accumulation and persistence of chlorobiphenyls, organochlorine pesticides and faecal sterols at the...

ELSEVIER

Environmental Pollution 88 (1995) 207-217 8 1995 Crown Copyright

Printed in Great Britain. 0269-7491/95/$09.50

ACCUMULATION AND PERSISTENCE OF CHLOROBIPHENYLS, ORGANOCHLORINE PESTICIDES AND

FAECAL STEROLS AT THE GARROCH HEAD SEWAGE SLUDGE DISPOSAL SITE, FIRTH OF CLYDE

A. G. Kelly Scottish Ofice Agriculture and Fisheries Department, Marine Laboratory, Victoria Road, Aberdeen, UK, AB9 8DB

(Received 20 October 1993; accepted 29 March 1994)

Abstract The sediment concentrations of organic carbon, faecal sterols, individual chlorobiphenyl congeners and organochlorine pesticides have been measuied in seabed cores from the sewage sludge disposal area at Garroch Head in the Firth of Clyde. The measurements confirm the accumulative nature of the site with high levels of sedimentary faecal sterols (152 mg kg-’ coprostanol). Levels of chlorobiphenyls, DDT compounds and dieldrin in surface sediment were elevated by factors of 12, 40 and 120, respectively, over those observed at a site remote from the eflects of dumping. Total chlorobiphenyl levels of 515 kg kg-’ Arochlor 1254 in surface sediment were comparable to levels found in other areas heavily contaminated with sewage sludge. The 20-cm depth of heavily sludge- contaminated sediment overlays a mixed sludge/basal sediment layer some 10 cm in depth. Levels of organochlorine contaminants were elevated to depths of 90 cm in the sediment, suggesting that the surface layer is a source of contaminants to the deeper sediment. Within the upper 15-20 cm sediment in the disposal area, chlorobiphenyls are conservative, the variation in their concentration with respect to depth being related to historical input. Lindane and possibly dieldrin, and hexachlorobenzene are not conservative. Faecal sterols are removed in sub-surface sediment, in contrast to conservative behaviour previously found at other sewage polluted sites.

INTRODUCTION

Marine dumping has been the preferred means of disposal of Glasgow’s sewage sludge since 1898. The dumping operation and its environmental impact have been reviewed (Pearson, 1985; McKay, 1986; Harper & Greer, 1988). Since 1974, dumping has taken place at a site some 7 km south of Garroch Head in the Firth of Clyde. The site is distinguished amongst United King- dom sludge disposal sites by its accumulating nature (Topping, 1987). Water movements in the area are small, peak currents of less than O-5 km h-i (Dooley, 1979) result in a large proportion of the dumped mate-

rial being deposited locally on the seabed. The sediment in the disposal area has been found to have elevated trace metal levels up to 2 km from the centre of the site and down to 20-30 cm depth (Clark 8z Davies, 1989; Rodger et al., 1992). Sediment organic carbon levels are similarly elevated (Pearson, 1986).

The sludge dumped at Garroch Head contains a large number of persistent organic contaminants, such as chlorobiphenyls, DDT compounds (DDTs), dieldrin and constituents of technical chlordane (McIntyre & Lester, 1984). During sewage processing these hydrophobic compounds associate with the sewage particulate phase and are concentrated in the sludge solids. The degree of removal in the primary sedimentation stage has been estimated to be 33-75% for Arochlor 1260, 41-54% for dieldrin and 17-50% for 4$-DDE, (McIntyre et al., 1981). Secondary digested sludge may also contain organochlorine (OCs) residues at elevated levels, (Bergh & Peoples, 1977). The faecal sterol coprostanol is highly hydrophobic (Saad & Higuchi, 1965) and has been shown to associate with sewage solids and be removed during primary treatment in a similar manner to the OCs (Murtagh & Bunch, 1976; McCalley et al., 1981). As sewage is the pre-eminent source of coprostanol in the marine environment, this sterol is an effective measure of the degree of sewage related contamination (Vivian, 1986).

The accumulation of organochlorine compounds in sediment at Garroch Head is evident from the results of the annual statutory monitoring by Strathclyde Regional Council, in and around the disposal area, carried out for the Scottish Office Agriculture and Fisheries Department. This accumulation is of concern in relation to the long term health of the local environment, since such compounds are generally recognised as being persistent, toxic to marine life, and bio-accumulating. It is therefore important to understand the processes controlling their accumulation, distribution and fate within the sediment. This study seeks to quan- tify the degree of contaminant accumulation at the Garroch Head site and to explore the persistence of these contaminants in the sedimentary environment.

207

208 A. G. Kelly

EXPERIMENTAL

Sampling Sediment samples were collected by RV Scotia in June 1988 from the sludge disposal area and at a ‘remote’ site at Pladda, south of Arran. The disposal site was defined in 1974 as an area one nautical mile in diameter centred on 55’39.48’N, 05’00.48’W. This is some 7 km south of Garroch Head in a water depth of 80 m. Five Craib cores were taken at the Garroch Head site. Drift- ing of the vessel between cores resulted in sampling at varying positions within the area (Fig. 1).

Short cores were obtained with a 5-cm diameter Craib corer (Craib, 1965) and long cores with a IO-cm diameter gravity corer. Short cores were sectioned into 5-cm slices and long cores into IO-cm slices. Subse- quent examination of the contaminant depth profiles in short and long cores indicated that the upper 12 cm of sediment had been lost from the gravity core. This phe- nomenon has been noted previously (Murdoch et al., 1981) for a narrow bore (5 cm) gravity corer, and may be related to the differing modes of action of the two samplers. Therefore in interpreting the data it has been assumed that the first gravity core section corresponds to a depth of 12-21 cm, and the deepest section to a depth of 82-91 cm.

Pretreatment The sediment from each core section was dried in a cir- culating air oven at 3540°C for 2-3 days. The total sample was then homogenised in a mortar and pestle. A sub-sample was taken for carbon/nitrogen analysis and further ground in an electric ball mill. A portion of this sediment (50-100 mg) was analysed for organic

56’ N

55’30’N

Fig. 1. Location of sampling sites within the sludge disposal area.

carbon and nitrogen using a Perkin-Elmer 240 Elemen- tal Analyser, subsequent to treatment with hydrochlo- ric acid to remove inorganic carbon, For organic contaminant analysis, dried sediment (l-20 g) was extracted by soxhlet for 4 h with dichloromethane. Soxh- let thimbles were pre-extracted with dichloromethane for 2 h before use. For reference purposes sewage sludge was obtained from Strathclyde Regional Coun- cil. This comprised a monthly composite of daily sam- plings from the Dalmarnock and Paisley sewage treatment works and was a mixture of primary and activated sludges. Homogenised sewage sludge (10 g) was extracted using a standard method (HMSO, 1984). Clean-up and class-separation were based on alumina/ silica chromatography (Wells et al., 1985) with adaptions to allow isolation of a sterol fraction and removal of sulphur (Jensen et al., 1977).

Analysis The gas chromatography instrument conditions are listed in Table 1. GC calibration was by the internal standard method with two bracketing standards for all determinands except the chlorobiphenyls. A combined solution of the National Research Council of Canada (NRCC), standards CLB-lA,B,C and D, was diluted by 100 for the calibration of the CBS. The linear range of the ECD detector had been established previously. The 35 chorobiphenyl congeners quantified were IUPAC Nos 28, 52, 49, 44, 40, 103, 121, 60, 101, 87, 77, 154, 151, 118, 105, 114, 153, 141, 137, 138, 129, 183, 128, 185, 173, 200, 180, 191, 170, 201, 189, 207, 194, 205 and 206. Other congeners in the NRCC solu- tions were excluded due to co-elution on the CPSil-8 column. The chlorobiphenyl concentration presented in the results section is the sum of these 35 congeners. This sum accounts for 45.08% of an Arochlor 1254 and 51.42% of an Arochlor 1260 type composition (Arochlor data from Schulz et al., 1989). The identity of major organochlorine determinands was confirmed by the use of negative-ion chemical ionisation mass spectrometry utilising a Hewlett-Packard 5989A mass spectrometer. Confirmation of sterols was performed using a Finnigan 5100 quadrupole mass spectrometer operating in electron impact (EI) mode. The unusually high levels of CB52 measured in Garroch Head sediment may be an

Table 1. GC analysis conditions

Chlorobiphenyls-Varian 3500 with ECD (33O”C), automated on-column injection (0.5 PI), injector temperature programme 12OT for 1 min then lO”C/min to 270°C. CP-SILS column, 50 m X 0.22 mm programmed from 8O’Wl minWC min/ 270°C. Carrier gas hydrogen at 45 cm/s.

Organochlorine pesticides-Pye 304 with ECD (33OT). Auto- mated Grob-type splitless injection (1 ~1) at 275°C. Column CP-SIL19 50 m X 0.22 mm programmed from 80°C for 1 min then 3Wmin to 270°C. Carrier gas hydrogen 45 cm/s.

Sterols-Carlo Erba 4160 with FID (320°C). Automated Grob- type splitless injection (1 ~1) at 29OT. Column either CP-SIL8 or CPSIL19 50 m X 0.22 mm, programmed from 80°C for 1 min then 4Wmin to 300°C for 30 min. Carrier gas hydrogen 45 ads.

Chlorobiphenyls, organochlorine pesticides and faecal sterols 209

analytical artifact resulting from co-elution of an unknown contaminant on the CPSil-8 column used for quantification. Analysis on a more polar CPSil-19 column gave partial resolution into 213 components. GUMS investigation in both EI and negative CI modes failed to reveal the identity of these co-elutants. Data for this congener must be regarded as compromised.

The quality of the data was monitored by an ongo- ing laboratory quality control scheme. The within- batch coefficient of variance derived from analysis of duplicate samples was 10% for chlorobiphenyls, 14% for pesticides and 10% for sterols. The between batch coefficient of variance utilised for control of the method was 21% for chlorobiphenyls, 33% for pesticides and 22% for sterols. No problems of contamination were found in any method blanks. In addition, regular analyses of a reference material certified for chlorobiphenyls by NRCC was performed to check the accuracy of the method. The laboratory reference material used in this study was a bulk sediment sample from the Garroch Head dump site which had been oven dried, ground in a ball-mill and sieved through a 180-pm filter (Wells & Kelly, 1991).

RESULTS AND DISCUSSION

Statistical examination of the data The greatest effects of the sludge dumping on benthic communities, sediment geophysical parameters (Pearson, 1986) and accumulation of heavy metals (Rodger et al., 1992) are found within 2 km of the centre of the disposal site. Craib core samples taken in this study were dis- tributed within this area and thus represent the most heavily contaminated zone. Five short cores were taken to enable statistical analysis of the variation in sediment contaminant concentration horizontally within the disposal area and vertically within the sediment (Kelly et al., 1994). Significant differences were found with respect to depth, but not between cores for the same depth. The data from the five cores has therefore been averaged to provide a mean concentration at each sampling depth (Tables l-3). The resultant depth/concentration profiles of the major determinands are illustrated in Fig. 2.

Sludge constituents in surface sediment

Organic carbon Organic carbon enrichment is evident in the physical appearance of the sediment, and confirmed by the total organic carbon (TOC) concentrations which were between 7.4% and 10.3% in the upper 15 cm (Table 2). These TOC measurements exceed the highest organic carbon loading previously reported from a sludge-disposal ground of 4.1% TOC in the New York Bight (West & Hatcher, 1980).

Faecal sterols The mean value of 152 mg kg-’ coprostanol in surface sediment (Table 2) is the highest concentration in a ma-

0

-100 0 20

0

-20

g -40

f DDTs

P -60

i;

(PS W’ ) D

-80

-100 -

0 8

ii

Dieldrin

(KS kg-’ )

1

c PADS

(pet kg-’ )

I.__-

0 80 0 1600 12

Fig. 2. Depth profiles of sediment concentration of major determinands.

rine sediment reported so far, and compares with previous values of 5.2 mg kg-’ in the New York Bight (Hatcher & McGillivary, 1979), 2.5 mg kg-’ adjacent to a sewage outfall (Pierce & Brown, 1984), and 3-5 mg kg-’ at the former Garroch Head dump site (some 5 km north of the present site) (Goodfellow et al., 1977). The higher concentration of coprostanol at the present site compared with that found at the former dump site is unlikely to be due to higher inputs of coprostanol in contemporary sludge. Coprostanol levels in the sludge samples analysed in this survey (1065 mg kg-’ coprostanol), were comparable to the 1283 mg kg-l previously found in Glasgow sludge (Goodfellow et al., 1977). The increase may be due to more accurate posi- tioning of the dumping operation resulting in less dispersion of sludge over the seabed. Prior to the operations at the new dump site, in 1974, the disposal practice led to the sludge being spread over a wide area (Mackay, 1986).

Chlorobiphenyls and organochlorine pesticides The chlorobiphenyl concentration in surface sediment at Garroch Head was 23 1.9 lug kg. ’ (Table 2), (515 pg kg-’ in terms of an Arochlor 1254 equivalent). This is substantially lower than the maximum of 2890 pg kg-’ Arochlor 1260 found at the former Clyde disposal site,

Tab

le 2

. C

once

ntra

tion

of d

eter

min

ands

in s

edim

ent

core

s fr

om t

he G

arro

ch H

ead

sew

age

slud

ge d

ispo

sal

site

and

Pla

dda

rem

ote

site

(dr

y w

eigh

t ba

sis)

Dep

th

U/o

%

I C

opro

stan

ol

Cho

lest

erol

H

CB

G

HC

H

Chl

orda

nes

Die

ldrin

4,

4’-D

DD

4,

4’-D

DT

(cm

) C

arbo

n N

itrog

en

mg

kg-’

mg

kg-’

pg

kg-’

cLg

kg~’

cL

g kg

’

m k

g ’

/s k

g- ’

e kg

’

GA

RR

OC

H

HEA

D

(Dum

ping

ar

ea)

SHO

RT

CO

RES

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5-10

9.

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15

14

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0 3.

3

1.08

15

2.2

75.4

5.

5 12

.8

14.1

13

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39.1

15

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13.3

26

.8

231.

8 51

5 9.

6 5.

0 0.

4 1.

1 0.

82

62.9

27

.0

6.3

6.2

15.1

64

.9

31.1

11

.1

14.8

17

.5

330.

4 72

1 11

.4

4.0

0.3

0.7

0.61

28

.6

18.0

3.

6 5.

4 15

.7

84.1

32

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11.5

12

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21.1

40

9.7

900

11.9

2.

9 0.

3 0.

9 0.

28

I.5

5.6

1.1

3.9

8.9

72.3

22

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13.1

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5 11

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210.

4 46

3 12

.0

2.6

0.6

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LON

G

CO

RE

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2 4.

4 22

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2 42

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9 82

-92

0.9

0.34

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0.17

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65

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0.13

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0.

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< 0.

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< 0.

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<

0.05

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DD

A

(Rem

ote

site

) SH

OR

T C

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3

co.0

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26.9

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1 13

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03

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7.

71

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1.

83

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1.63

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<

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0.41

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<

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9.

2 0.

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1.23

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0.17

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< 0.

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14.3

10

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0.2

1.5

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<

0.08

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0.55

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0.73

0.

26

12.5

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0.1

< 0.

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< 0.

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1.

15

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0.

53

0.10

<

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0.08

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0.08

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0.08

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0.08

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0.08

11.3

24

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0 2.

1 58

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1287

8.5

8.0

5.1

4$-D

DE

PAD

S SU

M

AR

OC

HLO

R

CR

\I

cLg

kg-’

cLg

kg-’

Cop

rost

anoV

D

DT/

D

DTI

C

BS

1254

R

atio

C

hole

stan

ol

DD

D

DD

E

pgkg

-’ pg

kg’


Table 3. Comparison of concentration of organocMori~~ contaminants in surface sediment from !Scottisb waters (pg kg’ dry wt)

Organic carbon %

HCB Lindane Dieldrin DDT CBS Archolar 1254 Reference total total equiv

“Sum 35 congeners. ‘Sum 7 ICES congeners.

(Halcrow et al., 1974) despite the more localised nature of the present dumping operation, as described above. The decrease appears to be due to a reduction in the chlorobiphenyl concentration in dumped sludge, lower- ing the overall chlorobiphenyl input by a factor of two to four over 14 years (Fig. 4(a)). Levels of chlorobiphenyls at Garroch Head in 1988 are similar to those at the New York Bight sludge disposal site where 2200 pg kg-’ Arochlor 1254 was measured in 1975 (West & Hatcher, 1980).

The relative concentrations of organochlorine pesticides in surface sediment at Garroch Head reflect the industrial base in the Clyde Valley (Table 3). Two of the major components, dieldrin (73.7 pg kg-‘) and polychloro-2-aminodiphenyl ethers (PADS) (26.9 pg kg-‘) result from use as moth proofing agents in the local textile industry. The major DDT component in Garroch Head sediment and in the dumped sludge is 4,4’-DDD, the mean concentrations of 4,4’-DDD, 4,4’- DDE and 4,4’-DDT in the upper sediment layer being 39.7 kg kg-‘, 13.3 pg kg-’ and 15 pg kg-‘, respectively. 4,4’-DDD is the product of microbially mediated reductive dechlorination of 4,4’-DDT, and may be formed from DDT in anoxic marine sediment (Zoro et al., 1974). Lindane (-y-HCH) is the major hexachlorocy- clohexane isomer present with smaller amounts of the more environmentally stable a-isomer. Several components of technical chlordane were present in the sediment.

For all determinands, levels in the surface O-5 cm depth range at Garroch Head were considerably elevated over those at ‘clean’ marine sites, such as the Moray Firth and offshore North Sea sites (Table 3). Comparison with St Abb’s Head and Bell Rock in the Firth of Forth, which are dispersive sewage sludge disposal sites, illustrates the accumulative nature of the Garroch Head site. Levels of chlorobiphenyls, DDT compounds, lindane and dieldrin in sediment were elevated by factors of 12, 40, 64 and 120, respectively, over those at the Pladda site in the south of the Firth of Clyde, remote from the effects of dumping (Table 2).

Persistence of chlorinated sludge constituents in seabed sediment The concentration/depth profiles of major determinands are illustrated in Fig. 2. These profiles suggest a

Firth of Clyde Garroch Head (CL5 cm) 10.3 5.5 12.8 13.7 68 232.0” 515 This study Pladda (O-5 cm) 1.2 co.03 0.2 0.6 2.9 18.8” 38.0 This study Firth of Forth St Abbs Head 0.24.6 0.04-0.21 0.2-l .4 1’ l-26 Borewell et al. Bell Rock 0.2-0.5 0.05-0.41 04-2.5 1.1-26 1986 Moray Firth 2.0 0.2 0.0 0.2 0.6 1.8’ 4.6 SOAFD 1991 North North Sea 1.5 CO.05 <0.004 0.1 0.1 0.36 0.7 SOAFD 1991

heterogeneous stratified structure with depth of sediment. Levels of total organic carbon, chlorobiphenyls, dieldrin, PADS and the DDTs are high in the upper 0 to 15-20 cm, and fall rapidly over 15-20 to 30 cm depth to reach constant low levels at 50-90 cm depth. These profiles may be interpreted as representing an upper sediment layer 15-20 cm in vertical thickness originating primarily from dumped sludge, overlying a zone of mixed sludge/basal sediment 5-10 cm in vertical thickness. The presence of elevated levels of organochlorine contaminants at the base of the sludge layer suggests that degradation of organochlorine compounds is occurring to a limited degree only. The persistence of chlorinated and non-chlorinated determinands can be explored by two means. First, input information is available for certain compounds. These data can be compared with the concentration/depth profiles of these compounds in the sediment. Secondly, changes in the ratios of related compounds with depth of sediment can be examined.

Historical input of contaminants Contaminant concentration/depth profiles, divergent from input profiles can be interpreted in terms of post- depositional biological or chemical processes. However, such interpretation requires some assessment of the effects of mixing on the vertical distribution of contaminants. The heterogeneity of the upper 30 cm of sediment suggests a low degree of sediment mixing. A model to quantitatively predict the effects of mixing is illustrated in Fig. 3. This is a simple box model, with nine successive inputs of 100 ktonnes of sludge over the period 19741988. Following each input, a degree of mixing takes place between all successive layers. The input data for chlorobiphenyls, dieldrin and lindane in sludge dumped at Garroch Head (Scottish Office Agri- culture and Fisheries Department Annual Dumping Reports 1974-88) has been used for the model. These data may not represent the true loading to the sediment, as a proportion of the input of organochlorine contaminants will diffuse away from the dump site in the aqueous and fine particulate phases. This does not affect the validity of the model as the same relative proportion of the input should impact the sediment after each dumping event.

212 A. G. Kelly

Predi umping

surfac

bdiment

20% i---I First input (1974/5)

se :e

Last input (1987/8)

Fig. 3. Model to represent mixing effects in sludge deposited at Garroch Head. The model simplifies the input oveI the period 1974-88 to nine successive inputs, each of 100 ktonnes of wet sludge. Following each input, 20% of all adjacent layers are mixed.

Concentration/depth profiles were then calculated with various degrees of mixing. The most successful which fitted the actual sediment data, used a mixing factor of 20% between layers. The observed and predicted concentrations in the upper 30 cm of sediment are shown in Fig. 4. For the organochlorine compounds examined, the fit is good for chlorobiphenyls, and poor for dieldrin and lindane, suggesting that degradation of dieldrin and lindane may be occurring. This finding matches the known relative stabilities of chlorobiphenyls and lindane, but not dieldrin which is relatively stable. Reduction of both lindane and hexachlorobenzene has been documented during anaerobic sludge digestion (Kirk & Lester, 1988; Fathepure et al., 1988), whilst dieldrin, 4,4’-DDE and Arochlor 1260, were not reduced. The processes by which lindane is being removed may be similar to that occurring during sludge digestion, i.e. reductive dechlorination. If so, then hexachlorobenzene whose concentration/depth profile resembles that of lindane may also be undergoing reduction in the sediment. Dieldrin is generally regarded as stable in sediment, but in one study involving incubation of natural sediment a 14% loss of dieldrin was observed after 56 days (Walker, 1977). The present data suggests some degree of dieldrin degradation is occurring.

Given that the gross composition of the sewage sludge is unlikely to change greatly, constant inputs of faecal sterols and organic carbon may be assumed. Fig- ure 4 illustrates the observed and predicted concentration depth profiles of coprostanol and sedimentary organic carbon. The concentrations of all sterols decrease exponentially with depth. These data do not fit the predicted profile for coprostanol and suggests degradation of sterols is occurring. Such non-conservative behaviour contrasts with that found in previous studies of cores from sewage sludge impacted sediment, where faecal sterols persisted for over 30 years (Hatcher

& McGillivary, 1979) and 180 years (Muller, 1979), and may reflect the high level of biological activity in Gar- roch Head sediment.

0

-10

E f -20 E

0

-30

-40 4

-L

a) Chlorobiphenyls b) Dieldrm

L ,

0 100 200 300 400 500 600

c) Lindane

I

I

) 50 100 150 200 250

d) Oganic carbon

d /

r’ I

f

! I 5 10 15 20 25 30 0 2.5 5 7.5 10 12.5

e) Coprostanol

./

,’

/-

f

f

A I

0 50 100 150 200

--- Actual

- Predicted

Fig. 4. Actual and predicted depth profiles of sediment concentrations of (a) chlorobiphenyls, (b) dieldrin, (c) lindane,

(d) organic carbon and (e) coprostanol, based on the mixing model and actual input data.

Chlorobiphenyls, organochlor pine pesticides and faecal sterols 213

Both sedimentary organic carbon and nitrogen ex- hibit a decline in concentration over the upper O-15 cm followed by a more rapid decrease to baseline values of approximately 1% at 35-45 cm depth. This profile fits well with that predicted, suggesting that the extent of removal of organic matter by chemical or biological processes is small in comparison with the total loading.

A second means of exploring stability, applicable to those organochlorine compounds for which historical input data is not available is examination of changes in the ratio of related compounds down the sediment cores. Considering 4,4’-DDT and its metabolites 4,4’-DDE and 4,4’-DDD the ratios of 4,4’-DDT to both varies lit- tle in the upper 15 cm, suggesting these compounds are stable in Garroch Head sediment (Table 2). The increase in both ratios below this depth may reflect higher input of 4,4’-DDT in the early seventies, prior to legislation banning its use. The distribution of specific chlorobiphenyl congeners does not alter in the upper 20 cm (Fig. 5) suggesting that no preferential degradation of specific congeners is occurring. These data confirm the conservative behaviour of these compounds sug- gested above, and contrasts with reported environmental degradation of chlorobiphenyls at a marine site highly contaminated by industrial input (Brown et al., 1987).

The possible presence of coprostanol synthesis at Garroch Head is of interest, as non-anthropogenic sources of coprostanol can contribute to presumed anthropogenic input in studies of sewage dispersion. Coprosfanol has been shown to be a minor product and cholestanol to be the major product of anaerobic microbial reduction of cholesterol, (Gaskell & Eglinton, 1975; Mermound et al., 1984; Teshima & Kanazawa, 1978). At Garroch Head there is a steady decrease in

the coprostanoYcholestano1 ratio from 5 : 1 in the upper 5 cm of sediment to 2.5 : 1 at 15-20 cm depth, which may indicate that reduction of cholesterol to predominantly cholestanol is occurring. However, this transformation is confused by rapid decreases in the levels of all stenols and stanols with depth.

Some further evidence to validate the idea of limited sediment mixing at Garroch Head is provided by faecal sterol measurements. Faecal sterols can provide an esti- mate of the contribution of sewage input to observed contaminant levels. In the context of the Garroch Head disposal site, the relative contribution of sludge to total sediment can be calculated by comparison of the contribution of sludge derived sterols to total sterols in sediment and sludge (Hatcher & McGillivary, 1979). This approach estimated that 44% of the sediment around the New York Bight sewage sludge dump site was derived from sludge. In this study, the faecal sterols coprostanol, epi-coprostanol and coprostanone comprised 45,549.5% of total sterols in surface sediment and 37% of total sterols in the sludge sample analysed. This suggests that all the surface sediment is derived from sludge and that there is limited mixing of the deposited sludge with resuspended natural sediment or with deeper sediment in which coprostanol levels are greatly reduced.

Transport of contaminants to deep sediment The long sediment core from Garroch Head contained measurable concentrations of OCs to a depth of 90 cm. For example, in core sections from 41-92 cm depth, the average concentrations were 8.9 pg kg-’ chlorobiphenyls, 1.4 pg kg-’ DDTs, 0.6 pg kg-’ dieldrin and 0.56 pg kg-’ lindane. The less persistent faecal sterol, coprostanol, was detected only to a depth of 41-52 cm. Given a

CB CONGENER DISTRIBUTION IN UPPER 20CM OF GARROCH HEAD SEDIMENT

28 52 49 44 40103121 60101 87 n15416111s1061141~1411911~1181831281051~~1~1~11~1189207184#)(5208

CB CONGENER

Fig. 5. Chlorobiphenyl conger distribution in upper 20 cm of sediment.

214 A. G. Kelly

??? ?‘. ... ., .. .. .‘.’ ... ..: ‘:w::.::::_:_ .:: ::-:::g:. . . :..::=: ::: * ,,I....,,...,,, * q CB 201

CB 206

CB 101 CB 52

CB 26

CB 153 CB 116

CB 160

0 20 40 60 80 100

SEDIMENT DEPTH (cm)

Variation in the relative abundance of chlorobiphenyl congeners with sediment depth Garroch Head gravity core sample

Fig. 6. Variation in the relative abundance of chlorobiphenyl congeners with sediment depth in Garroch Head long core.

natural sedimentation rate for the Arran Deep of 0.3 cm year’ (Baxter et al., 1980) sediments at 60 cm depth correspond to the natural deposition laid down in the mid-nineteenth century, i.e. prior to the introduction of modern organochlorine chemicals. The concentration of these chemicals observed in deep sediment may therefore originate by vertical transport from the overlying sediment which contains high levels of these compounds. Alternative explanations for the presence of these compounds at depth are (a) contamination by surface sediment during the sampling procedure; or (b) post-sampling contamination. The latter hypothesis was tested by comparison of the gravity core samples from Garroch Head with those from the remote site at Pladda. Post-sampling contamination should similarly effect both sets of samples. This is not the case as the deepest section from Pladda contains only 1.4 pg kg ’ chlorobiphenyls as against 12.9 pg kg~’ at Garroch Head. Similarly, the deep core sections from Garroch Head have appreciable concentrations of total DDT, dieldrin and lindane, whilst detectable levels were present only in the surface section of the Pladda core.

Contamination of the deeper core sections by ‘dirty’ surface sediment during sampling should result in no change in the relative amounts of contaminants down the core, as all sections will reflect that composition present in the ‘dirty’ surface section. An examination of the concentration/depth profiles of chlorobiphenyl congeners in the Garroch Head core reveals the existence of a depth related compositional change. The proportion relative to total chlorobiphenyls of the more chlorinated congeners IUPAC numbers 153, 201 and 206 decreases with depth, that of 118 and 180 remains con-

stant, whilst that of the less chlorinated congeners 28, 52 and 101 increases (Fig. 6). Thus the measured contaminant levels in deep sediment are not sampling arti- facts but must result from pre-sampling transport from surface to depth. Possible transport mechanisms are physical mixing of sediment by benthic macrofauna (Pearson, 1986), and solubilisation in pore water, (Chiou et al., 1986; Carter & Suffet, 1982; Brusseau & Rao, 1989), with subsequent diffusion down the concentration gradient. Physical mixing does not seem likely for, as stated above, this will result in contaminants being transported in proportion to their concentration in the source sediment. The observed vertical changes in relative concentration of the chlorobiphenyl congeners is inversely related to their aqueous solubili- ties and ability to partition into lipophilic media (Nir- malakhandan & Speece, 1989). Such a distribution could originate from ingestion and excretion of sediment by infauna, predominantly polychaetes, with selective retention of more lipophilic contaminants. However the reported maximum depth of penetration of 60 cm (Pearson, 1986) is less than the depth of contaminated sediment. Alternatively, mobilisation in pore water should result in differential transport rates based on the octanol-water partition coefficients of each contaminant, and the aqueous organic carbon content of the system. In a predominantly two phase water-sediment system, preferential solubilisation of the less lipophilic compounds will lead to increased relative levels of these compounds with depth (e.g. Oliver et al., 1989). Conversely, when aqueous organic carbon levels are high, lipophilic compounds in the aqueous phase are bound to organic colloids and their aqueous phase


Transport Degradation

1 Water I

70

60

Diffusion of OC contaminants, Dare water or

C&p$ carbon

Fig. 7. Physico-chemical processes operating in sediment at Garroch Head.

concentration becomes independent of partition coefficient (e.g. Brownawell & Farrington, 1986; Pankow & McKenzie, 1991). The observed pattern suggests the latter. Low sedimentary organic carbon levels at these depths support this conclusion.

CONCLUSIONS

This work confirms previous studies of the Garroch Head disposal site as an area of substantial contaminant accumulation (Pearson, 1985; Clarke & Davies, 1989). The limited water movement at the site and the low rate of mineralisation of organic matter has resulted in the formation of a layer of sludge impacted sediment of 15-20 cm depth (Fig. 7). Surface sediment contaminant concentrations at Garroch Head are higher than those found at a remote site in the Clyde Sea by a factor of 12 for chlorobiphenyls, 40 for DDT compounds and 120 for dieldrin. The sediment concentration of chlorobiphenyls is the highest yet found in Scottish waters being greater than the mean level of 93 pg kg-’ found at the Piper Alpha site (Wells et al., 1989), but is comparable to that found at other sewage- impacted marine sites such as the New York Bight (West & Hatcher, 1980). The effects of these high levels of sedimentary OCs on marine biota may be mitigated by high sediment organic carbon levels in accordance with equilibrium partitioning principles (di Toro, 1991) i.e. equilibrium levels of organochlorine residues in biota and water associated with the sediment will be around 10 times lower than those resulting from a sediment with equivalent organochlorine levels and ‘back- ground’ organic carbon levels of 1%.

There appear to be few mechanisms acting to reduce these contaminant levels (Fig. 7). In the short term, di- lution with clean sediment does not occur, as measurements of faecal sterols show that the upper sediment layer is composed of essentially pure sewage sludge.

Vertical mixing of sediment in the upper 30 cm is limited and not of sufficient extent to destroy a distinct stratigraphic structure, reflecting the historical input of persistent contaminants. Additionally, the chlorobiphenyls and other organochlorine pesticides are resis- tant to degradation in this sedimentary environment, occurring at levels comparable to those in the dumped sludge. Vertical biological or aqueous diffusive transport of contaminants will lead to movement of contaminants out of the heavily sludge contaminated layer. However, this flux of OCs is likely to be small in comparison with the total sediment burden. The sludge deposit may therefore remain in the long-term, as an area of elevated sediment organochlorine concentration in the Firth of Clyde.

ACKNOWLEDGEMENTS

The author would like to acknowledge Dougal Fin- layson who undertook the CHN analysis, Dr John Robson for assistance in part of the GUMS work and Dr David Wells for helpful advice and comments during the course of this work.

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