Temporal and spatial survey of dissolved 226Ra in coastal waters of the eastern Irish Sea

the !JcIence ofthe Told-t An--la-*lre.kFa-llUn-tiM.” The Science of the Total Environment 168 (199.5) 233-247

Temporal and spatial survey of dissolved 226Rain coastal waters of the eastern Irish Sea A.J. Poole*, D.J. Allington, D.C. Denoon Mini&y

of Agriculture,

Fisheries

and Food, Directorate of Fisheries Research, NR33 OHT. UK

Fisheries

Laboratory,

Lowestofr,

Suffolk,

Received13September 1994;accepted15November1994

__Abstract Phosphogypsum wastehasbeendischargedfrom the Albright and Wilson Whitehaven phosphoricacid production plant, under authorisation,into the easternIrish Sea since 1954.The operators changedtheir working practices in June 1992,i.e. started importing phosphoricacid rather than phosphateore, and theseresulted in a large reduction in 226Raand other radionuclidesdischarged.In the 6 months immediately following the changesat the plant, the concentration of 226Rain filtered seawaterfell by 91% at the dischargepipeline. A further reduction in concentration of 21% took placebetweenDecember1992and May 1993.Backgroundconcentrationsof 226Rain areasof the Irish Seaunaffected by industrial inputs of 226Raaverage2.0 mBq l- ‘. Ninety-eight percent of the survey area had 226Raconcentrations > 3 mBq 1-r prior to June 1992;this had fallen to 12% in December1992and then to < 1% by May 1993, although concentrations throughout most (96%) of the survey area remained above the natural background level. The background corrected inventory of 226Rain the surface waters of the survey area fell exponentially from a maximumvalue of 1.46 GBq prior to June 1992to a value of 0.06 GBq in December 1993, yielding an effective half time of 70 days. The variable concentration of 226Rain the surfacewaters of the survey area resulted from dispersionaway from the outfall and the survey area in the year following cessationof large dischargesof 226Ra.The estimated enhancementof 226Rain excessof background levels, and resulting from dischargesfrom the Albright and Wilson plant, were 7% of estimatedbackground levels by December 1993.It is possiblethat this excessis a consequenceof continued, much reduced discharges,that were equivalent to 6% of the backgroundwater column inventory in the survey area during 1993. Keywords:

226Ra. , Seawater;Eastern Irish Sea;Phosphoricacid plant; Radionuclidedischarges

1. Introduction Waste containing low levels of artificial radionuclides has been discharged, under authorisa-

* Corresponding author.0 Crowncopyright1995. Elsevier Science BV. SSDf

0048-9697(95)

04562-2

tion, into the eastern Irish Sea from BNFL Sellafield since the early 1950s. However, BNFL Sellafield, BNFL Springfields and the Albright and Wilson Whitehaven plant have also been discharging radioactive wastes containing natural series radionuclides into the eastern Irish Sea and

234

A.J. Poole et al. / The Science of the Total Environment

Table 1 Relative discharges of uranium (tonnes) from BNFL Sellafield and the Albright and Wilson Whitehaven plant, 1972-1992

Year 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992

Mean 1972-1992

_..-...___----

Sellafield Albright Cumulative total and Wilson Albright and Wilson 3 3 3 4 4 5

1 1

36 36 36 38 42 40 34 17 18 12 18 34 36 37 32 37 55 46 46 44 44

3

35

11 6 5 5 6 3 2 2 2

1 1 I

1

36 72 107 14.5 188 228 262 279 298 310 328 362 398 436 468 505 560 606 651 695 739

the Ribble Estuary during this time (MAFF, 1993). The mean annual discharge of U into the eastern Irish Sea from both BNFL Sellafield and the Albright and Wilson Whitehaven plant together between 1972 and 1992 was 38 tonnes, i.e. approximately 0.47 TBq year - ’ 238U (Table 1). The Albright and Wilson Whitehaven plant has been identified as the major source of enhanced levels of natural series radionuclides in the waters, sediment and biota of the eastern Irish Sea (McDonald et al., 1991, 1992; McCartney et al., 1990, 1992; Rollo et al., 1992; MAFF, 1993; Poole et al., 1994a). Albright and Wilson ceased importing phosphate ore, from Morocco, North Africa, during the summer of 1992 and have subsequently reduced large volume liquid discharges of phosphogypsum waste, derived from phosphoric acid processing, through the introduction of an effluent treatment plant (B.R. Donaldson, Albright and Wilson, personal communication). These two changes have resulted in a large reduction in discharges of U-series radionuclides into the east-

168 (1995) 233-247

ern Irish Sea (Fig. 1). The production of phosphoric acid causes disequilibrium of the 238U series. Radium-226 is primarily associated with the phosphogypsum waste and not the acid product as it co-precipitates with Ca in the gypsum (CaSO,. 2H,O; Roessler et al., 1979; Lardnoye et al., 1982; Roessler, 1990; van der Heijde et al., 19901, thus, cessation of the ore processing resulted in a large reduction in 226Ra discharged (Poole et al., 1994b). Sea water is generally undersaturated in gypsum (Becker, 1989) so that the fine grained phosphogypsum waste will readily dissolve releasing 226Ra into the water column where it will tend to remain due to its relatively low K, (Kd in coastal sediments = 5000; IAEA, 1985). As discharges of 226Ra from the plant, since June 1992, are < 1% of those prior to June 1992 (Poole et al., 1994b), the temporal and spatial variation in concentration of the 226Ra in the dissolved phase should be a good indicator of any water exchange in the area of the Albright and Wilson outfall. This paper describes the concentration of dissolved 226Ra in the surface waters of the eastern Irish Sea at peak discharges, prior to the changes in plant operations in June 1992, and at 5 months, 11 months and 17 months after changes in the process had been made. This work represents a small portion of a much larger study focused on the change in concentration of radionuclides in the surface waters and sediments following alterations to the operations at the Albright and Wilson Whitehaven plant. 2. Study area This study focuses on Saltom Bay, between Whitehaven and St Bees Head, into which Albright and Wilson discharge their effluent (Fig. 2a,b). The survey area covers 418 km’ and stretches from Harrington in the north, to Seascale in the south, and extends approximately 14.4 km west into the Irish Sea (Fig. 2b). The bottom sediments are generally sandy near the coast becoming more muddy to the south-west (Fig. 3); the only exception to this general trend was recorded prior to June 1992 when a patch of sandy mud, dominated by fine grained phosphogypsum slurry, was located close the pipeline, i.e.

A.J. Poole et al. / The Science of the Total Environment

168 (1995) 233-247

235

9.0 8.0

1.0 0.0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 Four weekly periods (December 1991 - March 1994) Fig 1. Albright and Wilson uranium discharge data, January 1992-March 1994 (B.R. Donaldson, Albright and Wilson, personal communication). Note: arrow refers to June 1992 when phosphate ore processing at the plant ceased.

MAR23 (Fig. 2b); this had largely dissipated by December 1992 (Fig. 4). The water column in the eastern Irish Sea is generally well mixed (Dickson, 1987) and the tides dominate the dynamics of the eastern Irish Sea (Howarth, 1984). The low frequency surface current pattern in the eastern Irish Sea is complex and highly correlatable with the wind stress. The non-wind driven currents and mean wind driven currents flow in opposite directions, south-eastwards and north-westwards, respectively, but with similar magnitudes (Howarth, 1984). The residual surface currents in the survey area move to the west to the north of St Bees Head and to the south-east to the south of St Bees Head (Dickson, 1987; Fig. 2a). The water depth in the survey area does not exceed 36 m (Fig. 2b). 3. Sample collection and analytical

techniques

The samples for the baseline survey, prior to changes in the discharges from the plant, were

collected during February and May 1992. Further samples were collected during December 1992, May 1993 and December 1993. The water samples were collected aboard the MAFF Research Vessel Ciroha and charter vessels using portable self-priming pumps. Approximately 17 1 of surface water were collected and filtered through a 0.45 pm filter paper on each visit to each site. Radon222 (a short lived daughter product of 226Ra) was then stripped from the water sample by means of the Rn emanation technique (Ivanovich and Murray, 1992). The 222Rn gas was then trapped in scintillation cells and counted using a photomultiplier tube. The samples were stripped three times over a period of 3 weeks to allow consecutive determinations of 222Rn ingrowth to be made. Two surface water samples were collected directly into glass bottles and analysed for salinity using an Autosal salinometer, whilst three 1-litre surface water samples were filtered through preweighed 0.45pm filter papers for suspended load determination.

236

A.J. Poole et al. / The Science of the Total Environment

f

I68 (1995) 233-247

2 IRISH

/

SEA v

S&ace dissolved

water

aa6 Ra (m Bql-‘)

A 6 c D E F G !Y

LONGITUDE

0

= = = = = = =

1.720.2 2.22 0.1 2.1 f 0.1 2.12 0.3 1.920.2 1.8* 0.3 1.820.1 3O

(V&T)

Fig. 2. (a) Map showing the location of the survey area in the eastern Irish Sea, general surface water circulation pattern and the positions, and concentrations, of background data.

4. Results The concentration of dissolved 226Ra in surface water samples collected during February and May 1992, i.e. when the plant was still processing phosphate ore and crude phosphoric acid, ranged from 46.4 rt 0.8 mBq 1-l close to the pipeline (MAR23) to 2.7 -t 0.1 mBq l-‘, at a site (MAR461 14.4 km west of the outfall (Table 2; Fig. 5a). The average concentration of dissolved 226Ra in the surface waters of the survey area in the period prior to

changes in processing operations at the plant was 10.2 mBq 1-l. The 226Ra data obtained from samples collected during December 1992 show reduced concentrations at all sites when compared with those samples collected prior to changes in the operations at the plant. The largest reduction, 93%, occurred at MAR24, a location 0.6 km south of the pipeline, whilst the smallest reduction, 4%, occurred at MAR46 14.4 km from the pipeline (Table 2; Figs. 5b,6). The maximum 226Ra concen-

A.J. Poole et al. / The Science of the Total Environment

. l MAR 27

IRISH

\\

168 (1995)

231

233-247

MAR 29 0

.

.

\

\ \ \ \

SEA

l MAR 46

Albright and Wilson

MAR 48 l

MAR 67

l

-

3cm. \

\

\

\ \

. MAR 75

\ \

I

\ \

\ \

\

l MAR73

1 ‘, ‘\ \ \

Fig. 2. (b) Detail of survey area, sampling points, bathymetry (m) and the location of the Albright and Wilson outfall. Note: Survey area marked (b) in Fig. 2. (a).

tration during December 1992 was 4.0 mBq 1-l and the average value was 2.9 mBq 1-l (range = 4.0 + 0.2-2.0 ? 0.1 mBq 1-l at stations MAR23 and MAR48, respectively), a 72% reduction on the average value of the samples collected during February and May 1992, i.e. prior to changes in the operations at the plant. The concentrations of dissolved 226Ra in the samples collected during May 1993 do not show a

uniform reduction at all stations, unlike the values obtained during December 1992 (Table 2; Fig. 5~). The maximum concentration of 226Ra during May 1993 was 3.3 + 0.1 mBq 1-r (at station MAR 29) and the minimum value was 1.8 + 0.1 mBq 1-l (at station MAR 67; Table 2; Fig. 6). The average concentration for all stations sampled during this period was 2.3 mBq l-‘, a 21% reduction compared with the value obtained

238

A.J. Poole et al. /The

Science

of the

Total Environment

168 (1995)

233-247

3”40’

Longitude

(West)

Fig. 3. Spatial distribution of seabed sediments - % < 63 pm.

for the December 1992 samples and a 77% reduction compared with samples collected prior to changes in the operations at the plant (Table 2). The concentrations of 226Ra in the samples collected during December 1993 averaged 2.2 mBq 1-l and ranged from 2.7 k 0.4 mBq 1-l to 1.9 + 0.1 mBq 1-l (stations MAR24 and MAR48 and 67, respectively; Table 2; Figs. 5d,6). The average value for samples collected during December 1993 does not show any statistically significant variation from that value obtained during May 1993.

5. Discussion

Concentrations of dissolved 226Ra in the Irish Sea surface water samples collected at locations distant from known industrial inputs of U-series radionuclides range from 1.7 + 0.2 to 2.2 + 0.1 mBq 1-l and have an average value of 2.0 mBq 1-l (taken from data shown in Fig. 2a). These values are well within the range of 1-4 mBq 1-l recorded for dissolved 226Ra concentrations for coastal seas and estuaries elsewhere (Iyengar, 1990). If a concentration of 2.0 mBq 1-i

A.J. Poole et al. / The Science of the Total Environment

239

168 (1995) 233-247

80.0

70.0 I

m

Jun-92

0

kc-92 May-93

60.0 1

m

k-93

-Ma

2 50.0 V

5 40.0 w ‘Z g 30.0 20.0 10.0 0.0 I

I 1

3

I 5

I 6

I

/

8 11 13 15 17 18 19 21 22 23 24 27 29 31 40 46 48 51 53 62 65 67 73 75 Sampling station (MAR)

Fig. 4. Temporal variation of seabed sediments at each of the sampling stations.

2’6Ra is used as a background value, the spatial and temporal variation in 226Ra in the survey area occurring as a consequence of inputs from the Albright and Wilson plant can be discussed. Prior to June 1992 and during December 1992, the whole of the survey area showed enhancement of 226Ra above the background level of 2.0 mBq 1-l. Seventy-eight percent of the 418square km area of the survey showed enhancement above background levels during May 1993 (Fig. 5~). However, there was a large fall in the concentrations of 226Ra in the survey area over the period February-May 1992 to May 1993 as concentrations of 226Ra > 3 mBq 1-l were seen over 98% of the area prior to June 1992, compared with 12% of the area in December 1992 and < 1% of the area in May 1993 and 0% of the area in December 1993 (Table 2; Figs. 5a-d). The fall in concentration of 226Ra across the survey area is not consistent as sites close to the outfall (e.g. MAR 23 and MAR 24) show the largest initial decrease in concentration whilst those fur-

ther away have smaller reductions (e.g. MAR 27 and 46; Table 2; Figs. 6,7). The inventory of 226Ra in the surface water, i.e. the top 1 m of the water column, of the survey area has been calculated for each of the surveys, assuming a water volume of 4.18 X lo9 m3. The nominal background inventory was calculated from the average background concentration of 2.0 mBq 1-l (Table 3). The enhanced inventory decreased from 175% of background prior to June 1992 to 40% in December 1992, 11% by May 1993 and 7% by December 1993. When the inventory values are plotted against time since cessation of large volume discharges and the preJune 1992 data are used to represent time zero, an exponential curve best fits the data (Fig. 8) such that: Y= 1.42

x

exp(-“~olXX) + 0.04

where Y = inventory of 226Ra in excess of background levels in the surface water (GBq); X

Average

4.0 3.3 4.5 3.7 1.9 2.0 1.7 3.5 2.0 1.0 0.3 0.0 0.6 17.0 9.9 5.9 7.4 14.4 9.7 5.2 7.5 11.6 10.2 17.0 14.1 15.7

MAR01 MAR03 MAROS MAR06 MAR08 MAR11 MAR13 MAR15 MAR17 MAR21 MAR22 MAR23 MAR24 MAR27 MAR29 MAR31 MAR40 MAR46 MAR48 MAR51 MAR53 MAR62 MAR65 MAR67 MAR73 MAR75

salinity

Depth Cm)

“hRa,

Distance from PLZ (km)

Table 2. Dissolved

16 20 24 25 18 10 14 23 10 7 7 7 7 28 19 6 23 29 26 8 18 8 24 33 21 30

41.3 52.1 37.9 42.5 17.7 2.0 5.5 39.0 25.7 1.4 2.0 36.8 1.4 26.7 36.3 1.7 32.6 10.5 24.9 2.2 44.3 1.9 35.5 7.3 35.3 68.5 10.2

1.7 1.5 1.5 2.3

5.4

0.4 50.6 0.4 2.5 4.1 0.4 3.5 1.1 4.3

4.0 10.1

21.7

11.4 2.8 2.7

SD.

4.4 5.3 4.1 8.2 5.1 5.9 5.4 7.4 8.5 11.9 35.0 46.4 43.8 3.0 6.4 8.1 6.7 2.7 4.0 8.0 8.2 4.9 5.2 5.9 4.0 7.3

Pre-June 1992

Ra-226

load values

grainsize

% < 63 pm

Average

and suspended

2.9

0.4 0.2 0.1 0.2 0.4 0.3 0.2 0.2 0.5 1.0 1.3 0.8 1.0 0.6 0.1 0.3 1.3 0.1 0.1 0.1 0.5 0.1 0.1 0.1 0.3 1.2

f

(mBq/l) May 93

2.3

0.3 0.3 0.1 0.2

2.1 2.7 2.8 2.9

2.1 1.8 2.2 2.1

0.1 2.5

2.8

2.8 3.1 2.8 2.6 2.0

1.1 2.3 0.2 2.9 0.3 3.1 1.9 0.1 3.3 0.4 2.3 0.3 2.0 0.2 2.0 0.1 2.2

0.2 2.4 0.4 2.2 0.2

1.0 2.5 0.5 2.1 0.3 2.3

?

collected

3.1 4.0 3.0

3.0 2.9 3.1

3.8 2.3 3.2

Dee 92

for samples

Dee 93

0.1 2.5 2.6 2.7 2.0 2.2 2.6 2.6 2.1 1.9

2.2

0.3 0.1 0.1 0.3

2.1 1.9 2.2 2.1

0.2 2.0

2.3 0.1 0.4 0.1 0.1 0.1 0.1 0.1 0.1 0.1

0.4 2.2 0.1 2.3 2.2 0.2

0.3 2.3 0.1 2.0 0.1 2.3

f

during

--___ 0.1 32.00 0.1 32.20 0.4 32.35 32.21 32.15 0.1 31.66 0.1 31.99 31.89 32.10 31.96 0.1 31.98 0.1 31.98 0.4 32.00 0.1 32.75 0.1 30.94 0.1 31.44 0.2 31.42 0.4 32.97 0.1 32.55 32.10 0.1 31.75 31.90 0.2 32.25 0.1 32.34 0.4 32.32 0.1 32.17

f

31.99 32.57 31.94 31.90

31.88

31.22 29.80 31.22 32.05 30.74 30.43 31.91 32.71 31.79

31.27 30.76 31.42

30.36 31.17 31.18

Dee 93 10.34 12.26 3.60 0.19 0.10 7.20 3.51 3.13 8.50 7.83 4.20 5.17 3.97 1.87 2.63 3.57 2.90 1.30 1.50 0.13 4.71 0.31 2.25 2.04 2.17 3.10

Pre-June 1992

Dee 92

+_

5.95 7.62 3.79 2.51 3.61 3.37 2.79 2.19 3.28

0.08 0.44 0.19 0.19 0.05 0.23 0.05 0.05 0.12

3.73 0.10 3.59 0.10 2.93 0.18

0.13 0.19 0.07 0.17

2.76 1.85 2.95 2.69

0.08 0.11 0.07 0.05

0.38 8.78 0.13

0.30 0.24 0.29 0.62 0.12 0.62 0.84 0.71 0.35 0.00 0.24 0.30 0.00 0.20

0.24 3.09 0.10 4.37 4.08 0.05 0.17 2.74 0.05

&

load (mg/l)

1993 and December

Suspended

1992, May

32.55 33.04 32.83 3.42 3.60 32.51 33.00 32.90 33.09 32.53 32.39 32.57 32.33 32.31 32.15 32.52 32.82 33.51 32.44 32.78 32.45 32.72 32.25 32.89 32.83 33.60 32.48 33.13 4.63 32.43 33.08 5.17 32.28 32.93 32.94 33.52 32.44 33.01 32.78 33.15

32.44 32.52 32.51 32.96 33.10 32.44 32.61 32.59

May 93

1992, December

Salinity (%o) ~. .-Pre-June Dee 1992 92

pre-June __

rt

Dee 93

2.93 1.36 2.17 2.89

9.78 0.19

0.17 0.19 0.84 0.34 0.63 0.72 1.45 0.23 0.35 1.04 0.42 1.26 0.17 0.00 0.51 1.25 0.00 0.15 0.05

f

0.10 9.33 0.00 0.04 4.04 0.14 0.24 14.19 0.17 0.39 5.44 0.11

3.83 0.00

4.90 0.14 39.01 4.50 0.24 11.44 3.67 0.24 23.89 15.37 38.85 6.80 0.19 41.07 5.17 0.24 69.48 3.28 0.16 30.04 33.11 35.33 12.92 0.12 24.61 25.42 0.04 31.18 16.14 0.04 27.67 2.31 0.04 6.75 4.92 0.35 21.78 8.87 0.28 37.72 3.92 0.51 19.33 0.71 0.20 4.44 2.90 0.07 8.41

May 93

1993

A.J. Poole et al. /The Science of the Total Enuironment 168 (1995) 233-247

241

:d)

I\

I

I

I 3-40’

\

I/

I

I

I 3-40’

Longitude (West) Fig. 5. Contour maps of the distribution of dissolved *6 Ra in surface waters: (a) prior to June 1992; (b) during December 1992; (c) during May 1993; and Cd) during December 1993. Note: the contour interval in Fig. 5a is 3 mBq 1-l.

242

A.J. Poole et al. /The Science of the Total Environment 168 (1995) 233-247

5.0 0.0 1

3

5

11

13

15

21

22

23

24

29

27

31

40

46

48

53

65

67

73

Station number (MAR) Fig. 6. Temporal changes in the concentration of dissolved 226Ra in surface waters at each survey station.

l

Pm-June 1992

50.0 45.0 --

.

40.0 -s 35.0 ---. ?I ; 30.0 -.o i 25 .0 -8 % 0 20.0 -?i $ 15.0 -. 10.0 --

.

5.0 --

n

:

..

.

.

.

.

.

.

.

.

. 8

860 0.0

,

0.0

89

s8

0

E0

w

8

;ie

8

8

I

I/

/

1I

I

/

I

/I

I

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

Distance from Albright and Wilson outfall (km) Fig. 7 Changes in concentration of total dissolved 226Ra with distance from the discharge outfall during each of the sampling pcriodh

A.J. Poole et al. / The Science of the Total Environment 2.0

r' = 0.00 Y = l.42%xp'Q""x' + 0.04 Half time q 70 days

1.8 1

1.0 1.4 = 1.2 1.0 0.8 0.0 0.4 0.2 0 0

50

100

100

200

Time since discharges

, 200

I 34Q

of phwphogypum

I 350

I 400

, 450

I 500

.

I 550

waste ceased (days)

Fig. 8. Change in inventory of dissolved 226Ra in the surface in excess of background concentrations with time.

waters

= time since discharge of phosphogypsum waste ceased (days). This curve has an associated correlation coefficient (r2) of 0.99. As the half-life of 226Ra is 1620 years, the effective half-time of the activity in the survey area can be calculated directly from the curve and gives a value of 70 days (one significant figure). These data are in reasonable agreement with 13’Cs half-time results, achieved using discharge data from BNFL Sellafield, which were estimated to be 30 days close to the Cumbrian coast and 200 days in the eastern Irish Sea (Jefferies et al., 1982). These data, do not, however, take account of the continuing discharges of 226Ra which contribute to the inventory and continue to account for the elevated concentrations of 226Ra close to the outfall (Table 2; Fig. 5d). The estimated inventory of 226Ra discharged into Saltom Bay from the Albright and Wilson plant during 1993 was 1.2 GBq (Poole et al., 1994b). This value is approximately 6% of the total background inventory of 226Ra in the survey area, assuming that the background concentration of “‘Ra is 2.0 10.1 mBq 1-l and that the total volume of water in the survey area is 1 X 1013 1. The sediments potentially act as another source of 226Ra to the water column. The observed build-up of phosphogypsum waste close to the pipeline (MAR23) sampled in February 1992 had dissipated by December 1992 and the concentration of 226Ra in the sediment had decreased from 680.6 + 25.9 Bq kg-’ to 16.6 + 0.7 Bq kg-’ between February 1992 and May 1993. This reduction may have

168 (1995) 233-247

243

been due to the dispersion of the waste on the seabed or dissolution into the water column. Preliminary interpretation of the concentration of total 226Ra in surface sediment samples collected show that the total inventory of 226Ra in the survey area did not change consistently in the 11 months following cessation of large volume discharges as the estimated inventories calculated for samples collected prior to June 1992 and during May 1993 are very similar, i.e. 0.23 TBq and 0.25 TBq, respectively. These data show that, although there may well have been some local reduction in the surface sediment concentration of 226Ra close to the pipeline which may have been released to the water column, there is no evidence to suggest that this phenomenon has been consistent across the whole of the survey area. There was a clear relationship between the concentration of 226Ra in surface seawater and distance from the Albright and Wilson outfall for the samples collected before June 1992. A drop in concentration of > 30 mBq 1-l is seen within 2 km of the plant, beyond which there appears to be a linear drop in concentration to the most distant sampling locations (Fig. 7). The concentration of dissolved 226Ra at points distant from the plant, assuming a background concentration of 2 mBq l-‘, indicate that the activity has been transported preferentially towards the north, north-north-west, south-west, south-south-west and along the coast to the south and the north (MAR 31, 29, 67, 75, 73 and 62). Furthermore, concentrations at distant points to the west and north-west (MAR 46 and 27), although showing enhancement, do so to a lesser extent (Table 2; Fig. 2b) Although these data take no account of the variable nature of the reduction in concentration of 226Ra at sites intermediate between the outfall and distant locations, they do give a good indication of the overall pattern of dispersion and dilution of 226Ra away from the outfall. This results in the 226Ra in the surface water leaving Saltom Bay and dispersing most rapidly out into the Irish Sea in an arc between south-west and north-west. This finding is in good agreement with 137Cs and nutrient data collected in the Sellafield and St. Bees Head areas which show that

244

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168 (1995) 233-247

A.J. Poole et al. /The

Science of the Total Environment

dispersion of 137Cs, resulting from Sellafield discharges, is greatest to the west, south-west and south, and that the dispersion of phosphate and silicate, which is derived in part from discharges from the Albright and Wilson plant, is greatest to the west and south-west of St Bees Head (Jones and Folkard, 1971; Jefferies et al., 1982). There appears to be no clear relationship between the concentrations of dissolved 226Ra in the surface waters and either surface seabed grainsize or the suspended load concentrations for any of the sampling periods: the concentration of 226Ra was highly variable, but uncorrelated in the period prior to June 1992, and has remained fairly constant and independent of these two variables during December 1992, May 1993 and December 1993 (Fig. 9). There is, however, a inverse linear relationship between salinity and 226Ra concentrations for samples collected during December 1992, May 1993 and December 1993 (Fig.

I68 (1995)

245

233-247

10). This is in general agreement with those 226Ra data from estuarine and continental shelf studies elsewhere which show a similar relationship within the salinity range 20-40%0 (Li et al., 1977; Li and Chan, 1979; Elsinger and Moore, 1980; Moore, 1981; Moore and Todd, 1993). This relationship can arise from desorption of 226Ra from the particulate material as the ionic strength of the water increases. The changes in 226Ra concentration with salinity found elsewhere (ibid.) are not as great as those seen in the eastern Irish Sea (Fig. 10) suggesting that the trend identified here is enhanced as a consequence of the combination of: (i) the continued input of fresh water effluent containing 226Ra that is currently being discharged into Saltom Bay, albeit in much reduced levels, from the Albright and Wilson plant, as shown by a value of 29.8%0 in December 1992 at MAR23 close to the pipeline; and (ii) the increase in salinity of the water offshore, as the influence

100.0 7

* mm .

-

.

.

FTC-June 1992 - SL

0 Dec92-SL

.

hay93-SL

l

0 Dec93-SL ’ Re-June *

. :

-5%

l

May93-GS

0

L&93-GS

I

. :

1992 - GS

A Dec92-GS

.

A A

.

I

I

I

1

I

I

20.0

30.0

40.0

50.0

60.0

70.0

Average grainsize (% < 63um) and suspended load @g/l) Fig. 9. Variation in total dissolved **“Ra in the surface waters with: seabed grainsize (GS), and surface water suspended load (SL), for each of the sampling periods.

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m Dee-92

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q

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l

Dee-93

.

o Otherwork *

0

I

/ I

I I

/ /

I I

t /

i

28.00

30.00

32.00

34.00

36.00

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Salinity (%) Fig. 10. Variation in total dissolved 226Ra in the surface waters with surface salinity for December 1992, May 1993 and December 1993. Note: Other work = Li et al., 1977; Li and Chan, 1979; Elsinger and Moore, 1980; Moore, 1981; Moore and Todd, 1993.

of fresh water running off the land and mixing with the seawater diminishes. 6. Conclusions The variation in the concentration of dissolved 226Ra in the area of Saltom Bay has previously been largely dependent on discharges from the Albright and Wilson Whitehaven phosphoric acid production plant. The estimated inventory of ‘26Ra in the surface waters in the 418-km* area studied here was approximately 175% of background levels whilst the plant was processing phosphate ore rather than crude phosphoric acid; this value had decreased to approximately 7% of the background inventory by December 1993, 17 months after the cessation of phosphate ore processing, giving an environmental half-time in the survey area of approximately 70 days. It should be noted that discharges of 226Ra from the plant during 1993 were approximately equal to 6% of the ‘natural’ background inventory of 226Ra in the survey area; this probably accounts for the

above background levels of 226Ra recorded in December 1993. The variation in the concentration of dissolved 226Ra in this area is independent of variations in surface seabed grainsize or suspended load but is dependent on salinity concentrations and the distance from the Albright and Wilson outfall. The concentrations of dissolved 226Ra in the surface seawater dropped by > 30 mBq 1-l over a distance of 2 km whilst the plant was still discharging large quantities of 226Ra. The activity appears to have been preferentially transported along the coast to the south and north and out into the Irish Sea to the north-north-west and south-west and south-south-west. Acknowledgements We thank the following: Albright and Wilson, and especially Dr Brian Donaldson, for providing data and information pertinent to this paper; Alison Reeve and Jenny Taylor for suspended load and salinity analyses; Dr Den Woodhead and

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