Fixation of 137Cs by soils and sediments in the Esk Estuary, Cumbria, UK

?~e Science of the Total Environment, 132 (1993) 71-92 E~sevier Science Publishers B.V., Amsterdam

71

Fixation of 137Cs by soils and sediments in the Esk Estuary, Cumbria, UK K.S. D a v i e s a'* a n d G . S h a w b

almperial College Centrefor Environmental Technology, 48, Princes Gardens, London, SWT, United Kingdom bCentrefor Analytical Research in the Environment, Imperial College at Silwood Park, Ascot, Berks. SL5 7TE, United Kingdom (Received October 29th, 1991; accepted January 24th, 1992)

ABSTRACT Enhanced levels of 137Cs are observed on the Cumbrian coast (UK) due'to the discharge of low level waste by BNFL (British Nuclear Fuels Ltd) at Sdlafield. Sorption of this radionuclide onto estuarine sediments is of considerable strategic importance as these areas represent a secondary source of transfer to inland areas. The study reported here was undertaken to determine, both quantitatively and qualitatively, the characteristics of the sorption and fixation of "TCs in sediments and soils sampled from within the Esk estuary. This was carried out by means of a detailed set of laboratory analyses on samples collected from the area. These involved a sequential solvent extraction to determine the relative pro!~ortions of ~3~Cs associated with exchangeable, reducible, oxidiseable and residual fractions of various substrates; a series of batch sorption experiments was also carried out to deter~nine the sorption characteristics of a selection of the substrates sampled. To complement ~:hese investigations grain size determinations coupled with X-ray diffraction analyses were employed to determine the mechanical and mineralogical properties of each sample collected.. ~3~Cswas most strongly associated with the residual phase as def'med by the sequential leaching scheme. The more consolidated, clay rich substrates appeared to sorb the greatest quantities of ~37Cs within the residual phase although there was no obvious correlation between the quantity of radiocaesium associated with this phase and the relative proportion of illite and <2/~m material within the substrate. Constants derived from the batch sorption isotherms, however, correlated well with the sequential leaching data and it is suggested that the Langrnuir model may be useful as a simple predictive model of the cap~tcity of substrates within the st,dy area to fix "~Cs. Data from the batch sorption study also suggested that caesium binding was either biphasic or monophasic and that, to some extent, this was controlled by the relative proportions of sand and clay-sized materia| within the samples.

Key words: radiocaesium; 137Csfucation; sorption isotherms; Langmuir model; sequential extraction; X-ray diffraction; radioecoiogy; Ravenglass; Esk Estuary; Sellafidd *Present address: Dept. of Earth Sciences, Universityof Oxford, Parks Road, Oxford, OXI 3PR, UK. 0048-9697/93/$06.00

© 1993 Elsevier Science Pubtishers B.V. All rights reserved

72

K.S. DAVIES AND G. SHAW

INTRODUCTION

The most significant releases of man-made radionuclides to the environment within the UK take the form of liquid discharges from the Sellafield reprocessing plant, Cumbria. A majoi" source of this waste effluent is storage pond water from the Magnox decanning process which contains ~37Cs(halflife 30 years) as its most abundant gamma-emitting radionuclide (Hamilton and Clarke, 1984). This nuclide may be associated with relatively large (80-300 ~m) 'hot' particles, the origin of which can be traced to storage tanks at the BNF site at Sellafield (Hamilton, 1989); the degree of,association of radiocaesium with such particles is greatest immediately following discharge. These liquid wastes are discharged to the north-eastern Irish sea through pipelines extending 2.1 kms beyond low water mark (Hunt, 1989) and subsequently contaminate sediments of estuaries such as that of the Esk and surrounding pasture lands which are subject to tidal inundation. With the introduction of 'SIXEP' (the site/on exchange effluent plant) in the mid 1980s the discharge of ~37Csto the north-eastern Irish Sea h~.s been reduced by up to a factor of ten (BNFL, 1988). However, during the early part of the last decade there was evidence of .~ctive ~37Csaccretion along certain areas of the C~rnbrian coast (Horrill, 1983) and the radioactivity levels and associated dose rates of many tide-washed pastures remain higher than those further inland. Indeed, saltmarshes and estuarine areas within the region contain the largest deposits of radionuclides and have a unique strategic importance in that they represent a secondary source of transfer to inland areas (HorriU, 1986). An important transfer process in these environments involves resuspension of fine particulate matter giving rise to the possibility of exposure of human populations to 137Csby both direct inhalation and via the foodchain. In the marine envirom,tent caesium remains soluble in the water column (Assinder et al., 1985) and is readily transported to nearby estuaries. Physicochemical conditions in estuarine environments, however, may result in the element becoming more strongly associated with sediments (Santschi et al., 1983), most probably due to a decreased concentr~ion of exchangeable co-ions such as potassium which compete with caesium for sorption sites on mineral surfaces (Cheng and Hagamuchi, 1968). Furthermore, the nature of the sediment is of great importance in controlling the sorption of ~37Csonto estuarine substrata; numerous s~udies (Aston and Stunners, 1982; Hetherington and Jeffries, 1974; Lomenick and Tamura, 1965) have indicated a positive correlation between the percentage of clay within marine, estuarine and lacustrine sediments and the quantity of radiocaesium 'fixed'. More specifically, the proportion of illite and micaceous minerals in soils and .~ediments has been found to be a decisive controlling factor in 137Cs sorption (Frissel and Penders, 1983; Evans et al., |983).

LEVELS OF 13?Cs IN SOILS AND SEDIMENTS IN CUMBRIA, UK

73

Fixation of ~37Csin soils and sediments underlying tide-washed pastures leads to generally low transfer into foodchains (Howard and Livens, 1991). There is evidence, however, that when exposed to changing concentrations of competitive ions such as K + and NH4 ÷ radiocaesium may be remobilised from marine sediments (Patel et al., 1978), an effect which will result in a greater likelihood of environmental transfer within estuaries and adjacent pastures. The study reported here attempts to clarify the factors responsible for the fLxation of 137Csin various :;ubstrata collected from the Esk estuary, near Ravenglass in Cumbria and thereby improve our understanding of the processes which may lead to sorption or desorption of this nuclide within the estuarine environment. It had three aims: first, to assess the degree of binding of radiocaesium by broadly defined chemical phases within sediments from various locat;,ons within the estuary; secondly, to characterise the potential for radiocaesium fixation in terms of particle size distribution and mineralogical properties; and thirdly, to determine both qualitatively and quantitatively the sorption characteristics of the sediments in the light of the above. MATERIALS AND METHODS

Area of study The estuary at Ravengiass is situated approximately 10 km south of So,llafield and, due to its highly elevated inventory of man-made radionuclides, has been the site of many previous investigations (recently reviewed by Howard and Livens, 1991). It encompasses the estuaries of the rivers Irt, Mite and Esk which drain the hinterland of the western Lake District, Cumbria and exhibits a wide variety of environments within a small area. These range from low lying flood plains (used as pasture land) to salt marshes, tidal fiats and sand banks protected from the sea by a dune complex (Hamilton and Clarke, i984); consequently the estuary contains a considerable range of substrate types.

Sampling strategy Following an initial laboratory and field-based radiometric survey (during late April, 1990), in ~vhich the spatial distribution of the radiocaesium inventory was found to be highly variable (Davies, 1990), three transect lines were delineated. These ran perpendicular from the fiver Esk (Fig. 1) and encompassed se,~fimentscomprising estuarine sand banks, tiaal fiats, salt marsh and flood plain pasture (Table 1). Samples were taken in May 1990 at intervals defined by visible changes in the type of sediment and associated vegetation. Each sample consisted of approximate!ly 500 g of substrate taken by scraping

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the surface sediment to a depth of 10 cm; this was wrapped in polythene and transported to the laboratory. Analyses were performed promptly and, apart from maintaining samples in a cool, dark environment, no special storage precautions were taken: Slight oxidation of some samples was evident during the storage period but it is unlikely that this had any significant direct effect on the behaviour of radiocaesium, although an indirect effect as a result of NH4 + oxidation is not ruled out (Pardue et al., 1989).

Laboratory studies Sequential leaching A four stage sequential leaching scheme was adopted based on that of Tessier et al. (1979). This was modified to improve the yield of 'labile' caesium by employing a cation exchange resin in conjuction with a liquid eluent. The full scheme was as follows: Fraction 1:20 g of fresh substrate were suspended in 30 ml of 0.5 M ammonium chloride solution containing 20 g of H + saturated resin (Dowex 50-100 mesh contained in a net bag). The suspensions were allowed to stand over night then shaken for 2 h on an end-over-end shaker after which time the resin was removed from the extracting solution, washed in double deionised water and assayed for gamma activity. The remaining suspension was centrifuged for 15 min at 4000 rev./min and the supernatant discarded. The soil pellet was then rinsed by resuspending in an excess of deionised water, centrifuging again for 15 min at 4000 rev./min and then discarding the supernatant. Centrifugation was used rather than filtration so as to prevent sample loss (Valin and Morse, 1982). Fraction 11: The soil pellet was resuspended in 0.04 M hydroxylamine hydrochloride in 25% (v/v) acetic acid. This suspension was maintained for 8 h at 95°C in a water bath with occasional agitation. Alter cooling the suspension was centrifuged at 4000 rev./min for 15 min and the supernatant drawn off tor radioassay. The soil pellet was then rinsed with deionised water, as above. Fraction III: The pellet was resuspended in 40 ml of 30% hydrogen peroxide and 20 ml of 0.02 M nitric acid. After standing overnight the suspension was heated at 85°C for 8 h with occasional agitation, cooled and then centrithged at 4000 rev./min for 15 min. The superna~ant was drawn off for radioassay and the pellet rinsed as above. .Fraction IV: The remaining pellet was dried for 48 h at 75°C, ground to homogeneity and packed into a plastic container for radioassay. Fractions I, II and III were each assayed for gamma activity using a germanium-lithium detector coupled to a multichannel analyser (Nuclear Data Systems) whereas fraction IV was assayed in a well-type NaI detector

LEVELS OF 137Cs IN SOILS AND SEDIMENTS IN CUMBRIA, UK

77

(LKB Wallac Compugamma 1282). Absolute specific activities of 13?Cs within the sample fractions (expressed as kBq kg -~ dry wt) were determined from standards obtained from the National Physical Laboratory, UK.

Mechanical and mineralogical analyses Grain size distributions within each sediment sample were determined using a Buoyoucos hydrometer following oxidation of the organic fraction by heating at 400°C for 24 h. The samples were dispersed in a sodium hexametaphosphate ('calgon') so!ution prior to determinations of specific gravity at intervals of 5 min and 5 h. Readings were temperature corrected to give estimates of sand (>0.06 nun). silt (0.06-0.002 ram) and clay fractions (<0.002 mm) expressed as percentage abundance on a dry weight basis. X-Ray diffraction (XRD) analyses were performed on all samples. These were initially rinsed with double deionised water and centrifuged for 1 h this process was repeated seven times in order to remove all sea water salts. Each sample was filen centrifuged for 4 min at 1000 rev./min so that the clay fraction remained in suspension. Some of the soil samples required the addition of 10 ml of 5% v/v ammonia solution as a dispersant to facilitate deflocculation. The isolated clay fraction was then applied in vacuo to ceramic tiles to give a uniform coating; coated tiles were left overnight in a desiccator to dry. XRD patterns were obtained using a Phillips XRD unit PW 1830 with vertical goniometer, monochromator and divergence slits. To distinguish between clay minerals four patterns were obtained sequentially (using Cu K ct radiation) following treatment of the clay, as follows: (a) air drying in a desiccator overnight; (b) addition of ethylene glycol to distinguish smectitic clay minerals; (c) heating to 400°C; and (d) 2eating to 500°C (which fuses kaolinite); Peak positions determined after these treatments were compared to previously prepared standards and corresponded to illite-chlorite (6°), illhe (8°) and kaolinite (12°). The relative l~Toportions of these in each sample were calculated for the illite-chlorite mi,~, ~br illite alone and for kaolinite alone. Data from mechanical analyses were combined with these mineralogical data to give an estimate of the percentage abundance of illite, chlorite and kaolinite within each sediment sampie.

Batch sorption studies These were carried out on all samples from transect X which was ide~itified as the transect in which most variability in substrate occurred, although one

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LEVELS OF 137C$IN SOILS AND SEDIMENTS IN CUMBRIA, UK

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sample froi~ transect Z was also selected which represen*~ed the most mature soil found during sampling. Prior to the batch sorption experiment pH was determined in a 2:1 deionised water slurry of each sample to ascertain whether there was significant variation between samples; this was an important pre~'equisite before any comparison of the sorption data was attempted. PrecCnditioning of the samples by saturating adsorption sites with a single species of cation was avoided as this practice can lead to significant changes in the natural composition of the sediment (Gutierrez and Fuentes, 1991). Small (0.025 g) samples of oven dried soil were placed in polyethylene tubes to which we~'e added 10 ml of CsCI solution. These were prepared as a dilution series from a stock solution of 1 mM ~3Cs to which had been added 10 ~tCi o~,i~~'~'~" ~ (carrier free). The final concentration range employed was 1-100/tM mCs with a constant specific activity of 37~ Bq s37Cs ~tmol-m ~33Cs. The suspensions were shaken for 2 h on an et~d-over-end shaker at a constant temperature of 20°C. Following centrifugat~,~n ~l 500-it! aliquot was taken from the supernatant of each suspension and, following radioassay by NaI detector (see above), the loss of activity due to sorption on to the sediment was determined by comparison with a similar aliquot taken from the original dilution series. The quantity of radioc~esium sorbed during the experiment was expressed as Bq 137Cs kg -~ dry weight.

RESULTS

Sequential leaching Figure 2 indicates that in the case of all transects the bulk of radiocaesium activity was associated with the residual phase within each sediment. This phase held approximately 70-90% of the total radiocaesium inventory at each sampling point. The labile fraction accounted for approximately 10% of this inventory and was quantitatively the second most important fraction i~ tenns of caesium retention. Oxidiseable and reducible fractions were quantitatively of least importance in terms of caesium retention, with t~e oxidiseable fraction tending to account for a greater part of the caesium inventory than the reducible fraction. These patterns of chemical distribution of caesium were generally constant across each transect. Exceptions to the rattern were seen at points 109 m along transect X, 48 and 85 m along transect Y and 74 m along transect Z. Table I indicates that these sites consisted of either sand banks, santly channel sediment or sandy river terrace sedin~ent.

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Mechanical and mineralogical analyses Table 2 shows the results of grain size dete~'minations. Percentages of sand, silt and clay were highly variable across transects X and Y. In the ca~e of these two transects the percentage of silt and clay increased with distance from the channel and this was mirrored by a decrease in the percentage of sand. Grain size distribution across transect Z was more constant and can be attributed to this transect being situated at a position higher upstream where the dominant substrate was an established pasture soil. Table 2 also shows the percentage distribution of the clay minerals illite, chlorite and kaolinite as determined by XRD. The pattern of distribution of these ndnera!s is highly constant me'an values given in the table show that illite represented 33% of the <2~tm fi'action and kaolinite 12%, with an illitechlorite mix representing 54°/'o (there was no significwnt variation in these proportions within the data set (P 2:, 0.997, n = 25)). This compares -~'ith an average mineralogical composition determined for <2~m material in lake sediments in the north-west of England of 42% illite and 58% chlorite (Bocock, 1981). Table 2 include~ a combination of the illite data with the grain size distribution data and gives estimates oF ~he abundance of this mineral as a percentage of the total sediment dry v,~eight. The percentage of illite in each transect shows an approximate increase ~vith distance from the -

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TABLE 3 Values of the binding constants K and b for Cs sorption to soils and sediments from various sites within the Ravenglass estuary. T h e ~ were derived by fitting the data from bat~:h sorption experiments to the Langmuir equation (eqn. 1). The subsc~=t~ I and 11 refer to constants derived for readily saturated and non-readily saturated sites, respectively. Sample no.

pH

KI

bl (kBcl kg -l)

K.

b. (kBq kg -l)

Partition conc. O,M 133Cs)

X1 X2 X3 X4 X5 X6 X7 X8 X9 Z7

7.26 7.31 7.94 7°67 7.75 8.14 7.93 7.96 -6.96

0.291 0.348 0.884 0.452 1.117 0.298 0.498 0.657 0.158 0.798

998.5 634.7 288.8 583.7 365.1 608.9 45 ~.8 392 5 578.1 592.4

0.003 0.032 0.072 0.087 0.7~ 0.027 0.029 --0.007

2049 ! 326 1273 1435 654.4 2043 ! 395 --12 126

! 0-20 10-20 5-10 5 5-10 i 0-20 ! 0-20 --10-20

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FiB. 3. (a) Specimen isotherm for a sample (X8) which displayed monophasic sorp~ion of caesium. The fitted relationship is a plot of eqn. I using constants derived fT,'omthe linearised plot (eqn. 2). The simply lin~r plot in (b) implies that sorption of Cs by this sediment was due to a singl~ ~ype of bind:/,ng site.

LL~V'~LSOF 137C'sIN SOILS AND SEDIMENTS IN CUMBRIA, UK

83

channel. This is also true of ~!ite-chlorite in transects X and Y although in transect Z the pattern for this niixture of minerals is less clear. Batch sorption studies

Table 3 shows pH values obtained in the sediment samples used in the batch sorption exveriments. The mean value of the nine determiner.ions made was 7.66 - - associated ~ t ~ this was a 95% confidence interval of ± 0.305 pH units, representing a variation ~tround the mean of less than 4- 4%. Due to this low variability in inherent pH b~tween sediment samples it may be assumed that its effect on caesium sorption behavio~r during the experiments is likely to have been negligible. An initial analysis of the batch sorption 0~ta indicated that sorption isotherms were non-linear. The data was subsequently applied to the Langmuir model (Veith and Sposito, 1977) which relates the quantity of a solute adsorbed to a surface to its ambient concentration in a fluid medium bathir:,g the surface: x = m

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1 +KC

whe~:ex/m is the weight of a~sot ba~.e pvr unit weight of adsoi~nt, K is a consts,nt relating to the initial slope of the so~fion isotherm*, C is the eq'ai~.ibrium concentration of adsorbete in the fluid phase and b is the maxi~m~ amount of adsorbate that can be adsorbed (assumed by this model to !~e ,~ complete monomolecular layer). The goodness of fit o~' d~ta to this model is n~rmally assessed by fitting the data t~, ,he ii~earised form of the equation. C x/m

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With the current data two distinct types ~f linearised curve were apparent-those whic,h conformed to the simple linear equation (Fig. 3) and those which exhibited two linear components separated by a distinct discontinuity (Fig 4). In the former case sorption of caesium on to the ~diment can be considered to be the i,:~ult of binding on to a single type of site whereas in the latter case sorption on to two distinct types of site is inferred (Bohn et al., 1979). As a result either a single pair of binding constants (K and b) or two *The relatively large value of Ki in relation to Kn indicates that the sorption sites yielding con~tants subscripted I will reach saturation at a lower equilibrium Cs concentration than those subsc~pted !i - these sites are tberefore reierred to in the text as readily saturated and non-readily saturated, respectively.

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pairs of constants, indicating differential affinities within a single sediment type, can be derived for the sorption of caesium on to sediments in this study. These data are shown in Table 3 which also indicates, for the samples in which biphasic sorption was evident, the concentration of caesium at which partition between sorption phases was seen to occur. It can be seen from this table that two of the nine samples examined exhibited only single phase (monosite) sorption; the simple Langmuir model was directly applicable to data derived from these sites whereas in all of the other samples the model was only applicable if biphasic sorption of Cs was envisaged. DISCUSSION

That ~-adiocaesiummay be strongly fixed by a 'residual' geochemical phase within soils and sediments has been demonstrated by several authors using a variety of techniques including sequential leaching (Wilkins et al., 1986; Livens and Baxter, 1988b) and more specific sorption studies (Cremers et al., 1988). The c~rrent study demonstrates that even within the highly variable conditions of the Esk estuary the principle mechanism governing Cs behaviour within soils and sediments is one of 'fixation' by a mineral component from which it is difficult to extract radiocaesium by a non-destructive liquid elution technique. It has b ~ n postulated that this fraction most likely represents secondary silic~tte minera!~ of wh/ch iUite in particular has been idemified (Frissel and Penders, 1983). Sequential leaching schemes akin to that employed in the ¢mTent study are open to several criticisms and for a comprehensive critique of these techniques the reader is directed to the re~:ent review by Kirst~n and F6rstner (1989). Their use as a relatively rapid and repeatable means of characterising the broad geochemical behaviour of elements such as Cs is generally accepted, however, as long as it is borne in mind that the final interpretation of the d0ta obtained is restricted to the operationally defined conditions associated w,;th tl,~e specific extraction sequence employed. The use of the Fig. 4. (a) Specimen isotherm for a sample (ZT) which displayed biphasic sorption of caesium. Plot (b) indicates that the isotherm can be resolved into two distinct linear components for which tgo sets of constants can be derived (see Table 3). The resulting composite isotherm in (a) can then be described by the expression

[Cs]~o~ =

KllCsIbl Ku[Cs]bl~ + 1 + K!fCs] 1 + Kn[Cs]

This impfies that sorption of Cs by this sample was due to two discrete types of binding site, although caution must be exercised when applying this interpre,tation (see Discussion).

86

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88

K.S. DAVIES AND G. SHAW

leaching scheme in this study has indicated that at several points along the selected transects fixation of ~37Cs by the residual fraction is somewhat reduced. From Table 2 this effect can broadly be correlated with the mechanical and mineralogical properties of the soil or sediment under study, viz. percentage abundance of clay-sized particles and illite. When examined more closely, however, the affinity of 137Cs for residual phases within sediments is rather poorly correlated with both the percentage of clay sized (< 2/zm) material and the percentage of illite within those sediments (Fig. 5). It therefore seems unlikely that a simple quantification of these parameters in soils and sediments within the estuary will provide a reliable estimate or" the capacity of the substrate to fix radiocaesium in an unavailable form. Secondary factors such as the ambient activities of the K + and NH4* ions may play a crucial role in controlling ~37Cs fixation in estuarine substrata. In the case of the latter ion the high degree of variability in substrate flooding and drainage is likely to affect Cs sorption via its effect on redox potential and soil nitrifying status, particularly in sites transitional between 'estuarine' and 'terrestrial', such as tide-washed pastures. The percentage composition of secondary silicate minerals is highly constant throughout the entire range of substrates sampled and it is clear that, taken on its own, this cannot be a factor controlling the considerable variation in mobility of radiocaesium observed within the estuary. When expressed as a percentage of total sediment dry weight (i.e., combining both mineralogical data and grain size data), however, the variation in mineralogical composition between sites becomes both systematic and highly significant (Table 2). It is possible to argue from this that sediment grain size is I;.kcly to provide a more important overal! control on Cs fixation than mineralogical composition per se. This effect has previously been recognised for soils and sediments in the Cumbrian region generally (Livens and Baxter, 1988a). Cremers et al. (1988) determined that the existence of both trivial an,~ specific sorption sites (associated with the frayed edges of hydrous micas such as illite) within four soils of widely differing textural properties led to ~; their exhibiting differential affinities for the Cs + ion which can vary widely according to such factors as the mineral:organic ratio. By quantifying the number of specific sorption sites within a given soil Sweeck et al. (1990) derived a measure of the ability of that soil to retain radiocaesium in a bound form unavailable for uptake by organisms, This they referred to as the 'Specific Radiocaesium Interception Potential' (SRIP), which Cremers et al. (1990) subsequently applied to eight Cumbrian soils with high (_>23%) organic contents. This rigorous approach has, to the authors' knowledge, never been applied to the question of radiocaesium contamination of the soils and sediments along the Cumbrian coastal fringe.

LEVELS OF 13?Cs IN SOILS AND SEDIMENTS IN CUMBRIA, UK

89

While the current study has not quantified the SRIP as defined by Sweeck et al. (1990) it has characterised the gross sorption behaviour of caesium on a range of substrates sampled from a complex estuarine environment. The data shown in Table 3 are derived for both trivial and specific sorption sites within the sediments and, in comparison to the studies reported by Sweeck et al. (1990) and Cremers et al. (1990), are rather crude. While the effects of differential pH of each of the substrates can be ruled out as a complicating factor in the interpretation of this data (see Table 3) the test samples were not conditioned prior to experimentation and so some competition for nonspecific sorption sites between Cs ÷ and other 'native' ions such as C a 2÷ and M g 2+ may well have occurred. When the constants obtained for the nonreadily saturated sorption sites, Ka and bmi, are plotted against the percentage of radiocaesium associated with the residual fraction, however, (Figs. 6a,b) the resulting correlation coefficients are high (r 2 _> 0.80, P <0.001). This suggests that the batch sorption technique used in the present study has given a reliable estimate of the potential for a particular substrate to fix radiocaesium. In the case of Fig. 6b a strong, positive correlation between the constant bn and the percentage of Cs associated with the residual phase indicates that a relatively simple determination of this constant could usefully be used to estimate the 'radiocaesium carrying capacity' of sediments within the estuary. The establishment of both monophasic and biphasic sorption isotherms for the substrates within this study gives a suggestion of the operation of two distinct sorption mechanisms of differing affinities for caesium (i.e. sorption by trivial and specific sites, sensu Cremers et al.). Biphasic isotherms (the majority) were evident for sediments in which a significant percentage of < 2~m material occurred whereas simple monosite sorption was evident in substrates dominated by sand sized particles. Sposito (1989), however, has emphasised that adsorption isotherm models such as the Langmuir equation should not be used to infer the operation of specific mechanisms. As tempting as it may seem, therefore, the authors will not attempt to draw any conclusion Iurther than suggesting that, in this instance, the Langmuir model has provided a sound empirical model of caesium sorption with some potential for predicting the fixation behaviour of ~37Cswithin the comp~e~ mixture of substrates that comprise the estuafine environment of the River Esk. CONCLUSIONS

Twenty-five ~oil and sediment samples have been collected from a variety of contrasting locations within the Esk estuary and have all been found to be. contaminated with ~37Cs, the source of which is most probably the near~ by Sellafield reprocessing plant. In all samples > 50% of this ia.diocaesium

90

K.S. DAVIES AND G. SHAW

aFpears to be firmly bound within a 'residual' phase, as defined by a sequential ~eaching scheme based on that of Tessier et al. (1979). From much literature evidence this fixation of m37Csis most probably due to specific binding by an illitic component of the < 2/zm mineral fraction, although the results from the sequential leaching study do not correlate significantly with the proportior~ of either the clay-sized fraction or the illite content, as determined by X-ray diffraction. This suggests that secondary factors such as the ambient activities of the K ÷ and/or NH4 + ions may also be important in controlling radiocaesium fixation processes. As a result the most reliable means of estimating the ~37Cs 'carrying capacity' of a soil or sediment within the estuary may be the establishment of sorption isotherms for small, non-conditioned samples. In the present study these correlated well with the observed data from sequential extractions. ACKNOWLEDGEMENTS

We would like to thank Dr. S. Bradley and Dr. D. Jackson of BNFL's Environmental Protection Group at Sellafield for supervising the fieldwork and for advice and assistance regarding the original radiometric survey of the study sites; also Martin Gill of the Royal School of Mines, Imperial College, London, for advice and assistance with XRD analyses. The MSc project from which this work derives was funded by BNFL; KSD also acknowledges the support of SERC through an advanced course studentship. REFERENCES Assinder, D.J., M. Kelly an0 S.R. Aston, 1985. Tidal variations in dissolved and particulate phase radionuclide activities in the Esk estuary and their distributioa coefficients and particulate activity fractions. J. Environ. Radioact., 2: 1-22. Aston, S.R. and D.A. Stanners, 1982. Gamma emitting fission products in the surface sediments of the Ravenglass Estuary. Mar, Po!!ut. Bull., 13 (4): 135-138. BNFL, 1988. Annual Report on Radioactive Discharges and Monitoring of the Environment. Health and Safety Directorate, Risley. Bocock, K.L., 1981. Radionuclides in Terrestrial Ecosystems. Institute of Terrestrial Ecology, Natural Environment Research Council. Published by HMSO, UK. Cheng, H. and H. Hagamuchi, 1981. Studies on the Adsorption of Marine Sediments. Health ~hys., 14: 35~-3~2. Cremers, A., A. Elsen, P. De Preter and A. Maes, 1988. Quantitative analysis of radiocaesium retention in soils. Nature, 335: 247-249. Cremers, A., A. Eisen, E. Valcke. J. Wauters, F.J. Sandalls and S.L. Gaudern, 1990. The sensitivity of upland soils to radiocesium contamination. In: G. Desmet, P. Nassimbeni an~ M. Belli (Eds.), Transfer of Radionuclides in Natural and Semi-natural Environme~ts. • Proceedings of a CEC workshop, Udine, Italy, September 1988. Elsevier, pp. 238-248. Davies, K.S., 1990. Qualitative and quantitative aspects of ~37Cs fixation by soils and sediments in the Ravenglass estuary, Cumbria. MSc thesis, University of London, UK.

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Evans, D.W., J.J. Albezts and R.A. Clark, 1983. Reversible ion exchange fixation of ~37Cs leading to mobilization from river sediments. Geochem. Cosmochem. Acta, 47: 1041-1049. Frissel, M.J. and R. lenders, 1983. Models for the accumulation and migration of 9°St, 137 Cs, 239~pu and 24JAmin the upper layer of soils. In: P.J. Coughtrey assisted by J.N.B. Bell and T.M. Roberts (Eds.), Ecological Aspects of Radionuclide Release, Special Publication Series of the British Ecological Society, Number 3. Blackwell Scientific, Oxford, pp. 63-73. Gutierrez, M. and H.R. Fuentes, 1991. Competitive adsorption of cesium, cobalt and strontium in conditioned clayey soil suspensions. J. Environ. Radioact., 13 (4): 271-282. Hamilton, E.I., 1989. Radionuclides and large particles m estuarine sediments. Mar. Pollut. Bull., 20 (12): 603-607. Hamilton, E.I. and K.R. Clarke, 1984. The recent sedimentation of the Esk estuary, Cumbria, UK: the application of radiochronology. Sci. Total Environ., 35: 325-386. Hetherington, J.A. and D.F. Jefferies, 1974. Distribution of some fission products of ~adionuclides in sea and estuarine sediments. Netherlands J. Sea Res., 8: 319-338. Horrill, A.D., 1983. levels of point to point variation of radionuclideburden in Cumbrian salt marsh environments. In: P.J. Coughtrey assisted by J.N.B Bell and T.M. Roberts (Ed.), Ecological Aspects of Radionuclide Release, Special Publication ~ries of the British Ecological Society, 3 edn. Blackwell Scientific, Oxford, I:.P. 199-217. Horriil, A.D., 1986. Radioactivity in terrestrial ecosystems. In P. Ineson (Ed.), Pollution in Cumbria. ITE Symposium No. 16, Institute of Terrestrial Ecology, Natural Environment Research Council. HMSO, UK, pp. 42-47. Howard, B.J. and F.R. Livens, 1991. Radioactive contamination of tide-washed pastures: a review of current knowledge of contamination levels of radionuclides in tide-washed pastures and their implications for radionuclide levels in food products from these areas. •MAFF, UK. Hunt, G.J., 1989. Radioactivity in Surface and Coastal Waters of the British Isles, 1988. Aquatic Environmental Monitoring Report 20. Ministry of Agriculture, Fisheries and Food. Directorate of Fisheries Research, Lowestoft. Kersten, M. and U. F6rstner, 1989. Speciation of trace elements in sediments, in G.E. Batley (Ed.),Trace Element Speciation, Analytical Methods and Problems. CRC Press, Florida, pp. 245-318. Livens, F.R. and M.S. Baxter, 1988a. Particle sge and radionuclide levels in some west Cumbrian soils. Sci. Total Environ, 70: !-17. Livens, F.R. and M.S. Baxter, 1988b. Chemical associations of artificial radionuciides in Cumbrian soilsn .L Environ. Radioact., 7: 75-86. Lomenick, T.F. and T. Tamura, 1965. Naturally occurring fixat|on of '~TCson sediments of Lacustrine origin. Soil Sci. Am. J., 29: 383-387. Pardue, J.H., R.D. DeLaune, W.H. Pa:rick, Jr and J.H. Whitcomb, 1989. The effect of redox potential on fixation of '37Cs in lake sediments. Health Phys, 57 (5): 781-789. Patel, B., S. Patel and S. Pawar, Desorptiou of radioactivity from nearshore sediment. Estuar. Coastal Mar. Sci., 7: 49-58. Santschi, P.H., Li Yuan-Hui and D.M. Adler, 1983. The relative mobility of natural (Th, Pb and P,~) and fallout (Pu, Am and Cs) radionuclides in the coastal marine environment: results from a model ecosystem (MERL) and Narrangansett. Geochem. Cosmochem. Acta, 47: 201-210. Sposito, G., 1989. The Chemistry ,'f Soils. Oxford University Press, New York, Oxford. Sweeck, L., J. Wauters, E. Valck,: ~nd A. Cremers, 1989. The sensitivity of upland soils to

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radiocesimn contamination. In: G. Desmet, P. Nassimbeni and M. Belli (Eds.), Transfer of radionr~clides in natural and semi-natural environments'. Proceedings of a CEC workshop, Udine, Italy, September 1988. Elsevier, pp. 249-258. Testier, A., P.G.C. Campbell and M. Bisson, 1979. Sequential extraction procedure for speciation of particulate trace metals. Anal. Chem., 51: 844-851. Van Valin, R. and J.W. Morse, 1982. An investigation of methods commonly used for the selective removal and characterisation of Trace Metals in Sediments. Veith, J.A. a~LdG. Sposito, 1977. On the use of the Langmuir equation in the interpretation of adsorption phenomena. Soil Soc. Am. J., 41: 697-702. Wilkins, B.T., N. Green, S.P. Stewart and R.O. Major, 1986. Factors that Affect the Associations of Radionuclides with Soil Phases. In: R.A. Bulman and J.R. Cooper (Ed.), Speciation of Fi:;sion and Activation Products in the Environment. Elsevier, pp. 101-113.