Impact of Bottom-Sediment Removal on 137 Cs 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 137 Cs dynamics in an urban pond in Koriyama City, Fukushima Prefecture, Japan, temporal changes in 137 Cs concentrations in bottom sediments and pond water were investigated before and after bottom-sediment removal. Post-decontamination, 137 Cs inventories in bottom sediments decreased by 46–89%. 137 Cs 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 137 Cs concentration in suspended solids (SS) and the mean dissolved 137 Cs concentration in pond water decreased by 52% and 5%, respectively. Even after decontamination, the normalized 137 Cs concentrations in SS and in water, which were calculated by dividing the 137 Cs concentrations by the mean 137 Cs inventories in each area, were higher than those in rivers, dam reservoirs, and ponds elsewhere in Fukushima. The positive correlations between δ 15 N values, an indicator of the source contribution to bottom sediments, and 137 Cs concentrations in the upper 5 cm of bottom sediments after decontamination implied that SS from urban areas gradually increased the 137 Cs inventories in the pond. The results underline the importance of secondary inputs of 137 Cs from highly urbanized catchments.