Determination of 239,240Pu in Caspian Sea Water Using the Sorption–Diffusion Model of the Radionuclide Uptake by Bottom Sediments

The content of global ^239,240Pu in Caspian sea water (1996–2056) was calculated using the sorption–diffusion model of the radionuclide uptake by bottom sediments with the distribution coefficient K_d = 50 × 10³ and diffusion coefficient D = 0.1 × 10^–7 cm²/s. The ^239,240Pu global fallout on the sea was assumed to be equal to the experimental value for the mid-latitude belt of Russia, 60 Bq/m². At the plutonium fallout density on the sea surface of 58 Bq/m², its inventory in the sea Q as of the year 1964 is 21.9 TBq. In 1996, the experimentally determined ^239,240Pu concentration in Caspian sea water was ~20 μBq/L, and the calculation by the model gives 17.8 μBq/L. During the ~30-year migration of global ^239,240Pu, about 93% of the radionuclide passed from the aqueous phase to bottom soils. According to the calculations, the ^239,240Pu concentration in the seawater in the period 1996–2056 will decrease from 17.8 to 10.5 μBq/L, and the ^239,240Pu inventory in the seawater, from 6.3 to 3.7%, of the fallout value. The results were verified using an independent method for ⁹⁰Sr monitoring in Caspian sea water and determining the ^239,240Pu/⁹⁰Sr concentration ratio in the water. The results of estimating the ^239,240Pu concentrations in the seawater (2017–2020) by these two methods reasonably agree with each other.