Journal of Coastal Research Fort Lauderdale, Florida Winter 1996

Organochlorine Contamination of UK Essex CoastSalt Marsh Sediments

M.D. Scrimshaw, J.M. Bubb and J.N. Lester

Environmental and Water Resources EngineeringDepartment of Civil Engineering

Imperial College of ScienceTechnology and Medicine, London SW7 2BU

ABSTRACT .

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SCRIMSHAW, M.D.; BUBH, ,LM., and LESTER, J.N., 1996. Organochlorine contamination of UK Essexcoast salt marsh sediments. Journal of Coastal Research, 12(1), 246-255. Fort Lauderdale (Florida),ISSN 0749-0208.

Subsequent to an initial survey of sediments from five salt marshes along the UK Essex coast, sampleswere taken from a further two sites in Essex and one in North Norfolk. Sites were selected to assess theimpact of sediment sources, particle size distribution and organic carbon content upon organochlorinecontamination. The determination of average and median concentrations of insecticides and PCB insediment from cores to a depth of 35 em identified a degree of contamination that may be consideredbackground in this area. Two sites, at Two Tree Island (in the Thames estuary) and at Bridgewick onthe Dengie peninsula, were found to exhibit enhanced degrees of contamination.

There was no evidence to link elevated sediment organochlorine concentrations with higher organiccar bon content or particle size distribution of the sediments. Distribution of contaminants within sedimentparticle size fractions, was however, related to the proportion of organic carbon in the <64fJm fraction.The most significant factor influencing the degree of contamination with OeL and PCB is likely to bethe source of sediments deposited on the salt marsh. Results imply that distance from the Thames estuary,associated sewage sludge dumping areas in the Barrow Deep and the effects of the mass movement ofwater into the southern North Sea via the English Channel may determine the relative enhancement oforganochlorines in sediments of the Essex salt marshes.

ADDITIONAL INDEX WORDS: Insecticides, polychlorinated biphenyl, micro-pollutant, littoral, pol­lution.

INTRODUCTION

A recent survey of five salt marshes situatedalong the UK Essex coast has identified the pres­ence of a number of organochlorine insecticides(OCL) and polychlorinated biphenyls (PCB) inthe sediments (SCRIMSHAW et al., 1994). The con­centrations of OCL and PCB varied by up to twoorders of magnitude between sites and such dif­ferences in contaminant loadings within sedi­ments were attributed to the influence of inputsources.

Synthetic organic micropollutants, such as theOCL and PCB do not occur naturally. Any tracesof these compounds detected in the environmenttherefore indicate anthropogenic input. Sourcesof such contaminants include urban, industrialand agricultural run-off, atmospheric deposition,municipal and industrial effluents and leachatesfrom waste disposal sites (BEDDING et al., 1982;MEAKINS et al., 1994). Organochlorine com­pounds have been detected in remote areas such

94089 received 10 October 1994; accepted in revision 3 February 1995.

as the Pacific Ocean (IWATA et al., 1993) and thepolar regions (HARGRAVE et al., 1988). Their ubiq­uitous occurrence implies that there may now beconcentrations of OCL and PCB in certain facetsof the environment, such as sediments, oceans andthe atmosphere, that are the norm rather thanthe exception. Salt marsh sediments along the UKEssex coast may display such a background. Wheresediment concentrations of OCL or PCB occurabove or below this value, local factors may sig­nificantly influence the degree of contamination.Accretion and erosion rates, proximity to contam­inant sources such as agricultural or industrialuse, effluent discharge or sewage disposal are like­ly to influence the relative contamination of in­dividual salt marshes.

Within the hydrosphere, due to their insolu­bility in water and high adsorptive tendencies,organochlorine compounds are predominantly as­sociated with particulate matter. The organic car­bon content and particle size distribution of sed­iments have been demonstrated to be the majorfactors influencing the sorption onto sediments(HIRAIZUMI et al., 1979; LARA and ERNST, 1993a,

Journal of Coastal Research, Vol. 12, No.1, 1996

Elevated Sediment Organochlorine Concentration 247

1993b) and subsequent partitioning back(KARICKHOFF et al., 1979; HORZEMPA and Dr TORO,1983) of OCL and PCB into the aqueous phase.

The objective of extending the study of UK saltmarsh sediments was to identify a level of con­tamination with DCL and PCB that may now beconsidered a background value, influenced byglobal factors such as atmospheric deposition andlocal accretion of sediments.

MATERIALS AND METHODS

Sampling of sediments using the field corer de­veloped at Imperial College (FLETCHER et al., 1994)and analytical methodology used for the deter­mination of OCL and PCB (SCRIMSHAW et al.,1994) were employed to ensure consistency of re­sults. Core locations and site descriptions are giv­en below. At all three sites, transect lines were setup from datum points situated at convenient lo­cations. Levelling of the sites along the transectsto obtain the elevation of core positions aboveOrdnance Datum (OD) Newlyn was carried outusing a Zeiss Tilting Level.

Site and Core Descriptions

Oakley, Hamford Water

This was once a large area of continuous, shel­tered marsh; it has suffered extensive erosion inthe past two decades resulting in large areas re­verting to mud fiat, interrupted by vegetatedmounds (Figure 1, Site 2). The vegetation can beclassified as SM13 Puccinellia maritima saltmarsh, according to the National Vegetation Clas­sification (NVC) (RODWELL, 1994). Other speciespresent included Suaeda maritima, Atriplex por­tulacoides, Limonium vulgare and Aster tripo­lium. Potential local inputs to the site are throughrun-off from the sluice exit, and the sample tran­sect was located near a sluice in anticipation ofobserving the greatest impact from this source.The site was sampled in January 1993. A channelcarrying fun-off from adjacent agricultural landwas used as a starting point for the transect. Thefollowing cores were taken:

Core 1 20 m from the datum point, 30 em indepth, mud flat environment.

Core 2 40 m from the datum point, 50 em indepth, vegetated with P. maritima only.

Core 3 105 m from the datum point, 45 em indepth, mud flat environment.

Core 4 141 m from datum point, 50 em in depth,vegetated with P. maritima only.

Dengie peninsula, Bridgewick Farm

This site (Figure 1, Site 3) is situated in anextensive area of open coast marsh, currently un­dergoing rapid lateral erosion at the marsh edgeand displaying vertical accretion on the uppervegetated areas. The area is a wide stand of SM14salt marsh (NVC), with Halimione portulacoidesand P. maritima exhibiting co-dominance. Tran­sition from mud flat to upper marsh is very abruptwith only narrow fragmented Spartina zones. Theedge of the marsh displays characteristic 'mudmounds' where wave energy is dissipated (PE­THICK, 1992). The sample transect was, as at Oak­ley, situated adjacent to a sluice draining localagricultural land. The salt marsh at Dengie wassampled in March 1993, with a transect con­structed from south of a major creek receivingsluice discharges. The following cores were taken:

Core 1 60 m from the datum point (on the sea­wall), 40 em in depth, vegetated with pre­dominantly H. portulacoides.

Core 2 250 m from datum point, 40 cm in depth,vegetated with predominantly H. por­tulacoides.

Core 3 375 ill from datum point, 60 em in depth,muddy bank of a large creek with exten­sive shell banks in the surrounding area.

Core 4 475 m from datum point, 20 em in depth,mud flat environment 70m from theeroding marsh edge.

Stiffkey, North Norfolk

Stiffkey is an extensive mature marsh (Figure1, Site 1). The upper marsh is well vegetated andcan be classified as SMI3, SM14 (NVC). There isa gradual decrease in elevation traversing sea­wards and a sandy shingle bar acts as a naturalboundary between the upper and lower marsh,which displays a sparser and less diverse vege­tation dominated by Spartina spp. and Salicor­nia spp. There is no sea wall at the landward edgeof the marsh, which follows the 5m contour. Localagricultural land therefore drains diffusely ontothe marsh at this point. Stiffkey was sampled inJune 1993. The transect line ran to the west of adisused sewage outfall pipe that previously serveda small residential area. Mean High Water (MHW)is defined by a sand and shingle ridge which risesapproximately 0.8m above the surrounding marsh.The following cores were taken:

Core 1 183 m from the datum point, 30 ern indepth, higher marsh with varied vege-

Journal of Coastal Research, Vol. 12, No.1, 1996

248 Scrimshaw, Bubb and Lester

Sluice

SCALE

/,..

- --~

Indication ofcirculation of

the North Sea ( I)

HamfordWater_~V ~6~ 4 I \)'V'V~

7~ \~i~~f8~ \ I

\ I

"1.

l ~~ \ ' -S~fflCY: <~, r=-.,';HW

" 1. ' . 'L .(2) j{

JJ

100m

~r- Vegetated salt marsh 106' Mud mounds _ rills • 4 Walt on Backwaters

Legend 'oM . • 5 SalcottAgncultural land Mud tlat and creeks .6 Tollesbury Wall

(I) DELHEZ and MARTIN ( 1992).

Figure 1. Locat ion of sampling sites and transects showing location or core positi ons.

• 7 South Woodham• 8 Tw o Tree Island

Qy Sediment core

tation including Salicornia spp. and P.maritima.

Core 2 614 m from the datum point. 45 em indepth, mid-marsh with varied vegetation

including L. vu lgare and Armeria mar­itima.

Core 3 720 m from the datum point, 25 em indepth, lower marsh densely populated

J ournal of Coastal Research , Vol. 12, No.1 , 1996

Eleva ted Sed iment Organ ochlori ne Concentration 249

Table l. Com pa rison 0/ orga nochlorine insect icide and PCB concentrat ions in sa lt marsh sedime nt cores as concentration ran gesand percentage occurrence.

Oakley Dengie Stiffkey

OCL Compou nd Ran ge (ng g ') 'h, Ran ge (ng g ' ) ~~i.l Rang e (ng g ') (1/, 0

HCB < 0.1- 0.7 23 < 0.1- 0.3 54 < 0.1- 0.2 3:1a HC H < 0.1-0.2 :I nd 0 nd 0

/lHCH nd 0 < 0. 1- 3.8 14 < 0.1- 0.:1 25, HCH < 0.1 :1 .7 47 < 0.1- 2.2 6 1 nd 0Aldrin nd 0 < 0. 1- 1.3 50 < 0.1- 0.1 8

Dield rin < 0.1- 0.7 41 < 0. 1- 0.7 7:> < 0.1- 1.4 67Endrin < 0.1- 0.:1 34 nd 0 < 0. 1- 2.5 33up I ppD DT < O. I--4.H :14 < 0.1- 17.8 75 < 0.1- 1.2 83op + ppDD E < 0. 1- 2.4 59 < 0.1- 1.7 86 < 0.1- 1.8 58op + ppDDD <0 .1- 2.4 63 < 0. 1- 2. 1 86 -c0.1- 3.2 75PCB < 1-6 32 < 1-242 79 1- 8 100n 29 2H 12

n = number of samples ana lysed

with lower marsh vegetation includingSpart ina spp.

Core 4 855 m from the datum point, 30 em indep th , predominantly mud but spa rselyvegetated with Sparti na spp.

Physicochemical Parameters

Readings of redox potential were taken in-situadjacent to, bu t prior to extracti on of the coresusing a 1m pr obe (Russell pH Ltd ., Auchter­mu ch ty). T emperature and pH readings weretaken on site at 5 cm intervals along sedi mentcores immediate ly up on ext rac t ion. All cores weredivided by depth into 5 cm sections. Particle sizedistribution was determined by wet sieving com­plete sec tions of cores from Oakl ey and Dengie.Size fra ctions in th e ran ges of > 2000 urn, 2000­600 /-lm, 600-200 um, 200-64 /-lm and < 64 urn weredetermined to allow classification according to theUdder-Wentworth scale (T UCKER,1981). Sampleswere washed through nylon mesh es (Lockertex,Warringt on ) under vacuum with a known volumeof native sea wat er. The percentage of each frac­tion was calculated from mas ses remain ing on th emeshes afte r allowance for the volumes of eachfrac t ion retained for determination of OCL andP CB. Distribution of particles in th e < 64 urn frac­t ion was determined using a Malv ern 2600/36 00Laser Particle Sizer (Ma lvern Instruments Ltd .,Malvern) utilising a 300 mm lens accord ing to themanufacturers instructions .

Samples of total sed iment (pri or to particle sizefract iona tion) were ana lysed for OCL and P CB.Wher e analytes were found to be present abovedet ection limits, and th e < 64 /-lm fra cti on com-

prised < 95% of th e sedi me nt tota l, this fractionwas also analysed for OCL and PCB. Blank de­terminations were carried out on native sea water,and all glassware was ace to ne rin sed prior to use.Det ection limits wer e 0.1 ng g- I and 1 ng g-I (d ryweight) for OCL and PCB respe ctiv ely based ona sample conta in ing 20-25 ng dry weight of sed­iment. AJI results are corrected for recoveri es aspreviously reported (SCRIMSHAWet al ., 1994).

T otal organic carbon (T OC) in bulk sedimentand particle size fractions were determined usin ga simple wet chem ical oxid ation procedure andredox titration as described by GAUDETTE et al .(1974). Agreement of thi s method with resultsobtained through analysis of replicate samples byan elemental analyser were considered acce pt­able.

RESULTS

Inter-Site Distribution of OCL and PCB

The observed conce ntra tion ranges in sed i­ments and th e percentage of samples in whichana lytes occurred a bove det ection limits are pre­sented in Table 1. Although factors such as depthof cores, accretion rates and frequency of inun­dation make direct comparison of different sitesdifficult, th ese valu es demonstrate t he ubiquitousnature of th e OCL and PCB within th ese saltmarsh sed iments . The concentrat ions of OCL andPCB observed at Oakl ey and Stiffkey were similarwith the presence of DOE and DOD , both break­down products of DDT, at conce ntra tio ns com­parable to th ose of th e parent compound. Thesefindings infer th at degradation of th e parent com-

Jo urna l of Coastal Resear ch, Vol. 12, No. I , 1996

250 Scrimshaw, Bubb and Lester

Mud flat/creek Vegetated

5 10 15 20 25

234 5

5 10 15 20 25

2 345

5 10 15 20 25

Dengie

Oakley

2 345

0-55-1010-1515-2020-25

E 30-35(,) 1 2 3 4 5.I£ad}

0-505-1010-1515-2020-2530-35

5 10 15 20 25

Concentration, ng g -1 dry weight

Figure 2. Concentration of sum of DDT (~DDT) with depth in sediments from mud flat/creek and vegetated areas at Oakley andDengie.

pound has occurred and that DDT concentrationsmay have been greater in the past than observedhere.

The pattern of organochlorine contaminationat Dengie, however, differed from that of the othertwo sites. The maximum observed concentrationsof PCB and DDT were up' to an order of magni­tude above those in sediments from Little Oakleyand Stiffkey, reaching concentrations of 240 ngg-l PCB at 35-40 cm depth in core 1 and 17.9 ngg-l DDT (5-10 cm, core 1). The presence of en­hanced sediment concentrations of PCB associ­ated with industrial sources at Dengie implies thatinputs to this site are not solely agricultural. Therelative distance of Dengie from any urban or in­dustrial area implies that more diffuse sourcesmay account for their presence in sediments atthis site. In contrast to observations at Oakleyand Stiffkey, DDT was prevalent in preference toits metabolites, DDE and DDD in sediments fromthe vegetated marsh at Dengie. This may indicatean inability of sediment microbial or invertebratepopulation to initially metabolise the DDT, orthat DDE and DDD have shown subsequent, rap­id metabolism in this environment.

The microbial and invertebrate populations ofthe sediments may, however, be influenced throughchanges in physicochemical parameters such asredox potential and pH. Differences between siteswere apparent in these factors although a caus­ative link between degradation pathways and ki­netics could not be formulated. Sediments from

vegetated cores at Dengie, for example, were morealkaline (pH 6.7-8.1) than those at Oakley (pH6.2-6.7). This difference is most probably ex­plained by the organogenic nature of the salt marshat Oakley where the decay of organic detritus islikely to result in a more acidic environment. ThepH values of sediments from mud flat/creek areasreflected those of the vegetated marsh, althoughthe minimum values recorded were 0.3 units lowerat both sites. Redox values in mud flat/creek sed­iments were less than -100 mV by 5 cm depth.Vegetated cores taken from Oakley displayed re­dox values from +56 mV at the surface, rapidlyfalling to -187 mV by 20 cm depth. However, atDengie vegetated marsh sediments were oxicthroughout at the time of sampling, with redoxvalues remaining at > +100 mV down to 40 cmdepth.

Intra-Site Variation of OCL and PCB

At all three sites, the maximum observed con­centrations of contaminants occurred in vegetat­ed marsh as opposed to mud flat sediments. Thisis illustrated in Figure 2 which compares char­acteristic sediment concentrations of the sum ofthe DDT compounds (DDT, DDE and DDD),};DDT, with depth from a mud flat/creek area andvegetated area of marsh at both Oakley and Den­gie. Sediments from vegetated areas displaymarked variations in concentration with depthwhich are not apparent in mud flat/creek cores,possibly due to reworking of sediments by tidal

Journal of Coastal Research, Vol. 12, No.1, 1996

Elevated Sediment Organochlorine Concentration 251

Table 2. Elevation above OD Newlyn (OD) and average TOe(0-35 cm) for sediment cores taken from Oakley, Dengie andStiffkey.

• Core 1Q] Core 2

25-30

30-35 _-••••••-

35-40M-~~~!!!1~."'~I11111~~~

5-10

10-15

5 15-20._••_-5g 20-25111•••

Oakley Dengie Stiffkey

Core ODm %TOC ODm %TOC ODm % TOC

1 0.40 1.9 2.24V 1.9 3.08v

2 1.78v 2.3 2.23v 1.6 3.14v

3 0.82 1.1 0.61 1.8 2.79v

4 1.67v 3.0 0.16 1.5 2.42v

vVegetated salt marsh

Figure 3. Distribution of PCB with depth in both vegetatedcores (1 and 2) from Dengie.

salt marshes do not appear to be solely attrib­utable to sediment carbon content. The % TOCcontents of sediments at Oakley and Dengie arereported in Table 2. The organogenic nature ofthe marsh at Oakley is reflected in the greaterpercentages of organic matter in cores taken fromvegetated marsh than observed at Dengie. Vari­ation of % TOC within individual cores was min­imal and could not account for the observed en­hancement of, for example, ~DDT at 5-10 cmdepth in vegetated cores from Dengie (Figure 2).

Particle size analysis of core samples from Den­gie and Oakley revealed that sediments at bothsites were predominantly composed of clays andsilts of <64 ~m in particle size. At Oakley, sedi­ments from vegetated areas comprised up to 8 %by dry weight of organic detritus of > 2 mID. insize, with the remaining material being <64 ~m.

Organic detritus was less evident at Dengie; how­ever, the marsh was characterised by an increas­ing proportion of fine sand (64-200 ~m) from corestaken farthest from the sea wall. Core 3 at Dengiewas taken at a creek edge, by a shell bank and at15-20 cm deep comprised 24 % by weight of shellmaterial.

Determination of the distribution of the OCLand PCB between particle size fractions was ham­pered by the small percentage of particles of > 64~m and by the presence of most determinands ator below detection limits. However, due to theinfluence of the organic detritus at Oakley andthe increasing percentage of fine sand at Dengie,it was possible to relate the percentage of the totalTOC contained in the < 64 ~m fraction of a sample

Concentration, ng g-I dry weight.

action and mobilisation of contaminants by morefrequent inundation or re-working of material byinvertebrates. At Oakley, ~DDT was predomi­nant in the upper layers (0-20 cm) of the vege­tated marsh, possibly reflecting historical depo­sition; however, sediments from the vegetatedmarsh at Dengie exhibited the presence of~DDTto greater depths (6.7 ng g-l at 30-35 cm) withfluctuating concentrations (Figure 2).

Polychlorinated biphenyl distribution withdepth in the vegetated cores from Dengie reflectedthat of the ~DDT, in that it exhibited no distinctpattern (Figure 3). The presence of both ~DDTand PCB (242 ng g-l at 35-40 cm) at depths of30-40 cm may be due to reworking of sedimentsthrough wave energy on the open-coast environ­ment, rather than a reflection of sedimentationover the previous two to three decades followingpeak usage and discharge to the environment ofthese groups of compounds.

Variations in concentrations of~DDTand PCBwith depth at Dengie may also be influenced bydegradation mechanisms. Redox values in core 1became negative (-19 mV) at approximately 45cm depth, and it may be that reductive dechlo­rination of DDT to DDD and the subsequent me­tabolism and mineralisation of such secondarymetabolites has removed traces of~DDTcontam­ination at such depths. PCB may also undergoreductive dechlorination in anaerobic environ­ments. Oscillation between oxic and anaerobicconditions, allowing for breakdown pathways tofollow different routes, combined with sedimentreworking along the high energy open coast, mayaccount for the depth of, and variations in, sed­iment concentrations of OCL and PCB at Dengie.

The Influence of Particle Size and Total OrganicCarbon (TOC)

Differences in the observed accumulation of theOCL and PCB within sediments at the different

20 40 60 RO 250

Journal of Coastal Research, Vol. 12, No.1, 1996

252 Scrimshaw, Bubb and Lester

100 • ppDDEB o Dieldrin 0 .~ 80car:: 0 oe~.g 0

~~0 0

0'::: 60 .bO~ ..~v 40 08r::"- 0

~ 20

20 40 60 80 100

Percentage of the total TOC in <64 fraction

Figure 4. Percentage of the total dieldrin and ppDDE in the< 64#Lm fraction against the percentage of the TOC within thatfraction.

to the percentage of ppDDE and dieldrin on thatfraction (Figure 4). The two values show a weakcorrelation (r2 = 0.53, p = 0.016) but no otherrelationship was apparent.

DISCUSSION

The comparison of data acquired from sedi­ment cores taken from different salt marsh sitesis complicated by the influence of local factors.Samples taken from the same depth below thesurface may not have accr'eted at the same timeand the degree of tidal inundation (hence plantcover, invertebrate activity and physicochemicalparameters) differs for sites of similar elevation.This is illustrated by comparing elevation (Table2) and plant cove~. Core 4 taken from Stiffkey

(elevation of +2.4 mOD) had mainly lower marshspecies present; however, the lower (+ 2.24 m) Core1 from Dengie was typical of mature vegetatedmarsh, SM14, implying that the relatively lowersite is less frequently inundated by tides.

To facilitate inter comparison, the maximum,mean and median values obtained from the anal­ysis of 5 cm core sections to a depth of 35 cm (orless where cores were shorter) are shown in Table3. Although certain measured values (such as 242ng g-l PCB maximum observed at Dengie at depth> 35 cm) are excluded from such a data set, thisapproach avoids comparing data from cores ofgreatly unequal depth and highlights differencesbetween sites.

The maximum concentrations of both OCL andPCB detected in sediments from Dengie were lessthan those previously observed at Two Tree Is­land in the Thames Estuary (SCRIMSHAW et al.,1994); however they remain within the same orderof magnitude (Table 3). Results from analysis ofsediments from Oakley and Stiffkey fall withinthe range previously reported for both OCL andPCB in other Essex salt marsh sediments. Manyof the OCL and PCB occur at low « 1.0 ng g-l)but detectable concentrations in salt marsh sed­iments' and this may be true for up to 100% ofsamples for some sites. Certain sites, however,appear to show enrichment of one or more con­taminants.

The average values for sediments from vege­tated salt marsh cores (Table 3a) indicate thatboth Two Tree Island, which has previously been

Table 3a. Maximum (max), average (av) and median (med) concentrations of contaminants in cores from vegetated salt marshsediments (0-35 cm depth).

Two Tree Island* Salcot* Walton* Tollesbury*

Max Av Med Max Av Med Max Av Med Max Av Med

~HCH 7.8 3.0 2.2 18.2 6.6 5.6 1.3 0.7 0.6 0.5 0.1 0.0Dieldrin 9.1 4.2 4.9 1.2 0.4 0.3 1.8 0.7 0.4 1.1 0.5 0.6~DDT 21.1 10.0 10.4 4.1 1.3 0.6 2.3 1.2 1.3 4.1 1.4 0.9~OCL 33.9 17.1 17.9 22.2 8.3 8.4 4.7 2.6 2.3 4.1 2.0 2.0PCB 116 34 19 4 1 0 2 1 0

n 12 10 5 10

South Woodham* Oakley Dengie Stiffkey

Max Av Med Max Av Med Max Av Med Max Av Med

~HCH 0.5 0.1 0.0 3.9 0.7 0.2 4.6 1.2 0.3 0.3 0.1 0.0Dieldrin 1.4 0.1 0.0 0.7 0.1 0.0 0.7 0.4 0.5 1.4 0.4 0.4~DDT 8.0 1.4 0.0 5.0 1.7 0.9 20.3 6.8 5.2 4.6 1.6 1.0~OCL 8.0 1.6 0.0 7.7 2.6 2.2 21.2 8.3 7.2 5.7 2.1 2.0PCB 5 0 0 4 1 0 59 20 16 8 3 2

n 14 12 13 11

Journal of Coastal Research, Vol. 12, No.1, 1996

Elevated Sediment Organochlorine Concentration 253

Table3b. Maximum (max ), auerage (au) and median (me d) concentrations of conta minants in cores from mud {lat /creek sedime nts(0- 35 em depth) .

Two Tree Island'

Max Av Med

~HCH 0.8 0.2 0.2Dieldrin 0.7 0.4 0.5mOT 5.9 2 . ~ 1.7WCI. 7.2 3.0 2.6PCB 48 20 19n 12

South Woodham'

Max Av Med

~HCH 2.6 0.2 0.0Dieldrin 0.2 0.0 0.0~DDT 2.0 0.5 0.0~OCL 2.6 0.7 0.3PCB 2 0 0n 11

Max

0.20.41.21.76

Max

1.10.62.33.56

Salcot"

Av

0.00.10.40.6I

10

Oakley

Av

0.20.21.21.66

12t

Med Max

1.2 1.00.1 0.90.1 II.:J0.3 12.31 17

Med Max

0.0 0.10.2 0.61.4 1.91.8 2.36 12

Wallon'

Av

0.40.42.63.4

6

13

Dengie

Av

0.00.20.70.94

10

Med

0.30.42.12.85

Med

0.00.20.71.01

Max

0.40.91.02.01

Max

Tollesbury'

Av

0.10.40.50.91

10

Stiffkey

Av

nil

Med

0.00.40.40.91

Med

'Data fromSCI<IMSH AW et al . (1994)tP CB values based on only2 samplesn = number of samples; s = interference due to sulfur

identified as displaying enhanced concentrationsof OCL and PCB (SCRIMSHAW et al., 1994) andDengie exhibited elevated conc ent ra t ions com­pared to the other marshes studied . Furthermore,the groups of organochlorine compounds contrib­uting to the total OCL burden within the sedi ­ments are clearly identified, with ~DDT beingubiquitous and most significant at Dengie andTwo Tree Island. Dieldrin was significant only atTwo Tree Island and the HCH isomers were pres­ent at Two Tree Island, Salcott and to a lesserextent at Dengie and Walton. Sediments fromStiffkey, North Norfolk, indicate a relatively lowdegree of enrichment but one which is within therange of that observed in Essex, with PCB exhib­iting a degree of enhancement. The ratio ofLHCHto ~DDT at Salcott is unusual and may be dueto local inputs of HCH at this si te.

Sediments from mud flat/creek areas (Table3b) exhibit a lower degree of conta minat ion thanthose from vegetated areas. Mud flat sedimentsfrom Two Tree Island reflect values observed invegetated sediments at the site, wh ereas those a tDengie give no indication of the contaminationpresent in the vegetated marsh ar eas . This maybe a result of greater scour of mud flat areas dueto higher tidal energy experienced on the opencoast marsh at Dengie compared to that at TwoTree Island,

There appears to be an average ba ckground

contamination of both OCL and PCB within theEssex salt marsh sediments, with the LDDT com­pounds being predominant in the OCL group. Thiscor responds to average LDDT concent rat ions of1.5 to 3.0 ng g-I and of - 1 ng g-I for PCB invegetated salt marsh sediments at up to 35 emdepth. The differences in median values (Table3) between sites follow the same pattern as theav erages, with sediments from both Dengie and,to a greater extent, Two Tree Island exhibitingenhancement. In general, however, medians arelower than average values. This indicates that thereis a degree of positive skew to the data which isexpected due to previou s ob servations in both thiswork and SCRIMSHAW et al. (1994) of sub-surfaceenrichment of contaminants.

The occurrence of maximum OCL contamina­tion with that of the PCB (at Two Tree Islandand Dengie) ind ica tes that common sources maybe responsible for the presence of these com ­pounds. These are unlikely to be local, due to theabsence of any industrial or related activity nearDengie to account for the pr esence of PCB. Theeffects of atmospheric deposition would not beexpected to result in observed differences in sed­iment concentrations at the seven Essex saltmarshes, as the sites are distributed over a rela­tively small region. It would, therefore, not beunreasonable to assume that sediment inputsources playa s ignifican t role in determining the

Journal of Coastal Research, Vol. 12. No. I , 1996

254 Scrimshaw, Bubb and Lester

relative degrees of inter site enrichment for theOCL and PCB in the Essex salt marshes.

Oakley, situated in Hamford Water, and thesalt marsh at Dengie are equidistant from theThames estuary sludge dumping grounds of theBarrow Deep. The disposal ground is only 20mdeep and strong tidal currents are likely to resultin rapid dispersal of material. Both HOLDEN (1970)and FISH (1983) estimate that dumping in theThames has accounted for 5 million tonnes ofsewage sludge per annum, with annual PCB in­puts estimated at 1000 kg (HOLDEN, 1970). Morerecent estimates of UK inputs of PCB into theNorth Sea through sludge dumping are between21-557 kg per annum (KLAMER et al., 1991).

The prevailing currents in the area are likelyto transport sediments from the Thames estuarynorthwards along the coast past Dengie and Ham­ford Water (Figure 1). Upstream movement ofbed material from the outer Thames estuary(INGLIS and ALLEN, 1957) may contribute to con­tamination at Two Tree Island, however, directinputs from the River Thames may be a moresignificant source of contaminants to this site. Theannual mass of PCB transported to the southernNorth Sea from the Thames is estimated to bebetween 47-63 kg per year, while inputs to theNorth sea via the English Channel are estimatedto be 1000 kg per year (KLAMER et al., 1991). Acombination of these input sources and watermovements in the area may account for enhancedconcentrations of OCL and PCB at Two Tree Is­land and Dengie, with distance from the estuaryleading to a decrease in the degree of con tami­nation.

Sediments from the Thames estuary and ma­terial from the Barrow Deep are likely to have aless significant impact on the sites at South Wood­ham Ferrers, Tollesbury Wall, Salcott and Oak­ley. This is due to their location in ebb-dominatedrivers with limited marine sediment inputs (SouthWoodham Ferrers, Tollesbury and Salcott) orwithin the sheltered environment of HamfordWater with subsequent low energy tidal inputs ofsediment. Polychlorinated biphenyl concentra­tions in sediments at Stiffkey may be the resultof inputs to the North Sea from the HumberPlume; data produced by KLAMER and FOMSGAARD(1993) indicates that this major estuary influencessediments deposited in the south of the Wash.

The information on particle size distributionand organic carbon content of sediments fromDengie and Little Oakley revealed little difference

between the sediments at the two sites. This wasnot reflected, however, in the degrees of contam­ination with both OCL and PCB. Differences inobserved concentrations of OCL and PCB be­tween the two sites are not, therefore attributableto sediment composition but to input sources.

CONCLUSIONS

The results of an extensive survey of salt marshsediments from the Essex coast indicate that con­tamination with DCL and PCB is ubiquitous. Av­erage or median concentrations of contaminantsin sediment cores to a depth of 35 em have enableda degree of background contamination to be as­sessed. Certain sites have been identified as ex­hibiting enhancement of contaminants above thisbackground.

The pattern of contamination of Essex saltmarsh sediments indicates that deposition of ma­terial from the major input sources in the region(the River Thames, dumping of sewage sludge,and the mass movement of water from the EnglishChannel) may influence the level of enhancementof OCL and PCB.

There was no evidence to suggest that particlesize distribution or organic carbon content werefactors that influenced observed differences incontamination between sites, although distribu­tion of chlorinated hydrocarbons between the sed­iment particle size fractions in each sample wasrelated to the proportion of TOC on the < 64 ,urnmateriaL

ACKNOWLEDGEMENTS

The authors are grateful to the National RiversAuthority (Eastern Region) and to the Essex Lo­cal Flood Defence Committee for funding the workand their permission to publish.

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