Variability in estimates of oil contamination in the intertidal zone of a gravel beach

Marine

Ahlrme I'olhlm.i Bullenn Vt,I I~ No I I pp 4 1 2 - 4 t 6 Printed In Gr~-at Britain

Polluhon

Bulletin

01)2~-~26X 84 S~ 0 0 + 0 ()(I © 1984 PtJ t~amon P ~ , s Lid

1984

Variability in Estimates of Oil Contamination in the Intertidal Zone of a Gravel Beach EDWARD H OWENS

Woodward-Clyde Oceaneermg, Abbotswell Road, West Tullos, Aberdeen ABI 4AD, UK Estimates of the surface cover of stranded oil on a gravel beach ranged between 25 and 51%. This variability is attributed primarily to wet weather conditions that altered the colour of the surface substrate, thus making identification of oil or oiled sediments difficult. Estimates and the observations from three surveys in wet weather ranged between 25 and 46%; whereas on a dry day the range from four surveys was only 42-51%. Within this small study area (7200 m 2) the same observer provided estimates of the surface oil cover that ranged from 1800 to 3 6 8 0 m 2. This wide variability for a small section of shoreline would probably be magnified during reconnaissance surveys where long sections of coast are surveyed to determine the degree of contamination. Surveys of beach contamination for cleanup decisions or for damage assessment should be conducted with regard to the environmental (weather) conditions and the degree of accuracy that is required from the survey. In situations where oil spills contaminate coastlines it 1s frequently important to establish the amount of oil that IS present at the coast in order to develop spill-response actions or to carry out damage-assessment surveys Except in rare circumstances, it IS unlikely that precise calculations of the amount of oil that is stranded are possible (ITOPF, 1983) However, it IS frequently necessary to establish an oil budget and in ttus respect estimates of the surface-oil coverage or the volume of stranded oil are required In particular, it is necessary to determine the amount of oil that resides in the shore zone in order to determine the area a n d / o r volume of stranded oil that would have to be removed for a beach cleanup operation Several problems exist in attempting to estimate the amount of stranded oil, these include the presence of dark-coloured bands of seaweed on the beach surface or heavy minerals in beach sediments, or hchen on bedrock or boulders In addmon, accuracy levels will vary depending on the amount of time available for a shoreline survey and on the size of the area to be covered Following an experimental oflspill in arctic Canada in 1981 (Blackall & Sergy, 1983), a series of shorehne studies were undertaken to provide a budget of the oll contamination in a small bay adjacent to the spill site A total of 15 m e of aged Largo Medlo crude oil was dincharged on the surface of the nearshore waters, within 200 m of a gravel beach (Dickens, 1982) In the summer of 1983 a series of tests were carried out to survey the amount of oll that remained In the shore zone These tests 412

involved both ground and aerial estimates, as well as detailed observations along surveyed profile lines in the intertidal zone The tests were c a m e d out to evaluate the varlabihty of different estimation techniques and also to assess any differences that might occur between two expenenced observers An additional factor was introduced by undertaking surveys in both wet and dry weather conditions during the test period

Field Site The tests were carried out on a beach adjacent to Ragged Channel on C a p e Hatt, northern Baffin Island, Canada (72 ° 31 ' N, 79 ° 50' W) The shore zone is a pocket beach approxtmately 400 m m length and with an intertidal zone that vanes in width between 20 and 35 m (Fig 1) The location is a very sheltered environment with an open-water season of 2 - 3 months each year (Owens et al, 1983a) The intertidal sediments are poorly sorted and range from silt/sand to gravels that are characterized by pebble and cobble-sized materials in a sand matrix The site, which was designated as 'Bay 11' in this senes of experiments, was surveyed m 1982 and 1983 to determine changes in both the topography of the intertidal zone and also the surface oil cover (Owens et al, 1982, 1983b)

Survey Design A total of seven surveys were conducted under different conditions and by different observers over 3 days in August 1983 A d~stInction is drawn between (1) estimates, which were obtained as a single visual assessment of the surface oll cover, and (2) observations, which were recorded along nineteen profile lines at 2 m intervals across the intertidal zone An estimate was made from the air, in a helicopter at approximately 100 m elevation, on a d a m p day following a period of rain On the same day a ground-level estimate was made from a rock outcrop at the northern end of the beach, approximately 5 m above the high-water mark Ground estimates were obtained subsequently 3 days later, on a dry day, simultaneously and independently by two observers standing on the rock outcrop at the northern end of the beach at the same location as the wet day estimates

Volume 15/Number 1 l / N o v e m b e r 1984

F~g 1 Aerml vtew to the north at low tide (approximately 100 m altitude) of study beach on the east shore of Ragged Channel, Cape Hatt, northern Baffin Island, N W T , Canada (14 August 1983)

These aerial and ground estimates are typical of the type of survey that ts undertaken during an od spill when an extensive length of coasthne has been contaminated by oll It was anuctpated that more accurate data could be obtained by recording detailed observations along 19 profile lines which were marked by a series of stakes in the backshore area The first set of observations was taken by a single observer on the same damp day as the hehcopter estimate and the first ground-level estimate From a fixed mark above the highwater line the observer paced 2 m and recorded an observation of the percentage surface off cover m the area 1 m ahead, 1 m behind and 10 m to each side of the profile line A second set of observations was made along the profile hnes by two observers, stmultaneously and independently, on the same dry day as the ground estimates were made by both observers Although both observers had discussed the survey destgn and also the general degree and type of contamination m the shore zone, no attempt was made to devise speofic criteria upon which the estimates and observattons would be based Thus the results would represent mdependent assessments of the surface oll cover in a situation that would replicate either of the observers conductmg such a survey alone m a real spill situation The total area of the intertidal zone m Bay l l is approximately 15 000 m 2 Initially (August 1981), 9500 m E were covered with oil but by August 1982 the contamlnated area increased to 11 300 m 2 (Owens et al, 1983b) At the time of this study (August 1983) the oil was confined to the northern half of the Bay 11 beach and covered approximately 3700 m e Beach sediment

samples were collected as part of the long-term momtormg programme for total hydrocarbon analysis The techntques used for this analysis and the results are presented elsehwere (Humphrey, 1984)

Results All estimates and observations were taken during the low-tide slack period either on a damp day with conlanuous drizzle, 12 August 1983, or on a day of clear skies with sunshine, 15 August 1983 Constdermg first of all the estimates of the surface oll cover (Table l a), both observers recorded slmdar estimates on the ground on the dry day A difference of only 5% is not considered slgmficant when attempting to estLrnate the surface oil cover over approxmaately 10 000 m 2 from a single locaaon The difference between the ground estimates of Observer A on wet and dry days is tnghly significant The differences m hght quality and m the colour of the intertidal substrate, due to surface wetness, resulted m an estLrnate on the wet day of only half TABLE 1

Estimates and observations on the per cent surface oll cover m the m t e m d a l zone Aerial (wet)

Observer A Ground Ground (wet) (dry)

(a) EsUmates of surface oil cover 35% 25

Observer B Ground (dry)

50

45

(b) Summary of surface od cover observauons 46 51

42

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Marine Pollutmn Bulletin

the surface off cover that was estimated 3 days later during dry weather conditions Assunung that the dry day esUrnates are more accurate than those from the wet day, the results suggest that a low-level aerial survey is hkely to be shghtly more accurate than a ground survey under wet or damp conditions. However, there ~s a sigmficant difference between the wet day esamates, both ground and aerial, and the dry day estimates A summary of the observations made along the profile hnes (Table lb) indicates a tugh level of consistency (+%5) between the three sets of results The number of observations of no oil recorded by Observer A decreased from 97 on the wet day to 88 on the dry day (Table 2) The difficulty m recording small quantities of oil on a pebble or cobble beach is an understandable source of vanablhty and flus is compounded when the surface of the sediment is wet The factor of surface wetness can also account for the larger number of observations by Observer A m the 7 5 - 1 0 0 % range on the wet day Observer B was apparently more conservative m l u s estimates of the surface oil cover and recorded almost 80% of the observations m either the 0 or the 7 5 - 1 0 0 % categones This bias towards the ends of the scale resulted m relatively few values having been recorded m the range 1 - 5 0 % (Table 2) The results show a high degree of conSlstency by each observer when comparing the ground estimates and the ground observations on the dry day (Table 1)

TABLE 2 Comparison of number of observations of the surface oll cover by category

Ollcover 0% 1-24 25-49 50-74 75-100%

Obse~er A Ground Ground (wet) (dry) 97 88 34 37 16 28 16 14 17 13

Observer B Ground (dry) 105 20 4 14 37

Discussion Two assumptions have been made m assessing the results of these surveys The first is that in this very sheltered, low-wave energy environment there was no difference m the surface oil cover between 12 August and 15 August 1983 The second assumption is that the observations and estirnates obtamed on the dry day are more accurate than those obtmned on the wet day, due to differences m light quality and the surface wetness of the intertidal substrate The most significant point that emerges from this field experiment is that the results of the surface oil cover surveys range between 25 and 51% In applying tins result to real-world spill situations the lmphcation is that surface od cover esttmates m the intertidal zone may vary significantly depending upon the type of survey conducted and the environmental 0 e weather) conditions at the time of that survey The first conclusion that may be drawn from the 414

surveys is that ground estimates on a wet day are hkely to provide low values m terms of the percentage surface oil cover on a contaminated mtertldal zone Figure 2 illustrates the visual differences that erasted durmg this survey in the intertidal zone on the wet and dry days These black and white photographs provide less information than do the original colour slides but nevertheless tlhistrate the difference m the appearance of the intertidal zone Aerial esttmates would appear to be better than ground estLrnates on a wet day, but both aerial and ground estimates are considerably less accurate than the more detailed series of ground observations that can be obtained from transects or profiles surveyed across the intertidal zone on a wet day On sand beaches the contrast between the colour of the sediments and the oil is usually much greater than on gravel or cobble beaches The varmbdlty of results on these beaches by reconnmssance surveys is less ltkely to be a problem, even on wet days Each of the two observers recorded the estimate of surface oil cover on the dry day before conducting the more detailed set of observations For both observers this brief experiment suggests that a ground estunate may be as good as a more tmae-consumlng series of observations for most purposes However, it is to be expected that different observers will use shghtly different citeria or standards in their esttmates or observations F r o m this experiment it cannot be Lmphed that one observer is more accurate than another but only that the ground observations by Observer A are probably more accurate on the dry day than that individual's observatmns on the wet day As part of the overall study, samples of beach sediments were collected to deternune the oil in sediment by weight and the geochermcal character of the oil (Owens, 1984) These detailed measurements of the oil-msediment by weight provide data on the exact amount of oil in a sample of beach material However, the distribution of stranded od IS usually extremely variable, particularly on gravel beaches, so that the results of sample surveys should be used in conjunction with broader surveys that estimate the surface oil cover The integration of one data set, in which analyses are conducted to accuracies of ppm, wRh another that has accuracies m the order of 5 or even 10%, would appear to be a mismatch. In reahty the two data sets are complementary and can prowde a valuable input to declsmns regarding the area of an oiled beach that would be cleaned and the volume of contaminated material that would be removed

Conclusions The contaminated northern section of the study beach is relatively small in area, approximately 7200 m 2, but the different surveys yield surface oll cover values that range between 1800 and 3680 m E This vanabthty has serious tmphcatlons if the data are to be used for the development of beach cleanup decisions or for the assessment of damage potential Large underestimates or overestimates of the volume of oil remammg on a

Volume 15/Number I I/November 1984

(a)

(b)

Ftg 2 (a) Intertidal zone of the study beach on wet day (12 August 1983) (b) Same view on dry day (14 August 1983) The arrow and circle locate identical features

beach can lead to errors m evalualang the cost or the length of time that imght be required to treat the shore zone A e n a l reconnaissance surveys or ground eslamates on damp or wet days prowde only general information on the surface Oli cover of gravel or cobble beaches If data are reqmred that will be apphed to cleanup decisions or

to damage assessment then ground observations or measurements would be necessary to meet these more accurate requirements The choice of method for a survey of the contaminated area will depend upon the ultimate use of the data Accuracy is a function of both the level of effort as well as the envaronmental conditions During wet weather condmons, aenal or ground esta415

Marine Pollution Bulletin Baffin Island Od Spill Working Rept 81-9, Environment Canada, Edmonton, Alberta Humphrey, B (1984) Baffm Island Off Spill Island Project--Chermstry Component Volume I Summary of Field Work and Shorehne Hydrocarbon Analyses Final Report to Environmental Protectmn Service by Seakem Oceanography, S~dney, B C ITOPF (1983) Recogmtmn of Oil on Shorelines Tech Informatmn Paper No 6, Int Tanker Owners PoUutmn Fed Ltd, London Owens, E H (1984) Shoreline Countermeasures--1983 Study Results (BIOS) Baffin Island Off Spill Working Rept 83-4, Emaronment Canada, Edmonton, Alberta (m press) Owens, E H , Harper, J tL & Foget, C R (1982) Shorehne Countermeasures--1981 Study Results (BIOS) Baffm Island Od Spill Working Rept, 81-4, Enwronment Canada, Edmonton, Alberta Owens, E H, Harper, J R, Foget, C R & Robson, W (1983a) Shoreline experiments and the persistence of off on arctic beaches Proc 1983 Od Spdl Conf, San Antomo, TX, Amer Petr Inst pub No 4356, pp 261-268 Waslungton, D C Owens, E H, Harper, J R & Foget, C R (1983b) Shoreline Countermeasures--1982 Study results (BIOS) Baffin Island Off Spill Working Rept 82-4, Envtronment Canada, Edmonton, Alberta

mates on gravel beaches are hkely to be considerably lower than smular dry day estimates On dry days a single, careful ground estunate may be considerably less timeconsuming, yet may be within the same accuracy range as a detailed mte~dal survey

The study was conducted as part of the Baffin Island Oil Spill ProJect (BIOS), an mternatmnal joint industry and government research programme The loglsUc support and funding prowded through this project is gratefully acknowledged

Blackall, P J &Sergy, G A (1983) TheBIOS project--anupdate Proc 1983 Od Spdl Conf, San Antomo, TX, Amer Petr Inst pub No 4356, pp 451-455 Washington, D C Dickens, D F (1982) Discharge Systems--1981 Study Results (BIOS)

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I I pp 4 1 6 - 4 1 8 1984

Trace Metals and Organochlorines in Plankton from the Southern Baltic Plankton samples, collected in the southern part of the Baltic Sea in 1983, were analysed for their trace metals (Cd, Pb, Cu, Zn, Mn, NI and Fe), organochlorIne pesticides (HCB,)~BHC and FDDT) and polychlonnated biphenyls (PCBs) content Trace metals, organochlorlne pesticides and polychlorlnated blphenyls levels in plankton were measured as a part of a pollution picture programme carried out in the southern part of the Baltic Sea in 1983 (Fig 1) 15" i

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Plankton samples were obtained by Nansen net (200 ~tm) between 29 August and 12 September 1983 during the cruise of R V Profesor Sledleckl The results of trace metal analyses based on a wet weight and on a dry weight basis, and water content of plankton samples, are given in Table 1 Table 2 gives the results of organochlorlne analyses (on a wet weight and on a hpld weight basis), water content and petroleum ether extractable lipid content of plankton Plankton samples analysed consisted mainly of zooplankton, but in nearshore samples collected phytoplankton was also present Trace metal concentration of plankton usually differs due to the dtfferences in spectes composition, plankton density, and environmental factors temperature, salinity, J

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