Long-term investigations of post-Chernobyl radiocaesium in fallout and air in North Croatia.

The long-term behaviour of 137Cs activity concentrations in air and fallout has been studied in the city of Zagreb for the post-Chernobyl period (1986–2006) as a part of an extended monitoring program of radioactive contamination of human environment in Croatia. Annual mean 137Cs activity concentrations in air and annual total deposition fluxes (wet plus dry) decreased from 2.8 × 10−4 Bq m−3 in September 1986 to 3.0 × 10−6 Bq m−3 in last quarter of 2006 and from 6,410 Bq m−2 year−1 in 1986 to 2 Bq m−2 year−1 in 2006 respectively. By fitting the measured 137Cs activity concentrations to the theoretical curve the ecological half-lives of 137Cs in air and fallout were estimated with respective values of 0.46 and 0.54 years for immediate post-Chernobyl period, increasing to 5.52 and 3.97 years afterwards. Using the data on 137Cs activity concentrations in air and fallout total caesium deposition velocity of (3.34 ± 3.13) × 10−2 ms−1 was estimated with median value being 2.13 × 10−2 ms−1. Such relatively high 137Cs deposition velocities compared with pre-Chernobyl ones, are characteristic for the post-Chernobyl period and, according to Stokes’ settling law, indicate that the diameters of aerosol particles associated with 137Cs originated from the Chernobyl accident are pretty large, i.e. >1 μm. 134Cs/137Cs activity ratio in fallout and in air has been found to be similar to the theoretically predicted values, initial value being about 0.5 and decreasing according to differential radioactive decay. The similar ratio has been observed in most of the other environmental samples. [ABSTRACT FROM AUTHOR]