Impact of Bottom-Sediment Removal on Cs-137 Contamination in an Urban Pond

Many irrigation ponds in Fukushima Prefecture were decontaminated due to the contamination of radiocesium released from Fukushima Daiichi Nuclear Power Plant. To evaluate the impact of decontamination on Cs-137 dynamics in an urban pond in Koriyama City, Fukushima Prefecture, Japan, temporal changes in Cs-137 concentrations in bottom sediments and pond water were investigated before and after bottom-sediment removal. Post-decontamination, Cs-137 inventories in bottom sediments decreased by 46-89%. Cs-137 inventories in bottom sediments were relatively high in fine sediments before decontamination, and were also high at points near the water inlet after decontamination. Following decontamination, the mean Cs-137 concentration in suspended solids (SS) and the mean dissolved Cs-137 concentration in pond water decreased by 52% and 5%, respectively. Even after decontamination, the normalized Cs-137 concentrations in SS and in water, which were calculated by dividing the Cs-137 concentrations by the mean Cs-137 inventories in each area, were higher than those in rivers, dam reservoirs, and ponds elsewhere in Fukushima. The positive correlations between delta N-15 values, an indicator of the source contribution to bottom sediments, and Cs-137 concentrations in the upper 5 cm of bottom sediments after decontamination implied that SS from urban areas gradually increased the Cs-137 inventories in the pond. The results underline the importance of secondary inputs of Cs-137 from highly urbanized catchments.