The Europaen Commission The Commission on the Protection of the Black Sea Against Pollution
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Report Contents

Preface Chapter 1A Chapter 1B Chapter 2 Chapter 3 Chapter 4 Chapter 5 Chapter 6 Chapter 7 Chapter 8 Chapter 9 Chapter 10 Chapter 11 Chapter 12
List of Tables List of Figures

State of Environment Report 2001 - 2006/7

Chief Editor, Prof. Dr. Temel Oguz, Institute of Marine Sciences, Middle East Technical University, Erdemli, Turkey

Chapter 4 - State of Radioactive Pollution

State of the Environment of the Black Sea - 2009

CHAPTER 4 THE STATE OF RADIOACTIVE POLLUTION

4.1. Introduction

Due to its geographical location and limited water exchange with the rest of the World Ocean, the Black Sea has been one of the marine basins most contaminated with artificial radioactivity. Anthropogenic radionuclides originated primarily from two sources: the large-scale atmospheric nuclear weapons tests carried out before 1963 and the Chernobyl Nuclear Power Plant accident in April 1986 (Buesseler and Livingston, 1996). The Black Sea received relatively high levels of atmospheric deposition from nuclear weapons testing, the global fallout reaching its maximum in the 40 o -50 o N latitude band (UNSCEAR, 2000), which runs across the Black Sea. The Chernobyl accident further led to direct contamination by fallout on the sea-surface. Secondary contributions from the deposition of radionuclides released to the atmosphere on the drainage basin entered the sea through river discharges, principally through the Danube and Dnieper Rivers (Livingston et al., 1988; Polikarpov et al., 1992). 137 Cs and 90 Sr are the most significant radionuclides reaching the Black Sea from these sources, due to their inventories, half-life and dosimetry. Additional relatively long-lived (e.g. Pu isotopes, 241 Am) or short-lived radionuclides (e.g. 95 Zr/ 95 Nb, 103 Ru, 106 Ru, 110m Ag, 125 Sb, 131 I, 134 Cs, 140 Ba/ 140 La, 141 Ce, 144 Ce) have been reported and were traceable to the above-mentioned sources. There is no official record and no environmental evidence of radioactive waste dumping into the Black Sea (IAEA, 1999) as an additional pollution source. Monitoring of 137 Cs and 90 Sr in water, sediment and biota and significant research on marine radioactivity were carried out in the Black Sea since the early 1960s. In the period 1986-2005, within the framework of various international and national field campaigns and monitoring programmes, the Black Sea riparian countries collaborated in numerous studies. The present chapter provides an overview of these studies and assesses recent levels of radioactive pollution.

4.2. Concentrations and inventories of radionuclides in the water column

The regionally-averaged vertical profiles of 137 Cs and 90 Sr concentrations for the central deep basin (Fig. 4.1) indicate a three layer structure formed by high-concentrations in the surface mixed layer, decreasing concentrations in the gradient layer, and low concentrations in the deep layer underneath. This structure has evolved by the progressive penetration of radionuclides to greater depths and the decrease in surface concentrations.

The atmospheric fallout deposited on the surface of the entire Black Sea during the first days of May 1986 was estimated at 1700-2400 TBq of 137 Cs, which corresponded to nearly 2% of the total 137 Cs release into the environment following the Chernobyl NPP accident (Nikitin et al., 1988; Polikarpov et al., 1991; Egorov et al., 1993). Shortly after the accident, the 137 Cs inventory in the surface 0-50 m layer reached 2700 TBq, exceeding its pre-Chernobyl value by a factor of 6-10. This inventory decreased abruptly to 1600 TBq in 1987 and then more gradually to around 500-600 TBq in 1998-2000 and 350?60 TBq in 2001-2004 (Fig. 4.2). The 137 Cs input of 26 TBq from the Danube and the Dnieper Rivers over the period 1986-2000 was negligible in comparison with the direct contribution of atmospheric fallout (Voitsekhovych, 2001). The outflow of 137 Cs through the Bosporus Strait was 250 TBq over the period 1986-2000 (Egorov et al., 2002, 2005).

Fig. 4.2. Temporal evolution of the 137 Cs inventory in the 0-50 m water layer of the Black Sea after the Chernobyl NPP accident: estimates based on measured water concentrations (circles) and modeling (solid and dashed lines) (Egorov et al., 1993). A corresponding average environmental half-life of 5-7 y can be estimated for 137 Cs in surface waters.

The contribution of Chernobyl-origin 90 Sr deposition on the sea-surface was estimated to be 100-300 TBq, which resulted in a rapid increase of concentrations in the surface mixed layer (Egorov et al., 1999). The Dnieper and Danube Rivers added around 160 TBq of 90 Sr into the sea during 1986-2000, comparable in magnitude to the amount introduced by fallout after the Chernobyl NPP accident (Voitsekhovych, 2001). The 90 Sr outflow through the Bosphorus was 110 TBq over the period 1986-2000 (Egorov et al., 2002, 2005). Estimates given by Egorov et al. (2006) indicate a 90 Sr inventory of 1770?790 TBq in the waters of the Black Sea in 1998-2000, 20% of which are attributable to the Chernobyl NPP accidental release, the majority of the 90Sr inventory being contributed by global fallout.. An area of particular interest due to secondary releases through the Dnieper from flooding of contaminated areas in the vicinity of the Chernobyl NPP, constituting an additional regional contamination source for the Black Sea, is the Dnieper-Bug estuary area. NW Black Sea annual mean surface concentrations of 16-21 Bq m -3 90 Sr were reported for 2001-2005, representing an increase as compared to the 1998-2000 annual means of between 10-15 Bq m -3 90 Sr (Egorov et al., 2006).

The evolution of 137 Cs and 90 Sr levels in coastal waters is illustrated by the values reported for the North-Eastern Black Sea (Table 4.1). Higher maximum post-Chernobyl levels in water were recorded in Sevastopol Bay, reaching up to 815 Bq m -3 and 157 Bq m -3 for 137 Cs and 90 Sr respectively (Fig. 4.4). As compared to initial levels measured after the Chernobyl accident, variations in average 137 Cs concentration values reported for 2001-2006 in coastal surface water narrowed down considerably, being mostly attributable to seasonal freshwater inflow and generally correlating well with salinity. Values between 12-21 Bq m -3 were reported for Varna, Bulgaria in 2002-2004 (Veleva, 2006), 11-26 Bq m -3 in 2003-2005 at the Georgian coast (Pagava, 2006), 15-36 Bq m -3 in 2001-2005, with a single value close to 50 Bq m -3 in September 2004, for Constanza, Romania (Puscasu and Dima, 2006), being similar to those reported for the Russian coast (Table 4.1).

Relatively few data were published recently on Pu isotopes in Black Sea water. Values reported for 239+240 Pu? over the period 1989-1998 range between 3 mBq m -3 in surface water and 13 mBq m -3 at 150-200 m depth.

Table 4.1. The dynamics of 137 Cs and 90 Sr levels (Bq m -3 ) in surface waters of the North-Eastern Black Sea near the Russian Coast.

Year

137 Cs

90 Sr

Range

Average

Range

Average

Before Chernoby (l)

NPP accident1

Homogeneous distribution

18.5

Homogeneous distribution

22.2

1986, June-July (2)

250-470

360?50

74-100

86?8

1986, October (2)

104-159

127?17

22-37

28?4

1987, June (2)

48-59

56?4

-

-

2000-20013

20.0-28.0

23.5?3.0

12.3-16.3

14.5?2.0

2004

20.0-23.5

21.7?1.8

10.3-11.0

10.7?0.4

2005

20.2-22.6

21.4?1.2

11.5-12.9

12.2?0.7

Sources: (1) Vakulovsky et al. (1980, 1994 ), (2) Nikitin et al. (1988), (3) IAEA (2004)

4.3. Concentrations and inventories of radionuclides in sediment

The highest 137 Cs concentrations measured in 1992-1994 in the upper 5-cm layer of NW-W Black Sea shelf bottom sediments were found near the Danube Delta, the Dnieper-Bug Estuary, and around the Tarkhankut Cape of the Crimea peninsula (Egorov et al., 2006). A similar pattern of contamination was found in 2003-2004 (Voitsekhovych et al., 2006), reflecting the contributions of the post-Chernobyl initial atmospheric deposition, further river inflow and sediment transport and deposition. Total inventories of 137 Cs in bottom sediments near the river mouths in 1990-1994 were in the range 10-40 kBq m -2 , one order of magnitude higher than at shelf break (2-5 kBq m -2 ) and two orders of magnitude higher than at the continental slope and deep-water basin (0.2-0.3 kBq m -2 ) (Egorov et al., 2006). Pre-Chernobyl inventories of 137 Cs in bottom sediments ranged between 1-8 kBq m -2 in coastal and shelf areas and were below 0.2 kBq m -2 in deeper areas (Vakulovsky et al, 1982). Maximum pre-Chernobyl 137 Cs concentrations in superficial bottom sediments of about 100 Bq kg -1 d.w. were reported by Vakulovsky et al. (1982). Maxima in the range of 500 Bq kg -1 d.w. 137 Cs were reported offshore the Danube mouths in September 1986 (Osvath et al., 1990), with current maxima reaching up to 100 Bq kg -1 d.w. 137 Cs according to Voitsekhovych et al. (2006).

Well-preserved 137 Cs profiles in the deep basin and NW Black Sea bottom sediments (Egorov et al., 2006; Voitsekhovych et al., 2006) showed two subsurface peaks attributable to global fallout from atmospheric nuclear weapons testing and the Chernobyl accident. The Chernobyl origin of the upper peak in the 137 Cs activity profile was documented using the activity ratio of 134 Cs/ 137 Cs, as the activity ratio of the short-lived 134 Cs (T 1/2 = 2.06 years) to the longer-lived 137 Cs (T 1/2 = 30.17 years) is known to be 0.53 in the Chernobyl release (Pentreath, 1988).? A further differentiation of the pre- and post-Chernobyl sediments was carried out using the activity ratio 238 Pu / 239+240 Pu, that was of about 0.04 for the pre-Chernobyl global fallout (Fig. 4.3) compared to 0.47 in the Chernobyl release (Pentreath, 1988). 210 Pb dating for well preserved cores, corroborated with indications from markers such as 137 Cs and Pu isotopes, was used for evaluating contributions from both radioactive and non-radioactive contamination sources and also sediment mass accumulation rates (Voitsekhovych et al., 2006; Gulin et al., 2002; IAEA, 2004).

Post-Chernobyl 239+240 Pu concentrations up to 0.4 Bq kg -1 d.w. were reported for sediments from both coastal and deep basin areas, depending on location and sediment composition (Egorov et al., 2006), with estimated 238 Pu / 239+240 Pu activity ratios varying in the range 0.105?0.165. These analyses indicated that about 75% of the total plutonium contamination in the Black Sea bottom sediments was caused by global fallout.

90 Sr being a soluble radionuclide, its levels in sediments remain generally low in the Black Sea. Pre-Chernobyl maxima of around 45 Bq m -2 90 Sr inventory and 1.3 Bq kg -1 d.w. 90 Sr concentrations were reported by Vakulovsky et al. (1982) for superficial bottom sediments. Following the Chernobyl accident, in the Black Sea at large concentrations of 90 Sr in superficial bottom sediments remained in the same range as their pre-Chernobyl levels. Higher values were reported for the NW Black Sea, in particular offshore the Dnieper-Bug estuary area, where Mirzoyeva et al. (2005) reported concentrations of 90 Sr in the 0-5 cm layer of bottom sediments ranging up to 45 Bq kg -1 d.w. in 1986, 80 Bq kg -1 d.w. in 1989, 45 Bq kg -1 d.w. in 1990-1996 and 150 Bq kg -1 d.w. in 1997-2000. They relate the differences observed to river input, as previously mentioned, periods of high water inflow through the Dnieper and, to a lesser extent, through the Danube, resulting in increases of 90 Sr levels in offshore superficial sediments.

Radionuclides in beach sand are typically reported for radioprotection purposes. Concentrations in the past years range between roughly 0.5-12 Bq kg -1 d.w. for 137 Cs, 0.2-10 Bq kg -1 d.w.? for 90 Sr and are below 0.2 Bq kg -1 d.w. for 239+240 Pu (IAEA, 2004).

4.4. Radionuclides in marine biota

The results of radioecological monitoring of the Sevastopol bays have shown that the increase of 137 Cs and 90 Sr concentrations in seawater recorded in May 1986 were followed by increases in the respective concentrations in marine biota (Fig. 4.4). Egorov et al. (2002) approximated the decrease recorded during the following years with exponential functions and estimated environmental half-lives for 90 Sr of 8.4 y for water, 4.9 y for seaweed, 6.7 y for mussels; and for 137 Cs 6.1 y for water, 4.7 y for seaweed, 7.5 y for mussels in the Sevastopol Bays. The doses delivered to the Black Sea biota by the anthropogenic radionuclides 90 Sr and 137 Cs after the Chernobyl NPP accident did not exceed the chronic exposure levels and the post-Chernobyl 90 Sr and 137 Cs contaminations did not introduce significant effects on biota in the Sevastopol Bay (Polikarpov, 1998; Mirzoyeva and Lazorenko, 2004). Maximum dose rates from anthropogenic radionuclides recorded in 1986 were found around 17%, 5.5% and 20% of the doses received from the natural 210 Po by fish, molluscs and seaweed, respectively.

Coastal monitoring results in countries around the Black Sea for the period 2000-2005 (Nikitin et al., 2006; Patrascu, 2006; Veleva, 2006; Pagava, 2006) indicate low levels of anthropogenic radionuclides in seaweed, molluscs and fish (Table 4.2). 239+240 Pu concentrations up to 17 mBq kg -1 w.w. in seaweed, 2.4 mBq kg -1 w.w. in mollusks and 1 mBq kg -1 w.w. in fish were reported at the NE Black Sea coast in 2000-2005 (Nikitin et al., 2006). Variations are observed between species and also depending on location, age etc., however, levels are generally low, with no radiological significance either for the biota themselves or for the human populations consuming edible species of marine biota.

4.5. Conclusions

Chernobyl atmospheric fallout deposited 1.7-2.4 PBq of 137 Cs into the Black Sea surface, which temporarily increased the 137 Cs inventory of the 0-50 m surface layer by a factor of 6-10 in comparison with its pre-Chernobyl value. The contribution of Chernobyl-origin 90 Sr (0.1-0.3 PBq) from atmospheric fallout was lower in comparison with that of 137 Cs. A subsequent 90 Sr input from the Danube and the Dnieper Rivers (about 0.16 PBq) was an important contribution to the budget of this radionuclide in the Black Sea, but the riverine 137 Cs input (0.02-0.03 PBq) was insignificant. The decrease of the 137 Cs inventory in the surface layer after Chernobyl has been mainly controlled by vertical mixing, loss through the Bosphorus Strait, and radioactive decay. The loss through the Bosphorus accounted for 2-2.5% of the 137 Cs inventory.? In the case of 90 Sr, these processes have been compensated by river inputs from the Dnieper and Danube Rivers up to 1994-1995 and partially after 2000. The vertical mixing of 137 Cs and 90 Sr was mainly effective within the 0-200 m layer. Sediment inventories of 137 Cs in the Danube and Dnieper delta regions exceeded with one order of magnitude the values in the slope zone and two orders of magnitude those in the deep basin. Marine biota along the coastal areas of the Black Sea presented very low levels of anthropogenic radionuclides.

Although the Black Sea was ranked at the level of the year 2000 amongst the marine regions of the World Ocean as the 2 nd highest in terms of 90 Sr concentrations in surface seawater (after the Irish Sea) and 3 rd highest in 137 Cs concentrations (after the Baltic and Irish Seas) (IAEA, 2005), the levels of anthropogenic radionuclides found in the Black Sea environment associate insignificant radiological doses to human populations.

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