Caesium contamination and vertical distribution in undisturbed soils in Croatia

The deposition and vertical distribution of 137Cs and 134Cs in the first 25 cm of undisturbed soil profiles were studied at more than 50 locations in Croatia. Relative contributions of fallout from weapon-testing and the Chernobyl accident-derived fallout to the total 137Cs deposition were estimated on the basis of the 137Cs/134Cs activity ratio. Total weapon-testing-derived deposition can be generally treated as uniform. Significant variation of contamination caused by Chernobyl accident can be explained by relatively short contamination period and great variability in activity and quantity of rainfall. The changes of caesium activity through vertical soil profiles are described by a general equation. Caesium penetrates fastest into deeper soil layers of siltous soils, slowest in the soils developed on flysch sediments. The vertical migration and distribution of Chernobyl-derived 137Cs are practically the same in terra rossa and loam-podzol soils two and half months after contamination. The average 134Cs activities of about 1.1 Bq were found in terra rossa, loam podzol and siltous soils in the soil layer between 7.5 and 8.7 cm. Soil samples were collected in July 1986 and both Chernobyl-derived caesium isotopes had only short time to migrate downwards. In soils developed on flysch sediments, loam-podzol and terra rossa weapon-testing-derived caesium activity decreases throughout the whole profile, while in siltous soils it increases in the first 3 cm of soil depth. The soil type influences the rate of vertical caesium migration downward soil profile. The results obtained reveal the following order of caesium vertical migration rate: siltous soil>terra rossa>loam-podzol>soils developed on flysch sediments.