ASSESSMENT OF THE ADRIATIC SEA-	WATER MEAN RESIDENCE TIME

USING FALLOUT 90Sr AS RADIOACTIVE TRACER

FRANIĆ Z.

Institute for Medical Research and Occupational Health, Radiation Protection Unit

Zagreb, Ksaverska cesta 2, PO Box 291, Croatia

E-mail: franic@imi.hr

In physical oceanography 90Sr can be used as radioactive tracer to study circulation of sea water. As a part of an extended monitoring programme, 90Sr has been analysed in sea water samples and fallout on the Adriatic since 1963. Samples of surface sea water have been collected on four locations of the Adriatic Sea: towns of Rovinj and Rijeka in North Adriatic, town of Split in Mid- Adriatic and town of Dubrovnik in South Adriatic. Fallout samples have been collected monthly in the city of Zadar (Mid-Adriatic). 90Sr activity concentrations are found to be approximately equal on all locations observed, not differing from the rest of the Mediterranean Sea [1]. It was estimated that approximately 85% of all man-made radioactive contamination in the Mediterranean comes from fallout [2]. Consequently, in the Adriatic, 90Sr activity concentrations are in good correlation with the fallout activity, the coefficient of correlation being 0.72 [1]. The elongated shape (some 800 by 200 km) and almost landlocked position of the Adriatic sea play an important role in dynamics of its water. The Adriatic sea is the major source of the densest water in the Eastern Mediterranean, the Eastern Mediterranean Deep Water (EMDW) [3]. Meteorological conditions favourable to dense water formation occurring during Bora events (the dominant winter wind blowing from Northeast), cause rapid mixing of surface waters with the intermediate water layer, which can therefore be considered as a single water reservoir (83% of the total volume). This is especially true for the North Adriatic since it is relatively shallow, the deep not exceeding 100 m. Together with the surface waters 90Sr sinks to deeper water layers and eventually, due to the general circulation pattern, leaves the Adriatic. Assuming that most of the 90Sr comes into the Adriatic sea by fallout, the 90Sr circulation in the Adriatic sea can be described by the simple mathematical model:

where

As(t)	is the total, time-dependant 90Sr activity (Bq) in reservoir consisting of surface and intermediate water, obtained by multiplying the observed activity concentration data by the volume the reservoir (29,000 km3)

is the decay constant for 90Sr (y-1),

1/ks	is the mean residence time of 90Sr in a reservoir (y) and

I(t)	is the total annual 90Sr input in to the Adriatic sea (Bqy-1).

In order to solve the equation (1) several assumptions were made. It was assumed that the intermediate water entering the Adriatic from the Mediterranean (Ionian sea) does not significantly contribute to total 90Sr inventory in the Adriatic due to a slower mixing rate of surface and intermediate waters. Also, it was assumed that the input of 90Sr by the Po river is proportional to the direct fallout input. Therefore, the total input of 90Sr to the Adriatic sea was estimated by multiplying the observed fallout data by the area of the Adriatic sea (138,600 km2) and multiplied by factor 1.2 to adjust for the run-off contribution form the Po river. Finally, the fallout data obtained from the measurements in the city of Zadar were assumed to be representative for the whole Adriatic area. The long-term fallout data were fitted to the equation:

where:

Io	is the initial input of 90Sr in the Adriatic sea and

kf	is a constant.

By fitting the fallout experimental data for the 1963-1996 period (Figure 1), for kf is obtained 0.39 y-1 and for Io=I(1963) 135 TBq (812 Bqm-2). After combining equations (1) and (2) and solving for A(t), with initial conditions Io=I(1963) and As(0)=As(1963) the following solution is obtained:

By fitting the experimental data to a theoretically predicted curve (3), the unknown parameter ks was calculated to be 0.21 y-1 (Figure 2). Although the model (3) is simplified representation of the real situation, the fit is reasonably good, the coefficient of correlation between experimental and predicted data being 0.92. The mean residence time of 90Sr in the reservoir consisting of surface and intermediate sea water layers being the reciprocal value of ks was estimated to be approximately 4.75 years. This value is comparable to the result of approximately 5 years, obtained by studying water flows of the Adriatic sea water through the strait of Otranto.

REFERENCES

[1]	FRANIĆ Z. and BAUMAN A. Activity of 90Sr and 137Cs in the Adriatic Sea, Health Physics, 64:162-169;1993.

[2]	United Nations Environment Program (UNEP). Mediterranean Action Plan. Assessment of the state of pollution in the Mediterranean sea by radioactive substances. Athens, UNEP 1991.

[3]	ORLIĆ M., GAČIĆ M. and LA VIOLETTE P.E. The currents and circulation of the Adriatic sea. Oceanologica Acta, 15(2):109-123;1992.