Factors controlling dissolved ¹³⁷ Cs activities in Matsukawa-ura lagoon, a semi-closed estuary, after the Fukushima accident

Abstract. The spatial and seasonal dynamics of 137Cs were investigated from 2021 to 2023 in Matsukawa-ura lagoon, a semi-closed estuarine area approximately 40 km north of the Fukushima Daiichi Nuclear Power Plant (FDNPP), Japan. The weighted mean dissolved 137Cs concentrations in the lagoon ranged from 5.3 to 19 Bq m−3, which were 2.4–8.6 times higher than those in the surrounding coastal seawater and inflowing river waters. Furthermore, dissolved 137Cs concentrations in the lagoon were higher in summer than in winter and showed a strong positive correlation with water temperature. Simplified box-model estimation indicates that continuous terrestrial input 137Cs are unlikely to contribute to the spatiotemporal variability of dissolved 137Cs concentrations in the lagoon. Instead 137Cs deposited in bottom sediments during the early stages of the FDNPP accident is gradually released as pore waters are exposed to seawater entering the lagoon, thereby sustaining elevated dissolved 137Cs concentrations. These results indicate that warmer summer conditions enhance the dissolution of 137Cs from bottom sediments and highlight the importance of sediment–pore water processes in controlling 137Cs dynamics in the coastal environments of Fukushima Prefecture.