA) coastline (Figure 1) (11, 12). These trendsimply ever slower loss rates for subsurface oil still present.

Declining oil loss rates arise mainly from geomorpho-logical variation among oiled beaches. Within the spill-affected region, a surface layer of cobbles to boulders oftenprotects underlying finer-grained sediments from physicaldispersion, thus providing the stability needed for oilpersistence (11-13). Variation in substrate-conferred stabilityaccounts for much of the variation in oil persistence atdifferent sites, with the most vulnerable oil dispersed morereadily. By 2001, subsurface oil presumably remained inespecially well-protected sediments where future dispersiondepends on abnormal disturbance events, and suggestedloss rates even slower than 20% yr-1. To test this, we evaluatedspatial changes in the area contaminated with subsurface oiland associated oil mass from 2001 to 2005 on 10 beaches inPWS. We also analyzed the composition of oil remaining atsites inside PWS and outside along the coast of the Gulf ofAlaska, to determine the extent of weathering in subsurfaceoil deposits.

Materials and MethodsTen beaches were randomly selected from 42 PWS beachesthat had subsurface oil in 2001 (Figure 1). At each beach, theupper intertidal was partitioned into contiguous rectanglesbounded above and below by tidal elevation strata, and twoquadrats were randomly located within each rectangle. Tooptimize the number of beaches and sampling intensity,larger rectangles were used in 2005 resulting in lowersampling density of quadrats (0.96 quadrats m-1 of alongshoredistance in 2001 compared with 0.3 in 2005). In the field, oilwas detected visually or by smell, sampled, and the ExxonValdez source confirmed by analysis of alicyclic biomarkersusing identification criteria given in ref 14. Our estimates ofoiled beach area derive from oil encounter rates, estimatedas the ratio of oiled (k) and total (N) randomly located 0.25m2 quadrats excavated to ∼0.5 m depth, within identicalbeach segments and tidal elevation strata both years. Thearea containing subsurface oil on each beach was estimatedas the sum of the rectangle areas containing oiled quadrats,where only half the rectangle area was included if only oneof the two quadrats contained oil (see ref 9 for details). Tofacilitate estimation of associated oil masses, we also classifiedthe intensity of oiling as heavy [HOR], medium [MOR], andlight [LOR] oil residues based on criteria given in ref 15. Theproducts of oiled beach area and the mean amount of oil perunit area for each oil class were summed across oil classesto estimate the mass of oil remaining on each beach. Onebeach (Site 3, Table 1) had only a trace of oil in 2001 but lightresidues were found in 2005.

We used a Bayesian approach (16) to distinguish the lossrate of oil from sampled beaches, as measured by the

* Corresponding author phone: 907-789-6065; fax: 907-789-6094;e-mail: Jeff.Short@noaa.gov.

† National Marine Fisheries Service.‡ U.S. Geological Survey.§ Institute of Arctic Biology.⊥ Auke Bay, Alaska.| Payne Environmental Consultants, Inc.# Seattle, Washington.

Environ. Sci. Technol. 2007, 41, 1245-1250

10.1021/es0620033 CCC: $37.00 2007 American Chemical Society VOL. 41, NO. 4, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 1245Published on Web 01/19/2007

disappearance of oiled quadrats with time, from the associ-ated background of inherent sampling variability (i.e.,sampling error). If the unknown proportions of a beach areathat remained oiled in sampling years 2001 and 2005 (a spanof 4 years) are denoted by p1 and p2, respectively, then p2 ep1 and p2 ) p1 × θ4, with θ being the annualized survival rateof oil at the beach, i.e., θ ) (p2/p1)1/4. If N quadrats weresampled from the beach at a particular time when theunknown proportion oiled was p, then the observed numberof oiled quadrats on the beach (k) is assumed to be a binomialrandom variable, i.e., k ∼ binomial (p, N). If the numbers ofquadrats sampled in 2001 and 2005 are denoted by N1 andN2, respectively, and the corresponding numbers of sampled

quadrats that were oiled were k1 and k2, then the observedannualized survival rate is θi

obs ) [(k2/N2)/(k1/N1)]1/4. Thisobserved survival rate is subject to considerable samplingerror when the portion of the beach included in the sampleis fairly small as for the present study. Further, this observedsurvival rate is undefined when k1 is zero and provides littleinformation on the annualized loss rate (1 - θ) when k2 iszero. To better estimate the annualized survival rates amongthe beaches sampled, the data from all beaches are analyzedsimultaneously under a single overarching model. Therandom changes in observed oiled proportions at the beaches(a subscript i is added to identify each particular beach) aremodeled as normally distributed following logit transforma-tion, specifically the model structure is logit(p1i) ) log [p1i/(1- p1i)] ) µi, logit(p2i) ) µi - δi

2, and δi is normally distributedwith mean d and precision parameter τ ()1/σ2). Use of δi

2

ensures that the model is constrained to losses of oiled beacharea only.

The Bayesian model was fitted to the data in Table 1 usingGibbs sampling (17), which provided estimates for thefrequency distribution of the annualized change θi, fromwhich the mean θhi and the value corresponding to the 10thpercentile of the distribution θi

(0.10) were estimated. Thisapproach allowed for variation in oiled proportions amongbeaches within and between years. Uninformative priors wereused for the unknowns, specifically d ∼ normal(0, 10 - 6), τ∼ gamma(10 - 3, 10 - 3), µi ∼ normal(0, 10 - 5), i ) 1,... ,10

A burn-in of 50 000 samples was discarded, and the next30 000 samples were used to describe the posterior distribu-tion of the unknowns.

Comparison of oil compositional changes provides a moresensitive indication of oil loss rates, reflecting in situweathering processes acting to remove oil components.Biodegradation typically acts first on n-alkanes, followed bybranched and alicyclic alkanes and PAH (18). To assess the

FIGURE 1. Locations of beaches sampled in Prince William Sound and the Gulf of Alaska. Curved lines indicate spill progress in 1989.

FIGURE 2. Estimated annualized loss rates of subsurface oil duringthe periods indicated.

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FIGURE 3. Proportion of total PAH (upper panel) and total n-alkanes (lower panel) remaining in samples collected from Prince WilliamSound and elsewhere in the Gulf of Alaska from 1989 to 2005.

TABLE 1. Ratio of Oiled (k) and Total (N) Quadrats Sampled, and Estimates of Oiled Beach Areas, Oil Mass, and AnnualizedSurvival Rate of Beach Area Oiled (Observed, θi

obs; Posterior Mean, θhi; and 10th Percentile, θi(0.10)) on 10 Beaches Sampled in

2001 and 2005 in Prince William Sound, Alaska (Annualized Loss Rate ) 1 - θ)

k/N oiled beach area (m2) oil mass (kg)

site 2001 2005 2001 2005 2001 2005 θiobs

1-θhi 1 -θi(0.10)

1 2/12 2/6 40 126 24 201 1.19 0.03 0.062 6/48 2/18 156 119 171 100 0.97 0.03 0.093 0/90 2/30 0 234 0 139 - 0.03 0.064 6/96 2/30 221 154 131 91 1.02 0.03 0.085 4/84 3/30 147 355 216 210 1.20 0.03 0.066 1/96 1/30 23 54 24 32 1.35 0.03 0.087 3/46 1/18 94 72 72 43 0.96 0.04 0.108 1/94 0/30 25 0 37 0 0 0.04 0.119 3/32 1/18 44 68 26 40 0.88 0.04 0.11

10 8/96 1/30 200 75 230 82 0.79 0.04 0.13

total 34/694 15/240 950 1260 931 938(95% CI) (586-1390) (627-2010) (440-1400) (377-1580)

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effects of these processes, we analyzed 43 samples ofsubsurface oil collected at intervals from seven locations alongthe GOA outside PWS from 1989 through 2005, and comparedthese with 38 samples collected from inside PWS in 2001(Figure 1). We estimated the proportions of total n-alkanes(Σn-alk) and total PAH (∑PAH) remaining by comparingconcentrations normalized to the chrysenes (the mostpersistent PAH readily detectable among those analyzed) inthe samples with normalized concentrations in oil collectedfrom the sea surface 11 days following the spill. The ratio ofthese normalized concentrations provides an indication ofhydrocarbon persistence relative to the chrysenes, discount-ing evaporative losses, which were nearly complete by 11days (19). Normalization is necessary to account for variableinclusion of sediments and water in the oil samples.

Normal alkane analytes included C10 through C34 analyzedby gas chromatography with a flame ionization detector, andPAH included 2- to 4-ring un- and alkyl-substituted homo-logues analyzed by gas chromatography with mass selectivedetector (GCMS; see ref 20 for analysis details). Alicyclicbiomarkers were determined by GCMS following ref 14.

Results and DiscussionDirect comparison of results from 2001 and 2005 indicatesa slight apparent increase of oiled beach area and associatedoil masses (Table 1). Mean oil encounter probabilitiesincreased from 0.049 in 2001 to 0.063 in 2005. The estimatedcumulative oiled beach area and oil mass similarly increasedfrom 950 to 1260 m2 and from 931 to 938 kg, respectively,increasing nearly as often as decreasing at individual beaches(Table 1). The percent of oiled quadrats classified as HOR,

MOR, LOR was 6, 41, and 53% in 2001, compared with 7, 20,and 73% in 2005, a change that was not statistically significant(ø2 ) 2.09, P ) 0.35, df ) 2).

The apparent increases in oiled beach areas and massesbetween 2001 and 2005 reflect sampling error because nocrude oil had been added to these beaches, and the highviscosity, high adhesion, and low concentration of the oil inthese sediments preclude substantial diffusive spreading (10,21). For these reasons, we assumed that the actual proportionof beach area that is oiled, p, is constrained to declinemonotonically with time.

Our analysis shows that the most likely rate of decline ofoiled beach area within PWS from 2001 to 2005 is 3-4% yr-1

(1 - θ; Table 1). The probability that the actual decline ratesexceed 10% per year is less than about 10% (1 - θi

(0.10), Table1). These rates are considerably slower than those noted priorto 2001 (Figure 2), confirming that physical dispersion rateshave since slowed.

Degradation rates of PAH and n-alkanes were comparableat the GOA sites, but the rates of n-alkane degradation at thePWS sites were generally higher. The Σn-alk at the GOA sitesoften contained ∼50% or more of the burden in the 11-dayoil, whereas it was usually below 10% at the PWS sites (Figure3). However, well-preserved n-alkanes and PAH persistedfor over a decade at some sites both inside and outside PWS(Figures 4 and 5), and oil in samples containing these alkanesremains semi-liquid and highly adhesive on contact withdry surfaces. The PAH at the GOA sites were even more

FIGURE 4. Aliphatic chromatograms of Exxon Valdez oil collectedfrom (A) the sea surface 11-day following the incident, (B) subsurfacesediment in Prince William Sound, Alaska, 12 years later, and (C)subsurface sediments from Cape Douglas in the Gulf of Alaska, 16years later. Symbols: C no., n-alkane with no. of carbon atoms; d-Cno., deuterated n-alkane standard; DCH, dodecycylohexane internalstandard; Pr, pristane; Ph, phytane.

FIGURE 5. Total ion current chromatograms of PAH in Exxon Valdezoil collected from (A) the sea surface 11-day following the incident,(B) subsurface sediment in Prince William Sound, Alaska, 12 yearslater, and (C) subsurface sediments from Cape Douglas in the Gulfof Alaska, 16 years later. Symbols: d-no., deuterated PAH; HMB,hexamethylbenzene internal standard.

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persistent; with one exception (Cape Gull), subsurface oilretained from ∼25-75% of the ∑PAH through 1999 that werepresent in the 11-day oil, and were still ∼25-50% by 2005(Figure 3). Somewhat lower ∑PAH were evident in the 2001PWS samples, which ranged from ∼10-50%. Overall, theseresults show remarkable preservation of n-alkanes at somesites, and imply PAH degradation rates on the order of ∼3-10% per year at most sites.

Although anoxia may account for long-term persistenceof biodegradable oil components elsewhere (2, 3), it is unlikelyto be a factor at our study sites. Intertidal sediments of PWSand the GOA usually contain <5% total organic carbon (22),and sedimentary oil refuges armored by boulders are stable,porous and well-flushed by ∼5 m tides and wave exposure,especially in the GOA (11, 12, 23). Low concentrations ofnitrate in seawater during warm seasons (24) may also retardbiodegradation. Extensive degradation at some sites (e.g.,Cape Gull, Figure 3) suggests that cool temperatures are aminor factor, because ocean surface temperatures are highlyinter-correlated across the spill-affected region (25), yetextensive degradation occurred at some sites.

More importantly, emulsification leading to “mousse”formation inhibited weathering processes directly (11, 26),and caused the viscosity of the oil to increase substantiallyas it was transported along the GOA coast. Within PWS duringthe early stages of the spill, less viscous oil readily percolatedinto the generally finer-grained beaches (10). As the spillprogressed along the GOA, the often greater porosity of moreexposed beaches allowed the increasingly viscous oil topercolate into them as well (11, 12). Inherent with the trapped,viscous oil parcels, low ratios of surface area to volumeprovided comparatively little habitat for oil-degrading mi-crobes. Low oil surface to volume ratios may also have playeda role in the persistence of n-alkanes following oil spillselsewhere (2, 27).

Our results indicate that the remaining subsurface oil maypersist with little change for decades, even in sediments thatare not anoxic. Such persistence can pose a contact hazardto intertidally foraging sea otters, sea ducks, and shorebirds(10, 28), create a chronic source of low-level contamination(29), discourage subsistence in a region where use is heavy,and degrade the wilderness character of protected lands (11,12).

AcknowledgmentsThis study was supported in part by the Exxon Valdez OilSpill Trustee Council and the Prince William Sound RegionalCitizens’ Advisory Council, but the findings and conclusionsare those of the authors and do not necessarily reflect theviews or positions of these organizations.

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Received for review August 20, 2006. Revised manuscriptreceived November 30, 2006. Accepted December 5, 2006.

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