137Cs, 239,240Pu and 241Am in bottom sediments and surface water of Lake Päijänne, Finland.

Abstract The concentrations and vertical distribution of 239,240 Pu, 241 Am and 137 Cs in the bottom sediments and water samples of Lake Paijanne were investigated. This lake is important, since the Paijanne area received a significant deposition from the Chernobyl fallout. Furthermore Lake Paijanne is the raw water source for the Helsinki metropolitan area. In addition no previous data on the distribution of plutonium and americium in the sediment profiles of Lake Paijanne exist. Only data covering the surface layer (0–1 cm) of the sediments are previously available. In the sediments the average total activities were 45 ± 15 Bq/m 2 and 20 ± 7 Bq/m 2 for 239,240 Pu and 241 Am, respectively. The average 241 Am/ 239,240 Pu ratio was 0.45 ± 0.14. The 241 Am/ 239,240 Pu ratio is lowest in the surface layer of the sediments and increases as a function of depth. The 238 Pu/ 239,240 Pu ratio of the sediment samples varied between 0.012 ± 0.025 and 0.162 ± 0.079, decreasing as a function of depth. The average activity in water was 4.9 ± 0.9 mBq/m 3 and 4.1 ± 0.2 mBq/m 3 for 239,240 Pu and 241 Am, respectively. The 241 Am/ 239,240 Pu ratio of water samples was 0.82 ± 0.17. 239,240 Pu originating from the Chernobyl fallout calculated from the average total activities covers approximately 1.95 ± 0.01% of the total 239,240 Pu activity in the bottom sediments. The average total 137 Cs activity of sediment profiles was 100 ± 15 kBq/m 2 and 19.3 ± 1.4 Bq/m 3 in water samples.