Assessment of 137Cs outspread in the Lithuanian part of the Baltic Sea

477

Assessment of 137Cs outspread in the Lithuanian part of the Baltic Sea L. Davuliene* , N. Tarasiuk, N. Spirkauskaite, G. Trinkunas, L. Valkunas Institute of Physics, LT-02300 Vilnius, Lithuania Abstract Results of analyses of 137 Cs in the Lithuanian part of the Baltic Sea and in the Curonian Lagoon in 1999–2001 are presented. According to these observations, the total 137 Cs activity concentration in Baltic Sea water has been the highest when compared with data for the world ocean. Sediment samples were collected in the Lithuanian part of the Baltic Sea for the first time after the Chernobyl accident. It is shown that the main 137 Cs accumulation zone in the area under consideration is located in the depth below 50-meters isobar. In the coastal water not influenced by the fresh water discharge from the Curonian Lagoon the 137 Cs was found mostly in the soluble form. The 137 Cs activity of the particulate phase was less than 10% of the total 137 Cs activity. Measurements revealed a linear relation between the 137 Cs concentration and salinity in the fresh water and seawater mixing zone. Validation studies of the circulation model for the Lithuanian marine were also carried out. The hydrodynamic model is able to assess spatial variations of 137 Cs concentrations with good accuracy (uncertainty of about 15%). Keywords: 137 Cs, Activity concentration, Fresh water, Seawater, Sediment, Hydrodynamic model, Curonian Lagoon, Baltic Sea

1. Introduction The Baltic Sea is one of the most contaminated seas with anthropogenic 137 Cs (NATO, 1998; Osvath et al., 2001; IAEA, 2005). The origin of this contamination is diverse. It is caused by global fallout as a result of nuclear weapons testing, by releases from nuclear reprocessing plants, by releases from nine nuclear power plants, which use seawater for reactor cooling in countries surrounding the sea, by the river run-off and the fallout right after the Chernobyl Nuclear Power Plant accident in 1986. As a result of the Chernobyl accident the long-lived radionuclide 137 Cs has had the most significant impact on the radioactive contamination of the sea. Measurements of 137 Cs activity concentration in the Baltic Sea near the Lithuanian coast revealed inhomogeneous distributions and remarkable fluctuations depending on meteorolog* Corresponding author. Address: Institute of Physics, Savanoriu 231, LT-02300 Vilnius, Lithuania; phone: (+370) 5 2661640; fax: (+370) 5 2602317; e-mail: [email protected]

RADIOACTIVITY IN THE ENVIRONMENT VOLUME 8 ISSN 1569-4860/DOI 10.1016/S1569-4860(05)08038-1

© 2006 Elsevier Ltd. All rights reserved.

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ical conditions (Styro et al., 2001). However, no correlation between 137 Cs and salinity was found in surface waters of the area. The Nemunas River is the third largest tributary of the Baltic Sea. Its drainage basin covers nearly the whole Lithuanian territory. The Nemunas River first flows into the Curonian Lagoon and then, through the Klaipeda Strait, it flows into the Baltic Sea. The influence of the water exchange between the Curonian Lagoon and the Baltic Sea on the 137 Cs activity distribution in the Lithuanian coastal waters has not been considered yet. Presence of the operating nuclear power reactor in Ignalina, Lithuania, which is similar to the Chernobyl-type RBMK reactor, makes the transport studies of radioactive substances via the Nemunas River in a case of severe accident of great importance. For this reason a circulation model of the North and Baltic Seas (known as BSHcmod) developed in the BSH (Bundesamt fur Seeschiffahrt und Hydrographie, Hamburg, Germany), has been applied to the south-eastern part of the Baltic Sea at the Lithuanian coast (Kleine, 1994; Dick et al., 2001; Davuliene et al., 2002a). This model with higher resolution has been extensively used in the Baltic Sea region, delivering daily forecast of water currents, water temperature and salinity, as well as water level and ice cover parameters, for combating the oil spill problems. It is the three-dimensional model of the Lithuanian marine waters covering both the Lithuanian part of the Baltic Sea and the Curonian Lagoon. This is also important for modelling the water exchange through the Klaipeda Strait. The applicability of this model to describe the distribution of 137 Cs, which is found in both soluble and particulate forms in the Lithuanian marine waters, is discussed in this paper. The model after successful validation can be used for the analysis of the heterogeneous distribution of 137 Cs in the Lithuanian waters, as well as for the assessment of the Nemunas River influence on the Lithuanian marine waters. In the years 1999–2000 and 2001–2002 two projects “Assessment of radionuclide migration in the Lithuanian part of the Baltic Sea environment” and “Assessment of radionuclide migration in the Lithuanian part of the Baltic Sea and Curonian Lagoon” were implemented. This paper will present the main outcomes of the projects concerning assessment of the 137 Cs outspread in the Lithuanian part of the Baltic Sea and in the Curonian Lagoon. Results of analyses of 137 Cs in the Lithuanian part of the Baltic Sea during 1999–2001 will also be presented. These data were used for validation of the hydrodynamic model of the Lithuanian marine waters using 137 Cs as a passive tracer.

2. Materials and methods 2.1. Study site The Curonian Lagoon represents a fresh-water shallow basin separated by the Curonian Spit from the Baltic Sea (Fig. 1). It is a transitory water basin collecting the run-off waters from the Nemunas (98%) and other minor rivers. Annually about 24 km3 of water from the Curonian Lagoon is transmitted into the sea. The interconnection between the Baltic Sea and the Curonian Lagoon occurs through the narrow Klaipeda Strait, which is artificially deepened to 14 m. Seawater is mostly observed in the northern part of the Curonian Lagoon. However, in some rare cases it penetrates 40 km into the lagoon, reaching its central part (Dubra and Dubra, 1998).

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Fig. 1. Location of sampling stations in the Baltic Sea and the Curonian Lagoon during investigations in 1999–2000 and 2001 (black rectangles).

The seawater mixes with fresh water of the Curonian Lagoon in the upper layer, less than 5–7 meters deep. Fresh water is mostly carried in the N-NW direction and can be traced as far as 7–10 nautical miles (nm) with salinity decreasing to 3h at the Klaipeda Strait (Dubra and Dubra, 1998). In the deeper waters (>60 m) of the study area, i.e. in the open sea, the salinity in the water column above thermocline is nearly uniform and reaches 7.5h. It should be admitted that the study area covering the sea is quite shallow, 30 m depth in average. Therefore, the formation of the sea-bottom profile and the structure of sedimentation material are highly affected by waves. The sedimentation material at the Lithuanian coast is mainly composed of sediments transported from the Curonian Lagoon, from the Sambian peninsula with along-shore current and of the corrosion products of submerged moraine deposit outcrops, accumulated mainly to the north from the Klaipeda Strait (Gavrilov et al., 1990). Coarse sand prevails near the coast. While the moraine loam, silt and alevrites prevail in deeper places and between boulders, in canyons and pits. The sea-bottom profile of shallow coastal water changes remarkably during storms. The transfer of resuspended sedimentation material offshore takes place, resulting in formation of radionuclide accumulation zones in bottom sediments in the deeper sea places. The Curonian Lagoon is a shallow water basin with the mean depth of 3.8 m. Therefore, sediments are periodically resuspended and mixed due to the wind-induced waves and water currents. The measurement data show variations in 137 Cs concentrations, characteristic for the whole Curonian Lagoon. They are site-dependent due to differences in sediment compo-

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sition and resuspension intensity (Tarasiuk et al., 1995). The 137 Cs concentration in the whole Curonian Lagoon depends on the seasonal and annual variations of the Nemunas River discharge and water exchange between the Curonian Lagoon and the Baltic Sea, as well as on meteorological conditions. Several 137 Cs accumulation zones in the Lithuanian marine waters can be indicated. 137 Cs accumulation zones of anthropogenic origin in the Baltic Sea at Lithuanian coast have developed near the deep-water pipe-lines. Here the coagulation process is important due to enforced interactions of radionuclides in seawater with the drainage water or the industrial water rich of organic substances (Tarasiuk and Spirkauskaite, 1994). 137 Cs accumulation zones of anthropogenic origin are also found in the Klaipeda Strait and the Curonian Lagoon (Tarasiuk et al., 1995; Lujaniene et al., 2005). According to the measurements, the Klaipeda Strait can also be indicated as a considerable 137 Cs accumulation zone. Due to continuous dredging the sediment distribution in the strait is changing. The dredged sediment material is brought to the dumping place in the vicinity of station No. 20a in the Baltic Sea (Fig. 2). The sedimentary material transport studies were done by Galkus and Jokšas (1997).

Fig. 2. Topography of the modelling area of the Baltic Sea and the measured 137 Cs massic activities in sediments.

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137 Cs

in seawater is found both in soluble and particulate forms. Suspended matter binds particle reactive radionuclides on their way from the Nemunas River mouth to the Curonian Lagoon, and in the coastal zone of the Baltic Sea. The first zone is the Curonian Lagoon itself which contains large amounts of organic (phytoplankton, algae, fish products, etc.) materials, that accumulate radiocaesium. The second zone is the fresh–seawater mixing area in the Baltic Sea coastal zone. 2.2. Experiment Investigations of the distribution of 137 Cs in the Lithuanian part of the Baltic Sea were accomplished in 1999–2001 (Tables 1–3). Water and bottom sediment samples were collected at the national and Helsinki Commission (HELCOM) monitoring stations in the Baltic Sea and the Curonian Lagoon. Measurements were carried out at the background stations in Preila and Juodkrant˙e during the campaigns on board of the R/V “V˙ejas” (at the sea) and of the cutter “Gintaras” (in the lagoon). In 2001, measurements were carried out at stations located on the 20 × 10 grid (Fig. 1). In Preila and Juodkrant˙e water samples were also collected on the beach. 3. Results 3.1. Seawater According to the field data obtained in May 1999, the 137 Cs activity concentration in surface water of the open sea was almost uniform. The measured 137 Cs concentration was 73– 79 Bq/m3 . In October 2001 it decreased to 60–67 Bq/m3 , i.e. about 20% (Fig. 3). The lowest 137 Cs concentrations in the sea, 39–53 Bq/m3 , were found in the samples collected at stations No. 4, 4a and 16, which are located in the close vicinity of the Klaipeda Strait. Thus mixing of the fresh and seawater is substantial. In May 1999 the highest 137 Cs concentrations (75–98 Bq/m3 ) were observed at the Preila background station. However, in October 2001 no increase was observed (Tables 1 and 2). The 137 Cs activity concentrations measured in the Baltic Sea were linearly correlated with the measured salinity of water (Fig. 4). In 1999, the increase in the salinity up to the mean value for the open sea surface water (about 7h) was followed by the increase in the 137 Cs activity concentration to 82 Bq/m3 , and in 2001 reaching up to 66 Bq/m3 . The correlation coefficients between the measured activity concentration and the salinity for the two periods were 0.94 and 0.90. During the field campaign in May 1999, the total 137 Cs activity concentration was measured. The activity concentration of 137 Cs bound with suspended matter varied from 0.9 to 9.7 Bq/m3 in the coastal zone of the Baltic Sea (Table 1). It comprised up to 10% of the total 137 Cs activity. The 137 Cs concentration in the open waters of the Baltic Sea was below the detection limit (Tables 1 and 2). Large variations in 137 Cs activity concentrations were observed in the northern and central parts of the Curonian Lagoon, including the Klaipeda Strait, during the storm in October 1999. The total 137 Cs activity concentration varied from 8 to 74 Bq/m3 with the highest values observed at the Klaipeda Strait, and then decreasing towards the central part of the Curonian

Date

Coordinates N

E

1999.05.17 1999.05.17

43 46a

56◦ 42 56◦ 03

19◦ 52 19◦ 24

1999.05.15

1b

56◦ 02

20◦ 50

1999.05.15

65

55◦ 53

20◦ 20

1999.05.15

64

55◦ 46

20◦ 54

1999.05.15

16

55◦ 45

21◦ 02

1999.05.15

4

55◦ 44

21◦ 03

1999.05.16

6b

55◦ 31

20◦ 34

1999.05.11 1999.05.13 1999.05.15 1999.05.17 1999.05.19 1999.05.21 1999.05.18 1999.10.27 2000.06.12 2000.06.12 2000.06.16 2000.06.16

Preila Preila Preila Preila Preila Preila 7 Preila Preila Preila Juodkrant˙e Juodkrant˙e

55◦ 21 55◦ 21 55◦ 21 55◦ 21 55◦ 21 55◦ 21 55◦ 19 55◦ 21 55◦ 21 55◦ 21 55◦ 32 55◦ 32

21◦ 01 21◦ 01 21◦ 01 21◦ 01 21◦ 01 21◦ 01 20◦ 57 21◦ 01 21◦ 01 21◦ 01 21◦ 06 21◦ 06

Activity concentration1 (Bq/m3 )

Sampling depth (m)

Volume (L)

Total

Dissolved

Particulate

Particulate loading1 (mg/L)

Surface Surface 73 Surface 25 Surface 47 Surface 30 Surface 14 Surface 12 Surface 65 Surface2 Surface2 Surface2 Surface2 Surface2 Surface2 Surface Surface2 Surface2 Surface2 Surface2 Surface2

50 50 25 50 30 50 30 50 30 50 30 50 30 100 40 50 47 52 50 50 50 50 50 50 50 50 50

79 ± 6 76 ± 7 77 ± 6 62 ± 6 69 ± 8 73 ± 2 67 ± 6 63 ± 6 72 ± 6 44 ± 4 59 ± 6 41 ± 3 52 ± 5 62 ± 3 73 ± 5 77 ± 6 98 ± 8 85 ± 5 82 ± 8 75 ± 7 84 ± 6 80 ± 6 89 ± 6 58 ± 3 59 ± 3 69 ± 3 63 ± 3

79 ± 6 76 ± 7 77 ± 6 62 ± 6 69 ± 8 73 ± 2 67 ± 6 63 ± 6 72 ± 6 44 ± 4 59 ± 6 41 ± 3 52 ± 5 62 ± 3 73 ± 5 71 ± 6 88 ± 8 84 ± 5 78 ± 8 73 ± 7 79 ± 6 80 ± 6 81 ± 5 56 ± 3 57 ± 3 67 ± 3 61 ± 3

b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. 0.2 ± 0.1 b.d.l. 6.4 ± 0.7 9.7 ± 1.1 0.9 ± 0.3 3.6 ± 0.7 1.8 ± 0.4 4.9 ± 0.6 b.d.l. 8.0 ± 1.2 2.0 ± 0.2 2.0 ± 0.2 2.0 ± 0.2 2.0 ± 0.2

0.08 0.06 – 0.18 0.07 0.13 0.06 0.1 0.05 0.39 0.12 0.55 0.17 – – 1.61 2.17 3.21 1.14 1.12 3.89 0.1 22.6 13.8 15.2 – –

Notes: b.d.l. – below detection limits. 1 Radiochemical analyses and estimation of 137 Cs activity concentration by dr. Galina Lujaniene (Institute of Physics, Lithuania). 2 From the beach.

Massic activity in bottom sediments1 (Bq/kg)

– – 86 ± 5 – 12 ± 1 21 ± 2 64 ± 3

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Station, No.

482

Table 1 137 Cs activities of water and sediment samples collected in the Lithuanian part of the Baltic Sea (1999–2000)

Table 2 137 Cs activities of water and sediment samples collected in the Lithuanian part of the Baltic Sea in October 2001 Date

N

E

2001.10.16

4

55◦ 50

21◦ 00

2001.10.16

5

55◦ 50

20◦ 40

2001.10.16

6

55◦ 50

20◦ 20

2001.10.16 2001.10.16

5I 4a

55◦ 46 55◦ 44

20◦ 56 21◦ 03

2001.10.16

7

55◦ 40

21◦ 00

2001.10.16

8

55◦ 40

20◦ 40

2001.10.16

9

55◦ 40

20◦ 20

2001.10.16

20a

55◦ 39

20◦ 44

2001.10.16

10

55◦ 30

21◦ 00

2001.10.16

11

55◦ 29

20◦ 40

2001.10.16

12

55◦ 30

20◦ 20

2001.10.16 2001.10.16

12a (I) 12a(II)

55◦ 37 55◦ 37

20◦ 20 20◦ 24

Sampling depth (m)

Volume (L)

Activity concentration (solube fraction) (Bq/m3 )

Surface 18 Surface 37 Surface 20 45 29 Surface 15 Surface 26 Surface 20 45 Surface 20 40 55 Surface 20 42 Surface 25 Surface 59 Surface 20 47 74 74

55 47 53 51 52 53 49.5 – 53 53 53 55 52 30 56 56 22 30 56 53 30 51 52 53 54 30 53 26 49 – –

54 ± 2 60 ± 3 61 ± 3 60 ± 3 66 ± 3 62 ± 3 61 ± 3 – 39 ± 2 55 ± 2 63 ± 4 62 ± 3 62 ± 3 61 ± 3 59 ± 4 64 ± 3 67 ± 3 62 ± 3 59 ± 3 60 ± 4 62 ± 3 64 ± 3 60 ± 3 62 ± 3 60 ± 3 64 ± 3 61 ± 3 66 ± 3 65 ± 3 – –

Sediment sample thickness (cm)

Massic activity in bottom sediments (Bq/kg)

137 Cs load

(Bq/m2 )

0.8

19 ± 1

0.9

36 ± 1

0.9 0.8

90 ± 1 20 ± 1

859 253

0.9

14 ± 1

203

0.9

23 ± 1

314

0.9

31 ± 1

400

2.4

175 ± 2

1377

0.9

22 ± 1

333

0.9

265

57

– 1171 609

483

Coordinates

Assessment of 137 Cs outspread in the Lithuanian part of the Baltic Sea

Station, No.

4.43

4.0 ± 0.2

0.5

18 ± 1

– 2.6 1.3

– 171 ± 5 202 ± 5

–

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(a)

(b)

Fig. 3. Measured 137 Cs activity concentration and salinity along with the simulated salinity distribution (a) on 15–16 May 1999 and (b) on 16–17 October 2001. (Salinity in the Lagoon is less than 3.5h.)

Fig. 4. Relationship between the salinity and 137 Cs activity concentration.

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Lagoon (Table 3). The westerly and northwesterly wind of 12 m/s resulted in seawater flow into the Curonian Lagoon. It should be noted that a considerable variation of the 137 Cs activities in the Curonian Lagoon was found for water-soluble and particulate forms during this period, ranging from 7 to 67 Bq/m3 for water-soluble, and from 0.7 to 20 Bq/m3 for particulate caesium (Table 3). The percentage of the particulate 137 Cs in the Curonian Lagoon water samples varied from 4 to 27% of the total activity. During the field campaign in the Curonian Lagoon on 13–18 June 2000 the meteorological conditions were rather calm with dominant southeasterly wind of 5 m/s. The total 137 Cs activity concentration measured in the central part of the Curonian Lagoon ranged from 1.1 to 2.3 Bq/m3 . It should be noted that a considerable variation of the 137 Cs activities in the Curonian Lagoon was found for caesium in the particulate form. The percentage of the particulate 137 Cs reached 87% of the total activity (Table 3, Nida station). Variations of the particulate loading showed only a slight correlation with the particulate 137 Cs activity concentration measured in the Curonian Lagoon. During the stormy and calm weather periods, the particulate loading in the northern and central parts of the Curonian Lagoon was about 35 mg/L. This is considerably higher particulate loading as compared with the measured in the coastal zone (22.6 mg/L, Preila station), and in the open sea (less than 0.1 mg/L, stations No. 43, 46a, Tables 1 and 3). The particulate loading in the Lithuanian coastal zone varied from 0.06 to 0.55 mg/L because of the influence of the fresh water outflow from the Curonian Lagoon. It is noteworthy that the particulate loading in the Klaipeda Strait during the stormy period in October 1999 reached 102.2 mg/L in the near-bottom water layer. 3.2. Sediment Field data on the 137 Cs massic activity of sea sediments showed scarce zones of 137 Cs accumulation in the eastern part of the Baltic Sea. The sea bottom was mainly covered with different types of sand – from the coarse type to alevritic (densities vary from 1.73 to 1.37 g/cm3 ), or sand with silt admixtures (station No. 6, depth 45 m, density 0.91 g/cm3 ). In the latter case, the 137 Cs massic activity of surface sediments was rather high, up to 90 Bq/kg dry weight (d.w.) (Table 2). At the Lithuanian coast of the Baltic Sea the lowest 137 Cs activity (8–21 Bq/kg) was observed in sandy sediment sampled in shallow water right down to the 5-metre isobar. The fine fraction of suspended particles in sediments increases with increasing water depth (15–25 m), and therefore a gradual increase in the 137 Cs activity in sediments is expected. Generally, the 137 Cs activity in the bottom sediments varied from 4 to 90 Bq/kg d.w. (Tables 1 and 2). The main accumulation zone of 137 Cs in the area under investigation was found in the region of the old bed of the Nemunas River (stations No. 9, 12a, Table 2). Vertical 137 Cs profiles had maximum (175–202 Bq/kg d.w.) in surface layers of the sediment cores, and showed decreasing activity with the sediment depth. 137 Cs loads in these soft sediments varied in the range of 5120–6180 Bq/m2 . The 137 Cs massic activities in the bottom sediments taken in the Curonian Lagoon varied from 0.7 to 90 Bq/kg. The high 137 Cs activity (150 Bq/kg d.w.) was characteristic for black silt sediments found in the Klaipeda port (station No. 2a, at the Klaipeda town sewerage outlet (Fig. 1, Table 3). However, the 137 Cs activities in the sediments near the fairway (Klaipeda port, station No. 3) decreased to 22 Bq/kg d.w. A comparatively high 137 Cs activity in bottom sediments measured in the vicinity of the settlements in the central part of the Lagoon (in

486

Table 3 137 Cs activities of water and sediment samples collected in the Curonian Lagoon (1999–2000)∗ Date

Total

Dissolved

Particulate

21◦ 17 21◦ 17 21◦ 01 21◦ 01 21◦ 08 21◦ 08 21◦ 09 21◦ 09 21◦ 08 21◦ 06 21◦ 06 21◦ 01 21◦ 01 21◦ 04 21◦ 04 21◦ 09 21◦ 09

Surface 1.5 Surface 3.5 Surface 2.0 Surface 2.0 13.0 Surface 12.0 Surface 4.0 Surface 4.0 Surface 8.0

100 – 100 – 100 – 100 – – 100 58 100 – 150 – 100 –

8±1 – 23 ± 2 – 15 ± 2 – 74 ± 7 – – 52 ± 5 74 ± 7 2.3 ± 0.4 – 1.1 ± 0.2 – 53 ± 2 –

7±1 – 16 ± 1 – 11 ± 1 – 67 ± 6 – – 46 ± 4 54 ± 5 0.3 ± 0.1 – 0.5 ± 0.1 – 52 ± 2 –

0.7 ± 0.1 – 7±1 – 4±1 – 7±1 – – 6±1 20 ± 2 2.0 ± 0.3 – 0.6 ± 0.1

Coordinates N

Nemunas River mouth, 12 Nemunas River mouth, 12 Nida Nida 8 8 Klaipeda port, 3 Klaipeda port, 3 Klaipeda port, 2 Klaipeda port, 1 Klaipeda port, 1 Nida Nida Preila Preila Klaipeda port, 3 Klaipeda port, 3

55◦ 20 55◦ 20 55◦ 19 55◦ 19 55◦ 25 55◦ 25 55◦ 39 55◦ 39 55◦ 42 55◦ 44 55◦ 44 55◦ 19 55◦ 19 55◦ 21 55◦ 21 55◦ 44 55◦ 44

∗ Radiochemical analyses and estimation of 137 Cs activity concentration by Dr. Galina Lujaniene.

1.0 ± 0.2 –

Particulate loading (mg/L) 4.6 – 32.9 – 32.6 – 31.6 – – 48.1 102.0 37.9 – 36.7 – 9.00 –

Massic activity in bottom sediments (Bq/kg) – 0.7 ± 0.1 – 58 ± 6 – 47 ± 3 – 22 ± 1 67 ± 6 – 30 ± 3 – 90 ± 6 – 71 ± 7 – 150 ± 11

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1999.10.27 1999.10.27 1999.10.27 1999.10.27 1999.10.27 1999.10.27 1999.10.27 1999.10.27 1999.10.27 1999.10.27 1999.10.27 2000.06.13 2000.06.13 2000.06.14 2000.06.14 2000.06.18 2000.06.18

Volume (L)

Activity concentration (Bq/m3 )

E

Sampling depth (m)

Sampling station, No.

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Preila – up to 71 Bq/kg d.w.) appeared to be related to the local zones of the anthropogenic organic contamination. The thick layer of bottom sediments of black silt was also taken in the vicinity of Nida. The 137 Cs activity in sediments varied from 58 to 90 Bq/kg d.w. (Table 3). 4. Model 4.1. Model implementation A detailed description of the BSH model is given by Kleine (1994) and Dick et al. (2001), and will not be presented here. The adapted circulation model for the Baltic Sea (Davuliene et al., 2002a) was used for calculating currents in shallow surface waters influenced by bottom topography, wind and river discharge. The model enables to gain information about the water level, water temperature and salinity, as well as about dispersion of passive substances in a real time. The area selected for modelling (with the upper left corner at 56◦ 20 45 N, 19◦ 55 25 E) covered the Lithuanian part of the Baltic Sea including the Curonian Lagoon, part of Latvian and Kaliningrad (Russia) coasts (Fig. 2). The 1 nm grid (or 1 × 1 40 ) was chosen and the water column was divided into five layers. Only one river input (the Nemunas River splitting into two branches called Atmata and Skirvyt˙e) was taken into account. For the western and northern open boundaries the BSHcmod simulation data were used. The applicability of the circulation model to simulate the 137 Cs activity concentration distribution as a passive tracer in the Baltic Sea near the Lithuanian coast should also be examined before applying the model to the case under consideration. 4.2. Validation of the hydrodynamic model As already discussed in the experimental part, the experimental data show a linear relation between the salinity and the 137 Cs activity concentration (Fig. 4). The salinity and the passive tracer distribution for the periods of expeditions were simulated in order to assess the applicability of the circulation model to simulate the 137 Cs activity concentration distribution in the Lithuanian marine waters (Fig. 3). The standard deviation between simulated and measured salinity for the two periods in 1999 and 2001 are 0.34 and 0.19h, respectively. This comprises about 10% of the salinity variation (about 3.5h) in the surface water of the area. The 137 Cs activity concentration can be calculated from the measured or simulated salinity data using the linear relationship. Correlation between the simulated distribution of the tracer and measured 137 Cs activity concentration during the periods of field campaigns was 0.93 and 0.96, with standard deviations of 6 and 2 Bq/m3 , respectively (Fig. 5). This indicates the applicability of the hydrodynamic model of Lithuanian marine waters to simulate the 137 Cs activity concentration within the error gap of about 15%. 4.3. Modelling results The mean value of the 137 Cs activity concentration in the Nemunas River mouth water is less than 2 Bq/m3 (Tarasiuk et al., 1999), therefore this is a diminishing factor for the 137 Cs

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(a)

(b)

Fig. 5. Measured and simulated 137 Cs activity concentration (AC) (a) on 15–16 May 1999 and (b) on 16–17 October 2001.

activity concentration in the southeastern part of the Baltic Sea coastal zone. The observed fluctuation of the 137 Cs activity concentration could be clearly attributed to the distribution of fresh water along the Lithuanian coast (Fig. 3). The fresh water flow in the Lithuanian coastal waters can be well followed due to the salinity decrease. In the reduced salinity zone the relation between the 137 Cs activity concentration and the salinity is most appropriate. The data sets of the measured 137 Cs activity concentration during the two field campaigns show different data scattering. As follows from the simulations of these two periods, the variation in the 137 Cs activity concentration could be explained by the fresh water distribution in the coastal zone. Several days before the field campaign on 11–14 May 1999 the east-southeast wind was dominating. The fresh water from the Curonian Lagoon was spreading along the Lithuanian coast in the northern direction from the Klaipeda Strait. However, during the field campaign the wind changed to the north-northwest. Because of that the fresh water spreading from the Curonian Lagoon was moving in the southwest direction from the Klaipeda Strait. This caused a decrease in the salinity followed by a considerable decrease in the 137 Cs activity concentration in the large area of the Lithuanian coast of the Baltic Sea during the period of 15–16 May 1999 (Fig. 3(a)). However, this was not the case during the field campaign on 16–17 October 2001. Due to the strong (up to 15 m/s) westerly wind dominating from 11 October 2001, the affluent evolved at the Lithuanian coast and the water outflow through the Klaipeda Strait was nearly stemmed. Although during the field campaign the wind direction changed (south-easterly) and the water outflow through the Klaipeda Strait was resumed, the measured salinity as well as variation of the 137 Cs activity concentration at the Lithuanian coast were rather weak (Fig. 3(b)). The simulations of the salinity distribution for the period of 1999–2001 have shown that the fresh water flow from the Curonian Lagoon modifies the salinity in the Lithuanian coast of the Baltic Sea in the area of 15 km from the seashore (Davuliene et al., 2002b). Because of dominating northerly and south-easterly winds at the Lithuanian coast, the obtained averaged simulated salinity distribution has the most reduced salinity values along the seashore to the

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north of the Klaipeda Strait. The simulated variability of the salinity was found negligible in the area approximately 30 km off the coast. In the open sea the measured salinity and the 137 Cs activity concentration can be considered as constant. The salinity in this area has a constant value down to the halocline. The measured 137 Cs activity concentration is fluctuating around its mean value in the range of uncertainties. Therefore, the determined values could be used for the model calibration as the background values, typical for the south-eastern Baltic Sea.

5. Conclusions The measured total 137 Cs activity concentration in the open Baltic Sea is determined by the self-cleaning process, which still persists. At present the 137 Cs activity concentration in the water of the southeastern Baltic Sea reaches 60–70 Bq/m3 , that is together with Irish Sea waters the highest concentration found in the world ocean (IAEA, 2005). Although in previous studies no correlation between the 137 Cs activity concentration and the salinity was found in the Lithuanian marine waters (Styro et al., 2001), our measurements revealed the linear relationship in the mixing zone of fresh water and seawater. The coefficient of proportionality between the 137 Cs concentration and the salinity depends on the average 137 Cs concentration in the Baltic Sea. The applied hydrodynamic model enabled to simulate the salinity distribution in Lithuanian waters and to evaluate the extent of the mixing zone. Measurements carried out in the Lithuanian part of the Baltic Sea and in the Curonian Lagoon in different seasons indicated that the 137 Cs concentration in the water depended on the hydrological situation, as well as on the water exchange between the sea and the lagoon. The observed fluctuations in the 137 Cs concentration in the Lithuanian part of the Baltic Sea could be clearly attributed to the distribution of the fresh water along the Lithuanian coast (reliability uncertainty of about 15%). The measured 137 Cs activity concentration of the surface water in the central part of the Baltic Sea (stations No. 46a and 43) in spring 1999 was on average by 5 Bq/m3 higher than at the Lithuanian coast (the offshore station No. 65). The caesium was found mostly in the soluble form in the open sea, and in coastal waters far from the fresh water and seawater mixing zone. The particulate 137 Cs activity concentration in the coastal water was less than 10% of the total 137 Cs activity concentration. The sediment samples were taken for the first time in open waters of the Lithuanian part of the Baltic Sea after the Chernobyl accident. According to the experimental data, the main 137 Cs accumulation zone in the area under consideration is located in the region of the old bed of the Nemunas River (below 50-meters isobar), which is in the reduced hydrodynamic activity zone below the thermocline (Fig. 2). The 137 Cs massic activity here is about 10 times higher than at the coast. The 137 Cs activity of particulate matter in the northern and central part of the Curonian Lagoon might reach during the period of calm weather 90% of the total activity. However, as the total 137 Cs activity concentration in the fresh water of the Curonian Lagoon is less than 5% of the total 137 Cs activity in the open sea, this fact is of minor importance when modelling the 137 Cs distribution in the Lithuanian marine waters. During the stormy period with the dominating northwesterly and westerly winds, the salt water intrusion into the Curonian Lagoon can occur. This is more often observed during the

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autumn season (Dubra and Dubra, 1998). The influence of this process on the salinity and on the 137 Cs distribution in the Curonian Lagoon has not been studied in detail so far. During the study period in October 1999 the 137 Cs activity concentration in the northern part of the Lagoon was measured during the intrusion of salt water. During this event, the total 137 Cs concentration in the northern part of the Curonian Lagoon increased remarkably and varied in a wide range. The largest values characteristic for the open sea were measured in the Klaipeda Strait. The particulate 137 Cs activity measured in the surface water of the Curonian Lagoon reached about 6 Bq/m3 and was nearly 5 times higher compared to the total 137 Cs activity observed during the period of calm weather. The resuspension may become of crucial importance for the total 137 Cs concentration during period of stormy weather. However, the contribution of all 137 Cs accumulation zones located in Lithuanian marine waters to the total 137 Cs concentration (depending on the meteorological situation) has not been studied yet. This would be the considerable task for the application of the sediment transport model. The high correlation found between the simulated and measured 137 Cs concentrations in the Lithuanian part of the Baltic Sea shows that the hydrodynamic model is able to assess spatial variations of 137 Cs concentration with good accuracy (uncertainty of about 15%). The variability in salinity was found negligible in the area approximately 30 km off the coast. Therefore, the 137 Cs concentrations measured at this distance could be used for the model calibration as the background value, typical for the south-eastern part of the Baltic Sea.

Acknowledgements This research was partly supported by the IAEA projects “Assessment of radionuclide migration in the Lithuanian part of the Baltic Sea environment”, LIT/2/002 (1999–2000), “Assessment of radionuclide migration in the Lithuanian part of the Baltic Sea and Curonian Lagoon”, LIT/7/002 (2001–2002), and by the Lithuanian State Science and Studies Foundation.

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