137Cs Distribution in the Mediterranean Sea: Recent Changes and Future Trends

IAEA-SM-354/10 XA9951140 137Cs distribution in the Mediterranean sea: recent changes and future trends

C. PAPUCCI and R. DELFANTI ENEA CRAM, P.O. Box 316, 19100 La Spezia, Italy

The major sources of 137Cs to the Mediterranean Sea are fallout from nuclear weapon testing in the early sixties and from the Chernobyl accident in 1986. From the ban of atmospheric nuclear testing in 1963 to 1986, radionuclide concentration in surface water have regularly decreased, mainly due to diffusion and convection processes that transported toward the bottom the input deposited at the surface. The resulting 137Cs vertical profiles were characterised by decreasing concentrations from surface to the bottom. The Chernobyl fallout produced a marked, but unhomogeneous input that was highest in the Eastern Mediterranean/Adriatic Sea. Recent studies showed that in 1995 significant changes were also occurring in the circulation of the intermediate and deep waters of the Eastern Mediterranean, a new type of water, coming from the Aegean Sea and characterised by higher salinity, temperature and CFC's concentration [1] occupied the deepest part of the Levantine Basin and had replaced 20 % of the deep and bottom waters of Adriatic origin. In order to understand the future trend in 137Cs distribution and inventory in the Mediterranean Sea, some key-questions have to be answered: 1. What is the present distribution of 137Cs in the Eastern Mediterranean and how is it related to the new water circulation? What are the expected trends in the exchanges at the Sicily Straits? 2. What are present exchanges at the Gibraltar Strait and what can we expect in the future? Our studies were then focused on the systems Eastern Mediterranean-Sicily Strait and Alboran Sea-Gibraltar Strait. In April 1995, in the framework of Cs-137 (mBq/L) the POEM (Physical Oceanography of the Theta Eastern Mediterranean) Programme, a campaign was carried out to determine the new distribution of 137Cs in the Eastern Mediterranean. As typical example, Fig. 1 shows the vertical profile of 137Cs, coupled with hydrological parameters and dissolved oxygen content, in a station located in the central part of the Levantine Basin. Surface 137Cs concentrations range in this area between 2.5 and 3.5 mBq/L, values very similar to those typical of the whole Mediterranean before the Chernobyl accident. As no surface water leaves the Eastern Mediterranean before being Salinity (psu) converted into Intermediate and deep * • i • i •—i waters, we can conclude that convection — O,(ml/L) processes have already transported most of !37 137 FIG. 1. Vertical profiles of Cs, salinity, the Chernobyl Cs to depth. In fact, the temperature and dissolved oxygen in the Central levels of this radionuclide are higher in the Levantine Basin (1995). Levantine Intermediate water (4 mBq/L)

16 and in the "new" bottom water of Aegean origin: 137Cs concentrations around 2.5 mBq/L characterise this water mass in the whole area. The changes in water circulation in the Levantine Basin, in the long term, will also affect radionuclide. distribution in the Ionian Sea and in the water flowing into the Western basin through the Sicily Strait: a fixed transect, crossing the Sicily Sill is in fact being regularly monitored to follow the time evolution of the process. The exchanges between the Atlantic • cs-137 (mBq/L) Ocean and the Mediterranean Sea are also examined. Till the mid-eighties, differences in 137Cs concentration in the water entering (5 mBq/L) and leaving (3 mBq/L) the Mediterranean, lead to a net 137Cs input of 0.12 PBq a"1 to the Mediterranean Sea [2]. Diffusion and convection processes have I progressively homogenised the concentration of this radionuclide in the water column, modifying 137Cs content in the waters flowing through the Strait. Present vertical profiles in the Alboran Sea 137 show i) a marked increase in Cs 38.5 concentration in the intermediate and deep water layers (ex. -1000 m: 2 mBq/L in 137 1994, compared to 0.5 mBq/L in 1974) and FIG. 2. Vertical profile of Cs and salinity at the ii) a marked decrease in surface water (2.5 Gibraltar Strait (1997). mBq/L in 1994 and 8 mBq/L in 1974). As a consequence, the 137Cs levels in waters entering and leaving the Mediterranean are now very similar (Fig. 2). In absence of substantial new inputs, the future expected trends are: a) the transport of l37Cs from the Eastern to the Western Basins through the Levantine Intermediate Water and the transitional Eastern Mediterranean Deep Water will be counterbalanced by the amount of cesium flowing into the Eastern Basin with the surface Modified Atlantic Water; b) the exchanges between Atlantic and Mediterranean will show the same balanced trend, as the input/output of 137Cs at Gibraltar are expected to be similar; c) the global 137Cs inventory in the Mediterranean Sea is thus going to decrease in the future, simply because of its physical decay.

REFERENCES

[1] Roether W., Manca B.B., Klein B., Bregant D., Georgopoulos D., Beitzel V., Kovacevic V., Luchetta A., "Recent changes in Eastern Mediterranean deep waters", Science 271, (1996) 333-335.

[2] Holm E., Fukai R., Whitehead N.E., "Radiocesium and transuranium elements in the Mediterranean Sea: Sources, inventories and environmental levels", Proc. Intn. Conf. On Environm. Radioactivity in the Med. Area, SNE Barcelona (1988), 601-617.