The effect of sodium dibutylphosphate on the remobilization of americium and plutonium from Irish Sea sediments

Volume26/Number 5/May 1993 Marine Pollution Bulletin. Printed in Great Britain.

Volume 26, No. 5, pp. 263-268, 1993.

0025-326X/93 $6.00+0.00 © 1993 Pergamon Press Ltd

The Effect of Sodium Dibutylphosphate on the Remobilization of Americium and Plutonium from Irish Sea Sediments J. HAMILTON-TAYLOR, M. KELLY, K. BRADSHAW and J. G. TITLEY* Institute of Environmental and Biological Sciences, University of Lancaster, Lancaster LA1 4YQ, UK *Current address: National Radiobiological Protection Board, Chilton, Didcot, O X I ! 0 R Q , U K

Laboratory experiments were carried out under environmentally realistic conditions in which Irish Sea sediment was resuspended into Irish Sea water for various time periods. In all cases, there was a significant (p generally < 0.05) increase in the dissolved ( < 0.22 [am) activity concentrations of 238pu, 239'24°pu and 241Am after mixing with the Irish Sea sediment. Dissolved Pu concentrations continued to increase throughout the maximum period employed (168 h), whereas dissolved Am concentrations showed no significant variation after the first sampling interval (24 h). The presence of 0.5 mg I-~ sodium dibutyiphosphate had no effect on the remobUization of Pu but appeared to produce a small increase in the dissolved 241Am concentration (i.e. a 25%, 0.35 mBq 1-1 increase). This was probably due to the solution complexation of Am by dibutyiphosphate. In context, the increase is too small to have any likely environmental importance.

Plutonium and americium are present in the authorized low-level radioactive waste from the British Nuclear Fuels plc reprocessing plant at Sellafield (Cumbria, UK), which is discharged into the NE Irish Sea at 20 m depth, 2 km offshore. From here Pu and Am have been dispersed throughout the Irish Sea by tidal, wind and density driven currents (e.g. Astonet al., 1985; Pentreath et al., 1984; Eakins et al., 1988; Carpenter et al., 1991). A strong association with natural particles is a key feature of the biogeoehemical behaviour of Pu and Am in the aquatic environment, influencing their dispersion, bioavailability and ultimate environmental fate. Therefore, it is important to assess the effect on solid-solution partitioning brought about by changes to the local chemical environment, especially where this involves a change in the composition of the waste stream discharged from the Sellafield site. Tributylphosphate (TBP) is an organic solvent widely used in the nuclear industry for its efficient extraction properties with regard to actinide elements (Seaborg,

1958). During nuclear fuel reprocessing at Sellafield, Pu and U are recovered by a solvent extraction process, employing TBP diluted with kerosene. A new Solvent Treatment Plant is under construction in which it is planned to hydrolyse the TBP to yield sodium dibutylphosphate (NaDBP). The NaDBP will be subsequently discharged to sea through the Sellafield pipelines. The pt/rpose of our study was to determine the likely effects of NaDBP on the remobilization behaviour of Pu and Am into seawater from Irish Sea sediments, contaminated by earlier discharges from the reprocessing plant at SeUafield. Methodology

Materials Sediment samples were collected on the 5 December 1990 from a water depth of - 2 0 m in the north-east Irish Sea, approximately 3 km offshore from Sellafield near the end of the discharge pipeline (54°24'N, 3°33'W). The muddy sediments in this area are known to be amongst the most radioactive in the Irish Sea (Pentreath et al., 1984). A sample of surface sediment was collected corresponding to - t o p 10 cm, by means of a towed dredge sampler. Approximately 20 g of the sediment was homogenized thoroughly and retained in a field moist state at 4°C. About 50 1 of seawater was collected on the same day by pump, after flushing for 5 min, in acid cleaned polyethylene containers, prerinsed with sample. The seawater was filtered through 0.22 ~tm membranes and then stored in the dark at 4°C. A second seawater sample was collected from the same site in February 1991 and treated in an identical fashion. Analysis Extraction and clean-up of Pu and Am were based on a combination of standard literature techniques (Lovett & Nelson, 1981; Holm et al., 1979; Faris & Buchanan, 1964; Kraus & Nelson, 1959). The ambient and experimental seawater filtrates were made 5% v/v in HCI, and then yield m o n i t o r s (242pu and 243Am) added. Pu and Am were extracted by calcium oxalate precipita263

Marine PollutionBulletin tion following the reduction of Pu(V, VI) to Pu(III, IV). The oxalate precipitate was recovered by filtration and ashed at 700°C for 30 min. The residue was dissolved in HC1 and the Pu and Am recovered by an Fe oxide precipitation step. The sediments were refluxed in hot 6M HCI for 5 h following addition of the yield monitors. The residual solids were separated by centrifugation and discarded. Pu and Am in the supernatant ~vcre again recovered by Fe oxide precipitation. The oxide precipitate recovered in both schemes was dissolved in conc. HC1, and then Pu and Am were separated and impurities removed by means of a series of anion and cation exchange columns. The Pu and Am fractions, recovered separately, were electrodeposited onto stainless steel planchettes (Talvitie, 1972) and the activity determined by a-spectrometry (Berkeley Nucleonics Alpha Spectrometer Model AP3). Counting times up to 6X105 s were employed and the results recorded on an Amstrad PC using 'Maestro II' software (EG&G ORTEC). The analytical errors, quoted for individual measurements in Tables 2 and 7, are composed of the following propagated errors: 1. counting errors of the 242pu and 243Am yield monitors, 2. counting errors of the 238pu, 239"24°pu and 24IAm, 3. errors arising from the standardization of the yield monitors, and 4. detector background errors; and they are given as _+2 standard deviations. Experimental conditions and methods The experiments were designed to simulate conditions in the Irish Sea. The maximum NaDBP concentration, 200 m from the Sellafield outfall, is estimated to be - 0.5 mg 1-~ during future operations (BNF plc, pers. comm.). This concentration was used in all experiments. The temperature (17°C and 10°C) of between 1 and 3.51 of Irish Sea water in polyethylene beakers was stabilized in a water bath. NaDBP was added together with a known amount of contaminated Irish Sea sediment, in its natural field moist state. The resulting suspensions were mixed continuously for an hour and thereafter intermittently by overhead stirrer for up to 7 days. Thus remobilization as a function of time was determined in a series of independent experiments. Control experiments were run in parallel but without NaDBP addition. Experiments were terminated by filtration through 0.22 ~tm membranes. Statistics The Student's t test was employed to determine the significance of the differences observed between means, where experiments were replicated. The variances in the data sets, thus defined, therefore include experimental as well as analytical errors. Pooled or separate estimates of variance were used as appropriate. A two-tailed test was used unless otherwise stated. The difference between two means is regarded as being significant only if the null hypothesis is rejected at the 0.10 probability level.

Results The sea water characteristics (e.g. salinity 33.2%o, pH 264

8.1, 9.5°C) at the time of sampling were typical for the Irish Sea. The initial activity concentrations of 238pu, 23924°pu and 241Am in the Irish Sea sediments are shown in Table 1. The concentrations are comparable with those reported for current surficial muddy sediments in the offshore and adjacent areas of the north-east Irish Sea (Hunt, 1990; BNF plc, 1991). The main series of remobilization experiments attempted to simulate 'worse case' conditions by employing a sediment concentration of 330 mg 1-~, corresponding to the maximum suspended sediment concentration in the north-east Irish Sea recorded by McKay et al. (1987), and a temperature of 17°C. The experiments were run in duplicate and the results are shown in Table 2 and Figures 1 and 2, as a function of time. The dissolved concentrations of all the radionuclides show a clear and significant increase, relative to the background seawater concentrations, following addition of the contaminated sediment (p < 0.05 in all cases but one, where p < 0.10). In the case of the 23~pu and 239'24°pu, dissolved concentrations increased continuously with time (Fig. 1), but no significant differences existed between experiments with and without NaDBP addition. Therefore the with and without NaDBP data were pooled and the resulting means compared (Table 3), showing that the observed time trends were statistically significant throughout the 168 h period. With 241Am, in contrast, there were no significant time trends after the initial rapid release of 241Am into the dissolved phase, but there was some suggestion that remobilization was greater in the presence of NaDBP. The time dependent data were therefore pooled to give two sets of results, i.e. with and without NaDBP, where the number of samples with respect to each mean is 6. The results show that there was a small but probably significant difference between the two means (Table 4). Therefore, there is evidence to suggest that 241Am remobilization was enhanced by the presence of NaDBP, although the absolute (0.35 mBq 1-~) and relative ( - 25%) extent of enhancement was small, especially taking into account that the NaDBP concentration employed in the experiments is the maximum likely to occur in the environment. Mean distribution coefficients (Kd) for the various homogeneous data sets identified above are shown in Table 5. K d is used here for convenience, although equilibrium is clearly not always attained in the experiments (e.g. with Pu). There is good agreement with previously published Kd data, based on comparable experimental studies on Esk Estuary materials and on observations in the Irish Sea. The Esk Estuary is located at the eastern margin of the Irish Sea, some 10 km south of Sellafield, and consequently has accumulated large quantities of Sellafield derived radionuclides via the Irish Sea. A second series of experiments was undertaken to determine the extent of remobilization over a 24 h period under more typical conditions, i.e. at 10°C, and 20 and 120 mg 1-~ suspended sediment concentration, representing low and mean sediment concentrations for the north-east Irish Sea (McKay et al., 1987). The results

Volume 26/Number 5/May 1993

are compared in Table 6 with the data from a repeated 330 mg 1-] experiment. They show that the type of Pu and Am remobilization described above occurred throughout the range of conditions likely to be found in the Irish Sea. Differences in the dissolved radionuclide concentrations were observed under the cxpcrimcntal conditions employed, the major controlling factor probably being the suspended sediment concentration. However, the differences were not great and even at low sediment concentration (20 mg l-i), Pu and Am remobilization was clearly apparent.

(Turner et aL, 1991). The model also provides a basis for explaining previously observed field data for the Esk Estuary (Kelly et aL, 1988).

Discussion

The effect of sodium dibutylphosphate on the activity concentrations of dissolved 238pu, 239.24°pu and 24IAm in Irish Sea waters after reaction with contaminated Irish Sea sediment (analytical errors shown).

TABLE 1 Activity concentrations of 239.24°Pu, 238Pu and 241Am in Irish Sea sediments (mean and its standard error, based on n replicate analyses),

1239.24"Pu1 (/Bq kg-t)

1238pul (/Bq kg-')

1172_+22 (n = 5)

124tAm] (/Bq kg-')

284_+8 (n = 5)

1535-+68 (n = 3)

[239.24"pu1 :[23~pu] 4.1 _+0.11 (n = 5)

TABLE 2

Considerable insight into the solid-solution behaviour of Pu and to a lesser extent Am in the Irish Sea area has been provided by a series of comparable laboratory experimental studies carried out on Esk Estuary materials (Burton, 1986; Hamilton-Taylor et aL, 1987; Mudge et al., 1988). In these studies, the release of Pu and Am from contaminated Esk sediments was rapid, with (quasi-) equilibrium being attained within - 15 min. It has been suggested (Hamilton-Taylor et aL, 1987; Mudge et aL, 1988) that the rapid kinetics and much of the other observed estuarine behaviour of Pu can be attributed to surface complexation reactions. This has recently been supported by the relatively successful application of a constant capacitance, surface complexation model to a series of experimental sorption data

Reaction [NaDBP] time(/h) (/mgl -t)

[239.24('PuI (/mBql -t)

[238Pu1 (/mBql-=)

[241Am] [239,24('Pu] (/mBql -]) :[23Spu]

Initial seawater 24 24 24 24 72 72 72 72 168 168 168 168

4.12_+0.16 4.13_+0.20 4.59_+0.28 4.73_+0.31 4.78_+0.29 5.47_+0.42 6.91 _+0.58 6.98_+0.56 6.28_+0.36 6.53_+0.35 8.55_+0.53 9.29_+0.53 7.73_+0.45 8.95_+0.46

0.96_+0.07 0.99_+0.08 1.12_+0.11 1.02_+0.12 1.16_+0.12 1.32_+0.17 1.68_+0.22 1.61_+0.21 1.53_+0.15 1.54_+0.15 2.18_+0.29 2.27+0.22 1.90_+0.23 2.13_+0.19

0.51+0.07 0.52_+0.07 2.01_+0.18 1.49_+0.28 1.75_+0.27 1.58_+0.28 1.26_+0.19 1.18_+0.18 2.46_+0,47 1.52_+0,17 1.13+0,13 1.37_+0,13 1.77_+0.28 1.50_+0.13

--0 0 0.5 0.5 0 0 0.5 0.5 0 0 0.5 0.5

4.3 4.2 4.1 4.6 4.1 4.1 4.1 4.3 4.1 4.2 3.9 4.1 4.1 4.2

10

Pu

g

8

239,240 without NaDBP

E

239,240 with NaDBP 6

Initial cone.

[] g

r"

.o

238 without NaDBP 238 with NaDBP

¢:

Initial corm.

e--

o o

8

2 !

0 0

100 Time / h

200

Fig. 1 The effect of sodium dibutylphosphate (0.5 mg 1-j) on the activity concentrations of dissolved 23Spu and 23'J24°pu in Irish Sea waters after reaction with Irish Sea sediment.

Am O" rn E

m

[] []

[]

?

co

[]

|

as

•

without NaDBP

[]

with NaDBP

I-1

Initial conc.

¢:

o O

o

0

I

0

|

,

100 Time / h

J

200

Fig. 2 The effect of sodium dibutylphosphate (0,5 mg 1-~) on the activity concentrations of dissolved -'4=Am in Irish Sea waters after reaction with Irish Sea sediment.

265

Marine Pollution Bulletin TABLE 3 A comparison of the mean dissolved 238pu and -'3',24t~puconcentrations at successive time intervals (n=2 for initial concentrations, =4 at all other times). Time

Sample

l/h)

size (n)

Mean 1239,24°Pu1 (/mBq 1-~)

Student's t test 2 tail. prob.

Mean 1238pu] (/mBq 1-~ )

Student's t test 2 tail. prob.

0 24 72 168

2 4 4 4

4.13 4.89 6.68 8.63

0.030 0.001 0.005

0.98 1.16 1.59 2.12

0.061 0.002 0.003

TABLE 4 A comparison of the mean dissolved 24~Am concentrations between the "with" and "without" NaDBP remobilization experiments (n=6 for each mean). [NaDBP] (/mg 1-)) 0 0.5

Mean [241Am] (/mBq 1-~) 1.41 1.76

Student's t test 2 tail. prob. 1 tail. prob. 0,101

0.051

The behaviour of Am observed in the present study (i.e. steady-state dissolved concentrations being achieved within 24 h) also indicates a rapid re-establishment of equilibrium conditions, quite probably as a result of similar surface exchange reactions. This is clearly not the case with Pu, as dissolved concentrations were still increasing after 168 h, suggesting a different o r an additional reaction mechanism. Preliminary modelling indicates that the kinetics of the 238pu and 239'24°pu remobilization reactions agree to within 10%, and that the data shown in Fig. 1 can be well-described (e.g. accounting for > 97% of the variance of the concentrations) in terms of first order kinetics. However, it is not possible to model the data definitively, and hence obtain rate constants, without knowledge of the reversibility of the reaction and the proportion of the sediment-bound Pu actually involved. With the Esk sediments, only 3% of the total Pu present directly participated in the rapid surface exchange reactions, the rest being more strongly bound (Mudge et al., 1988). The suggestion that 238pu and 239'24°pu behave in the same way is also supported by their constant ratio in the experimental solutions (Table 2), and it is noteworthy that the ratio is the same as that initially present in the Irish Sea water. There is a general lack of information as to the nature of the slow Pu remobilization reaction, but it may involve the slow release of more strongly bound Pu, for example, from oxide phases and organic matter. Application of the Tessier sequential leaching scheme to Esk sediment indicated that, of the total Pu, the amounts associated with the operationally-defined fractions were as follows: 'oxide', - 30%; 'organic matter', - 10%; residual H F / H N O 3 fraction, - 5 5 % (Mudge et al., 1988). A redox reaction may also be involved, since sediment-bound and seawater Pu are thought to occur predominantly as Pu (III, IV) and Pu (V, VI), respectively (Nelson and Lovett, 1978). Photochemical oxidation of adsorbed Pu(IV) was important in experiments with model phases (e.g. goethite) over similar time periods to that used in our experiments (KeeneyKennicutt & Morse, 1985; Morse & Choppin, 1986) and in the release of Pu(V, VI) to Esk Estuary waters 266

(Mudge et al., 1988). The present experiments were carried out under ambient light conditions in the laboratory. It has also been shown, again over similar time scales to those used in our experiments, that dissolved Pu(IV) is partly converted to Pu(V) in sea water in the presence of Irish Sea sediment and that the conversion rate increases with sediment concentration (McCubbin and Leonard, unpub, data). With Am, the most likely cause of the enhanced solubilization in the presence of dibutyiphosphate ions (DBP) is through the formation of a solution complex. The degree of hydrolysis and complex formation of the actinide elements is as follows (Seaborg, 1958): M 4+ >

MO 2+ >

M 3+ >

MO +.

Am is likely to be present in the (III) oxidation state, whereas Pu probably occurs as (IV) and (V) (see Morse & Choppin, 1986). Therefore, at first appearance, it might be expected that Pu(IV) would be complexed by DBP to a greater extent than Am(Ill). At the pH of seawater, however, Pu(IV) speciation is dominated totally by hydrolysis products, such that other complexes are invariably insignificant (see Morse and Choppin, 1986). Furthermore, Pu(V) complexation by DBP would be weak, compared to trivalent Am. Therefore it would appear to be reasonable for Am(III) to exhibit the greatest degree of enhanced remobilization due to DBP complexation. The experiments were designed to reflect worst case conditions with regard to the effect of NaDBP on actinide remobilization from contaminated sediments. The principal desorption events in the Irish Sea, especially those involving the slow release of Pu as identified in the present study, will occur when bottom sediments, labelled with actinides during earlier periods of higher discharges, are resuspended into seawater having the low concentrations seen at the present time. Such resuspension events will occur during storms, due to wave generated currents, and will therefore be relatively infrequent. Data from the north-east Irish Sea (Draper, 1967) indicate that wave conditions are likely to be capable of eroding fine sand over 20% of the year at 10 m water depth. However, this period will be considerably less for the more active mud sediments, for which the erosion conditions are unknown. How the elevated solution levels of actinides will evolve after such a resuspension event will depend on the dilution of the water mass during dispersion. The experimental data allow prediction of the magnitude of seawater actinide concentrations that might arise by desorption from sediment, following a resuspension event. Table 7 shows the predicted sea-

Volume 26/Number 5/May 1993 TABLE

5

Field and experimentally determined mean K d values for Am and Pu, involving Irish Sea water and contaminated Irish Sea sediment. 239'24tlpu K d

-~a1Am Ka

( / 1 0 * l kg-I)

1.2 1.0

238Pu Kd --0.24 0.13 0.23

---

0.24 0.14 0.18 2.0 ].4

1.1

--

0.14

--

Comments Reaction time 24-168 h, without NaDBP* Reaction time 24-168 h, with NaDBP* Reaction time 24 h* Reaction time 168 h* Esk Estuary sediment, Irish Sea water, reaction time l h, 800 mg l-j Esk Estuary sediment, North Sea water, reaction time 15-60 min, 10 g I-E Muddy sediments in the Irish Sea

Source This study This study This study This study Kelly et al. (1986) Burton (1986) Pentreath (1985)

*Sediment concentration of 330 mg I-~ . TABLE6 Activity concentrations of dissolved238pu, 239'24°puand 241Amin Irish Sea waters before and after reaction with contaminated Irish Sea sediment for 24 h in the presence of 0.5 mg 1-1 sodium dibutylphosphate. Temp. [Sediment] [239,2a°pu[ [23Spu] [241Am] (/°C) ( / m g l -~) ( / m B q l -~) (/mBq1-1) ( / m B q l -~) Initial conc. Final conc.

-10 10 17

-20 120 330

1.5 + 0.2"[ 0.4 ± 0.1 t 1.6 ± 0.6* 3.2 ± 0.5:1: 0.8 ± 0.2:1: 3.2 ± 0.7:1: 3.4±1.3:1: 0.9±0.3:1: 4.1±0.5:1: 7.0_+1.4" 1.7_+0.5" 4.2±1.3:1:

*Mean and its standard error, based on duplicated measurements or experiments. tMean and its standard error, based on triplicated measurements. :l:Analytical error associated with single experiment. TABLE 7 Predicted and measured activity concentrations of dissolved 239"24°pu and 24tAm in seawater. Measured Irish Sea concentrations* (/mBq I- i) 239.240pu 24EAm

Predicted concentrations (/mBq 1- i) 239,240pu

2a~Am

Seawater 24 h

Seawater 168h

Seawater

Seawater + NaDBP

60

I00

19

23

7

6

*BNF plc (1990).

water concentrations based on the experimental KdS and sediment activity concentrations of 14 kBq kg-J 239'24°pu and 23.5 kBq kg -1 241Am. These represent the maximum levels found in suspended sediment from the nearshore zone of the Irish Sea over the period 1981-84 (McKay et al., 1987), and are close to the highest levels found in offshore bottom sediments, e.g. 8 kBq kg-~ 239'24°pu and 9 kBq kg -I 241Am from close to the discharge point (Kirby et al., 1983). The predicted sea water concentrations exceed those currently measured in the area (Table 7) (BNF plc, 1990), but these conditions are unlikely to be commonly attained since suspended sediment activity concentrations are typically lower, e.g. the mean levels recorded from the site occupied by McKay et al. (1987) were 5 kBq kg -I 239'24°pu and 5 kBq kg -I 241Am. Furthermore, the effect of the presence of NaDBP in the discharges on the remobilization of Am is very small compared to the past observed variations in the range of seawater Am concentrations (Pentreath, 1985). Therefore the general conclusion is that the experimentally derived KdS for Pu and Am, with and without NaDBP (Table 5), do not predict an occurrence of conditions in the Irish Sea outside the range that has

already been experienced. In keeping with this, the experimental Kds fall within the range of Pu and A m KdS measured in the Irish Sea, e.g. 239"24°pu 3.7 × 105 +_0.4 × 105; 241Am 1.9 × 106 +_-0 . 2 × 106 in 1979 (Pentreath, 1975). We gratefully acknowledge the financial support of British Nuclear Fuels plc for this work. Aston, S. R., Assinder, D. J. & Kelly, M. (1985). Plutonium in intertidal coastal and estuarine sediments in the Northern Irish Sea. Est. CstL Shelf Sci. 20,761-771. BNF plc, 1990. Radioactive discharges and monitoring of the environment 1989. Annual Report on Radioactive Discharges and Monitoring of the Environment. BNF plc, 1991. Radioactive discharges and monitoring of the environment 1990. Vol. I. Annual Report on Radioactive Discharges and Monitoring of the Environment. Burton, P. J. (1986), Laboratory studies on the remobilisation of actinides from Ravenglass estuary sediment. Set. Tot. Environ, 52, 123-145. Carpenter, R. C., Burton, P. J., Strange, L. P. and Pratley, F. W. (1991). Radionuclides in intertidal sands and sediments from Morecambe Bay to the Dee estuary. AEA Technol Rep., AERE-R 13803. Caulcutt, R. and Boddy, R. (1983). Statistics for Analytical Chemists. Chapman and Hall, London. Draper, L. (1967). Wave activity at the sea bed around Northwestern Europe. Marine Geol. 5, 133-140. Eakins, J. D., Morgan, D. A., Baston, G. M. N., Pratley, F. A., Yarnold, L. P. and Burton, P. J. (1988) Studies of environmental radioactivity in Cumbria. Part 8. Plutonium and americium in intertidal sands of north-west England. AEA Technol. Rep., AERE-R 12061. Farts, J. P. and Buchanan, R. F. (1964). Anion exchange characteristics of elements in nitric acid medium. AnaL Chem. 36, 1157-1158. Hamilton-Taylor, J., Kelly, M., Mudge, S. and Bradshaw, K. (1987). Rapid remoblisation of plutonium from estuarine sediments. J. Environ. Radioactivi~ 5, 409-423. Holm, E., Ballestra, S. and Fukai, R. (1979). A method for ion-exchange separation of low levels of americium in environmental materials. Talanta 26,791-794. Hunt, G. J. (1990). Radioactivity in Surface and Coastal Waters of the British Isles, 1989. Aquat. Environ. Monit. Rep., No. 23, MAFF, Lowestoft. Keeney-Kennicutt, W. L. and Morse, J. W. (1985). The redox chemistry of Pu(V)O + interaction with common mineral surfaces in dilute solution and seawater. Geochim. Cosmochim. Acta 49, 2577-2588. Kelly, M., Hamilton-Taylor, J., Mudge, S. and Bradshaw, K. (1986). Transuranic radionuclides in the estuarine environment. Dept. of Environ. Rep., RW 87.039. Kelly, M., Mudge, S., Hamilton-Taylor, J. and Bradshaw, K. (1988). The behaviour of dissolved plutonium in the Esk Estuary, UK. In Radionuclides: A Tool For Oceanography, (J. C. Guary, P. Guegueniat & R. J. Pentreath, eds), pp. 321-330. Elsevier Applied Science. Kirby, R., Parker, W. R., Pentreat~, R. J. and Lovett, M. B. (1983). Sedimentation studies relevant to low level radioactive effluent dispersal in the Irish Sea, Part 111:An evaluation of possible mechanisms for the incorporation of radionuclides into marine sediments. Inst. Oceanographic Sci., Taunton. Kraus, K. A. and Nelson, F. (1959). Anion exchange studies of metal complexes. In The Structure of Electrolytic Solutions (W. H. Hamer, ed.), pp. 340-364. John Wiley, New York. Lovett, M. B. and Nelson, C. M. (1981). Determination of some oxidation states of plutonium in seawater and associated particulate

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Marine Pollution Bulletin matter. In Proc. Symp. Techniques for Identifying Transuranic Speciation in Aquatic Environments, 24-28 March 1980, Ispra, Italy, pp. 27-35. IAEA, Vienna. McKay, W. A., Pattenden, N. J. and Branson, J. R. (1987). Studies of environmental radioactivity in Cumbria. Part 10: Some radionuclides in near-shore seawater 1980-84. A E R E Rep., A E R E - R 11912. Morse, J. W. and Choppin, G. W. (1986). Laboratory studies of plutonium in marine systems. Mar. Chem., 20, 73-89. Mudge, S., Hamilton-Taylor, J., Kelly, M. and Bradshaw, K. (1988). Laboratory studies of the chemical behavior of plutonium associated with contaminated estuarine sediments. J. Environ. Radioactivity, 8, 217-237. Nelson, D. M. and Lovett, M. B. (1978). Oxidation states of plutonium in the Irish Sea. Nature 276,599-601. Pentreath, R. J., Lovett, M. B., Jefferies, D. F., Woodhead, D. S., Talbot, J. W. and Mitchell, N. T. (1984). Impact on public radiation exposure

of transuranium nuclides discharged in liquid wastes from fuel element reprocessing at Sellafield, United Kingdom. In Radioactive Waste Management. VoL 5. IAEA-CN-43/32, pp. 315-329, IAEA, Vienna. Pentreath, R. J. (1985). Radioactive discharges from Sellafield (U.K.). In: Behaviour of Radionuclides Released into Coastal Waters. IAEA Tech. Doc. 329, Annex 1, pp. 67-100, IAEA. Vienna. Seaborg, G. T. (1958). The Transuranium Elements. Yale Univ. Press. Talvitie, N. A. (1972). Electrodeposition of actinides for alphaspectrometric determination. Anal ('hem., 44,280-283. Turner, D. R., Knox, S., Penedo, E, Titley, J. G., Hamilton-Taylor, J., Kelly, M. and Williams, G. (1991). Surface complexation modelling of plutonium adsorption on sediments of the Esk Estuary, Cumbria. In Radionuclides in the Study of Marine Processes, (P. J. Kershaw & D. S. Woodhead, eds), pp. 165-174. Elsevier Applied Science.

MarinePollutionBulletin,Volume26, No. 5. pp. 268-272, 1993. Printed in Great Britain.

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Eutrophication and Gastrotrich Diversity in the Northern Adriatic Sea WAYNE A. EVANS*, M. ANTONIO TODAROt and WILLIAM D. HUMMON* *Department of Biological Sciences, Ohio University, Athens, OH 45701 USA t Department of Zoology, Louisiana State University, Baton Rouge, L A 90803, USA

Marine Gastrotricha of littoral and sublittoral sands at nine locations along the northern Adriatic coast were sampled during July, 1991. Three locations on barrier islands near the Laguna Veneta (Alberoni, San Nicol6, and Punta Sabbioni) supported a higher number of species in the littoral and shallow sublittoral zones (six each) than were reported by Hummon etal. (1990), who found no species at the same sites in 1989. The increased number of species may be related to colder temperatures during the winter previous to our study, which delayed formation of algal mats in the littoral zone, preventing the depletion of oxygen in the littoral sediments that can lead to gastrotrich mortality. Locations in the northeastern Adriatic, Bibione (13 species) and Foce Isonzo (15 species), evidenced a more diverse gastrotrich fauna than the more northwestern locations, and may serve as a source for colonizing species, moved with longshore currents, to westward populations subject to extinction due to local anoxic events.

In recent decades, benthic communities in the northern Adriatic Sea have come under increasing stress from acute dystrophic events such as anoxic bottom conditions, aggregates of mucous algae, and red tides resulting from eutrophication in the region (Crema et al., 1991; Ghiardelli & Specchi, 1989; Justic, 1987). Such 268

events can lead initially to mass mortalities in benthic populations and ultimately to reduced species diversity for macrofaunal (Brenco-Hrs, 1980; Crema et al., 1991; Justic et al., 1987) and meiofaunal (Gray, 1971; Hummon et al., 1990) components of bottom communities. Hummon et al., (1990) described a drastic reduction in the number of species of Gastrotricha in beaches and shallow, nearshore sublittoral sands of the northern Adriatic since Schrom first sampled there more than 20 years ago (Schrom, 1966a,b, 1972). Where Schrom had found 25 species of Gastrotricha, Hummon et al. reported only two, with an additional four species from an adjacent nearshore bar that was not sampled by Schrom. We decided to resample the locations common to Schrom and Hummon et al. and several additional locations to determine: 1. if the decline in gastrotrich diversity at the original locations continues to the present, 2. if new locations nearby exhibit similar levels of diversity, 3. if nearshore sublittoral sands serve as a diversity refugium for littoral habitats as Hummon et al. have hypothesized, and 4. if richer habitats in the northeastern Adriatic such as the Foce Isonzo (Hummon et al., 1990) could potentially supply species to more impoverished habitats near the Laguna Veneta via the westward longshore transport of sediments. Comparison with Schrom's data might also illuminate current diversity patterns within (littoral vs. sublittoral) and among locations. We also wanted to assess the relationship between