137Cs in freshwater fish and lake water in Finland after the Chernobyl deposition

The uneven deposition of 137 Cs from the Chernobyl accident caused a large variation in the activity concentrations of 137 Cs in Finnish lakes. The concentrations of 137 Cs in lake water had decreased by more than 90% until the end of 1986, and thereafter the decrease gradually slowed down. In 2002, activity concentrations of 137 Cs in lake waters varied from 4 to 330 Bq m -3 . The concentrations of 137 Cs in fish had been generally highest in 1987 or 1988 and decreased thereafter by on average about 70% until 2005. After the deposition, several environmental processes, such as the water flow, sedimentation and runoff, and other characteristics such as potassium concentration, other water chemical parameters and the water residence time of the lake, have changed the distribution of 137 Cs in the compartments of the aquatic environment. The results also show that the activity concentrations of 137 Cs decreased at various rates in different lakes; 137 Cs in fish decreased three times more rapidly in one lake compared to another lake. In 2000-2004, activity concentration of 137 Cs in fish ranged from 8 Bq kg -1 to 7800 Bq kg -1 f.w. (f.w. = fresh weight).