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Analysis of models assessing the radionuclide migration from catchments to water bodies.

In this paper the analysis of models for assessing the migration of radioactive substances from catchments to water bodies was carried out. Comparisons are made between the mathematical form of the experimental dissolved radionuclide transfer functions (Transfer Function = the amount of radionuclide flowing per unit time from upstream drainage basin to a water body following a single-pulse deposition of radioactive substance) evaluated for rivers in Europe contaminated after the Chernobyl accident, with the "Green Functions" (Green Function = the radionuclide flow per unit time from catchment to water body calculated by the model as a result of a single-pulse input deposition) characterizing some of the most common models. Generally transfer functions are the sum of some time-dependent exponential components. The analysis showed that two main components (a short-term and a long-term component) may be detected over a period of only a few years after the accident. The comparison of transfer functions and Green functions showed that a) models based on the traditional concept of kd (the radionuclide partition coefficient soil-water) do not explain the higher value of the experimental long-term effective-decay constant for 137Cs compared with 90Sr; and b) traditional models do not explain the nonlinear dependence of the flux (Bq s-1) of dissolved 90Sr migrating through a catchment as a function of the water flux. A semi-empirical model was developed to give reason for the above effects. This model is based on phenomena of water saturation in different soil layers and on the nonreversible processes responsible for the non-availability to migration of radionuclides.