  Systematic, long term measurements, starting in 1963, of 90Sr activity
concentrations in sea water have been performed at four locations (cities of
Rovinj, Rijeka, Split and Dubrovnik) along the Croatian coast of the Adriatic
sea. In addition, fallout samples were collected in the city of Zadar. 90Sr
activity concentrations are in good correlation with the fallout activity, the
coefficient of correlation being 0.72. After the nuclear moratorium on
atmospheric nuclear bomb tests in 1960s, 90Sr activity concentrations in sea
water exponentially dropped from 14.8 +/- 2.4 Bq/m3 in 1963 to 2.0 +/- 0.3
Bq/m3 in 2003. In the same period, the total annual 90Sr land surface deposit
in Zadar fell by three orders of magnitude, from 713.3 Bq/m2 in 1963 to 0.4
Bq/m2 in 2003. Using strontium sea water and fallout data, a mathematical model
was developed to describe the rate of change of 90Sr activity concentrations in
the Adriatic sea water and estimate its mean residence time in the Adriatic. By
fitting the experimental data to a theoretically predicted curve, the mean
residence time of 90Sr in the Adriatic sea water was estimated to be
approximately 3.4 +/- 0.4 years, standard deviation being calculated by Monte
Carlo simulations. As in physical oceanography 90Sr can be used as effective
radioactive tracer of water mass transport, this value also reflects the upper
limit for turnover time of the Adriatic sea water. The turnover time of 3.4
years for the Adriatic sea water is in reasonable agreement with the value
which was estimated, by studying water flows through the Strait of Otranto, to
be on the order of 1 year.
