Caesium contamination and vertical distribution in undisturbed soils in Croatia

The deposition and vertical distribution of Cs-137 and Cs-134 in the first 25 cm of undisturbed soil profiles were studied at more than 50 locations i n Croatia. The relative contributions of fallout from weapons-testing and t he Chernobyl accident-derived fallout to the total Cs-137 deposition were e stimated on the basis of the Cs-137/Cs-134 activity ratio. Total weapon-tes ting-derived deposition can generally be treated as uniform. Significant va riation of contamination caused by the Chernobyl accident can be explained by the relatively short contamination period and great variability in the a ctivity and quantity of rainfall. The changes of caesium activity through v ertical soil profiles are described by a general equation. Caesium penetrat es fastest into the deeper soil layers of silty soils, slowest in the soils developed on flysch sediments. The vertical migration and distribution of Chernobyl-derived Cs-137 are practically the same in terra rossa and loam-p odzol soils two and half months after contamination. The average Cs-134 act ivities of about 1.1 Bq per section were found in terra rossa, loam podzol and silty soils in the soil layer between 7.5 and 8.7 cm. Soil samples were collected in July 1986 and thus both Chernobyl-derived caesium isotopes ha d only a short time to migrate downwards. In soils developed on flysch sedi ments, loam-podzol and terra rossa, weapon-testing-derived caesium activity decreases throughout the whole profile, while in silty soils it increases in the first 3 cm of soil depth. The soil type influences the rate of verti cal caesium migration down the soil profile. The results obtained reveal th e following order of vertical migration rate of caesium: silty soil > terra rossa > loam-podzol > soils developed on flysch sediments. (C) 1999 Publis hed by Elsevier Science Ltd. All rights reserved.