Post-depositional redistribution and gradual accumulation of 137Cs in a riparian wetland ecosystem in Sweden

After the Chernobyl accident, high activity concentrations of 137Cs (>1 MBq m−2) were detected in a riparian swamp in the central-eastern part of Sweden. The objective of this study was to clarify the redistribution processes behind the accumulation of 137Cs in the wetland. A mass balance budget of 137Cs was calculated based on soil and sediment samples and reports in the literature. Results showed that accumulation occurred over several years. Of all the 137Cs activity discharged between 1986 and 2002 from the upstream lake, 29% was estimated to be retained in the wetland. In 2003, measurements showed that 17 kBq m−2 sedimented on the stream banks of the wetland. Continuing overbank sedimentation by spring flooding prolongs the time that the wetland will contain high activity concentrations of 137Cs. Consequently, organisms living in wetlands serving as sinks for 137Cs may become exposed over long time periods to high activity concentrations.