Radiocesium contamination of lake sediments and fish following the Fukushima nuclear accident and their partition coefficient

To evaluate the influences of the Fukushima nuclear accident in 2011 on lake sediments and fish and to understand the difference in their contamination levels, data on radiocesium concentrations (Cs-137 and Cs-134) were analyzed for sediments and 18 fish species (including one freshwater prawn) taken from 15 lakes in northeastern Japan. Temporal trends in radiocesium concentrations (referenced to radioactivity on 15 Mar 2011) were not clear during 2011-2013 except in a few species of fish. There was a decrease among topmouth gudgeon (Pseudorasbora parva), icefish (Salangichthys microdon), and pond smelt (Hypomesus nipponensis) but an increase among channel catfish (Ictalurus punctatus) and kokanee (Oncorhynchus nerka). Significant positive correlations between lake-averaged radiocesium concentrations of sediments and fish were observed for most fish species. The partition coefficients (PCs), defined as fish concentration divided by sediment concentration on a dry weight basis, clustered mostly in the range of 0.3-3 ([Bq/kg-dry]/[Bq/kg-dry]) and were high in large-sized types (e. g., channel catfish and brown trout [Salmo trutta]) but low in small-sized types (e. g., topmouth gudgeon and icefish). After normalization, the PCs of the respective lakes were analyzed together with lake characteristics related to water exchange and lake dimensions, water quality, and sediments and were found to be high in the lakes with high water retention time and vice versa, suggesting prolonged contact and equilibration between the water and the sediments. Finally, the efficacy and potential problems of using the PCs between sediments and fish are discussed.