Migration of fallout radiocaesium in a grassland soil from 1986 to 2001: Part I: Activity–depth profiles of 134Cs and 137Cs

The temporal changes of the vertical distribution of 134Cs (deposited by the Chernobyl fallout in 1986) and 137Cs (deposited by the Chernobyl and the global fallout) in the soil were investigated at an undisturbed Bavarian grassland site in Germany. At ten sampling dates between 1986 and 2001, the activity density of 134Cs and 137Cs was determined in various soil layers down to 80 cm depth. In 2001, the small-scale spatial variability of the radiocaesium activity was determined by sampling five plots within 10 m2 (coefficient of variation about 20% for the upper soil layers). Between 1987 and 1990, substantial changes of the activity–depth profiles were observed. The percentage depth distributions of 134Cs and 137Cs were rather similar. The 50%-depth of the accumulated activity increased from 2.4 cm in 1988 to 5.3 cm in 2001 for 134Cs and from 2.7 to 5.8 cm for 137Cs. This indicates that at the study site the migration data of Chernobyl-derived 137Cs can be estimated by those of total 137Cs. In the second part of this study, the activity–depth profiles will be evaluated by the convection–dispersion model [Schimmack, W, Feria Márquez, F. Migration of fallout radiocaesium in a grassland soil from 1986 to 2001. Part II: Evaluation of the activity-depth profiles by transport models. Sci Total Environ 2006-this issue].