Estimation of the Adriatic sea water turnover time using fallout 90 Sr as a radioactive tracer

Systematic, long term measurements, starting in 1963, of Sr 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. Sr 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, Sr activity concentrations in sea water exponentially dropped from 14.8 ± 2.4 Bq m in 1963 to 2.0 ± 0.3 Bq m in 2003. In the same period, the total annual Sr land surface deposit in Zadar fell by three orders of magnitude, from 713.3 Bq m in 1963 to 0.4 Bq m in 2003. Using strontium sea water and fallout data, a mathematical model was developed to describe the rate of change of Sr 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 Sr 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 Sr 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.