Mapping of 137Cs deposition over eastern France 16 years after the Chernobyl accident.

We observed the relationship between present-day 137Cs surface activity and the rainfall in May 1986 to establish a relevant map of 137Cs deposition over eastern France. This study was performed in an area of the lower Rhône valley where clayey soils are assumed to trap caesium efficiently. Based on the rainwater/airborne activity ratio deduced from this relationship, we verified that present-day activities, measured in clayey soils of this type, are relatively representative of the initial depositions. The interlocked contaminated areas of the resulting map can be related to rainfall occurrences, as is the case for all countries affected by the Chernobyl fallout. This map can be reasonably compared with those obtained for countries bordering France in terms of both activity levels and extent of homogeneous activity areas. Lastly, we demonstrate that it would not be possible to produce a coherent map of initial fallout by extending the campaign over the whole of the eastern territory based on present-day soil activity measurements alone. Hence, only the relationship between 137Cs deposition and the rainfall of the first week of May 1986 can be used to construct a map depicting the Chernobyl fallout over eastern France.