Abstract
A Chernobyl-derived 137Cs- fallout was associated with one or two rainfalls Because of that vast areas of the Europe affected by Chernobyl-derived fallout are characterized by non-uniform field of radionuclide contamination. It was assessed after detailed field investigation within few river basins of the Central Russia located in areas with different levels of Chernobyl contamination, that existing maps of radionuclide contamination composed during last two decades are not enough detailed for assessment of initial contamination field transformation by the lateral migration processes of the Chernobyl-derived 137Cs. This problem can be overcomed   if additional soil sampling are undertaken in reference locations for correction of exiting radionuclide contamination maps. However it is necessary to evaluate the optimal number of bulk samples which should be taken in each sampling point for receiving statistically correct results of radionuclide concentration. Special investigation was undertaken in few catchments (S= 2-50 km2) of the Central Russia, located in areas with different levels of initial Chernobyl contamination, for evaluation the optimal number of samples, which should be taken in each sampling point for the determination of Cs-137 concentrations error not exceed 30 % on 95 % confidence level.

Keywords: Cs-137, soil sampling, evaluation of contamination, small catchments, radioecological mapping

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