Organotin distribution in sediments and waters of selected east coast estuaries in the UK

Marine Pollution Bulletin :Varine I>olhltion Bulletin. Volume 24. No. 10, pp. 492 498. 1992. Printed in Great Britaitl.

0025-326X/92 $5.1)(}+0.00 © 1992 Pergamon Press Ltd

Organotin Distribution in Sediments and Waters of Selected East Coast Estuaries in the UK P. H. DOWSON, J. M. BUBB and J. N. LESTER Environmental and Water Resource Engineering Section, Department of Civil Engineering, Imperial College of Science, Technology and Medicine, Exhibition Road, London SW7 2BU, UK

An extensive spatial survey of butyl- and methyltin compounds in the sediments and water column of seven Suffolk and Essex estuaries was undertaken between June and August 1990 to determine the main areas of organotin contamination. A number of these estuaries contain important shelifisheries and all have a high degree of boating activity. Speciation of organotins was performed by a purge and trap/boiling point separation method and detected by Atomic Absorption Spectrometry. Despite the implementation of the 1987 UK retail ban on tributyitin (TBT) based antifouling paints on vessels <25 m in length, 7 of the 17 sites sampled exhibited TBT water concentrations above the Environmental Quality Target (EQT) set in 1987. Water column concentrations ranged from <3-71.2 ng I-t. Dredging had occurred at some of these sites indicating that desorption of TBT from the sediment may occur following sediment disturbance. Surficial sediment organotin concentrations displayed high spatial variability with TBT levels ranging from < 3 - 3 9 3 5 ng g-1. The highest organotin concentrations occurred at sites with high levels of boating activity.

Tributyltin has been used as a biocide in antifouling paints applied to boats, ships and docks since the late 1950s but has caused concern because these paints release the highly toxic tributyltin (TBT) directly into the aquatic environment. The environmental impact of organotins has been the subject of numerous investigations which have been summarized by Clarke et al. (1988). High concentrations of TBT have been shown to exist especially in water and sediment of marinas and yacht harbours. In addition to TBT, other organotin compounds such as monobutyltin (MBT), dibutyltin (DBT) and methyltins have been detected in the aquatic environment (Maguire et al., 1986; Valkirs et al., 1986; Cleary & Stebbing, 1987). Whilst butyltin compounds are exclusively of anthropogenic origin, methyltin compounds may be formed by biological or abiological methylation in the environment (Craig et al., 1982; Thompson et al., 1985). 492

Alzieu et al. (1982) proposed that TBT leachates from antifouling paints were the most likely cause of severe problems in culturing oysters in the Arcachon Bay. Similar chambering effects were later observed by Waldock & Thain (1983) in the Blackwater and Crouch estuaries of Eastern Britain. Toxicity studies have revealed acute effects of TBT for aquatic organisms at concentrations as low as 1 ~tg 1-1 (Lawler & Aldrich, 1987; Gibbs et al., 1987), whilst sublethal effects of TBT on the dog whelk have been found to occur at concentrations < 10 ng 1-J, imposex being initiated at a TBT water concentration of <0.5 ng 1-1, as Sn (Bryan & Gibbs, 1991). Thus TBT has shown to be damaging at levels far below those yet recorded for other marine pollutants. The bioconcentration of organotin compounds to toxic levels through the ingestion of contaminated particles or via bioaccumulation, increases the risk above that expected from ambient levels (Clarke et al., 1988). Since 1 July 1987 all antifoulings (utilized in the UK) whether they contain TBT or not, have been subject to the Food and Environmental Protection Act (1985) Part III. This replaced the voluntary notification scheme and requires that all formulations containing aquatic biocides have prior approval by ministers before they can be sold or used. In May 1987, a complete ban on the retail sale of TBT based paints for use on vessels <25 m was introduced in the UK under the Control of Pollution Act (1974) (Waldock et al., 1987). TBT is now on the list of 132 substances for EEC action in the Council Directive on 'Pollution Caused by Certain Dangerous Substances into the Aquatic Environment' (76/464/EEC) (Department of the Environment, 1986). TBT and its associated compounds appear in List II (the Grey List) of the Directive and National Environmental Quality Standards (EQS) have been set. The EQS as defined by the Water Research Centre for TBT, were set at 20 ng 1-t for freshwater and 2 ng I-~ for saline water (Water Research Centre, 1988). New research has shown that concentrations of the pesticide tributyltin (TBT) exceed the UKs official water quality standard in the surface layers of the North Sea. Such findings will add pressure for a global ban on the use of TBT based antifouling paints on large ships

Volume 2 4 / N u m b e r 10/October 1992

t R. Butler

Ipswich 9~

\

27 36

28

.~v,

18-22

Colchester 39

Harwich

Walton4 Backwaters

42~ e~ 46

38

North Sea

41

40 R .Crouch

44 R.I

11

47 DISTANCE (Kin)

Fig. 1 Location of sampling area.

(ENDS, 1991). A House of Lords Select Committee recommended a complete ban in 1990 (ENDS, 1991), but the Government responded by argueing that scientific evidence was insufficient to persuade other countries from introducing further curbs on the use of TBT antifoulants and international collaboration would be needed to make any moves effective. However, a world-wide ban was also called for last year by the Japanese delegation at a meeting of the International Maritime Organization's Marine Environment Protection Committee (MEPC). The move reflected concern about the accumulation of TBT and a sister compound; triphenyl tin (TPT), in fish and birds taken from bays and estuaries. Controls on both substances were introduced in Japan in 1988 (ENDS, 1991), and TPT based antifoulants were banned from use on all vessels this year. The chemical industry's European Federation (CEFIC), deflected these moves by claiming that the superior performance of TBT antifouling paints saved the world's shipping industry $2.5 billion, and 7.2 million t in fuel consumption every year. However, the Japanese claim that copper-based paints provide an acceptable antifouling performance

over 2 yr; the general dry docking interval for commercial vessels. Tin-free self-polishing paints are also expected to be available commercially in the near future. This study examines the spatial distribution of a range of organotin compounds including TBT from seven different estuarine systems on the east coast of the UK. The effect of the 1987 retail ban on reducing TBT levels in the aquatic environment is also discussed. Materials and M e t h o d s Between June and August 1990, an extensive spatial sediment survey was undertaken in seven estuarine fiver systems in Essex and Suffolk. This incorporated the Rivers Deben, Orwell, Stour, Colne, Crouch, Roach, and Walton Backwaters (Fig. 1). A number of these catchments contain important shell fisheries and are popular boating areas. Additionally, water column surveys were undertaken on the River Orwell and River Deben. Time constraints prevented the analysis of water from the other estuarine systems. All sediment and water samples were stored in a cool box and kept 493

Marine Pollution Bulletin

away from direct sunlight to minimize changes in sample composition prior to analysis. Water samples were taken by immersing 500 ml polyethylene containers approximately 20 cm under the surface to prevent the inclusion of the surface microlayer, as high concentrations of organotins have been found in the surface microlayer and its inclusion may exaggerate organotin concentrations in the water column (Cleary and Stebbing, 1987). Acid clean containers were rinsed three times with fiver water before sample acquisition to minimize contamination. Determination of total suspended matter concentrations was undertaken on all water column samples according to standard methods (Standing Committee of Analysts, 1980). Surficial sediment samples were taken with Teflon coated polypropylene scoops and placed in acid washed glass sample jars and stored in cool boxes. The top few centimetres of sediment were collected to ensure that only the most recent deposits were sampled. The sediment extraction procedure for organotins was initiated on the day of collection, immediately on return to the laboratory. Bulk sediment samples were homogenized by vigorously shaking for 1 min. Sediment (10 g) was placed into a preweighed centrifuge tube with 20 ml of Aristar acetic acid and shaken for 15 h at room temperature in the dark. Samples were then centrifuged at 2000 g for 20 rain (Dowson et al., 1992). A 2 ml volume of the supernatant was subsequently diluted with 200 ml of deionized water and 6 ml of concentrated Aristar acetic acid in the reaction flask prior to analysis. Water samples were analysed without pretreatment. Samples (200 ml) were acidified with 8 ml of concentrated Aristar acetic acid in the reaction flask. Organotins were speciated using a modified version of the 'purge and trap' boiling point separation method developed by Donard et al. (1986) and described by Dowson et al. (1992). Organotin hydrides were cryogenically trapped using liquid nitrogen and separated on a simple chromatographic column packed with Chromosorb G.HP 80/100 mesh, coated with 10% OV-101. Hydride species were sequentially desorbed in relation to their specific boiling points after heating of the column (-196-200°C). The hydrides were carried by a helium flow (140 ml min -1) with oxygen and hydrogen being introduced into the quartz cell as additive gases with respective flows of 20 ml min -~ and 500 ml min -~. Detection was performed by a PerkinElmer 1100 B atomic absorption spectrometer operating at a wavelength of 224.6 nm, using an TABLE 1

Site classification for levels of TBT in water and sediment samples.

Site classification (i) (ii) (iii) (iv) (v)

Uncontaminated Lightly contaminated Moderately contaminated Highly contaminated Grossly contaminated

Total no. of sites surveyed

494

Number of sites in each category TBT conc. Sediment Water (ng g-~ or ng l-I) sites sites <3 3-20 20-100 100-500 <500

25 5 11 1 5

10 0 7 0 0

47

17

electrothermally heated (900°C) quartz furnace and a tin EDL lamp (Perkin-Elmer Ltd).

Results The spatial survey undertaken between June-August 1990, identified the main areas of organotin contamination in estuarine systems throughout Essex and Suffolk. The severity of TBT contamination in waters and sediments in the study area has been categorized according to the criteria laid out in Table 1. The sediment organotin concentrations found at each site are listed in Table 2.

Spatial Distribution of Organotins in Surficial Estuarine Sediments The sediment survey covered 47 sites in 7 estuarine catchments. Over half the sites investigated contained undetectable levels of TBT (<3.0 ng g-l). Five 'grossly' contaminated sites were identified, of which Deben Yacht Club (site 4), Titchmarsh and Tollesbury marinas (sites 19 and 42 respectively) exhibited TBT concentrations in excess of 2000 ng g-~. Robertsons Boatyard (647 ng g-l) on the River Deben (site 1) and Eastend at Paglesham (site 46) on the Roach (1293 ng g-t) also contained gross levels of TBT in bottom sediments. Sites which were grossly contaminated coincided with areas of high boating activity. Concentrations of methyltins were low compared to their butyltin counterparts ranging from <0.2 ng g-1 (the analytical detection limit) to a maximum of 6.5 ng g-~, the mode being firmly placed at <0.2 ng g-t. Spatial Survey of Organotins in the Water Column A water column survey of the Rivers Deben and Orwell was undertaken in June 1990. This was designed to coincide with summer boating activities so the impact of pleasure craft on TBT water column concentrations and the effectiveness of the 1987 TBT retail ban could be evaluated. Results of the survey are listed in Table 3. These display surface water concentrations for individual river systems where detectable levels of TBT were found. Enrichment of TBT and associated compounds was very site specific, with the majority of locations displaying water column concentrations below the limit of detection (<3.0 ng l-L). For those sample locations where measurable levels of TBT were found (7 out of the 17 sites surveyed) all exhibited concentrations in excess of the EQS for saline and freshwaters (depicted in bold in Table 3). Detectable concentrations ranged from 21.1-71.2 ng 1-l. The occurrence of high TBT levels tended to coincide with the location of boat yards, marinas, or mooring facilities. For those sites where TBT was present in water samples in significant quantities, concentrations were such that T B T > D B T > M B T , with the exception of Woolverstone Marina (site 11) and Shotley Marina (site 16). The occurrence of TBT was usually substantiated by the presence of DBT and MBT; its degradation products. Methyltins did not form a significant contribution to total organotin since levels were generally

Volume 2 4 / N u m b e r 10/October 1992 TABLE 2

Summer 1990 spatial survey for organotin concentrations in Essex and Suffolk fiver surface sediments.

Site

Sample location

Total butyltin

River Deben Robertsons Boatyard Tidemill Marina Wisstocks Boatyard Deben Yacht Club Waldringfield Quay Ramsholt Quay

1. 2. 3. 4. 5. 6.

2263 82.4 143 5287 194 16.3

7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

River Orwell Ipswich Wet Dock Ipswich Lock Gates Foxes Marina Bourne Creek Woolverstone Marina Orwell at Woolverstone Pin Mill Levington Marina Collimer Point Shotley Marina Shotley Point

18. 19. 20. 21. 22. 23. 24. 25. 26.

Walton Backwaters Walton Yacht Club Titchmarsh Marina Twizzle, North Bank Walton Channel Hamford Water Kirby Creek Oakley Creek Dugmore Creek Landermere Creek

27. 28. 29. 30. 31. 32. 33.

Organotin concentration (ng g-t as Sn) DBT MBT TMT

TBT

647 42.2 65.1 2149 111 < 3.0

1500 30.3 69.0 2964 56.0 12.1

DMT

MMT

116 9.93 9.12 174 27.2 4.18

0.53 < 0.2 < 0.2 t.05 < 0.2 < 0.2

1.22 1.34 0.82 < 0.2 0.46 < 0.2

< 0.2 1.32 1.54 < 0.2 2.15 < 0.2

25.7 7.44 70.4 35.7 128 108 44.7 12 l < 5.0 166 10.9

< 3.0 < 3.0 46.2 17.1 85.4 84.1 22.9 56.3 < 3.0 89.7 < 3.0

23.6 7.44 19.4 15.2 36.8 21.4 16.5 60.4 < 1.0 69.6 10.9

2.13 < 1.(I 4.76 3.39 5.32 3.04 5.25 4.68 < 1.0 6.63 < 1.0

< 0.2 0.48 0.86 < 11.2 < 0.2 1/.77 < 0.2 < 0.2 < 0.2 < 0.2 0.40

< 0.2 < (I.2 1.06 1.22 < 0.2 < 0.2 < 0.2 < 0.2 < 0.2 < 11.2 0.46

< 0.2 < 11.2 2.77 1.26 0.80 < 0.2 < 0.2 < 11.2 < 0.2 1.54 1/.86

4.67 4483 54.3 61.1 4.89 11.8 15.8 52.4 < 5.0

< 3.0 3935 < 3.0 5.75 < 3.0 <3.0 < 3.0 < 3.0 < 3.0

2.74 665 8.64 31.7 < 1.0 <1.0 7.72 15.8 < 1.0

1.93 90.6 13.3 23.6 4.89 11.8 8.09 36.6 < 1.0

< 0.2 < 0.2 < 0.2 <0.2 < 0.2 <0.2 < 0.2 3.55 < 0.2

< 0.2 0.91 < 0.2 3.78 < 0.2 3.41 < 11.2 0.63 < 0.2

< 0.2 1.38 < 0.2 <0.2 < 0.2 <0.2 < 11.2 0.411 < 0.2

River Stour Swan Basin at Mistley Stone Point at Wrabness Erwarton Pier Dock River Stour Sailing Club Harwich Breakwater Harwich Haven

90.1 < 5.0 8.50 16.0 7.00 12.9 57.7

50.7 < 3.0 < 3.0 < 3.0 < 3.0 < 3.0 7.76

32.7 < 1.0 5.12 6.48 < 1.0 < 1.0 24.5

6.72 < 1.0 3.38 9.47 7.00 12.9 25.4

< < < < < <

0.53 0.2 11.2 0.2 0.2 0.2 0.2

< < < < < <

1.15 0.2 0.2 0.2 0.2 11.2 0.2

< < < < < <

2.08 0.2 11.2 0.2 0.2 0.2 0.2

34. 35. 36. 37.

River Colne Rowhedge Ferry Arlesford Creek Batemans Tower Pyefleet Channel

32.2 8.93 3.96 12.2

19.9 < 3.0 < 3.0 < 3.0

8.22 6.10 3.96 8.26

4.00 2.83 < 1.0 3.96

< 0.2 < 0.2 < 0.2 0.52

< < < <

0.2 0.2 0.2 11.2

< < < <

0.2 0.2 0.2 0.2

38. 39. 40. 41. 42.

River Blackwater Maldon Promenade Heybridge Basin Marconi Sailing Club Bradwell Marina Tollesbury Marina

23.3 < 5.0 < 5.0 79.3 4187

< 3.0 < 3.0 < 3.0 29.4 2835

10.4 < 1 .(/ 1.64 25.4 120

< 0.2 < 0.2 < 0.2 4.60 < 0.2

< 0.2 1.22 1.24 6.52 < 0.2

< 0.2 < 0.2 0.77 1.34 < 0.2

43. 44. 45. 46. 47.

River Crouch & Roach Hullbridge North Fambridge Royal Burnham Y.C. Eastend, Paglesham Monkton Quay

64.3 48.4 7.64 2225 16.5

40.9 32.2 < 3.0 1293 9.85

2.63 < 1.0 < 1.0 96.7 1.81

< 0.2 < 0.2 < 0.2 1.36 < 0.2

< 0.2 < 0.2 < 0.2 0.99 0.60

< < < < <

detection

the

mono-,

di- and tri-methyl forms were present in 5 of the analysed,

Methyltins

levels

ranging

in

either

20.8 16.2 7.64 836 4.74

below

17 s a m p l e s

limit.

12.9 < 1.0 < 1.0 24.5 1232

from

the

0.79-2.02

n g 1-~.

bodies, with enrichment yards

and

mooring

fouling paints were a major in the aquatic environment. Concentrations <3.0

Discussion Elevated systems TBT

ng

1-1

maximum concentrations

monobutyltin within

reflected

were

detected

Essex

and

boat

usage

of

tributyl-

in a n u m b e r Suffolk. The patterns

dibutyl-

and

of estuarine distribution

within

the

of

water

occurring

localities.

water

in the (the

water

analytical

that

column

ranged

detection

limit)

o f 7 1 . 2 n g 1-7. T h e s e l e v e l s c o m p a r e

column

1-7 a s

sites, boat-

implies

anti-

source of these compounds

concentrations

obtained

igations (Table 4). Concentrations Maguire

at marina

This

0.2 0.2 0.2 0.2 0.2

from to

a

well with

in other invest-

in excess of 1000 ng

have been quoted (Maguire et al., 1 9 8 5 ; et al., 1 9 8 6 1 b u t m o r e f r e q u e n t l y m a x i m a i n t h e

Sn

495

Marine Pollution Bulletin TABLE 3

Summer 1990 spatial survey of water column samples taken 20 cm below the surface microlayer. Site

SS

TBT

Organotin concentrations (ng 1-~ as Sn) DBT MBT MMT

1.28 2.08 2.61 2.17 1.94 2.23

23.7 < 3.0 40.2 44.8 < 3.0 < 3,0

10.9 2.58 25.3 23.3 < 1.0 < 1.0

3.31 < 1.0 13.8 8.93 < 1.0 < 1.0

2.08 2.01 3.35 2.64 2.63 3.39 1.91 3.28 4.80 4.67 2.62

< 3,0 < 3.0 25.9 < 3.0 71.2 < 3.0 <3.0 21.1 < 3.0 49.3 < 3.0

< 1.0 < 1.0 17.5 29.3 87.0 7.75 16.0 16.5 < 1.0 114 7.50

< 1.0 < 1.0 10.9 19.4 21.7 7.78 16.5 12.0 < t.0 56.3 8.40

Location

DMT

TMT

1.07 < 0.20 < 0.20 1.33 < 0.20 < 0.20

0.95 < 0.20 < 0.20 < 0.20 < 0.20 < 0.20

< 0.20 < 0.20 < 0.20 < 0.20 < 0.20 < 0.20

< 0.20 < 0.20 < 11.20 < 0.20 < 0.20 < 0.20 <11.20 1.33 < 0.20 <0.20 < 0.2/)

< 0.20 < 0.20 < 0.20 < 020 0.79 < 0.20 <0.20 1.12 < 0.20 2.02 < 0.20

< 0.20 < 0.20 < 0.20 < 0.20 1.12 < 0.20 <0.20 <0.20 < 0.20 <0.20 < 0.20

River D e b e n

1. 2. 3. 4. 5. 6.

Robertsons Boatyard Tide Mill Marina Whisstocks Boatyard Deben Y.C. Waldringfield Quay Ramsholt Quay River Orwell

7. 8. 9. 10. I 1. 12. 13. 14. 15. 16. 17.

Ipswich Wet Dock Ipswich Lock Gates Foxes Marina Basin Bourne Creek Woolverstone Marina Orwell, Woolverstone Pin Mill Levington Marina Collimer Point Shotley Marina Shotley Point

Values in bold exceed the Environmental Quality Standards set for fresh and saline waters (20 and 2 ng l-~ respectively). SS = Suspended solids concentration (mg 1-~). TABLE 4

Comparison of tributyltin concentrations in sediments and waters for a number of river and estuarine systems. Location

Water system

Walton Backwaters

Open channel Marina Estuary Estuary Estuary Estuary Estuary Estuary Rivers Harbour Marina Marina Estuary Seawater Rivers Estuarine Marina Seawater

R. Stout, UK R. Orwell, UK R. Deben, UK R. Colne, UK R. Blackwater, UK R. Roach, UK Norfolk Broads, U K R. Rhine, Germany Lake Lucerne, Switz. Arcachon Bay, Fr Auckland, NZ France R. Bure & Yare, UK UK (various) Ontario, Canada R. Crouch, UK

Water conc. (ng l-t)

< 3.0-71.2 < 3.0-44.8

0.7-2.3 245-571 < 1 200 nd-I 13 nd-600 nd- 1890 nd-900

Sediment conc. (ng g-~) < 3.0-6 39.1 3935 < 3.0-51 < 3.0-89.7 < 3.0-2149" < 3.0-20 < 3.0-2835* < 3.0-1293* < 3.[)- 1291 27-34 < 2-107 < 1.0-12 220 < 2.0-760

nd- 10 790

Reference Present study Present study Present study Present study Present study Present study Present study Present study Dowson etal., 1992 Schebek & Andreae, 1991 Fent & Hunn., 1991 Quevauviller et al., 1990 De Mora etal., 1989 Alzieu etal., 1989 Waite et al., 1989 Waldock etal., 1987 Maguire et al., 1986 Waldock & Miller, 1983

*High concentrations from marina sites. r a n g e o f 1 0 0 - 2 0 0 ng 1- j h a v e b e e n f o u n d ( S e l i g m a n et al., 1986; C l e a r y & S t e b b i n g , 1985). T h e E Q S f o r T B T in f r e s h w a t e r e n v i r o n m e n t s was set at 2 0 ng 1-~ f o l l o w i n g t h e d i s c o v e r y in 1 9 8 0 , that tributyltin leachates from antifouling paints were the m o s t likely c a u s e o f s e v e r e p r o b l e m s in c u l t u r i n g o y s t e r s o n t h e A t l a n t i c C o a s t in F r a n c e ( A l z i e u et al., 1982). A l t h o u g h s u b - l e t h a l effects o n a q u a t i c o r g a n i s m s h a v e b e e n o b s e r v e d at c o n c e n t r a t i o n s b e l o w 10 ng 1-~ ( L a w l e r & A l d r i c h , 1987). T h e w a t e r c o l u m n c o n centrations found within the Deben and Orwell river s y s t e m s e x c e e d the E Q S in s o m e i n s t a n c e s , b u t measurements obtained were based on bulk water samples and the accentuation of TBT levels by adsorption o n t o s u s p e n d e d solids w a s o r i g i n a l l y p r o p o s e d to explain these comparatively high water column loadings. H o w e v e r , little c o r r e l a t i o n was f o u n d b e t w e e n s u s p e n d e d solids a n d T B T c o n c e n t r a t i o n s f o r t h e w a t e r c o l u m n s a m p l e s . In all c a s e s t h e t o t a l s u s p e n d e d solids 496

c o n c e n t r a t i o n was b e l o w 10 m g 1-~, t h e level at w h i c h Q u e v a u v i l l e r & D o n a r d ( 1 9 9 0 ) r e c o m m e n d filtering bulk water samples. T B T is also k n o w n to c o n c e n t r a t e at t h e s u r f a c e m i c r o - l a y e r w h e r e films c o n t a i n i n g a l c o h o l s a n d fatty a c i d s t e n d to c o n c e n t r a t e l i p o p h i l l i c p o l l u t a n t s , s u c h as o r g a n o t i n s ( C l e a r y & S t e b b i n g , 1987). T h i s c a n i n c r e a s e T B T c o n c e n t r a t i o n s by f a c t o r s o f b e t w e e n 2 a n d 10 c o m p a r e d to a v e r a g e w a t e r c o l u m n l o a d i n g s . T h i s has m a j o r i m p l i c a t i o n s f o r i n t e r t i d a l a r e a s w h e r e d e p o s i t i o n o n t h e s h o r e o c c u r s at l o w t i d e ( Q u e v a u viiler et al., 1989). T h e o c c u r r e n c e o f h i g h w a t e r c o l u m n c o n c e n t r a t i o n s b e l o w the s u r f a c e m i c r o l a y e r at s o m e o f t h e s a m p l i n g l o c a t i o n s i m p l i e s that f u r t h e r enrichment of these toxic compounds may occur within t h e m i c r o l a y e r , t h e r e f o r e i n c r e a s i n g t h e e x p o s u r e risk o f a q u a t i c b i o t a t o w a r d s T B T a n d its d e g r a d a t i o n p r o d ucts ( C l e a r y & S t e b b i n g , 1987). T h e p r e s e n c e o f T B T in s o m e w a t e r s a m p l e s d e s p i t e

Volume 24/Number 10/October 1992

the retail ban implemented in 1987 implies that TBT may still have been in use during the Summer of 1990. A number of contaminated sites have however been dredged in the months prior to sample collection, these include Tidemill, Levington, Titchmarsh and Tollesbury marinas. Studies undertaken by Unger et aL (1988) have shown that TBT sorption onto sediment is reversible, indicating that TBT contaminated sediments can act as a major source for dissolved TBT. Dredging activities undertaken at the above sites may consequently have re-released organotin compounds back into the water column following desorption from the sedimentary compartment, thereby increasing water column concentrations. Certain sites (e.g. Shotley Marina) are also open to commercial shipping, which are not covered by the 1987 ban on the use of TBT antifouling paints. Additionally, Thames barges and marine craft >25 m use Shotley Marina. The highest levels of TBT were usually found however, not in areas of commercial shipping, but where small boats were abundant confirming reports by Cleary & Stebbing (1987). The fact that there appeared to be no substantial difference between the levels of TBT detected in waters used by ocean going vessels, such as on the River Orwell, and waters utilized exclusively by pleasure boats, implies that either the commercial sea shipping has little effect on the levels of organotins, or that the slow release copolymer TBT formulations utilized by the commercial shipping fleet are being used by pleasure craft as opposed to the free association paints which used to be predominant in the past (Waldock et al., 1987). Butyltin partitioning within the water column in a concentration sequence of TBT > DBT > MBT, is typical for the degradation of tributyltin forms in water and is consistent with findings reported by Stang & Seligman (1986) and Seligman et al. (1986). The presence of di- and mono-butyl forms in some water samples despite the absence of TBT, implied active degradation within the water column or the removal of TBT from water to sediment under certain environmental conditions. Laboratory studies have shown that a wide variety of agents are capable of cleaving the tincarbon bond (Blunden & Chapman, 1982). In the aquatic environment the most relevant processes are likely to be photochemical cleavage and biological cleavage by microorganisms (Maguire, 1991). Previous assessments of TBT degradation in the water column have indicated that the compound has a low persistence in the water column. However, as TBT shows a tendency to accumulate in sediments, TBT degradation processes in sediments are more likely to control the overall persistence of TBT in the aquatic environment (Stewart & De Mora, 1989). Concentrations of TBT within surficial sediments ranged from <3.0-3935 ng g-~, but over half the sites sampled contained indeterminable amounts of TBT. At those sites where TBT was identified, most were within the 20-100 ng g-~ range. Only five sites were classified as 'grossly' contaminated with concentrations in excess of 500 ng g-~, which tended to correspond to marinas and boatyard localities. Wet sieving of high TBT con-

taminated sediments has revealed visible paint flakes in the past (Dowson et al., 1992). The presence of TBT paint fragments in sediment samples could therefore create localized pockets of high TBT concentrations within the survey network which may form long term centres for the release of TBT. From Table 4 it can be seen that the maximum levels of TBT in sediments are generally quoted as 500-1000 ng g-~ as Sn (Maguire et al., 1985; Seligman et al., 1986) but concentrations as high as 10 780 ng g-J have been found (Maguire et al., 1986). TBT levels in bottom sediments at Titchmarsh and Tollesbury marinas (>2000 ng g-~) are high compared to results quoted by other investigators. This degree of enrichment may pose a threat to aquatic communities if TBT becomes available following sediment disturbance, particularly to benthic organisms which are constantly exposed to the contaminated strata. Methyltin concentrations in sediments were low compared to butyltin species but compared favourably with other published work (Tugrul et al., 1983; Maguire et aL, 1986; Dowson et al., 1992). The source of methyltins remains elusive; anthropogenic discharges, abiotic methylation reactions and biotic methylation have all been demonstrated (Thompson et al., 1985). Gilmour et al. (1985) found that mixed cultures of bacteria isolated from Chesapeake Bay sediments were capable of methylating tin under anaerobic as well as aerobic conditions and the anaerobes Desulfovibrio spp., are capable of accomplishing methylation. It is most likely that the relatively low concentrations of methyltins found in this survey arise through the in-situ methylation of inorganic tin.

Conclusions The major input pathway of TBT to the aquatic environment is via boat service facilities such as boatyards, marinas, and mooring sites. High water column concentrations in boating areas coincided with summer boat usage patterns implying that TBT was either being leached from boat hulls, 4 yr after the implementation of the retail ban or that desorption of TBT from contaminated sediments as a result of dredging activities was releasing organotins back into the water column. It is likely that aquatic sediments act as both a sink and a potential source of TBT to the aquatic environment, despite a reduction in direct TBT inputs. This work was carried out under contract to the National Rivers Authority (formerly Anglian Water), who gave approval for publication. One of the authors (P.D.) is grateful to The Science and Engineering Research Council for the award of a Postgraduate Studentship.

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