Radioecological modeling of the 131I activity dynamics in the pasture vegetation of Mazovia in the year of the Chernobyl accident: Reconstruction, verification, reliability assessments

A radioecological model, which is a system of linear differential equations describing the dynamics of the transport of 137Cs and 131I radionuclides along the food chain after their release into the atmosphere after the Chernobyl accident, was used to reconstruct “instrumental” data of the 131I activities in the grass pastures in the central part of Mazovia. Four atmospheric models were used for the reconstruction: direct calculation, homogeneous cloud – inhomogeneous rainfall, inhomogeneous cloud – homogeneous rainfall, and a model with recalculation of the 137Cs and 131I activities in the atmosphere. The “instrumental” data were reconstructed based on data from direct measurements of the 131I activity in lawn grass. It has been shown that the direct calculation and homogeneous cloud models lead to a better agreement of the calculated and reconstructed “instrumental” data than the inhomogeneous cloud model. The arithmetic mean ratio of the calculated and reconstructed “instrumental” data lie in a range of 0.84 to 0.95 for the direct calculation and homogeneous cloud models, and in a range of 1.7 to 3.0 for the inhomogeneous cloud model. The mean geometric deviation for all models is constant and equal to 1.7. Instrumental and reconstructed “instrumental” data show a significant decrease in the specific activity of 131I in grass due to its wash-off by continuous rainfall, both during rainfall and after most of the deposition takes place. Due to this effect, the coefficient of the 131I retention on grass in the form of the maximum activity ratio to the 137Cs deposition density decreases from 34 to 1.4 m2/kg while it increases from 1 to 29 kBq/m2 as the result of the rainfall growth from 0 to 40 mm.