Estimation of the Contribution of Chernobyl 137Cs to the Caspian Sea Pollution by 137Cs from Global Fallout

The concentration of 137Cs originating from the Chernobyl accident in the Caspian water was estimated by subtracting the concentration of global 137Cs from the total concentration of 137Cs radionuclides in water at the monitoring date (1996). Late monitoring of the Caspian waters in 1996 made it impossible to determine the “Chernobyl” 137Cs from the 134Cs/137Cs ratio because of the decay of the short-lived radionuclide. The concentration of global 137Cs in the seawater was calculated using the sorption–diffusion model of the 137Cs absorption by bottom sediments with sorption coefficients (Kd) and diffusion coefficients (D) equal to 2700 L/kg and 1.0 × 10–7 cm2/s, respectively. By the monitoring date (1995–1996), the inventories of global and “Chernobyl” 137Cs in the seawater were determined to be 312.6 and 93.8 TBq, respectively, with “Chernobyl” 137Cs accounting for 23% of the total amount. With low fallout of “Chernobyl” 137Cs on the Caspian Sea, 137Cs of global fallout dominated in the marine pollution. During the 56-year exposure (1964–2020), the concentration of global 137Cs in the Caspian water decreased from 23.7 to 1.8 Bq/m3, which is typical of the current level of 137Cs in waters of the world ocean.