IAEA-SM-354/40P XA"51287 A STUDY ON NATURAL AND ARTIFICIAL RADIOACTIVITY IN SELECTED AREAS IN THE AEGEAN AND IONIAN SEA - EASTERN MEDITERRANEAN

H. FLOROU National Centre for Scientific Research "Demokritos" (NCSR"Demokritos") Athens - Greece

1. Natural radioactivity

The natural gamma radiation status has been determined in selected areas of elevated levels of natural radioactivity in the Aegean Sea, the Milos and Ikaria islands. Milos island is located in the volcanic arc of the Cyclades Archipelagos (Papanicolaou, 1986) in the southeastern Aegean Sea, Greece. The levels of natural radioactivity in the environment of the island is of particular interest, taking into account that the volcanic areas are characterized by enhanced concentrations of natural radionuclides (Forstner, 1979).

The results of radiometry indicate the existence of some areas in Milos where the exposure rate exceeds 20 \iR h"1, which corresponds to dose rate of 123 nSv h"1. These are relatively high values if compared with the observations in other Greek regions, with a reported mean value of 80 nSv h"1 (Kritidis and Florou, 1989). Nevertheless, these values are significantly lower than the maxima reported for other regions of the world (Soc. of Nuclear Medicine, 1982).

The concentrations of 226Ra, 232Th and "°K in sediments show that for the three radionuclides Milos presents higher concentrations in comparison with the other Greek areas considered (Florou et al, 1988). These values range in the upper limit of the corresponding values for Ionian Sea, North Adriatic and Lingurian Sea. The concentrations of natural radionuclides in the marine organisms are comparable with those observed in the other Greek areas considered (Florou, 1996) and other Mediterranean areas (Nonnis-Marzano and Triulzi, 1994). Considering plants, the marine angiosperm Posidonia oceanica shows elevated values of 226Ra and228Tb, which, in general present the greatest bioaccumulation of Th in different parts of the plant (Florou et al, 1987).

Ikaria island is located in the Eastern Aegean Sea. In the littoral zone of the island there are several spas and in the sub-littoral zone some springs bubble up from the bottom as well. The results of gamma radiometry applied in the wide area of the island show that the measured exposure dose rate is higher in the vicinity of the spas (Trabidou et al, 1995).

The concentrations of 238U, ^Ra1232Th and 40K in sediments and of 226Ra, 228Ra, 222Rn and ^K in sea water are higher than those observed in the other Greek areas considered and in areas referred in the international literature (Florou, 1996).

The elevated concentrations of natural radionuclides in the abiotic materials of Ikaria spas are reflected in the concentrations of natural radionuclides in the examined organisms, as can be concluded from the observed elevated values in comparison with the respective typical values of the Greek environment and other Mediterranean areas (Dovlete and Bologa, 1986).

2. Artificial Radioactivity

As the Aegean Sea is adjacent by its north part to the Black Sea through the Straits of Dardanelles, the Black Sea water mass enter to the Aegean Sea via surface current circulation during the water exchange procedure between the two basins.

The Chernobyl nuclear accident on 26 April 1986 resulted in a deposition of 137Cs of 2400 TBq in the Black Sea (Egorov et al, 1994), whereas the respective amount for the Aegean Sea was 530 TBq (Kritidis and Florou, 1990). More than 10 years after, the Chernobyl Nuclear Power Plant area is the land-based source of radionuclides chronic pollution of the Black Sea through the Pripyat river and the

301 Dnieper river. In addition, radioactive pollutants of terrestrial origin are carried in by the Danube, Dniester and Dnieper river outflows and drainage system procedures. Since the river outflows deliver contaminated terrestrial material to the northwestern part of the Black Sea, whereas 137Cs is re- suspended from the Black Sea sediments an amount of 250 TBq is estimated to remain in the 0-50 m layer.

According to our measurements of surface sea water during 1993 (in the frame of MARINAMED project of the EU), it was shown that approximately 48 TBq of 137Cs was discharged to the Aegean Sea during 1993, due to the purification process of the Black Sea, throughout the Bosporus and Dardanelles (Florou et al, 1994).

Considering the generic inventory of 137Cs in the Black Sea - Aegean Sea during 1993, the average concentration of 137Cs in the Black Sea varies up to 130 Bq m"3. The respective value in the mouth of Dardanelles is 120 ± 8 Bq m3, whereas the estimated mean values for the Aegean Sea is 20.7 ± 14.7 Bq m"3 (9.3 ±. 4.2 Bq m'3 if measured maximum is excluded). This vaifle is quite high, compared to the pre-accident levels 2.6 ± 0.3 Bq m'3 or to the respective value for the Ionian Sea 9.2 ± 2.5 Bq m'3 (Florou and Kritidis, 1994).

It has also been shown that 90Sr is carried into the Aegean Sea by the same processes, resulting in a, more or less, similar distribution pattern (Polikarpov et al, 1991).

Thus, we assume that a more or less predictable amounts of l37Cs and '"Sr are discharged to the Aegean Sea - Eastern Mediterranean during the purification processes of the Black Sea through the Dardanelles channel, via surface current circulation.

REFERENCES

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