Floodplain contamination as a proxy for long-term changes in Chernobyl-derived ¹³⁷Cs in river basin

This paper discusses the opportunities and challenges of using vertical distribution of Chernobyl-derived ¹³⁷Cs in floodplain sediments to evaluate long-term changes of sediment-associated radionuclides in river system. It summarizes the results of more than a decade of research conducted in a lowland river (the Upa River) basin heavily contaminated after the Chernobyl accident. The overbank deposition of sediment-associated ¹³⁷Cs on floodplains is determined by several factors, which must be considered when interpreting the observed contamination patterns, the most important being topography and hydrological regime, which control frequency of floodplain inundation during high water periods. Frequent floodings are conducive to a better record of ¹³⁷Cs variations in sediment load. The ¹³⁷Cs vertical distribution in floodplain sediments can be altered by certain disturbances, which makes its interpretation more difficult. The contribution of local sources to sediment load should be considered to avoid misinterpretation of ¹³⁷Cs redistribution in upstream catchment. Downstream overbank deposition is impacted by management of river flow through artificial reservoirs, and therefore such floodplain sediments do not represent migration of ¹³⁷Cs in upstream catchment. Nevertheless, investigation of floodplains downstream can provide valuable data about transport of clay and silt particles carrying pollutants.