the relationship between sediment and plutonium budgets in a small macrotidal estuary: esk estuary,...

J. Environ. Radioactivity 13 (1991)55-74

The Relationship Between Sediment and Plutonium Budgets in a Small Macrotidal Estuary: Esk Estuary,

Cumbria, UK

M. Kelly, M. Emptage, S. Mudge, K. Bradshaw & J. Hamilton-Taylor

Environmental Science Division. Institute of Biological and Environmental Science, University of Lancaster, Bailrigg. Lancaster LA 14YQ. UK

(Received 24 July 1989; revised version received 28 December 1989; accepted 5 January 1990)

A BS TRA CT

During a spring tide, measurements were made of sediment and z+u'"4°Pu d£vcharges through a cross-section of the Esk estuary. These indicated that over the full tidal cycle the inner estuary had a net gain of ca. 18 t of sediment and ca, 85 M Bq of particulate phase 2Jv'"4° Pu, and a probable net loss of ca. I to 2 MBq of solution phase 2"~'24°Pu. Each of these was the net result of large gross discharges of sediment and plutonium into and out of the estuary/or which the sea was the main source, with eroded estuarine sediment providing an additional minor source of sediment, of particulate phase plutonium and, via desorption, of solution phase plutonium. A net input with each tide, of sediment and its associated radionuclides, is considered to be typical for the Esk estuary under the normal conditions of low river flows.

I N T R O D U C T I O N

Artificial radionuclides are discharged into the Irish Sea under iicence from the nuclear fuel reprocessing plant at Sellafield, Cumbria . The radionuclides are subsequent ly found in both the particulate and solution phases in seawater . Estuaries along the coast, therefore , receive inputs from the sea of radionuclides associated with particulate matter and in

55 1. Environ. Radioactivity 0265-931X/90/$03.50 t~) 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain

56 M. Kelly, M. Emptage. S.Mudge, K. Bradshaw, J. Hamilton-Taylor

solution. Sedimentary deposits within these estuaries can provide a sink for the particle-bound radionuclides and, consequently, the deposits act as secondary sources of radionuclides when physically reworked. This leads not only to the remobilisation of contaminated particles, but also to the release of radionuclides to solution as a result of particle/solution interactions. Discharges from Sellafield, and thus inputs to estuaries, began in 1952, peaked in the 1970s and have since decreased considerably (Stather et al. , 1984; BNFL, 1989). It is therefore possible that reworking of contaminated estuarine sediments may release radionuclides into coastal environments at higher levels than are derived from contemporary discharges.

The Esk estuary, which is a small estuary lying 10 km south of Sellafield, has received relatively high levels of radionuclide inputs from the sea and, consequently, has been the site of much related research (see later cited references). This report presents the results of a survey carried out to determine a plutonium budget for the inner Esk estuary over a single tidal cycle. More particularly, the aim was to establish whether, under normal conditions, the estuary was still a net sink or had become a net exporter of particulate plutonium and/or plutonium in solution.

METHODS

in order to obtain radionuclidc fluxes for the inner Esk estuary, velocity measurements and water samples were taken at five stations across the estuary at Ravenglass (Figs 1 and 2) on a 9.23 m spring tide (6 October 1987). The stations were occupied sequentially over a complete tidal cycle, using two boats.

On each sampling occasion, four to six velocity measurements were made, using a Braystoke current meter, in a vertical profile at intervals of 0.25 m above the bed or at logarithmically upwards-increasing multiples of this if the water depth allowed. One-litre water samples for suspended sediment, salinity and radionuclide analysis were taken at the same depths and times with a submersible pump. in order to reduce the number of samples for analysis, the water samples from a vertical profile were bulked in such a way as to produce two duplicate, 1 iitre discharge-weighted samples. This technique, of bulking fractions weighted according to the water discharge, allows the discharges of variables, such as ~uq)ended sediment or radionuelides, to be calculated from a single sample representative of the whole profile, whatever the vertical distribution of concentration of the variable.

The bulking procedure requires that the velocity and suspended

Sediment and plutonium budgets in a small macrotidal estuary, 57

54" 20"N

r ~

Ikm \ \

T.L. i'2sw

::lavenglass

New~iggin

3 '~;~0' W

Limit

Mean High Water -- Mean Low Water

Fig. I. Map of Esk estuary with site of survey cross-section.

1

e -

K 3 E3

4

Stat ions 5 4 3

Level ~ , , South

Distance (m)

2 1

/ North

Fig. 2. Survey cross-section with vertical profile stations.

58 M. Kelly. M. Emptage, S.Mudge, K. Bradshaw, J. Hamilton-Taylor

sediment concentrations, measured at individual points in a vertical profile, are taken as the mean values for an interval of height stretching above and below the sampling point. The limits used for each interval were halfway to the adjacent sample points, or to the surface, or close to the bed, as appropriate. The water discharge through the height interval corresponding to sampling point i in a given profile is then given by

qi = u, di (1)

where q, is the discharge per unit width, u, is the velocity, and di the height interval. The water discharge through the whole water column is then given by

O = ~ : q, (2) i=l

where Q is the unit width water discharge through the profile, and n is the number (4-6) of sampling points in the profile. Hence, the volume of water required from each sample to produce a representative sample for the entire water column is

V, = v t ( q , / Q ) (3)

where Vt is the volume of total sample taken, in this case 1 litre, and V i is the volume required.

From each profile, one of the bulked samples was filtered in the field through a 0-22/~m Mitliporc GWSP membrane filter. The filtrate was acidified with HNO3 and KzCr,O 7 was added as a holding oxidant. Subsequently, the two fractions were analysed for 23~Pu and z-~'~'z4tJpu by alpha-spectrometry following chemical separation by ion exchange methods. For the dissolved fraction only, two oxidation state categories (Pu(lll + IV) and Pu(V + VI)) were differentiated using the method of Lovett & Nelson (1981). The salinity was measured on the second bulk sample using an electrical conductivity salinometer, and the sediment concentration was determined by filtration through two superimposed 0-22 ~m Millipore filters, the lower one being used as a control.

The sediment discharge through each vertical profile was calculated from

S = C Q (4)

where C is the discharge-weighted mean sediment concentration in the bulked sample, and S is the unit width suslxmd~ sediment discharge through the profile. The discharges of radionuclides in the solution and particulate phases are given by

R. = A . O "~"

Sediment and plutonium budgets in a small macrotidal estuary.

8 ¢ , , , , ~

59

J

24(]

12¢

80

40

• , ' , , , ' , , ! T

-4 -2 0 2 4 6 8 Tm~e (hours)

Fig. 3. Distribution of discharges across the survey cross-section throughout the tidal cycle: (a) suspended sediment di~harge (contour units, g/s/m).

and

Rp = A r S (6)

where R, and R r are the unit width radionuclide discharges, and A~ and Ap arc the specific activities of the solution and particulate phases, respcc- tively.

The individual unit width discharge values were plotted on space/time diagrams and contoured to show the variation of the discharge across the section throughout the tidal cycle (e.g. Fig. 3). Data points, which are the same for all variables, are shown only in Fig. 3(a) and the waterline on the diagrams is positioned from the relationships between the measured water depths and the cross-section of the estuary surveyed at low water. From the contour diagrams, the following were calculated for the whole survey cross-section and for the deep water station: the instantaneous discharges, at successive times, of water, salt, sediment and radionuclides (Fig. 4), and the total discharges on the flood and ebb portions of the tidal cycle (Table l). Only the data for ~ ' 2 ~ P u are dealt with in detail.

6(I M. Kelly, M. Emptage, S.Mudge, K. Bradshaw. J. Hamilton-Taylor

24o.;

200~

1

. I

re. t u m ~ / ~ / I I I , ,-~-"~-- ~ \ re, tum,d

160

120

40,

-4 ~, 0 2 4 6 8 rc'tw (ho~xs)

I.'ig. 3--contd. (b) particulate p h a ~ 2"~q'2*JPu specific activity (contour units. Bqls/m).

TABLE I Di.~harges Through the Mouth of the Inner Esk Estuary for Survey Cross-section and the

Deep Water Channel Station

Component Units Flood tide Ebb tide Net (/tide)

(a) Cross-section Water Mm + 3-13 3.27 -0 .14 Salt kt 93.4 100.8 -7 .4 Sediment t 106.4 88.8 + 17-6 "-~Y'2"aJPu (particulate) MBq 254.6 169.7 +84.9 '3"'24~JPu (solution) Pu(ill . IV) MBq 7-3 7.4 -0-1 Pu(V, Vi) MBq 20-9 21.7 -0 .8

(b) Deep water channel Salt kt/m 0.88 1.28 -0 .40 Sediment t/m 1.34 1.72 -0-38 23'~.2"MJPu (particulate) MBq/m 2.9 2-9 0


w I I t g

61

240.

200

160!

I

120.

8O

, , , , . . . . , . ,

-4 -2 0 2 4 6 Time ibexes)

T a ~ ' t u r n e d

Fig. 3---contd. (c) solution phase 23~'2~pu specific activity (contour units, Bq/~m).

ESTU ARY C H A R A C T E R I S T I C S

The Esk estuary is a shallow, macrotidal estuary with a complex plan, having a major (lrt) and minor tributary (Mite) on its north side (Fig. I). On the day of the survey, with a medium spring tide, the tidal amplitude at Ravenglass was 4.6 m. At the survey section, the maximum depth at high water was 5-26 m and the width 272 m, whereas at low water these had reduced to 0-65 m and 37 m, respectively. The tidal cycle is strongly asymmetric with a short flood tide of 3.3 h and a prolonged ebb of 8.75 h. Such asymmetry is characteristic of shallow estuaries such as the Esk. Consequently, peak unit width water discharge (5-41 m3/s/m) occurs on the flood, 1.5 h before high water. The maximum on the ebb is not much lower (4-99 m3/s/m), as the estuary is short and empties rapidly. There- fore, for much of the extended ebb period, flow is confined to the low water channel, which is essentially a prolongation of the River Esk.

62 M. Kelly. M. Emptage, S.Mudge. K. Bradshaw. J. Hamilton-Taylor

~to

a a

A= o

e~

30

20

10

.10

.20 FLOg0

a) Survey cross section

Q Sediment kg/s

+ Particulate phase 239,240 Pu kBq/s

o Solution phase 239'24°pu kBq/s

EBB

Time relat ive to High Water/h

(;i)

~Jo

to O

I .g: U to

o

I ~ )

U

. I (X)

-2UI)

w

b) Deep water channel station

0 Sediment k g / s

+ Particulate phase 239,24°pu kBq/s

o Solution phase 239.240Pu kBq ls

, i , ,; , ~ ,

Time relat ive to High Water/h

(b)

Fig. 4. Variation of (fischaqlCs t lvougig~t the tidal cycle of suspended sedim0flt, paniculate and solution pime z~"~'eYu: (a) for the whole survey _c ~ ; (b) for the deep water channel station. O, Sedimeatt (kl~s); +. particulate phase "I~Po (kBq/s); ¢~,

solution phmm ~ ' ~ . ( k B q / s ) .

Sediment and plutonium budgets in a small macrotidal estuary. 63

Sedimentologically, the Esk is characterised by its channel having a complex distribution of fixed and mobile bed reaches and by the major depositional areas of intertidal banks being located in the middle and upper reaches. In the survey reach itself, the channel bed is largely stable, being formed of lag gravels and mussel banks. Locally, small relic areas of eroding intertidal muddy sands are exposed and a narrow strip of sand with mega-ripple (dune) bedforms on the south side is the only mobile bed area. Sediment transport through the reach, therefore, is predominantly in suspension (suspended wash load) and a bed load is virtually absent. Downstream, mobile sand bed reaches are extensive in the outer estuary, beyond the junction with the Irt and Mite tributaries, where over half the cross-section is occupied by sand mega-ripples. However, the deep water channel, which primarily feeds the inner Esk on the flood tide, has a fixed lag gravel and mussel bank bed. Upstream, above Newbiggin, a sand bed channel is bordered by mud and sand intertidal banks. In places, active erosion of the margins of these intertidal banks is taking place, especially by bank collapse. This provides an important means of remobilising estuarine sediment of any age, i.e. both pro- and post-commencement of Scllaticld discharges.

CROSS-SECTION BUDGETS

Water and salt budgets

From the distribution of water discharge in space and time at the cross-section, the net tidal volume, i.e. the discharge on the flood tide, was calculated as 3.13 Mm 3 (Table l(a)) and the additional volume discharged on the ebb due to river flow as 0-14 Mm 3. This compares with 0.34 Mm -~ derived from the hourly discharge data obtained from the North West Water Authority gauging station, 1 km above the tidal limit of the Esk. Although in part the error may lie in the gauging station data, it is useful to consider this discrepancy as an estimate of errors associated with the budget survey. As such, it represents an underestimate of the ebb discharge of 6,4%.

Consideration of the salt budget provides a further check on the overall accuracy of the procedures used, since salt should behave relatively conservatively, with near equal flood and ebb discharges. Calculations based on a contour plot showed this to be essentially the case, with values of 93.4 and 100-8 kt/tide for the flood and ebb, respectively (based on equating total dissolved solids concentration with the instrumentally measured salinity) (Table l(a)). In this case, the discrepancy may be due

64 M. Kelly, M. Emptage, S.Mudge, K. Bradshaw, J. Hamilton-Taylor

r~ 80

g 7o ~, ( , , ' )

60

E

._~ 40

0 u 20

E 10 ®

if'/

• 0 -5

H,gn Water

I i I I I

A B C , D I I ,

/',If ; / I ",,

-3 -I I 3 5 7

Time (hours)

Fig. 5. Variation of salinity and suspended ~diment concentration at the deep water channel station. I , Sediment concentration (mg/litre); +, salinity (%~).

to salt being inherited from earlier, higher tides due to trapping and slow release of water on salt marsh surfaces and in pore waters. As an estimate of errors in the survey, it would indicate an overeslimation of ebb discharges by 7.3%.

,~diment imdget

The distribution of sediment di~hargc is shown in Fig. 3(a). T h e flood tide sediment discharge for the whole survey section exceeds that of the ebb (Fig. 4(a) and Table l(a)), giving a net input of sediment to the estuary of ca. 18 t over the tidal cycle. This difference, equivalent to 20% of the ebb discharge, exceeds the magnitude of the errors suggested above, and is considered to be real. Tim imbalance in the sediment budget is due to the interplay between the discharge-weighte, d sediment concentration and the water discharge. Thus, although higher sediment concentrations occur on the ebb than on the flood, and last longer, they occur at a period of declining water discharge towards the end of the ebb (see Fig. 5).

Particulate phase plulomimm budget

The distribution of particulate 23'~'2'mpu specific activity is shown in Fig. 3(b). With a net input of sediment into the estuary over the tidal cycle, it is

Sediment and plutonium budgets in a small macrotidal estuary 65

not surprising that there is a related net input of particulate plutonium of 85 MBq (Fig. 4(a) and Table l(a)). However, this is proportionally higher than the sediment net input because the mean "3'~'2'mpu specific activity is higher for the incoming sediment (2-24_ 0.23 kBq kg - t (n = 12)) than the outgoing ( 1-73 _ 0-27 kBq kg- t (n = 20)). One possible explanation of the difference is that older, lower specific activity sediment has been eroded within the inner estuary. If this is the case, then the relative uniformity throughout the tidal cycle of the 238pu/23'~'24°pu specific activity ratio would require that the eroded sediment came predominantly from deposits predating the Sellafieid discharges, since the activity ratio in the discharges has changed during their history. Since grain size affects sediment specific activity, the observed variation in plutonium activities could be explained also by a relative increase in coarse grain sizes, although this is not predicted by the water velocities.

Solution phase plutonium budget

The distribution of solution phase "3'~'24"Pu specific activity is shown in Fig. 3(c). ] 'he distribution of the discharges for the individual oxidation state categories ("~" :"Pu( l l l , IV) and "-3"Z4°Pu(V, VI)) in solution wcre essentially similar. The budgets for both categories show slight net losses from the estuary over the tidal cycle (Fig. 4(a) and Table l(a)), but the losscs arc

10

6

E _)-

G 4 <

0 0

/ . . . . . . . . . /

° Pu (V.Vl) j/1

/

. . p u ( v . v o • "" . :

/

. . . . . . . . . . . . . . . . . . . . r • , , , , t • , , t , t l ,

4 8 12 16 20 24 28 32

S a h n l t y ( p p t )

Fig. 6. Distribution of solution phase plutonium specific activities with salinity compared with the range of theoretical dilution curves. E]. Pu(lll, IV): +, Pu(V, VI).

66 M. Kelly, M. Emptage, S.Mudge, K. Bradshaw, J. Hamilton-Taylor

less than the errors suggested by the water and salt budgets. However, net losses should be observed because it is known that a proportion of the plutonium in solution is derived from within the estuary, by remobilisation of plutonium from sediment in low salinity water (Assinder et al. , 1984; Eakins et al., 1985; Burton, 1986; Hamilton-Taylor et al., 1987; Kelly et al., 1988; Mudge et al., 1988). This is confirmed in the present study by the occurrence of solution plutonium activities above the theoretical dilution values (Fig. 6), i.e. by the nonconservative behaviour of both oxidation categories. Using the maximum dilution curves in Fig. 6, the minimum amount of remobilised plutonium present in solution at each sampling point can be obtained. From this, and the water discharge, the total discharge of remobilised Pu on the ebb tide was estimated as 1.9 MBq ~3'~'24°pu (0-8 MBq -'3'~'Z~°Pu(lll, IV) and 1. i MBq 23'9"24°pu(V, VI)). These figures exceed those derived directly from the discharge measurements, especially for Pu(lll , IV), suggesting that the water discharge at the end of the ebb is being overestimated. Both sets of figures, however, indicate that the remobilised Pu represents only a small fraction of the total Pu budget.

BUDGETS FOR DEEI' WATER Ct lANNEL STATION

it is notable that budgets calculated for the station in the middle of the deep water channel (station 3) are not indicative of the state of the net budget for the survey cross-section as a whole (Table I(a) and (b)). Thus, the budget for the unit width salt discharge at station 3 is markedly out of balance, with the tkx~l discharge being 69% of the ebb, compared with 93% obtained for the whole survey section. A similar difference is seen for the sediment budget, with a net output of sediment at station 3, compared with the net input shown by the whole section. For the particulate plutonium budget, the station 3 budget is balanced whereas the cross-section shows a considerable net input.

These differences can also be seen in the discharge/time curves for the whole survey section and the deep water channel station (Figs 4(a) and (b)). They show a displacement in the timing of the maximum sediment and particulate plutonium discharges on the ebb from 1.5 h after high water for the survey section to 4 h after in the deep water channel, which is clearly due to the passage of the suspended sediment concentration (turbidity) maximum (Fig. 6).

These results demonstrate that the flow in the channel and marginal sections can make different relative contributions to the discharge budgets of an estuary and that caution is needed in using measurements at a single station to estimate cross-section budgets. It is a deficiency of the present

Sediment and plutonium budgets in a small macrotidal estuary. 67

survey that the marginal areas were not more thoroughly sampled due to lack of resources.

S O U R C E S AND SINKS OF S E D I M E N T AND P L U T O N I U M

Sediment

The sediment budget of the Esk estuary over a tidal cycle has inputs from three major sources: from the external sediment sources of the sea and the river and the internal source of the estuarine deposits. The other potential internal sources of biological productivity and colloid aggregation are considered to be of relatively minor importance. The outputs are to two sinks: the sediment discharged to the sea and the sediment deposited in the estuary. The sediment load of the river is negligible under normal low flow conditions (ca. ! g/m3). The relative importance of the other components can bc estimated by examining the sediment discharges of the separate water bodies as detined by their salinity.

Four water bodies with different salinity characteristics pass sequentially through the mouth of the inner Esk estuary, i.e. a variable salinity water body and a full seawater body on the flood tide and the opposite on the ebb tide, as illustrated by the salinity curve for station 3 (A-D , Fig. 5). The early flood tide brackish water (A) comprises water which was present in the outer estuary channel at low water mixed with incoming seawater. The m~,in pz, rt of the flood, however, consists of undiluted seawater (B). On the ebb, this body of undiluted seawater returns to the sea (C), but with a reduced volume due to mixing of the seawater with river water in the inner estuary. Conversely, the volume of the ebb brackish water body in the inner estuary (D) greatly exceeds that of the flood brackish water body due to the addition of river water and dilution of sea water. With low river discharges, such as on the day of the survey, freshwater does not extend to Ravenglass at low water.

The sediment load of the undiluted seawater body on the flood tide (e.g. B, Fig. 5) can be considered as derived largely from the sea. It has a characteristically uniform spatial and temporal concentration (ca. 34.1 + 3-23 g m -3 (n = 7)) and consequently a constant ratio of sediment to salt concentration. If this ratio is assumed to have been the same also for the seawater input of the early flood tide (A), then it can be used to determine the theoretical marine sediment contribution to the sediment discharge of the water bodies, A, C and D, from their salt discharges:

Q,,,~d = Q~,l, x R (7)

68 M. Kelly. M. Emptage, S.Mudge, K. Bradshaw. J. Hamilton-Tayh~r

TABLE 2 Estimated Components of the Sediment and Particulate Pu Budgets for the Inner Esk

Estuary

Sediment :w'"'~'Pu (particulate)

t % MBq %

/npu/s (i) Marine source 1()2 90 244 96

(ii) River source <1 <t -- (iii) Estuarine source (outer) 4 4 11 4 (iv) Estuarine s o u r c e (inner) 7 6 -- --

Totals 113 100 255 I(X)

Outputs (v) To ~a (+ outer estuary) 8~ 72 170 62 (vi) Deposition 34 28 106 38

Totals 123 I(~} 276 100

Error (unaccounted for) 10 ~ 21 --

where Q,.~,d and Q,~t arc the theoretical marine sediment and measured salt discharges of the water body concerned, and R is the sediment to salt concentration ratio of the fl(md tide seawater body (B). A net sediment contribution from or to the estuarine sediments (Qc.~d) is then obtained from the difference between the measured sediment di.~hargc of the water body (Q.~d) and the calculated marine sediment contribution (Qm.~,J), with a negative value indicating deposition, and a positive one, erosion:

O~,,d -- (.),~,,- O,.~,, (8)

The actual values obtained by these calculations (Table 2), which are based on the full cross-sectional data, can be considered to be approximate only, because of the assumptions made and because they are based on the salt budget data, for which there is an apparent error. Allowing for this 'error ' in the salt budget removes the differences between the total inputs and outputs of sediment and plutonium in Table 2. The net changes that are calculated to have taken place within the estuary (deposition and erosion) do not record the actual amounts of sediment exchanged between a water body and its bed, i.e. the absolute amounts of sediment eroded and/or deposited are underestimated. Another weakness in the net figures is that the eroded quantity can include not only 'old" sediment, but also sediment deposited on the same tide by a different water body. Despite


these limitations, the figures are considered to be a reasonable assessment of the relative importance of the different terms in the sediment budget.

The most important sediment source for the inner estuary is the sea (90%) during the flood tide ((i) in Table 2), transported mainly by the seawater body (B, Fig. 5). This is supplemented by a lesser but significant amount produced by net erosion within the estuary (10%), of which about one-third ((iii) in Table 2) is transported into the inner estuary by the initial brackish water body of the flood tide (A). This, theoretically, includes sediment eroded in the outer estuary channel and sediment transported by the preceding ebb which is still suspended in water in the channel when the tide turns. The latter component may have been derived from the inner arms of the Esk, Mite and irt. In the inner estuary, an additional quantity of sediment is added to the brackish water body by net erosion during the flood and ebb tides ((iv) in Table 2).

The main sink for sediment is the sea (72%) and, to a minor extent, the outer estuary ((v) in Table 2). Its transport is mainly (60%) by the returning seawater (C) but a substantial part (40%) is by the brackish water body (D), because this water body has sediment added not only by net erosion while in the inner estuary, but also by mixing with seawater. The peak suspended sediment concentrations seen at the survey section were in this water body (77 g/m3). Typically, the maximum concentrations in the estuary at high water (the turbidity maximum) occur in the brackish water in the upper reaches of the estuary (Assinder et al., 1985). Net deposition in the inner estuary constitutes the other sink (28%) ((vi) in Table 2), equivalent to 34 t of sediment. This occurs from the seawater body (C) while in the inner estuary, resulting in a decrease of its suspended sediment concentration by one-third (to 22-6 + 3-0 g/m 3 (n = 11)) by the time it passed the survey cross-section on the ebb.

The association of net erosion with the brackish water body, which contrasts with net deposition from the seawater body, is due to the combination of shallow depths and high velocities, producing higher bed shear stresses during the passage of the brackish water. Despite the occurrence of this erosion, the calculated tigures show that the inner estuary as a whole was a site of net deposition during the surveyed tide. In practice, the two processes will be spatially separate, with erosion principally at the channel margins and deposition on the intertidal banks.

Particulate phase plutonium

The general conclusions arrived at about the sources and sinks for the sediment will apply to the plutonium associated with it. That is, the major source and sink was the sea, with estuarine sediments acting as a minor

70 M. Kelly, M. Emptage, S. Mudge, K. Bradshaw. J. Hamilton-Taylor

additional source and sink. The quantities of the particulate phase Pu inputs and outputs can be estimated by using the same technique as for the sediment, i.e. by calculating the marine component of the discharge for the different water bodies and determining the net difference between these and the measured particulate plutonium discharges. The figures show (Table 2) that the changes in the specific activity of the sediment between the flood and ebb tides enhance the importance of the marine source for particulate plutonium (96%) and result in a zero net input of particulate plutonium from the inner estuary. Similarly, the balance is changed between the outputs, with increased deposition (38%), although the sea remains the dominant sink (62%).

Solution phase plutonium

As described above for the solution phase plutonium budget, a quantity is supplied by an estuarine source, by remobilisation from sediments. The amount estimated above is equivalent to 7% of the total input to the inner estuary. The sea is the remaining source and the only quantifiable sink, although in practice there will be an exchange with sediment pore waters.

Study of the distribution of the remobilised plutonium in the estuarine waters of the Esk (Kelly el al . , 1988) has shown that the highest levels occur at salinities of less than 5%0. This has been attributed to the resuspension, into low ptl brackish waters, of estuarine sediments equilibrated with saline pore waters, it could be due also to rapid mixing into the brackish water of suspended sediment from the seawater body. The above calculations indicate that up to two-thirds of the sediment in suspension in the brackish water body could be derived from the seawater and one-third by erosion. This may not be representative of the lower salinity fraction, however, nor does it take into account sediment exchange between bed and water column.

DISCUSSION AND CONCLUSIONS

The components of the 23'~'2'aJPu budget of the inner Esk estuary for a spring tide are summarised in Fig. 7. The total input was estimated at 285 MBq, of which 89% was in the particulate phase and 11% in solution. About two-thirds of this total was returned to the sea and one-third retained in the inner estuary in association with deposited sediment.

Thus, the plutonium budget of the inner Esk estuary is dominated by the behaviour of the sediment-associated plutonium, which is in keeping with the known affinity of plutonium for sediment and its high Kd (Assinder et


Umts = MBq / t=cle

71

--I. ~70

L 29

Sea f- J

11 oulef estuary

2B

255

Inner t Esk i Estuary

Water Body

L

t I ,t erosion Sediment j desorpllon

deposiDon , [ <1 85 1-2

1 <0 I

<0 I

239.240 paniculate phase Pu 239,240 solution phase Pu

Fig. 7. Model of z~'~':~a~Pu budget components for the inner Esk estuary.

al., 1985; Burton et al., 1986). Consequently, the net plutonium budget depends largely on the net sediment budget, but is influenced also by changes in the specilic activity of the sediment. For a particular tide, the sediment budget is a function of the relative amounts of sediment transported from the external sources of the sea and river, and the net amount deposited or eroded in the estuary during the tide. The observed net input of sediment and plutonium into the inner Esk estuary is considered to be typical of conditions with normal (low) river flows, with the magnitude of the net input being proportional to the tidal amplitude in the spring-neap cycle. It is also thought likely that this is the situation for the lrt and Mite tributaries as well. Such net sediment inputs from the sea, which increase with tidal amplitude, are a feature of other UK estuaries (lnglis & Allen, 1957; Price & Kendrick, 1963), although others are considered to be in balance or showing net gains of sediment from riverine sources (Dyer, 1986).

The principal source of sediment and associated particulate-phase plutonium in the estuary was the sea, and the proportion of their discharge generated within the estuary by erosion of older sediments was estimated to be fairly small ( 10 and 4%, respectively). This fraction will increase with river discharge, which will prolong the period of high discharge on the ebb tide and give conditions likely to lead to net losses of sediment and particulate plutonium from the estuary. However, over the observed tide, the inner estuary gained activity due to sediment deposition exceeding

72 M. Kelly. M. Emptage, S.Mudge, K. Bradshaw, J. Hamilton.Taylor

erosion and the eroded sediment having a lower specific activity than that deposited. This change in the specific activity of the ebb discharge will not be a consistent feature, but will depend on the location of the areas of erosion.

Although it appears that the inner Esk estuary was a net exporter of plutonium in solution, due to its remobilisation from sediment in the estuary, the amounts involved were too small to be properly quantified. They were also small compared with the amount derived from the sea, and insignificant in the context of the total plutonium budget (ca. 0-3%). Despite the decrease in the activity of the plutonium discharges from the Sellafield plant, it is likely that the marine source will continue to contribute relatively high specific activity sediment to the estuary, being fed by the large reservoir of fine-grained contaminated sediments which occur offshore (Pentreath et al., 1984, 1985), where they can be resuspended by storms. It is therefore likely that the net gain of particulate plutonium and slight net losses in solution per tide seen in the survey will continue to be characteristic of normal tides for the foreseeable future. Previous conclusions that the estuary had become a net exporter of plutonium (Eakins et al., 1985; Burton et al., 1986; Burton & Yarnold, 1988) were based on extrapolations of data from a single station located in the deep water channel in the outer estuary. Although the data cannot be directly compared, it is significant that the net budgets calculated for the deep water channel station in this study show that they do not reflect the mean conditions fi~r the whole estuarine cross-section. However, it is highly likely that with certain less frequent combinations of tides and high river discharges there will be a net loss of sediment and plutonium from the Esk estuary.

It is apparent that proper definition of mass/activity budgets for estuaries requires a resource-intensive, muitistation sampling programme and is not yet clear to what extent this can be short-cut and still give an indication of the state of the budget. It is also desirable that in future studies a more comprehensive estimation of the errors is made than was able to be carried out in the present survey.

A C K N O W L E D G EMENTS

This work was carried out as part of a contract with the U K Department of the Environment, and the results are published with the agreement of the Department. The assistance of C. Allen, R. AIIott and L. Rosser is gratefully acknowledged.


R E F E R E N C E S

Assinder, D. J., Kelly, M. & Aston, S. R. (1984). Conservative and non-conservative behaviour of radionuclides in an estuarine environment, with particular respect to the behaviour of plutonium isotopes. Environ. Technoi. Lett., 5, 23-30.

Assinder, D. J., Kelly, M. & Aston, S. R. (1985). Tidal variations in dissolved and particulate phase radionuclide activities in the Esk Estuary, England. J. Environ. Radioactivity, 2, 1-22.

BNFL (1989). Radioactive Discharges and Monitoring of the Environment 1987. British Nuclear Fuels, Warrington, UK, 134 pp.

Burton, P. J. (1986). Laboratory studies on the remobilisation of actinides from Ravenglass Estuary sediment. Sci. Total Environ., 52, 123-45.

Burton, P. J., Eakins, J. D. & Lally, A. E. (1986). The significance of distribution coefficients in the remobilisation of actinides from intertidal sediments of the Ravenglass Estuary. In Application of Distribution Coefficients to Radiological Assessment Models, eds T. H. Sibley & C. Myttenaere, Elsevier, London, pp. 288--97.

Burton, P. J. & Yarnold, L. P. (1988). Remobilisation of actinides from intertidal sediments of the Ravenglass Estuary. Sci. Total Environ., 69, 239--60.

Dyer, K. R. (1986). Coastal and Estuarine Sediment Dynamics. Wiley, Chiches- ter, UK, 342 pp.

Eakins, J. D., Burton, P. J., tlumphreys, D. G. & Lally, A. E. (1985). The remobilisation of actinides from contaminated intertidal sediments in the Ravenglass Estuary. In Proc. CEC Seminar, Renesse, The Netherlands, 1984. CEC Publication X1[/380/85, CEC, Luxembourg, pp. 107-22.

llamilton-Taylor, J., Kelly, M., Mudge, S. & Bradshaw, K. (1987). Rapid rcmobilisation of plutonium from estuarine sediments. J. Environ. Radioactiv- ity, 5, 4119-23.

lnglis, C. C. & Allen, F. tt. (1957). The regimen of the Thames Estuary as affected by current, salinitics and river flow. Proc. lnstn Civil Engrs, 7,827-78.

Kelly, M., Mudge, S., Hamilton-Taylor, J. & Bradshaw, K. (1988). The behaviour of dissolved plutonium in the Esk Estuary, UK. In Radionuclides: a Tool[or Oceanography. Proc. Int. Syrup., Cherbourg, France 1987, eds J. C. Guary, P. Guegueniat & R. J. Pentreath, Elsevier Science Publishers Ltd, London, pp. 321-30.

Lovett, M. B. & Nelson, D. M. (1981). Determination of some oxidation states of plutonium in seawater and associated particulate matter, in Proc. Syrup. Techniques for Identifying Transuranic Speciation in Aquatic Environments. 24-28 March 1980, lspra, Italy. IAEA, Vienna, pp. 27-35.

Mudge, S., Hamilton-Taylor, J., Kelly, M. & Bradshaw, K. (1988). Laboratory studies of the chemical behaviour of plutonium associated with contaminated estuarine sediments. J. Environ. Radioactivity, 8, 218--37.

Pentreath, R. J., Lovett, M. B., Jefferies, D. F., Woodhead, D. S., Talbot, J. W. & Mitchell, N. T. (1984). The impact on public radiation exposure of transuranium nuclides discharged in liquid wastes from fuel element processing at Sellafield, UK. In Int. Conf. Radioactive Waste Management, Seattle, USA, 16-20 May, 1983. IAEA-CN-43, IAEA, Vienna, pp. 315-29.

7,~ M. Kelly. M. Emptage. S.Mudge, K. Brad, shaw. J. Hamilton-Taylor

Pentreath, R. J., Calmet, D., Guegueniat, P. M., Patel, B. & Patel, S. (1985). Behaviour of Radionuclides Released in Coastal Waters. IAEA-TECDOC-329, IAEA, Vienna, 110 pp.

Price, W. A. & Kendrick, M. P. (1963). Field and model investigation into the reasons for siltation in the Mersey Estuary. Proc. lnstn Civil Engrs, 24, 473--518.

Stather, J. W., Wrixon, A. D. & Simmonds, J. R. (1984). The Risks of Leukaemia and Other Cancers in Seascale from Rachation Exposure. NRPB-RITI. National Radiological Protection Board, Didcot, UK, 297 pp.

the relationship between sediment and plutonium budgets in a small macrotidal estuary: esk estuary,...

Documents